# 2 blade or 3 blade propeller



## ddilman (Oct 26, 2004)

I need help deciding between a 2 blade or 3 blade propeller
for a Catalina 36 with a wing keel. Planning to do little
or no racing and I''m non experienced in docking.
How practical is the 3 blade when docking? And is it worth
the extra drag when sailing? 
Thank You 
Danny


----------



## Mysun (Nov 4, 2004)

I have a three-blade on my Hunter 335 and can really notice the performance difference. More prop-walk with the three during docking. I don&#8217;t really notice any negative sailing difference. Let''s face it, if I was in a hurry I''d by a motor boat.

Ian


----------



## Jeff_H (Feb 26, 2000)

When you say that you don''t notice the difference, have you sailed a 335 that had a two blade prop or sailed your boat with its three blade propeller near a boat with a two blade prop. The difference between using a two blade vs a three blade shows up primarily in low windspeeds when it might make the difference in whether you can sail or not and at high windspeeds when it can result in greater heeling and more weather helm, and of course less controllable speeds. 

Jeff


----------



## Mysun (Nov 4, 2004)

Jeff,

We had a choice between a two blade and the three blade when we launched the boat for the first time after purchase and decided on the three blade right off the bat. We have not done comparisons between performance of the two vs. three directly on our boat. Compared to other boats we sail with we have absolutely no trouble holding our own. Keeping it in perspective, I''m an ex dinghy racer turned cruiser so - is my sailing ability overcoming the slight drag difference - I don''t know but it might? Most sailors we go out with have more of a set-it and forget it mentality than I do. I prefer to tweak sail trim more than most. Our marina is tight and our slip is a little awkward to get to. I really notice the performance of the prop when docking in heavier winds. The ability to &#8220;hit the breaks&#8221; when things get a little out of control is a real confidence booster. The weather helm issue you mention is one I have not experienced. The 335 is a fractional rig and it just simply works better with a reef in the main. I&#8217;m usually one of the first to reef and my wife prefers it if I keep the boat flat. (Like a dinghy) I hope this helps even if it&#8217;s not the science you were looking for.


----------



## Jeff_H (Feb 26, 2000)

I think that your post does help a lot. As an ex-dinghy racer I would think that you would be a bit more sensitive if the drag was noticably hurting your performance. With that in mind, the fact that from your perspective the three blade does not appear to noticably effect performance suggests that the impact of the three blade prop on the Hunter 335 is an obvious issue. 

I also found your observation on reefing interesting. I find that I generally have to reef less with a fractional rig (at least on frac''s with backstay adjusters and decent deck hardware) but that when there comes a time when reefing is necessary, the reef produces a boat that has very nice handling characteristics, pretty much like a cutter sailing under mainsail and staysail only. 

Thanks for your comments.
Jeff


----------



## sailingfool (Apr 17, 2000)

I still don''t get what the coversation of 2 versus 3 is supposed to be about...

My understanding is that thrust is a factor of blade area, and the proper blade area is determined by engine HP and boat displacement. You can get your blade area two ways - a larger prop with only two blades, or a smaller prop with three blades, but both with the same blade area, or your engine will not run at cruising RPM. Regardless, the performance or the props will be basically the same...you only NEED a three blade when the proper sized two blade would not have adequate clearance around it. If you buy a three blade when a two blade would fit, you have simply wasted some money to no benefit...
Which is a common enuff syndrome in sailing...


----------



## Jeff_H (Feb 26, 2000)

It is not quite that simple. You can get greater blade area in a number of ways. On a two blade prop you can increase the width of the blades without increasing diameter (often called a ''power prop''). This adds some power but is relatively inefficient since it does not increase the leading edge of the prop but does increase the tip drag. In order to get the full benefit of increased blade area out of a two blade prop, the two blade propellor needs to have a larger diameter. Because of the longer lever arm of the larger diameter prop it requires greater torque to turn a two blade prop of equal area to a three blade at the same RPM. Since the determinant of horsepower in a rotating shaft is rpm x torque (times a constant I believe, but that I can''t recall) the greater torque will require more horsepower for a given thrust. That is the theory. The reality can be a little different because the greater reciprocating mass and lower efficiency of a three blade can offset this increase in HP a little.

Respectfully,
Jeff


----------



## TSOJOURNER (Dec 16, 1999)

I am swinging a 17"x12" 2 blade fixed, which I need to replace. It produces significant drag and prop walk (which can be used on occasion but I would rather have less). 

I am looking into getting a new Campbell Sailor Prop, 3 blade fixed, mostly because of cost ($485) and this will be for cruising. This will reportedly give less drag, less prop walk, smoother action and more bite in forward and reverse. 

The boat is 38ft LOA, 31ft LWL, 22,000lbs. Perkins 4-108 engine, Hurth V tranny doing 2.13:1. 

Campbell is recommending a 16"x9" 3 blade fixed.

Does that sound in the ballpark?

My best to all.

John 
s/v Invictus


----------



## jack_patricia (May 20, 2001)

John, I apologize for backing you up a bit in your considerations but I notice you are going to reprop and then use your boat (a Hood 38, right?) for cruising.

Where are you thinking of cruising? And to what extent will you be wanting to use marinas, refuel and otherwise maneuver in close confines? And finally, do you have a bow thruster?

I mention all this because I find, more and more, that prop walk is far more often our ally than our opponent when maneuvering, and that this growing awareness has resulted from the cruising areas we''ve frequented (less so in the Caribbean, moreso in some parts of the NE USA and Europe). And our boat is not unlike your own (42 x 33 x 21,000#). WHOOSH does have a bow thruster - it''s my wife, with a boat hook - but even so, we pretty much have to rely on what we can make the boat do via the engine, and by being cagey about wind direction. INVICTUS is also a fair amount of boat for the engine she carries and I believe, like us, lacks the deep fin around which a boat might otherwise pivot a bit more sharply.

I''m not sure any of this argues against a Campbell prop. Guess I just wanted to stir some add''l reflection of the benefits of prop walk given the limitations we otherwise probably share. <g>

Jack


----------



## Jeff_H (Feb 26, 2000)

A couple minor points, a three blade prop of equal thrust generally will have significantly greater drag than the two blade under sail. In most regions PHRF gives six seconds a mile for a three blade fixed prop vs a two blade fixed and I have talked to owners who convert back to two blades for racing because they claim that they can''t sail to their rating with thier three blade props. Obviously it depends on the boat and the props.

I am also surprised at the big change in pitch. Since you are not changing the transmission, I would expect that the pitch should be roughly the same for both propellors since pitch is theoretically the amount that the boat would advance with one turn of the propellor assuming zero slipage. The slippage should be fairly similar between the two props in flat water and so I would expect the pitch to be much closer. 

Jeff


----------



## TSOJOURNER (Dec 16, 1999)

Hi Jack and Jeff, 

It is great to chat with both of you again. I appreciate your points, all good ones.

I too am surprised at the Campbell pitch and diam recommendation and so was wondering if anyone had made this specific change and had comparable numbers. Apparently though, Campbell says that they have a lot of experience with the engine, displacement and tranny in this range and likes these numbers best. Still, I would like to hear from anyone who has actually made this change.

I do think the campbell prop will have much less drag than a similar three blade and more than likely less drag than the existing 2 blade prop (it has broad blade surfaces). I am not that concerned with drag, it is a fact of life. I cannot afford and otherwise do not wish to have a folding or feathering prop. I want a fixed prop as it is simpler, cheaper and more reliable. 

Jack, completely agree with your points about using prop walk (and do not have a bow thruster). I do use it and there are situations in which it is great. But...invariably, there are more situations in which I would like to have much less. If need be, we could use a spring line to accomplish much the same as the walk. 

My cruising plans have changed a little bit due to family reasons. I have to stay closer to FLA and the east coast (so that I could hop a plane to FLA to take care of my folks if the need arises). So, I will be in FLA and the islands. Probably much of the caribbean and cruise up and down the east coast. Perhaps the Med later, that was my goal, but it will have to wait. Marina stays are likely and so backing into or out of a slip will be nicer with less walk. 

The bottom line really is that my existing prop was dinged by the previous owner and is probably out of tune as well as chipped. It has been ok, but I have budgeted a change at this point. Since a Campbell or MW sailor prop is so cheap, why not go that route. And 2 vs 3....all the experts I talk with suggest 3 blades as they run smoother so less stress on the trans. These new "sailor" props should have less drag than my existing prop. I do not have any speed or performance issues with the boat so I will be satisfied. I know that a feathering prop would likely be better, but it is just not in the budget and as stated, I feel a fixed is more robust and reliable. But really, cost is the greater driver.

Appreciate your thoughts always.

Your friend

John
s/v Invictus


----------



## jack_patricia (May 20, 2001)

John, I''d definitely recommend the 3-blade prop. Given your near-term but also long-term cruising plans, I suspect you''ll be happier with it despite its add''l drag.

Have you worked Campbell''s pitch & diameter recs against either of Dave Gerr''s books? (Prop Book and also the Nature of Boats). I think a lot of the latter and have heard the former recommended by savvy folks (Calder is one), and while neither book will produce an iron-clad conclusion for you, it might prove useful to have an ''independent, theoretical but expert'' view to compare against everything else you hear. (The Nature of Boats will also keep you out of the bars at night; it''s a very thoughtful, helpful read, if you haven''t already consumed it).

Understand completely about the ''better choice'' being the one you can afford. After struggling with the Euro and the Pound for the last two years, we understand. At least your near-term cruising will be done in the U.S. Dollar''s neighborhood.

Jack


----------



## TSOJOURNER (Dec 16, 1999)

Thanks Jack. I will have to look up Gerr''s books. I like his writing and if Nigel recommended them....all the more. Nigel is a great guy. He really likes Hood 38''s too, so...that colors my opinion.

I am trying to ''ground truth'' the prop specs recommendation from Campbell by using some net resources (though would really like real world examples still). I have written Michigan wheel to get their recommendation...they have a similar "sailor" prop. A 3 blade with thinner blades for less drag. They said about 10 days to get back to me. Whew. 

If your plans include more Med sailing this coming summer, check out La Maddelena. One of my favorite places and the one I''d like to get back to most.

Best

John
s/v Invictus
Hood 38


----------



## TSOJOURNER (Dec 16, 1999)

OK, just got an email from Mich Wheel. Sent them all my specs and asked for a quote. They recommend a 3 blade fixed....17x11. 

Interesting, Campbell also said 17x11 at first blush but then changed that to 16x9 when I sent them my trans specs later on. 

Hmmmm......Jeff????? Jack??? Anyone?

I am going to write campbell back and ask them to recheck. I can see 16", not a big deal. But going from 12" pitch to 9"? 

J


----------



## TSOJOURNER (Dec 16, 1999)

I looked into this same switch. A local prop shop shop said to go with the same dia w/ 2" less pitch. Think about it, 150% of the thrust area of a 2 blade, reducing the pitch takes the strain off the engine at the same rpm. The big difference that was explanied to me was with a 3 blade, two blades were always thrusting in less distrubed water from the keel, with a 2 blade there is only one.


----------



## Capesailor (Oct 11, 2012)

Hello,

I'm a new member. I'm buying a new Marshall Sanderling and am deciding on a two blade vs three blade prop for the 10 hp Nanni 2.10 diesel. Is there more drag with the 3 blade? Is the three blade quieter with less vibration? I am interested in getting max thrust in a tidal outlet to the Atlantic. 

Capesailor


----------



## Gene T (May 23, 2006)

Generally better to start a new thread than use one 8 years old. 3 will have more drag with lower vibration probably only felt on the tiller. Drag with a 3 blade is so significant most sailors will go with a folding or feathering 3 blade, and pay the difference. 

Gene


----------



## sailingfool (Apr 17, 2000)

Capesailor said:


> Hello,
> 
> I'm a new member. I'm buying a new Marshall Sanderling and am deciding on a two blade vs three blade prop for the 10 hp Nanni 2.10 diesel. Is there more drag with the 3 blade? Is the three blade quieter with less vibration? I am interested in getting max thrust in a tidal outlet to the Atlantic.
> 
> Capesailor


That's a small diesel, it certainly will work fine with a suitable size 2 blade. I would doubt they make a 3 blade to deliver so little thrust...


----------



## HMoll (Sep 18, 2007)

I'll tel you what's dragging: this thread! :laugher For cruising in heavy currents, on a small boat: 3 blade-folding. I went from feathering 2-blade maxprop to 3-blade folding flexofold. Gained a full knot motoring and maybe lost .2kts sailing, which is unmeasureable, 'cause I probably "lose" more than that just chillin' and enjoying the ride! Get 3 blades. No brainer.


----------



## Captin G (Mar 25, 2017)

How about RPM's verses changing prop. I have two issues. I can not seem to correct the vibration of my two blade. I removed and balanced. Then Aligned shaft. Still can't keep my cutlass bearing from loosening. The screws back out. I tried a little epoxy on a new cutlass and screws. lasted a few months. I thought a three blade may have less vibration. 
Also, with a little head wind and current on the nose I can only get 2 or 2.5 knots motoring. I have a 35 hp perkins 3 cylinder parama M35 in a Heritage West Indies 36 (17500 displacement). I have a manual and have asked around however I can't seem to get a straight answer on RPM's. I try not to go over 2000, 2100 max. I would like more speed and less vibration. Can I rev her up to 2500 rpms? Anyone?


----------



## Minnewaska (Feb 21, 2010)

75% of max rated rpm is usually a rough sweet spot. You need to know what your motors max rated rpm is. 

However, I don't think RPM is the cause of your vibration. Even if more seems to smooth it out, you should still figure out why its vibrating so much. You said you aligned your shaft, which is the first thing I would do. Were the engine mounts in good shape or could they be changing after you started the motor.


----------



## SanderO (Jul 12, 2007)

I have had a fixed 2 blade prop on Shiva since I got her 32 years ago. I have no basis for comparison. But it would be interesting to try a season with a 3 blade or even a folding or feathering prop... since I am quite familiar with performance on this boat with a 2 blade fixed.

I don't race... fractions of a knot are usually of no interest to me... I know this can add up on a long passage to actual time. But really... does it matter? I sail to escape the constraints of time and the clock and appointments and so on... I sail to enjoy the boat, the motion, the wind, the waves... the solitude... the wonder and so on.

The two blade pushes the hull at hull speed, backs well... and seems to not have noticeable drag. It's simple. The prop seems to align itself in a vertical position when sailing.... I don't know why... maybe the orientation has the least drag???


I am not motivated to spend thousands for an advanced prop.


----------



## colemj (Jul 10, 2003)

The governed rpm on the Perama 35 is 3600. Your cruising rpm should be ~2800, and there would be no worries running continuously at 3200.

Always operating the engine at 2000rpm is bad for it.

Mark


----------



## Tanski (May 28, 2015)

Does it vibrate through out the rpm range or just where you are trying to run it?


----------



## Minnewaska (Feb 21, 2010)

For under $20, you can buy a digital laser tachometer that reads RPM off whatever you apply a reflective piece of tape upon. Good idea to use one to check the accuracy of your hardwired tachometer.


----------



## overbored (Oct 8, 2010)

Captin G said:


> How about RPM's verses changing prop. I have two issues. I can not seem to correct the vibration of my two blade. I removed and balanced. Then Aligned shaft. Still can't keep my cutlass bearing from loosening. The screws back out. I tried a little epoxy on a new cutlass and screws. lasted a few months. I thought a three blade may have less vibration.
> Also, with a little head wind and current on the nose I can only get 2 or 2.5 knots motoring. I have a 35 hp perkins 3 cylinder parama M35 in a Heritage West Indies 36 (17500 displacement). I have a manual and have asked around however I can't seem to get a straight answer on RPM's. I try not to go over 2000, 2100 max. I would like more speed and less vibration. Can I rev her up to 2500 rpms? Anyone?


it's not because it is a two blade that it is vibrating. there are hundreds of thousands of two blade props that don't vibrate. you have a problem in the prop drive system. sounds like bent shaft or prop. you have excessive runout of some component. maybe corroded flang surface?
how did you do the alignment? Did you check the runout on the output flange? did you check prop shaft runout with a dial indicator? did you use a set of feeler gauges on the flange?
Your engine should cruise 2800 to 3200 rpm


----------



## travlin-easy (Dec 24, 2010)

For me, with a full keel boat, there are significant advantages of using a 2-blade prop over a 3-blade. The biggest advantage is in reverse, while trying to control the boat and backing up into the slip. The 3-blade prop does not have sufficient thrust in reverse because of the close proximity of the 6-inch thick keel, which is only a few inches away from the back of the prop. Much of the thrust is applied directly against the back of the keel, which is flat, thus it has no effect in backing up the boat. Additionally, the thrust generated against the keel is deflected outward, thereby interrupting what little thrust is toward the bow of the boat.

With a similar size 2-blade prop, which will be significantly longer, more of the prop projects beyond the margins of the keel, thereby allowing more water to be thrust toward the boat while backing up. 

I will be installing a 2 blade prop later this season, when the cash flow improves a bit. I was going to buy one this week, but after noticing the car had four bald tires, the tire store got the prop money, plus a lot more this morning. 

Now, I noticed the OP has a wing keel, so there really should not be a significant difference when motoring, either forward or reverse.

Good luck,

Gary


----------

