# Props 3 blade Vs 2 blade



## DulceSuerna (May 19, 2010)

Hello want some good advice here, prop shops all want to sell their product so I am asking sailors not prop dealers.
We currently have a 31' boat fairly heavy @12k Power is via a Westerbeke 4-91 (25hp) connected to a Hurth transmisson. We currently have a 2 blade 17" prop of no special make. The boat only sees about 4 knots @ 1500 rpm @2000 rpm not quite a knot gain. 
I am told by friends a 3 blade prop will geatly improve both speed and backing ability. 
My concern is am I trading better motoring for slower sailing due to more drag from an exta blade on the prop? I really feel the boat should be faster under power. (not that we are wanting to waterski)
Should we look at just a different pitch 2 blade? (unsure of current pitch)
The Hurth owners manual says to let the prop freespin under sail and not lock it in forward. It comments if I choose to lock it out to do so in reverse only.
I am not really in a position to afford a feathering or folding prop.
Thanks,


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## sailingdog (Mar 19, 2006)

If you got a feathering prop, like the Kiwi prop, you would have better backing, lower drag and likely more power in forward as well, though that can vary depending on the pitch, diameter, and gearing of your engine. It would help if you said what boat you had, since the hull speed is highly dependent on the waterline of the boat, and without knowing which 31' boat, it is hard to say what your boat should be able to do. A boat with a 25' waterline should be able to get 6.7 knots at hull speed...and your boat is badly under-propped by the sounds of it, since you're only making 5 knots.


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## trantor12020 (Mar 11, 2006)

Campbell Sailor Prop 3 blade should be good. Its not pricey. You have to ask Norm Ross (West by North) for correct diameter & pitch. You have to send him details of your engine/boat/2blade performance.


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## AirborneSF (Dec 14, 2010)

My 26' C&C just had a 3 blade 'sailboat' prop installed, to replace a 'powerboat' prop which a PO installed. I asked about a 2 blade,(so I could 'lock' it inline with the keel), and was told the drag of the 3 blade was not enough to worry about? My trans is in netural when the sails are up, this is all the info I can offer. It will go to hull speed with the 8hp yanmar at almost full power. I hope this helps somewhat?


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## john1066 (Feb 4, 2006)

On Adriana, Pearson Vanguard 33', I changed the two blade 13 x 15 for a three blade 13 x 13 in anticipation of a lot of windward motor-sailing going down island. This improved things somewhat, giving hull speed of 6.25k at 2400 rpm on a Yanmar 3GM30F. The two blade struggled to get hull speed at all.
You will lose some light airs sailing performance, no doubt. 
The Vanguard has the prop in an aperture and it was a tight fit to get the three blade on and off. You need to check this before buying a new prop.


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## sailingfool (Apr 17, 2000)

DulceSuerna said:


> .....My concern is am I trading better motoring for slower sailing due to more drag from an exta blade on the prop? I really feel the boat should be faster under power. (not that we are wanting to waterski)
> Should we look at just a different pitch 2 blade? (unsure of current pitch)
> The Hurth owners manual says to let the prop freespin under sail and not lock it in forward. It comments if I choose to lock it out to do so in reverse only.
> I am not really in a position to afford a feathering or folding prop.
> Thanks,


There are many threads on Sailnet about this subject. You might start with this one:
http://www.sailnet.com/forums/general-discussion-sailing-related/70216-3-blade-prop.html

First you should know and use the load RPM for your engine, that this the engine power at which you should be expecting whatever boat speed you expect. It is good for the health of the egine for it to be run a that power.

The power delivered by your prop is largely a factor of the blade area, as power demands go up for larger boats, they go to three blade props for more blade area as the diameter of bigger two blades wont fit under the hull.

Two blade and three blade propellers witht he same blade area will produce basically the same power, but the three blade costs more... Just adding a three blade without knowing how it fits your application would be a very bad idea, and wont fix any issues you have.


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## blt2ski (May 5, 2005)

You might look at ebay or equal for a used folder or equal prop if you go with a 3 blade. A 3 blaed will give you more bite if motoring in currents, and not slow you down as much. If you can find out the prop diam, some like my maxprop, the pitch can be changed, so if you found say a 14" diam 2 blade on ebay for a good price, it functions etc, but the wrong pitch, it can be change in about an hour on a flat work surface or even on the shaft itself. I am not familiar with other folding/feathering props, as I have a max prop. 

BUT, it does sound like the prop size or pitch is not correct for your application, and that would be the first thing to figure out, what prop/pitch you should have. 

I can do 6.5-6.6 knots with a 13" max prop at max rpm or about 2800 or so with my 16hp twin cyl yanmar, I am about 7000 lbs total. 2000-2500 I am in the mid5-6knot range. I do not have a tach, so I may very well be doing the same speeds as you. BUT, if you top out at 2000 rpms with you motor, you DO need a new prop to deal with this rpm to get you to hull speed.

Marty


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## Faster (Sep 13, 2005)

I'd agree that your current setup is probably not properly sized/pitched. We have had good success in the past with the 3 blade Campbell Sailor as a midpoint alternative to the costly folding/feathering option. A 3 blade prop will give you a much smoother ride, at some cost in drag, obviously, over the same 2 blade prop.

Our son's C36 with the 3 cyl 21 HP Universal does better than 6 knots at around 2200 - similar weight or heavier than your boat. A stiff headwind does slow them down, though.


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## travlin-easy (Dec 24, 2010)

My catalina 27 has a 2-blade prop, 30-HP Atomic-4 and with slack tide and a clean bottom she'll do 7.2 to 7.5. There are lots of myths about 2-blade props V/S 3-blade. There's also some common sense factors to consider as well.

If the boat has a relatively thick keel and the prop does not have a lot of clearance between the prop and keel, and prop and rudder, there is a turbulence issue to consider--particularly when backing. Under these circumstances the additional width of a 2-blade prop will provide about 20 to 30 percent more keel/rudder clearance, thereby providing more thrust against the water. A 3-blade prop, with its much smaller diameter doesn't have the needed clearance, therefore much of the prop thrust may be against the keel or rudder. 

I've always locked my prop using the transmission while sailing to prevent it from turning. I've read that this is important because to transmission is not being lubricated with circulating oil when the engine is not running. That made sense to me! Additionally, there's no noise and less chance of wrapping up a crab pot line because the prop isn't spinning. Granted, you could hook a crab pot line, but it's not going to wrap around the prop shaft if the prop isn't spinning.

Good Luck,

Gary


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## john1066 (Feb 4, 2006)

A speed of 7.5k is remarkable performance for a displacement boat with a LWL of under 24'.


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## sailingdog (Mar 19, 2006)

john1066 said:


> A speed of 7.5k is remarkable performance for a displacement boat with a LWL of under 24'.


I was thinking the same thing.


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## Boasun (Feb 10, 2007)

A three bladed prop has less vibration then a two blade and is more effective.
There are vessels that use five and six blade props and do just fine. but they are ships and not sail boats. Three blade is about the max for a sailboat. Properly sized... you may want to test a few props on your boat to see which one you want. That is better then taking someone's word for it.


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## travlin-easy (Dec 24, 2010)

john1066 said:


> A speed of 7.5k is remarkable performance for a displacement boat with a LWL of under 24'.


The boat is very fast, and I've been told that when I switch to the Morgan OI I just purchased I'm probably going to be very disappointed. The speed was measured using GPS at dead slack tide and was SOG. Of course, the bottom had just been sanded and painted and was clean as a whistle. There are very few boats that can keep up with me on the upper bay, and it's rare that I get passed by someone--even in a much larger boat.

The Catalina is currently up for sale on Craig's List Baltimore if anyone is interested.

Cheers,

Gary


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## GeorgeB (Dec 30, 2004)

You need to get with your local prop man/ repair yard pronto. A 17 inch prop diameter is wrong for your size engine and boat size. You never did mention your prop's pitch which is equally important. Are you producing white smoke? This size prop (unless it has a really, really flat pitch) is most likely lugging your engine, causing it to run at a lower RPM and utimately coke'ing up your exhaust elbow. Eventually you will burn through that elbow. This is a do it now sort of project. Unfortunately, all my prop calculations are in my home files so I can't help you with specifics.

When I was National Measurer for my class association, I did an extenstive data base study of prop blades and their effect on sailing speeds (20 years of fleet racing). I also received drag data from Michigan, Flex-o-Fold and Campbell. Our conclusion was the third blade had a tremendous effect on drag, something like 30-40 seconds a mile vs. a two bladed folding prop. The performance hit is on the order of a half knot. The impact from a two bladed prop aligned on the strut is less than half (more along the range of 10-12 seconds per mile, but unfortunately, we didn't have a large enough sample size to draw a more definitive conclusion).

A properly sized and pitched three blade prop develops more thrust than an appropriately size two blade. You will feel the impact most when motoring in heavy chop or against a prevailing wind. The other benefit of the pressure gradient against the hull as the blade passes by. The gradient drops less with a three blade. The impact being less vibration or throbbing under your feet when motoring. (I used to have to stand on a rubber mat when I was in delivering mode).


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## degreeoff (Oct 25, 2009)

I had to pipe up about the catalina 27.....they seriously hit a good hull shape there...my last boat was a C27 and it sailed faster than my c30......I have been told they were fast for their size


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## LakeSuperiorGeezer (Oct 8, 2010)

*Propeller and Speed*



DulceSuerna said:


> We currently have a 31' boat fairly heavy @12k Power is via a Westerbeke 4-91 (25hp) connected to a Hurth transmission. We currently have a 2 blade 17" prop of no special make. The boat only sees about 4 knots @ 1500 rpm @2000 rpm not quite a knot gain.


This is probably the prop that came with the boat if it has not been re-engined. The prop was matched to the transmission, engine, and boat when the boat was designed so I am thinking there is not much to gain with another propeller unless it is variable. If we know the make and size of boat, then maybe someone could give their experience with this boat and compare it to yours. If the bottom is dirty or the prop has corrosion or damage, this would slow the boat. There is almost twice the horsepower being developed at 2000 RPM than 1500 RPM so you are probably close to hull speed if you are only getting about another knot, but we do not know what the hull speed is for sure without the water line length. Also the weight of the boat with all that you have in it for cruising, determines the horsepower needed to attain a calculated speed.


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## travlin-easy (Dec 24, 2010)

One of the things I forgot to ask was when was the last time the bottom was blasted, painted, and the prop cleaned and polished? It doesn't take more than a couple dozen barnacles on the prop to really slow the boat to a crawl. And, if the boat's bottom and rudder has a fair amount of crud growing on it, it's almost like towing a couple five-gallon buckets.

Gary


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## DulceSuerna (May 19, 2010)

The boat is a 31 bombay clipper, 11' beam 3'6" draft. Bottom very clean, and just painted in April, I check it monthly. This is the original motor and has low engine hours (485) so most likely the original prop. Many other BC31 owners complain about not motoring very fast either, though many came with a smaller engine of 15 hp. The keel is very wide which may hurt the performance of a smaller diameter 3 blade prop. I am unsure of the pitch on my current prop, though it does seem very flat. As for white smoke, I get what seems like a tiny bit of steam in the exhaust but nothing billowy or alarming (though I am by no means an expert). The specs say the engine is rated for max of 2500 rpm but I do not feel comfortable running it quite that hard, The few times I have exceeded 2200 the speed never picked up. 
The spec sheet located at the link I provided shows the engine puts out maximum torque @ around 1700 rpm and good fuel economy as well. 
Maybe this is the cross I have to bear for having a heavy wide ship.
Great spec site for westerbeke 30 or 4-91 owners
http://www.westerbeke.com/productBrochures/w30.pdf


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## travlin-easy (Dec 24, 2010)

Sounds as if you are at hull speed if the speed didn't increase and the bottom and prop are clean. 

Good Luck,

Gary


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## sailingdog (Mar 19, 2006)

*According to the specs, a Bombay Clipper 31 has a LWL of 25.6' or so... which means that it should have a hull speed of 6.8 knots or so.* You should be able to get above 5.5 knots pretty easily, provided the boat was propped properly. Obviously, it isn't propped right... you need to talk to a good prop shop regarding this.

The problem is that your prop is probably cavitating, because it is so badly mis-matched to your boat. That would prevent you from getting anything close to the full speed out of your engine. 


DulceSuerna said:


> The boat is a 31 bombay clipper, 11' beam 3'6" draft. Bottom very clean, and just painted in April, I check it monthly. This is the original motor and has low engine hours (485) so most likely the original prop. Many other BC31 owners complain about not motoring very fast either, though many came with a smaller engine of 15 hp. The keel is very wide which may hurt the performance of a smaller diameter 3 blade prop. I am unsure of the pitch on my current prop, though it does seem very flat. As for white smoke, I get what seems like a tiny bit of steam in the exhaust but nothing billowy or alarming (though I am by no means an expert). The specs say the engine is rated for max of 2500 rpm but I do not feel comfortable running it quite that hard, The few times I have exceeded 2200 the speed never picked up.
> The spec sheet located at the link I provided shows the engine puts out maximum torque @ around 1700 rpm and good fuel economy as well.
> Maybe this is the cross I have to bear for having a heavy wide ship.
> Great spec site for westerbeke 30 or 4-91 owners
> http://www.westerbeke.com/productBrochures/w30.pdf


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## LakeSuperiorGeezer (Oct 8, 2010)

sailingdog said:


> *According to the specs, a Bombay Clipper 31 has a LWL of 25.6' or so... which means that it should have a hull speed of 6.8 knots or so.* You should be able to get above 5.5 knots pretty easily, provided the boat was propped properly.


Nigel Calder's Cruising Handbook starting on page 195 has calculations on propeller size. If I use a hull speed of 6.8 knots and a blade area of 55 square inches for a 17 inch two blade prop (from graph on page 201) and the formula on blade loading, we get about ten horsepower for this hull speed. If we look a the RPM vs. brake horsepower graph for the 91-4 westerberke from http://www.westerbeke.com/productBrochures/w30.pdf the propeller load at 1750 RPM gives 10 horsepower so looks like anything over this RPM is not going to give any more speed. About 1500 RPM would be an economical RPM with 7 horsepower expended and a fuel consumption of two gallons per hour. Looks like you have plenty of reserve power for bucking headwinds.


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## travlin-easy (Dec 24, 2010)

One more question: How are you measuring the boat speed? Are you using GPS, or are you using an impeller driven knot-meter? GPS speeds are accurate to within 1/100th of a MPH/KPH. The impeller devices are pretty good, but they tend to foul easily, which ultimately effects accuracy.

Good Luck,

Gary


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## LakeSuperiorGeezer (Oct 8, 2010)

travlineasy said:


> One more question: How are you measuring the boat speed? Are you using GPS, or are you using an impeller driven knot-meter? GPS speeds are accurate to within 1/100th of a MPH/KPH. The impeller devices are pretty good, but they tend to foul easily, which ultimately effects accuracy.


Good question, also if you have any tidal or river current, this will throw off the GPS as it measures speed over ground, which will equal speed over water if there is no current.

As it was pointed out earlier, if the propeller has a rough surface from collision damage, corrosion, sea growths or barnacles, this will degrade performance so need to check that propeller for a smooth (even pollished) surface


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## DulceSuerna (May 19, 2010)

A few answers but first off she averages .5 or less gph fuel consumption @ 1500 rpm, looking at the chart I did the math wrong and wound up with 2gph also then realized it was a decimal error, In reality she seems to get even better fuel economy. When we installed the tank (poly tank)we marked it so we new exactly how much fuel it had and could monitor.
We measure speed mainly with our GPS, the knot meter did not seem to be super accurrate but it raises a valid point as the last several times out we were always fighting against the tide or a headwind. I think that may be a small portion but think it could use a different prop all the same as even during a slack tide the performanceis not quite where it should be.


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## LakeSuperiorGeezer (Oct 8, 2010)

What say you want to do is to make your boat go faster by changing the propeller. To do this, you either want the engine rpm to go up to get maximum horsepower if it does not reach the rpm for maximum horsepower (propeller needs less pitch), or else the engine easily goes to its maximum governed speed and you want a greater pitch on the propeller so you get the engine running at maximum torque for the maximum rpm to get maximum horsepower to the propeller. You need a propeller with a greater pitch. This works OK with a hull that planes; however, with a displacement hull you are limited by hull speed. Your displacement to length ratio is 266 and a medium displacement is 250 to 320 so we can say yours is not a racing boat and cannot go any faster than hull speed of or 1.34 times the square root of the water line. The water line is 25.58 and the square root of this is 5.058 times 1.34 gives 6.78 knots. Now you have said you have run the boat at 2200 rpm and this does not increase the speed. You also say “The boat only sees about 4 knots @ 1500 rpm @2000 rpm not quite a knot gain.” What this is telling me is you are maybe at hull speed at 2000 rpm and certainly there at 2200 rpm. Anything greater than 2000 rpm and certainly 2200 rpm is only going to make a bigger bow wave. As a check on this, look at your bow wave and see where it comes at the end of the water line. Is that wave holding the stern up, or is it up hill at full power. My point is that you cannot make this boat go any faster by changing the propeller because of the limit imposed by the hull speed. Your engine and propeller gets the boat there or perhaps a little beyond hull speed, but with the graph of the horsepower versus boat speed going straight up beyond hull speed, it’s just a waste of diesel. All you would get by changing the propeller for optimum horsepower from the engine would be a bigger bow wave. You were saying that the beam of the boat at 11.5 feet might slow it down. Maybe so, but also I am thinking that there needs be a check on the knot meter, even if it is throwing little pieces of paper towel or tooth picks overboard, timing the distance traveled over some portion of the boat and calculating how fast you are going that way. Maybe it’s not worth the effort to check the accuracy of speed over water because you cannot do anything about it anyway. You have a powerful engine for the size of the boat and only under the most adverse conditions going into the wind with no sails are you going to come close to using that engine. The rule of thumb including adverse conditions is 3 to 4 horsepower per long ton (2240 pounds) including boat, provisions, and crew. One last thought is a two bladed propeller is more efficient than a three bladed because there is less water disturbance for the blades to bite into. If you really want another propeller, go with a variable pitch, but be aware that you will not go any faster, only perhaps get a little better fuel economy and a bit more thrust under adverse conditions.


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## sherrif (Feb 5, 2011)

Does anyone have an idea how to change a 3-blade prop on a Morgan 32? It appeas as if you have to remove the rudder--is this the case?

Wayne


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## sailingdog (Mar 19, 2006)

Unfortunately, if the prop is badly chosen, the fact that he's basically stopping at 5 knots @ 2000 may not mean a damn thing about the boat's top end speed. Far more likely is that the prop is now cavitating as it is being over-driven, and the boat can't gain any speed because the prop isn't biting. The boat's HULL SPEED is, as you calculated probably closer to 6.8 knots, and to get there, you need a prop that is of the proper diameter and pitch. The one the OP has is probably neither the right diameter nor the correct pitch.

Your mistake is that you're assuming the prop is somewhere in the ballpark of being correct, which I tend to doubt at this point.



LakeSuperiorGeezer said:


> ....Now you have said you have run the boat at 2200 rpm and this does not increase the speed. You also say "The boat only sees about 4 knots @ 1500 rpm @2000 rpm not quite a knot gain." What this is telling me is you are maybe at hull speed at 2000 rpm and certainly there at 2200 rpm. Anything greater than 2000 rpm and certainly 2200 rpm is only going to make a bigger bow wave. As a check on this, look at your bow wave and see where it comes at the end of the water line. Is that wave holding the stern up, or is it up hill at full power. My point is that you cannot make this boat go any faster by changing the propeller because of the limit imposed by the hull speed. Your engine and propeller gets the boat there or perhaps a little beyond hull speed, but with the graph of the horsepower versus boat speed going straight up beyond hull speed, it's just a waste of diesel.


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## LakeSuperiorGeezer (Oct 8, 2010)

What happens with cavitation is that water will boil at room temperature if the pressure is reduced enough. If the curvature of the propeller blade is high and the speed of the propeller tips are at a high speed, water will separate from the propeller to form vapor. This problem is more likely to occur if air bubbles are introduced into the water, as the propeller will speed up because there is less bite with the air bubbles. The water is actually boiling even though the water is cold. When the water moves to a part of the propeller where the curvature is less, the vapor collapses and reforms into the water that was vaporized. Water is heavy and this collapse onto the propeller surface causes the erosion from the impact, which is powerful and can be very fast with its destructive erosion. If the propeller does not show any erosion then I doubt there is enough cavitation to cause a performance problem. Also, the boat owner does not run the engine at maximum power for a high propeller speed so I think cavitation is even more unlikely. And further, if this is the original propeller, a marine architect would match the propeller to the engine and these folks are very aware of cavitation problems. If this is a typical installation for this engine and propeller, there is a 2:1 gearbox reduction in speed, and the pitch is 11 inches, not a lot of curvature.


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## sailingdog (Mar 19, 2006)

LSG-

I'm aware of all this, but you're the one making the assumption the prop is properly spec'd for the boat, which may not be the case. There are a lot of boats out there with badly mis-matched props on them. We don't know the history of the boat or when the prop was last changed out, and by whom.



LakeSuperiorGeezer said:


> What happens with cavitation is that water will boil at room temperature if the pressure is reduced enough. If the curvature of the propeller blade is high and the speed of the propeller tips are at a high speed, water will separate from the propeller to form vapor. This problem is more likely to occur if air bubbles are introduced into the water, as the propeller will speed up because there is less bite with the air bubbles. The water is actually boiling even though the water is cold. When the water moves to a part of the propeller where the curvature is less, the vapor collapses and reforms into the water that was vaporized. Water is heavy and this collapse onto the propeller surface causes the erosion from the impact, which is powerful and can be very fast with its destructive erosion. If the propeller does not show any erosion then I doubt there is enough cavitation to cause a performance problem. Also, the boat owner does not run the engine at maximum power for a high propeller speed so I think cavitation is even more unlikely. And further, if this is the original propeller, a marine architect would match the propeller to the engine and these folks are very aware of cavitation problems. If this is a typical installation for this engine and propeller, there is a 2:1 gearbox reduction in speed, and the pitch is 11 inches, not a lot of curvature.


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## asdf38 (Jul 7, 2010)

sailingdog said:


> *According to the specs, a Bombay Clipper 31 has a LWL of 25.6' or so... which means that it should have a hull speed of 6.8 knots or so.* You should be able to get above 5.5 knots pretty easily, provided the boat was propped properly. Obviously, it isn't propped right... you need to talk to a good prop shop regarding this.
> 
> The problem is that your prop is probably cavitating, because it is so badly mis-matched to your boat. That would prevent you from getting anything close to the full speed out of your engine.


Sailingdog, not to hyjack this thread, but what is a good prop shop in the greater Boston area? I'm in the market for a prop right now and am trying to figure out where to look.


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## Maine Sail (Jan 6, 2003)

asdf38 said:


> Sailingdog, not to hyjack this thread, but what is a good prop shop in the greater Boston area? I'm in the market for a prop right now and am trying to figure out where to look.


What type of prop? For prop work it is very, very tough to beat *Accutech Marine*. They also sell props. I have not seen many shops that can balance a prop like Accutech or who understand props and prop issues better.


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## Waltthesalt (Sep 22, 2009)

The easiest way to do this is go to the Michigan Wheel website and fill in the info for them to do a prop calculation. It’s free. 

The ultimate reference for doing you own calculations is Dan Spurr’s “Propeller Handbook”. It’s a bit technical but may be at your local library.. 

Basically your boat has a hull speed that you should reach when your propeller diameter and pitch is correct and your engine is fully loaded at max RPM. Diesels want to be run at about 85+% power. One of the biggest problems with sailboat diesels is they aren’t run at full power enough. It looks you’re your prop may not be matched well. Let’s just say your boat displaces 12,000 # loaded and the waterline is 26ft.. This gives you a Lb/HP of 480#HP. Your speed/length ratio is 1.25 so “hull speed” is 1.2x√26 or 1.2x5 or 7 kts. So at 4 kts your propeller is off or your engine is not up to power, or you need to scrap the barnacles.

So what about 2 blades or three? Two blades are more efficient in driving the boat under power and have less drag under sail. The reason to go to three blades is usually because you don’t have the room to fit the correct size 2 bladed propeller. You need a minimum amount of blade area to keep your prop from cavitation. You also need a minimum clearance between your prop tip and the hull. That’s about 15% of the prop diameter. When a two bladed prop with enough blade area doesn’t fit the space that you have you go to a three blade prop. More blades more area. 

Three blades are also a bit smoother. But that’s mostly when their two blade counterparts don’t have enough hull clearance. 

In 1995 some grad students evaluated sailboat props in a test tank at MIT. Their paper has a lot of detail on how different props compare. They showed about a 50% drop in drag by freewheeling a non feathering prop instead of leaving it engaged to the engine. Reverse power with a 3 blade also looked better. But bear in mind this stopping power is hrust while the boat is still going ahead.

Easy answer, ask Michigan Wheel to make a calculation


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## asdf38 (Jul 7, 2010)

Thanks Maine Sail and Waltthesalt. 

I guess I'm struggling with 3 things. First, how bad is my existing prop. There were a lot of barnacles on it when I pulled the boat out, however I don't recall there being a difference between early in the season and late in the season (I first test sailed the boat in early July and bought it in late July), the boat was always slow. I emailed the previous owner to see if he remembered cleaning off the prop over the previous winter for the off-chance that there were multiple years of growth that I've now cleaned off. Also, the prop seems as if it's mounted too close to the hull - there is very little space between the prop and the back of the keel and the keel is quite thick. Perhaps a better installation of the existing prop combined with the barnacle removal would mostly solve my problem.

The 2nd question is that assuming I want a new prop, what is the correct size. This one is strait forward to solve as there are plenty of resources online.

The 3rd question relates to my lack of experience in the marine world and is a reoccurring problem. If I decide to replace the prop and don't want to try it myself (I'm pretty sure about this one), how do I find a professional to do it? I'm not even sure what exactly to look up in the yellow pages. What's the best way to go about this? Of course I chat with people when I see them at marinas and so forth but in half a season I haven't build up many resources here. Thanks.


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## ByAirAndBySea (Apr 30, 2009)

sailingdog said:


> I was thinking the same thing.


If I'm not mistaken... if it is a true displacement boat.. that's actually impossible to go that fast for that size boat no matter how slick or how much power. No doubt the gps is giving boat + current speed.


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## SlowButSteady (Feb 17, 2010)

ByAirAndBySea said:


> If I'm not mistaken... if it is a true displacement boat.. that's actually impossible to go that fast for that size boat no matter how slick or how much power. No doubt the gps is giving boat + current speed.


Not impossible, just more difficult. There is nothing "magic" about theoretical hull speed. The commonly used formula (1.34 times the square-root of the waterline) is the speed at which the bow wave/wake is exactly the waterline length. Getting a boat to go faster than this means that the boat starts to "climb" its bow wave, or pitch up a bit. Since this takes a bit of energy, more power is required to move the boat. Think of how a planing motorboat behaves before it gets on to a plane: first it acts like a regular displacement hull, essentially sitting level as it moves forward; as the speed increases, it starts to pitch up (this is where it is a bit above hull speed and is "climbing" its bow wave); then, as it gets going faster it seems to "jump forward" and up onto a plane. A Catalina 27 with an A4 (nominally 30hp) has about three times as much power available as it needs to travel at hull speed with a clean bottom. So, if the engine was putting out about 1/3 to 1/2 of its rated hp (or about 10 to 15 hp of 30 hp), the boat would go at just about hull speed (about 6.3 kts for a Cat 27). The rest of the power output of the engine (15 to 20 hp) is needed to get it past hull speed and up to 7.5 kts.

My Cal 2-27 has about the same LWL as a Catalina 27, and it will go just a bit faster than hull speed with her Yanmar 2gm20 wide open (or at about 16 hp of engine output). If I had twice the power available, she could undoubtedly go another knot or so. Neither boat is particularly fast, specially my the standards of modern sport boats. But, similar to what used to be said about the F-4 Phantom ("Proof that with enough horsepower, even a brick can fly."), with a big enough engine a boat's displacement hull speed can be overcome, a bit. No magic, just fluid dynamics.


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## ByAirAndBySea (Apr 30, 2009)

Use 1.4 rather than 1.34 and you'll get closer to your real hull speed. as for climbing the wave....physics won't allow a displacement vessel to leave the wave trough. The reason motorboats can is because they ride on top of the water more or less. Unless u r a centerboard boat u will not be riding on the water. (Most centerboards at least)


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## SlowButSteady (Feb 17, 2010)

ByAirAndBySea said:


> Use 1.4 rather than 1.34 and you'll get closer to your real hull speed. as for climbing the wave....physics won't allow a displacement vessel to leave the wave trough. The reason motorboats can is because they ride on top of the water more or less. Unless u r a centerboard boat u will not be riding on the water. (Most centerboards at least)


The 1.34 "factor" is actually derived from a combination of a number of factors, including the density, viscosity, and surface tension of the water (which all vary with temperature and salinity), wavelength (_lambda_), and gravity (g). As it turns out, if we set everything but _lambda_ to reasonable constants (and discount surface tension, which is too small a factor to worry about at the relevant scale), the whole thing boils down to the common 1.34 times SQRT_lambda_. However, it will vary a bit with temperature and salinity.

Some folks, apparently you included, often add a bit to that 1.34 factor. This isn't terribly unreasonable, as the LWL is measured with the boat in a static condition and the dynamic waterline will increase a bit as the boat speeds up. However, there is a dramatic increase in the power required to propel a boat as it gets into the 1.34 to 1.5 times SQRT_lambda_ range. There is no actual inflection point in the curve, so we can debate as to exactly which factor is most appropriate. However, a displacement boat (any displacement boat) can "overcome" its hull speed with a large enough engine. This isn't to say that a Catalina 27 with an A4 running at full throttle will plane, but one look at the bow wave will demonstrate that the NEXT crest of that wave will be beyond the stern of the boat (i.e., the wave generated has a longer _lambda_ than the LWL of the boat). In this condition, the hull will be pitched slightly up, since the stern will be sitting in the trough of the bow wave and the bow will be perched (or "climbing up") that wave.

Note how the boat in the following pic is traveling a bit above hull speed (the stern is forward of the following crest), AND the hull appears to be pitched up slightly:


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## LakeSuperiorGeezer (Oct 8, 2010)

Since we last addressed the propeller topic, I have purchased the Propeller Handbook by Dave Gerr. Crouch’s Propeller Method works well for sailboats, as there is no need to get every last bit of efficiency as one would with a tugboat for instance. The Bp-δ method is exact. The Bp-δ method also allows calculation for cavitation. Let me look at the book and do a few preliminary calculations. I will eventually need a speed and RPM to calculate slip by your making a run in two directions and taking the average GPS speed reading. Do you have a clean propeller? If not I can estimate. Do the run with little wind and tidal influence. Tide is less important than wind. I could use the 4 knots @ 1500 rpm, but speed at two places would be nice, like 4.1 knots or something like that for instance at 1500 rpm.

If the boat is out of the water so that you cannot do a speed run, then I need the distance from waterline to bottom of boat, not including the keel, how far the boat sinks into the water and also the beam at the waterline. When you clean the propeller, try to get the number off the hub face, which is where the propeller shaft enters the propeller. The number is sometimes stamped on the hub where the blades attach to it. The number will start with the diameter, 17, then an "X" followed by another number. That other number is the pitch. You may have to remove the sacrificial zinc if it is bolted to the propeller shaft just behind the skeg to see the number, but you will need to replace the zinc anyway. The zinc could be attached to the propeller where the nut is that holds the propeller on the propeller shaft. Propeller sizing on this boat could best be done with the Bp-δ method because she is so wide, the assumptions needed to use Crouch’s Propeller Method could throw it off.

You will need a puller suitable for a two or three blade propeller. Most pullers are for three blades. Pullers are easy to use, just attach to the propeller and turn the bolt to pull the propeller. Sometimes you have to tighten quite a bit and the propeller will come off with a good pop sound. Loosen the nut holding the propeller on, but leave on loose to prevent the prop from falling off the propeller shaft onto the ground. If you have this done do not let them use a slide hammer to remove the propeller! Use a puller. A slide hammer can destroy the transmission with immediate failure of the case, or damage internally that causes failure at sea such as a bearing. Also while you are at it, you should check the cutless bearing. You could even take the flange loose at the transmission and pull the propeller shaft out some to check for crevice corrosion and pitting if you have a stainless steel propeller shaft.

The minimum distance from the skeg to the 17 inch propeller is 30% of the diameter or slightly more than five inches. For tip clearance to hull, a 15 percent value is suitable for sailboats because of the slower turning propellers. Take 15 percent of 17 inches and you get two and a half inches. By the way, no matter what the propeller size, the clearance must be at least 2 inches. For a propeller in an aperture, the value is 12 percent, but I believe the Bombay Clipper does not have an aperture. The distance from the propeller to the rudder must be at least fifteen percent of the diameter, which is again two and a half inches.


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## fallard (Nov 30, 2009)

If you are really concerned about boat speed under sail and reverse thrust is important, you might consider a feathering prop, like a MaxProp, that has no blade "cup". It will be less efficient motoring forward, but the reverse thrust is hugely more than with a fixed prop. 

A MaxProp has another advantage: you can "reprop" by changing the pitch, -- you just don't want to do it with the boat in the water. The downside is that they are expensive.


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## Waltthesalt (Sep 22, 2009)

Question 1,2
Do I need to replace the propeller? Sailboat propellers don’t wear out like on high speed outboards. Sailboat props only need to be replaced if damaged and vibrate. Even then a prop shop can usually straighten it out. Some people change from a fixed prop to a folding or feathering to get better speed under sail. But under power fixed props perform as well or better than feathering of folding ones. 

If the diameter and pitch of the prop is correct for your boat and engine you don’t need to replace the propeller. You can find this out by checking what’s stamped on the prop hub when you pull the boat for painting. Check this against calculations done for your boat/motor. If your boat is a class-type boat say an Erickson 31, you can ask that boat owners’ association or Sailnet what prop pitch and size they use. Prop shops can measure pitch from the prop and may be able to make some adjustment to solve the problem . If your engine is mechanically OK but can’t get up to rpm the prop is overloading your engine and you have too much pitch. A prop shop may be able to reduce the pitch. If the engine gets up to RPM but isn’t pushing you to speed which I think is your problem The engine is under loaded and maybe you have too little pitch and they may be able to increase the pitch.

Note on a gas engine a too little prop causes the engine to overspeed as the throttle directly controls the gas to the engine. Diesels are different as the throttle controls a governor which controls the flow of fuel to the engine. So at full throttle on a diesel you just go up to full rpm regardless of how hard the engine is pushing.

Cavitations is something else. It’s when the prop loses its bite on the water. Like spinning your car wheels. A heavily barnacled prop can case this. There are antifoling spray-can paints for propellers. There’s a recent Sailnet tread on this. Also a prop that’s too small can cause cavitation… not enough blade area. As mentioned before a larger diameter and/or more blades give your more area. 

Having the prop too close to the keel as you mention can also reduce it’s effectiveness. Usually the designer has this figured out and unless the shaft has been shortened or rearranged it should be OK. Anyway the thumb rule is that the the distance between the prop and keel or skeg should be at least 30% of the propeller at the middle of the propeller blade. So about 6 in for you. I’m not talking about a strut which is pretty streamlined we’re talking about a solid keel in front of the prop. Another point is that the aft edge of that keel should be fared and not blunt or square edged.

Questions #3
Ask around the marina or boatyard many boat mechanics don’t have the best of reputations. People will tell you if they’ve been burned.


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## LakeSuperiorGeezer (Oct 8, 2010)

Waltthesalt said:


> Question 1,2
> Do I need to replace the propeller? Sailboat propellers don't wear out like on high speed outboards. Sailboat props only need to be replaced if damaged and vibrate. Even then a prop shop can usually straighten it out. Some people change from a fixed prop to a folding or feathering to get better speed under sail. But under power fixed props perform as well or better than feathering of folding ones.
> 
> If the diameter and pitch of the prop is correct for your boat and engine you don't need to replace the propeller. You can find this out by checking what's stamped on the prop hub when you pull the boat for painting. Check this against calculations done for your boat/motor. If your boat is a class-type boat say an Erickson 31, you can ask that boat owners' association or Sailnet what prop pitch and size they use. Prop shops can measure pitch from the prop and may be able to make some adjustment to solve the problem . If your engine is mechanically OK but can't get up to rpm the prop is overloading your engine and you have too much pitch. A prop shop may be able to reduce the pitch. If the engine gets up to RPM but isn't pushing you to speed which I think is your problem The engine is under loaded and maybe you have too little pitch and they may be able to increase the pitch.
> ...


You make very good points here. The boat owner did say he thought this was the original engine and propeller. The marine architect is going to match engine to propeller as best he can and maybe even make some modifications to the original specifications after a trial run. I had not thought about cavitation and barnacles, but when you point it out it makes sense. There's going to be a lot of turbulence with changes in velocity around those barnacles and also a lot of energy not converted to thrust. Velocity with abrupt changes in flow direction is what causes cavitation. The boat owner is complaining about low speed. As you said, send the propeller to a good propeller shop and let them check it with one of their computers. Also, even a little damage to the leading edge of the propeller blade can really influence performance. From what I have read, a lot of manufacturers are not that careful about getting propellers just right. In Europe they are very careful about manufacturing unlike in the USA. Put the original propeller back on the boat and see what the improvement is, do a few test runs like you suggested. Could also on a calm day run directly true north and south for a nautical mile using GPS to get very accurate speed. With several RPM settings included with the speed, a lot can be figured out about what is going on with the propeller.

It's converting from say an Atomic 4 to diesel that a careful look at is really justified. I ran some numbers on an Alberg 30 on sailnet and found that only 5 horsepower was converted to thrust at full RPM. I think they put a speed boat propeller on it.


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## Moehorenfeldt (Feb 5, 2002)

*2 v.s 3 blade props*

I have had Cruising Design Performance Props on my Hughes 29 (S &S design) powered by Atomic 4, and now on my Cherubini designed 1981 Hunter 33, powered by a Yanmar 2GM. 3 Blade props are supposed to give you better efficiency and a smoother curve than 2 blade, but, I'll tell you the difference in performance with the CD prop is incredible. There is some "prop-walk" in reverse, but I use that to help me turn around in tight corners without touching the helm. Go to the CD website and check out their recommendation for the prop for your setup. Yanmar suggests locking the engine in reverse when under sail to keep the prop from spinning. Less wear on the cutless bearing, and stuffing box, you see. Also, the literature seems to indicate that a spinning prop creates more "drag" than one that is fixed. My $.02 anyway:hothead


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## Faster (Sep 13, 2005)

Moehorenfeldt said:


> .... Yanmar suggests locking the engine in reverse when under sail to keep the prop from spinning. Less wear on the cutless bearing, and stuffing box, you see. Also, the literature seems to indicate that a spinning prop creates more "drag" than one that is fixed. My $.02 anyway:hothead


Actually, Yanmar released a specific press release in 2008 directing owners not to lock the prop in reverse.



> *.pdf File (LINK)**
> 
> Advisory Number: MSA08-003:
> 
> ...


And also the spinning vs locked 'drag' debate is hotly contested, with Mainesail's personal research posted here taking the opposite view.
Take a look:

http://www.sailnet.com/forums/gear-maintenance/53628-fixed-vs-free-wheeling-prop-test-data.html


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## nauticalrich (Aug 31, 2001)

I switched to a 3 blade cambell prop on my Oday 31.
I would never go back. The boat runs smoother under power, almost no vibration as before, it's a little faster and I notice no difference under sail.

I miss the propwalk in reverse as I used it to my advantage in docking


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## wolfenzee (Jul 13, 2008)

I have a Vetus m4.14, a 4 cylinder 1415cc ( Mitsubishi K4E )
rated at 33hp max rpm 3000rpm, max torque 78Nm (57.5ft/lb) at 2100rpm
fuel consumption is listed at 260 g/Kw-h (gph?) at 2000rpm.
My boat is 30' w/25' lwl 15,000lbs(hull speed 6.75kt)
1000rpm=4kt, 2000rpm=6.5kt, 2200rpm=7kt (stern squatted down)
I have a 16"x9"" three bladed prop, because I have room for such a big prop could I replace it with a 2 bladed prop with more pitch and achieve the same thing...I could line up the prop with the keel while under sail and cut down drag).


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## geronimo69 (Jul 31, 2013)

travlineasy said:


> I've always locked my prop using the transmission while sailing to prevent it from turning. I've read that this is important because to transmission is not being lubricated with circulating oil when the engine is not running. That made sense to me! Additionally, there's no noise and less chance of wrapping up a crab pot line because the prop isn't spinning. Granted, you could hook a crab pot line, but it's not going to wrap around the prop shaft if the prop isn't spinning.


Hi Gary, I too have a C27 and need to replace the prop. I'm going to go back to a 2 blade prop but also wanted to ask, how do you lock the prop during sailing. Thanks, Jim


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## Pendragon35 (Jun 26, 2014)

Yanmar engines are not meant to be in gear when sailing. I have a 3gn30, the manual says trans in neutral for sailing engine off.


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## travlin-easy (Dec 24, 2010)

Jim, I always locked mine in forward, mainly because of the A4 transmission, which only locks in forward and not in reverse. I really don't think it makes much of a difference as long as the prop is not constantly spinning while the engine is not running. 

All the best,

Gary


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## CS Cruiser (Dec 5, 2011)

Pitch and diameter are in the fore front, but often blade area is over looked, and this can have dramatic effect in many scenarios. Deal with a respected prop shop to get the right advice for your setup. As far as what came with the boat even if original - don't consider for a moment it is optimum.


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## THEFRENCHA (Jan 26, 2003)

A good prop shop around Boston
H&S propellers in Salem Ma 
Very helpful and will work closely with you 

I changed from 2 blades to 3 
Not much difference in term of speed but real improvement on moving backward as I Need to zigzag between slips and go all the way through my marina and hate docking bow first


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## travlin-easy (Dec 24, 2010)

I believe that something is being overlooked in this discussion, and I also believe that manufacturers overlooked the same scenario. When a prop is positioned very close to a thick keel, such as on my Morgan 33 OI, this caused a problem with thrust in both directions, particularly reverse. The closer the prop is to the keel, the greater the problem caused by flow restriction around the keel. Take a look at this photo and you will see what I mean.










Anyone that has a heavy, full-keeled boat and has tried to back it up knows exactly what I'm talking about. When I had the Catalina, the fin keel was pretty far away from the prop, which essentially was not impeded by anything structural on the boat. When you backed the Catalina down to back into a slip, as soon as the boat began to move you had great steerage and control, regardless if the prop was two or three bladed. It was almost as if you were using an outboard motor to back the boat into the slip.

With the Morgan, as you can see by the photo above, while moving forward, water is drawn around the keel and thrust toward the rudder, which has some impediment, but not a lot. In reverse, that three bladed prop is thrusting directly against the flattened, back end of the keel, thus forcing water outward and negating much of the thrust toward the bow of the boat. Steerage is downright awful, even when you finally get the boat moving backward and you can flip a coin to whether the propwalk will be port or starboard, the boat just kinda makes up it's own mind. If that three bladed prop were a much wider two bladed prop, more of the thrust would be directed around both the keel and rudder.

I sincerely believe that manufacturers really didn't consider the motoring aspects of sailing when they designed most sailboats - just the sailing aspects. If I were designing my Morgan, I would have shaped both the thrust edges of the rudder and keel quite differently, with a much sharper edge that would provide better flow characteristics. I am still having serious thoughts about shaving off much of the back end of the boats keel and front edge of the rudder, but I'm just too damned old to undertake projects like this anymore. If I can find a suitable two blade prop for my Morgan 33 OI, and at a reasonable price, I definitely will give it a try and report the results.

All the best,

Gary


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## capt jgwinks (Sep 24, 2013)

What RPM can you reach with your current set up? If you can reach engine red line, and not reach hull speed, you are under propped. This is like running a five speed car on fourth gear all the time. Wastes fuel, limits speed, and with a diesel, is hard on the engine as it's not loaded heavily enough. A two blade with higher pitch, or a three blade of similar or slightly more pitch can help. If moneys tight, the cheapest fix is to have your existing prop repitched, add maybe two inches. Props should be tuned and balanced periodically anyway, adding pitch at the same time is a minimal cost. I did this on my last boat, a Columbia 8.7 (((29'), had the same problem as yours. Ran 5 knots at 3400 red line. Went from 9 to 11 inches of pitch, the boat would then hit 6.5 knots at 3000. Cruised 6 knots at 2800. Perfect.


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## smurphny (Feb 20, 2009)

I've considered changing my 2 blade for a three blade but one thing I wonder about is whether a three blade is more likely to spin when under sail. A two bladed prop will tend to naturally situate itself vertically behind the keel where it is at least partially blocked from water by the aft face of the keel (on a deep keel boat) whereas a three bladed prop will always have one blade exposed. Is this more likely to cause the annoying neutral prop spinning? This is related to Gary's post above.


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## capt jgwinks (Sep 24, 2013)

Probably is more prone to free wheeling. On both my last two boats, the pros were right at the back of the keel, with only an inch or two of exposed shaft. Perfect test bed for this. Previous boat had a two blade, it did not spin on its own, current boat had a large power boat type three blade, and it does spin. Anecdotal evidence, but it is logical.


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