# overpropped vs underpropped



## groundhog (Jun 27, 2006)

Hi,
I just had a new diesel engine installed, which seems to be working well.
I also have a max-prop propeller where you can change the pitch in 2 degree increments.

The target rpm at max throttle is apparently 3500-3600 for this engine according to the mechanic. I can make about 3260.

After signing off on the engine, and sending the warranty info to the mfr, the manufacturer is saying that I need to decrease pitch.

Doing the math from the prop manual, which reports about 7% rpm increase per degree, I would expect to be then underpropped, at about 3725? 

What is the point? I can hit close to the target above and below the target rpm.


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## chucklesR (Sep 17, 2007)

The math is the math, you would/should be able to over rev the engine, assuming your 3260 is under normal load. Personally, I would not change it unless it somehow voids the warranty. 
You should never run at more than 3/4 throttle continuously, and at WOT never for sure more than your engine manufacturer specifies (usually less than 30 minutes). 
If you are making hull speed, leave it be - if you are not making hull speed in reasonable wave action at 3/4 throttle then your engine is too small, IMHO.


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## Maine Sail (Jan 6, 2003)

groundhog said:


> Hi,
> I just had a new diesel engine installed, which seems to be working well.
> I also have a max-prop propeller where you can change the pitch in 2 degree increments.
> 
> ...


You need to listen to the manufacturer since they will be the ones to void or not void your warranty. Most engine manufacturers want to see you within 100 RPM of max rated.. I know Westerbeke/Universal are pretty strict on this.

Most would rather see it slightly under propped than over propped. Add a load like a HO alternator, cruising supplies, engine driven refrigeration, head wind or head current and you'll be glad you're not over propped..


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## groundhog (Jun 27, 2006)

well, the engine seems to effortlessly move the boat as set up.
It reaches hull speed well before full throttle.

I just dont understand what being 250 or so rpm off makes any difference.
If I was well over propped, I could see re-adjusting. But they are going to force me to re-lift the boat now, well after they signed off on the boat when we could have easily made an adjustment.


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## kellysails (Nov 1, 2008)

In my example my boat was underproped with a Maxprop. 

@20 degrees
WOT no load 3400rpm
WOT under load 3400rpm

@22 degrees
WOT no load 3400rpm
WOT under load 3250rpm

I gained a good 3/4 knot at cruising 2600rpm.


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## lancelot9898 (Dec 30, 2008)

I too have a max prop and a 4-108 Perkins that has a max continuous rating of 3600 rpm. One thing you need to check is the calibration of the tach using a photo tack. I found that mine was off quite a bit. 

The standard wisdom is to set that pitch so that you can achieve that rpm rating at WOT.....I have not followed that wisdom and not suggesting that you follow what I did. I'm just relating what I have done. After changing that pitch and seeing how the boat performed under various setting(it's a 37 ft Tayana), I decided on a pitch that let me cruise at 6 knots at 80% of WOT. However note that my WOT is only 3000 and not the 3600 ....The only thing IMHO that I'm doing is not achieving the max hp that is available if I could go to the 3600rpm. I fail to see how that I'm lugging the engine or glazing the cylinders. I could be wrong, but I've been doing this since the engine was new back in 1985 putting an average of 60 hours per year for the past 28 years.


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## lancelot9898 (Dec 30, 2008)

Just reread my reply and need to explain it better. I wanted to cruise at around 2500 rpm and lower pitch settings on the max prop would not let me achieve my desired 6 knots.


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## jrd22 (Nov 14, 2000)

The easiest way to explain why you don't want to be over propped or pitched is by comparing your boat to a large truck pulling a heavy load. Your boat is NOT like your car which glides effortlessly down the highway where you can select the highest gear available and just loaf along. Your boat is traveling through the water which has a good deal of friction, and it takes a set amount of horsepower to overcome that friction. Your boat is more like a truck climbing a grade, all the time, and like the truck you have to be in the correct gear (prop/pitch) for the engine to be at the correct rpm in order to develop the needed horsepower. If you are over propped or pitched it's the same as being in too high a gear which puts much more load on the engine and builds a lot more heat. Your engine will not last as long and will be much more prone to a catastrophic failure (bearing failure, head gasket problems, turbo failure, etc), especially if you get into a situation where you need to run at WOT for an extended time. If you have lots of horsepower you can get away with it, possibly for a long time, but sooner or later it will bite you. Deciding to set your pitch in order to achieve a desired speed through the water at a certain rpm is throwing everything known about engine/prop theory out the window. You can't expect an engine to climb a hill in overdrive (which is what over prop/pitch is). Ask any good marine mechanic if over propping or pitching is a good idea, they'll undoubtedly have numerous horror stories to tell you.


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## chef2sail (Nov 27, 2007)

jrd22 said:


> The easiest way to explain why you don't want to be over propped or pitched is by comparing your boat to a large truck pulling a heavy load. Your boat is NOT like your car which glides effortlessly down the highway where you can select the highest gear available and just loaf along. Your boat is traveling through the water which has a good deal of friction, and it takes a set amount of horsepower to overcome that friction. Your boat is more like a truck climbing a grade, all the time, and like the truck you have to be in the correct gear (prop/pitch) for the engine to be at the correct rpm in order to develop the needed horsepower. If you are over propped or pitched it's the same as being in too high a gear which puts much more load on the engine and builds a lot more heat. Your engine will not last as long and will be much more prone to a catastrophic failure (bearing failure, head gasket problems, turbo failure, etc), especially if you get into a situation where you need to run at WOT for an extended time. If you have lots of horsepower you can get away with it, possibly for a long time, but sooner or later it will bite you. Deciding to set your pitch in order to achieve a desired speed through the water at a certain rpm is throwing everything known about engine/prop theory out the window. You can't expect an engine to climb a hill in overdrive (which is what over prop/pitch is). Ask any good marine mechanic if over propping or pitching is a good idea, they'll undoubtedly have numerous horror stories to tell you.


Great analogy


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## lancelot9898 (Dec 30, 2008)

I think the analogy is flawed in the sense that as I drive my Jeep up a hill in 5th gear I can tell that I'm lugging the engine seeing the rpm reduced to below 1000 no matter how much throttle I give it. I then down shift to 3rd and the rpm goes up and more power is available. Now on a boat there are no gears to shift from considering that the tranny is set in my case at 2.1:1 and the pitch is pre set with no means of adjusting underway. Under calm conditions I can maintain 6 knots at 2500rpm but as the seas and headwinds increase my speed will be reduced but that rpm will remain at 2500 plus I can still wot it at 3000. I'm just not using the available hp if I had preset the pitch at a lower setting. My 2 cents.


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## jrd22 (Nov 14, 2000)

Lance- I'm not telling you what to do but using your analogy when you have your throttle wide open on the boat and you aren't able to achieve the manufacturers WOT rpm you ARE lugging the engine just like your Jeep when you have your foot in it and can't accelerate because of too much load. That extra load from too much prop/pitch is present throughout the rpm range but you can only tell when you try to reach WOT. Your engine is pulling it's guts out trying to get there but just can't (just like when you have to shift in your Jeep). By the way, your use of WOT in reference to your engine is incorrect. WOT is the manufacturers required maximum rpm under load, what you are referring to on your boat is the maximum rpm attainable with your over pitched prop at full throttle which is 600 rpm below WOT. Sorry, not meaning to go off on you, this is a very common misconception that results in various expensive engine problems. There would be absolutely no reason for the manufacturers to even bother to establish the WOT for their engines if it wasn't important.


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## lancelot9898 (Dec 30, 2008)

Thanks John for the discussion. I understand that my WOT term is not the WOT that the mfg uses. It's just the max rpm that can be achieved at a given prop pitch.

Let me try to explain in a different way. As I go up the hill in my Jeep without down shifting I will slow down with the rpm going down to zero yet when I go into a headsea/headwind that is not the case for the boat. The boat will slow down yet the engine will maintain 2500rpm. Why is that? The Jeep has no slippage in that the tires maintain firm contact with the pavement while the prop on the boat is "slipping'. In other words the pitch of the prop is set for an optimal speed. I set mine for achiveing 6 knots at 2500 rpm and can approach hull speed at "wot" of 3000 under calm conditions. I just don't want to hear the engine screaming at 4000 rpm which is the max intermittent rating and is what I can achieve under no load. While a max prop allows for setting different pitches it does not allow a variable pitch while underway. I guess we will have to agree to disagree on this and note that I'm not advocationg anybody follow me on this. 

This brings up another interesting point on engine operation. In my Perkins workshop manual, it mentions replacing a zinc in the heat exchanger. There is no zinc in that heat exchanger. I found a good perkins mechanic who agreed that there is indeed no zinc since the rubber end gaskets on the end of the exchanger isolated the core from the metal of the rest of the engine. My point is that even a workshop manual can be wrong...and in some cases even I. 

Best Regards


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## Minnewaska (Feb 21, 2010)

Just to be certain, you must check WOT rpm under load. If you try to do so at the dock, in neutral, you will get a different result and its inapplicable.

When you are overpitched, you will get too low a result. I will echo that you confirm the accuracy of the tach, as they are indeed notoriously wrong. Lancelot, it could even be the case with yours.

Backpressure from overpitch is just clearly bad for a motor over the long term. Some engines will be more finicky than others, but getting way with it is like deciding to take up smoking because you know of one 90 year old that smoked their entire life.

Ironically, overpitching the prop helps maneuverability around the dock. That will probably convince some to take their chances.  Being overpitched at cruising speed for a lengthy period of time, probably isn't that bad. But getting there is just like leaving a stop sign in 4th gear. Doing that thousands of times just ain't good.

Not sure why one would be fixated on cruising at a given rpm. It should be whatever the motor wants to be at proper settings. Don't like the noise? Well that's obvious, but there are no engine noises that I'm good with. Necessary evil.


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## kellysails (Nov 1, 2008)

Regarding engine noise, ref to my last post of the change I made.



kellysails said:


> @20 degrees
> WOT no load 3400rpm
> WOT under load 3400rpm
> 
> ...


FWIW 
My wife and I both noticed that after we made the change to the prop, the engine sounded slightly different at cruising rpm. It seemed a bit smoother, a bit lower sound tempo. Oddly we thought "it" sounded "happier", not so whiny


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## jrd22 (Nov 14, 2000)

Kelly- what is your engine manufacturers spec for WOT. Full governed rpm is the rpm at full throttle in neutral, the governor controls the maximum rpm to prevent damage to the engine. If you were able to reach the max governed speed under load you were either way underpropped, or you have a lot of hp. Sounds like by increasing pitch you actually put the engine under load which would make a difference in sound.


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## captbillc (Jul 31, 2008)

running with the greater pitch at cruising RPM gives better fuel economy & less engine wear. as long as the engine can keep the same RPM under heavier load conditions & without smoking it is not overloaded.


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## srah1953 (Dec 15, 2012)

I have a Yanmar 3JH5CE with SD50 saildrive. How do I know what RPM should be at WOT. The only figures I can find is that the manufacturer quotes a maximum output figure at the crankshaft which is 39hp @ 3,000 RPM. With the original fixed prop, I can get around 3,200 RPM. What does this mean in terms of being over- or under-propped?
Many thanks


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## amarinesurveyor (Feb 12, 2009)

jrd22 has it right. I see that many still have the opinion that putting a bigger prop or a steeper pitch in your prop will reduce engine wear and save fuel, and they choose to disregard engine manufacturers and mechanics. Everyone thinks they know better. You all can do whatever you want with your boat, but I think you'll find that with the correct or slightly under-pitched prop you will get the least engine wear and the best fuel economy. Yes, with an over-pitched prop you actually burn more fuel and can't develop full horsepower.
The analogy I like to use is: having a boat with an over-pitched prop is kind of like riding you bicycle in high gear (you know how hard it is on your legs, well it's like that for the engine that is over-propped).


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## Waltthesalt (Sep 22, 2009)

Difference between cars and marine diesels is that the diesel you control the governor and the governor controls the throttle, on a car you just control the throttle. So a marine diesel won't exceed the max governor setting. If you can't get up to RPM, also black smoke, too much pitch, if RPM gets up too quickly and hull speed is down too little pitch


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## kellysails (Nov 1, 2008)

FYI


jrd22 said:


> Kelly- what is your engine manufacturers spec for WOT. Full governed rpm is the rpm at full throttle in neutral, the governor controls the maximum rpm to prevent damage to the engine. If you were able to reach the max governed speed under load you were either way underpropped, or you have a lot of hp. Sounds like by increasing pitch you actually put the engine under load which would make a difference in sound.


Yanmar 4JH5E naturally aspirated (non-turbo). Yes, we are hitting the governed rpm rate (3400 rpm). Prior to making the prop pitch change we easily hit the governed rate under load. So our yard, who does all the warranty work for Beneteau, agreed that increasing the pitch would be a worth while activity. The two degree increase really helped my speed at cruising rpm. My Max Prop actually allows 1 degree pitch changes, the yard actually thinks it could go another 1 degree, but I am happy where we are. The engine sounds happy, so I am happy. So under load WOT is 3250 from a max no-load of 3400, sounds good to me.

FYI
4JH5E is 54 hp and we have an SD50 saildrive. Beneteau Oceanis 45. 23,500 lbs.


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## kellysails (Nov 1, 2008)

srah1953 said:


> I have a Yanmar 3JH5CE with SD50 saildrive. How do I know what RPM should be at WOT. The only figures I can find is that the manufacturer quotes a maximum output figure at the crankshaft which is 39hp @ 3,000 RPM. With the original fixed prop, I can get around 3,200 RPM. What does this mean in terms of being over- or under-propped?
> Many thanks


In neutral, push throttle all the way forward, record rpm. Then while in gear (out and about) do the same. IMHO (same with Beneteau warranty yard) if you hit the same rpm under load as you hit in neutral you might be under-proped. Just to make your self comfortable, ask the yard who does warranty work for your model/engine.


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## MedSailor (Mar 30, 2008)

groundhog said:


> well, the engine seems to effortlessly move the boat as set up.
> It reaches hull speed well before full throttle.
> 
> I just dont understand what being 250 or so rpm off makes any difference.
> If I was well over propped, I could see re-adjusting. But they are going to force me to re-lift the boat now, well after they signed off on the boat when we could have easily made an adjustment.


Sizing the prop is like picking the correct gear on your 10-speed bicycle. I'm sure you can reach maximum speed in 10th gear on a flat road, but you'll need to put more force into it and you could stretch the chain and deform the bearings.

If you are in 8th gear on a flat road, the force required is less, though you need to pedal more RPMs to do it. You still reach your maximum speed but you haven't stretched the chain, deformed the bearings, or worn out anything else in doing so.

Your engine manufacturer believes their engine will last longer if it isn't working so hard and the only way to know how hard it is working is if you are ABLE to reach max RPM at WOT.

I hope this makes sense.

MedSailor


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## TQA (Apr 4, 2009)

You are driving along at 65 mph in a car with a six speed gearbox.

Do you

A drive in 3rd
B drive in 4th
C drive in 5th
D drive in 6th

I hope you all came up with D [ No Ferrari owners need answer ]

Now ask yourself can I get to max rpm in 6th gear? No of course.You are pulling to high a gear but you WILL get more MPG at 65 mph than any other gear.

That is why on a cruising boat you select a prop that is overpitched and you should NOT be able to reach max rpm as quoted by the manufacturer.

This thread contains some inaccurate information. A diesel engine will be at it's most fuel efficient when run at it's peak torque RPM. This is basic mechanical engineering.

Assuming that bearings are designed to cope with the BMEP at that speed then the engine life will not suffer EVEN if it is running at full fuel setting to achieve that speed.

Accurate figures for diesel engine life are notoriously difficult to arrive at but there is a considerable body of empirical evidence that suggests that engines that are run below their max power rpm last longer.

So in practice know what RPM your engine generates max torque and prop it so at full throttle with NO WIND and FLAT WATER your RPM is a bit higher, say 20%.

Now if you know you are going to spend your time running the ICW then go 10%.

Or if you know you will be bashing to windward in heavy chop maybe 30%.

BTW Cruising sailboat diesel engines rarely see enough use to wear out.


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## groundhog (Jun 27, 2006)

Hi... 
Remember me?
I am not a mechanical engineer, but i am an engineer...

My situation seems like... lets go with the ten speed bike..

The bike rider should select gear 3 for overall optimum match to his legs over all terain.

Me being 240 rpm below target seems like I am in gear 3.1 on the ten speed bike.
Not in gear 10 and way overpropped.

Am I correct mechanical engineers?


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## jrd22 (Nov 14, 2000)

TQA- your analogy is incorrect. You can't compare a car that generally has way more maximum horsepower that it will ever need to a sailboat that generally only has a bit more hp than needed to achieve hull speed. You also can't compare a care with a lot of horsepower leisurely coasting along at 65 mph to a sailboat that is just below it's maximum speed due to hull design and high drag through the water at that speed. To make your comparison somewhat more realistic you should have the car going up a mountain pass at 65 mph, which gear are you going to have to be in to get the rpm up high enough for the engine to develop the hp required to maintain the 65? It isn't going to 6th gear, probably not even 5th if it's a steep hill. And it still isn't a good comparison because of the hp difference. They don't make cars that barely have enough hp to get to 65 (anymore) but that would be a fairer comparison. You're not even close to comparing apples to apples. If you want to run your engine at a higher load than is recommended by every engine manufacturer on earth and every marine mechanic that knows what he's talking about fine, but don't encourage others to follow your advice. Some of the older heavy duty engines might do OK running over propped but the modern, lighter, higher revving, turbo engines are a different story. How much fuel do you figure you have to save to even pay for a minor engine problem like a blown head gasket? Is the small amount you save by running a few hundred rpms lower even measurable? Seems penny wise, pound foolish to me.

Ground- not knowing what your recommended WOT is I don't know if 240 rpm is a big deal or not. I do know that on a Volvo that has a WOT of 3800 the factory mechanic told me that I was asking for trouble only being able to hit 3600. I had the prop re-pitched. I would call a couple of marine shops that work on your engine and run it by them for opinions.


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## MedSailor (Mar 30, 2008)

TQA, 

Not trying to pile-on here, but I also disagree with your analogy. The OP's problem is NOT being able to reach max rated RPMs whereas in your analogy any gear other than D would result in more RPM's.

Also, your analogy seems to point to fuel efficiency as the goal. No mechanic, or holder or a warranty gives a rats ass about MPG. I think they are worried about wearing out (or breaking) the engine prematurely, which is also what I'm worried that the OP will do. 

On the topic of MPG, for any given hull/boat, there is an optimum engine HP rating. In a perfect world you would choose an engine that is exactly sized and propped to reach hull speed (or your desired cruising speed) at the engine's maximum torque and efficiency curve. 

Now we enter the real world of the boats, props, and information that we have. The OP has an engine of a certain HP installed in his boat, and presumably can't/won't change that. Now he has a choice; he can either choose a prop pitch/size that will optimize MPG or engine life. 

I'm with jrd22. I think he should ask SEVERAL mechanics that are preferably familiar with his engine and ideally his boat. He should ask them exactly how close to rated rpm at WOT is close enough, and then prop to suit. 

From what he said earlier, he can reach hull speed without being anywhere near full throttle, so he likely has an engine that is well sized to his boat. Once he de-props it a couple inches he'll still likely be hitting hull speed easily and hopefully it'll be closer to where it should be on the mileage curve. 

On my boat I solicited the opinions of several mechanics, and the one I trust the most told me that the rating on my engine (4000rpm) is actually bogus and could back this up with some good reasoning. He recommended that I prop mine to hit 36-3800rpm at WOT and I've done that. My circumstance is a bit different as I have a screwy, older engine (Perkins 4.108) and this mechanic had messed around with my particular engine on my particular boat before. My engine is also undersized for my boat, but old, sturdy, and heard to break, so I've pitched for a slightly shorter lifespan in exchange for being ABLE to reach hull speed at 75% throttle. I mention this because I've given the topic a LOT of thought and time over the years. 

Speaking to the physics again, here is another thought experiment. Imagine the OP fitting a tugboat prop to his boat. He can hit hull speed at 500RPM and WOT gets him 550RPM. He looks at his fuel curve in his manual and shouts with joy because he's only supposed to be burning 0.5gal/hr at 500RPM. The only problem is that that consumption curve is assuming a regular load. What he will actually find is that he's burning more diesel than an appropriately size prop (due to inefficiencies in overly-rich combustion) and he will be putting so much stress on his engine as it labors with all that torque that he will wear out his engine in no time. 

TQA, in the interests of continuing our dialog, can you give us specifics on which information you feel is inaccurate? 

Medsailor

PS To the OP: all of these discussions about reaching WOT are intended for a boat with a CLEAN BOTTOM AND PROP. If your bottom and prop are foul don't re-pitch based on your experiments. That is, unless you expect to always have a foul bottom and prop.


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## TQA (Apr 4, 2009)

I don't want to get involved in a war over what is the right RPM to achieve with a boat diesel.

The OP asked a specific question related to satisfying the warrenty conditions on a new engine. Rather than tell him which way to go I was trying to give him some advice BASED on his expected boat and engine use.

But there are some basic facts about diesels that people who decide to change things need to know.

The throttle on almost all modern diesels is connected to the governor and sets the RPM, the governor in turn adjusts the fuel supply to achieve that rpm. This means that at half throttle the engine can easily be in a full fuel situation and producing all the power it can AT THAT RPM.

Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP

However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired. The only time I have seen this being a problem was an engine set up to drive a large refrigeration pump and a double alternator set up, when run just above tickover the rear cylinders ran hot. FORD 6D.

In my professional opinion as a engineer an engine installation should allow full fuel operation at ANY REASONABLE RPM without overheating. I would be happy to go to court on this.

Cooling an engine gets more complex if it has a turbo fitted. This is because the cooling is usually done by the engine oil. [ I would NOT have a turbo diesel in a sailboat although I am out of touch with some of the latest developments in the field where they claim that no cool downs are required]

An engine operated at it's max torque RPM will be running at is most fuel efficient RPM. Generally this would also be the most miles to the gallon in a boat. Look up Brake Specific Fuel Consumption.

Just to complicate things engines are more efficient when run hot.

My background is vehicle engineering but I am aware of the large body of research on maximising fuel economy and engine life on ships diesels.


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## Minnewaska (Feb 21, 2010)

TQA said:


> ....I would NOT have a turbo diesel in a sailboat although I am out of touch with some of the latest developments in the field where they claim that no cool downs are required]...


I have one and curse the bloody thing all the time. Utterly useless for a sailboat with our relatively small horsepower needs. In fact, my engine is a 150hp model, de-rated to 100hp, so any weight savings for powering up a smaller lighter engine is lost. Don't get me started.


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## MedSailor (Mar 30, 2008)

Minnewaska said:


> I have one and curse the bloody thing all the time. Utterly useless for a sailboat with our relatively small horsepower needs. In fact, my engine is a 150hp model, de-rated to 100hp, so any weight savings for powering up a smaller lighter engine is lost. Don't get me started.


Why don't you see if you can get it re-tuned to be 150HP???












MedSailor


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## Minnewaska (Feb 21, 2010)

MedSailor said:


> Why don't you see if you can get it re-tuned to be 150HP???


The bloody displacement hull keeps getting in the way.


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## Oaksail (Jan 6, 2011)

TQA said:


> Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP
> 
> However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired.


I test pumps and fluid flowrates for a living. Centrifugal pumps and fans operate with direct proportional flow rate vs rpm.
Decrease a pump by 15% rpm and you get 15% less flow.


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## SloopJonB (Jun 6, 2011)

MedSailor said:


> My circumstance is a bit different as I have a screwy, older engine (*Perkins 4.108*) and this mechanic had messed around with my particular engine on my particular boat before. My engine is also undersized for my boat, but old, sturdy, and heard to break, so I've pitched for a* slightly shorter lifespan *in exchange for being ABLE to reach hull speed at 75% throttle.


Your great grand children are going to be annoyed with you.


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## TQA (Apr 4, 2009)

Oaksail said:


> I test pumps and fluid flowrates for a living. Centrifugal pumps and fans operate with direct proportional flow rate vs rpm.
> Decrease a pump by 15% rpm and you get 15% less flow.


Dat not wot my text books and test results said.



> Notice that larger pumps are inherently more efficient and that efficiency drops rapidly at specific speeds of 1000 or less. Jacques Chaurette p. eng.


Primary coolant centrifugal pumps would be classed as small?

However I know I am not a centrifugal pump expert.


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## Oaksail (Jan 6, 2011)

TQA said:


> Dat not wot my text books and test results said.
> 
> Primary coolant centrifugal pumps would be classed as small?
> 
> However I know I am not a centrifugal pump expert.


Hi TQA, I can't scan my text books but this might work

Try this link Downloads & CDs
You can get to it through ITT Bell & Gossett website, click on "Online Calculators" scroll down and click on 'Flow / Head - Variable Speed Pumping Wizard" and try it out.

Start at 5 gpm at 10 ft of head and then look at what happens to flow at different speeds. Directly proportional.

Try 5000 gpm at 10, 100, or a 1000 ft of head.
Directly proportional.

I test small pumps and i test BIG pumps. I test them at different speeds.


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## Maine Sail (Jan 6, 2003)

Oaksail said:


> Hi TQA, I can't scan my text books but this might work
> 
> Try this link Downloads & CDs
> You can get to it through ITT Bell & Gossett website, click on "Online Calculators" scroll down and click on 'Flow / Head - Variable Speed Pumping Wizard" and try it out.
> ...


This is very easy to see with centrifugal bilge pumps. Drop the voltage and the output is also cut. It can be pretty dramatic...


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## Brezzin (Dec 4, 2006)

Back to the topic.
Having just replaced my three blade fixed with a four blade feathering. I find that I can only get to 2900rpm with the max WOT at 3200. But I can reach hull speed easily here.

Now I almost never run the engine at WOT. Probably 99% of the time at 2500 or less because I don't care for the motion of the boat over 8 knots. The engine is a Yanmar 4jh4HTE 

Is the 4 blade adding more stress to the drive train than the three and am I doing damage at this prop setting?


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## Minnewaska (Feb 21, 2010)

Brezzin said:


> ....Having just replaced my three blade fixed with a four blade feathering. I find that I can only get to 2900rpm with the max WOT at 3200. ....


This is confusing. If your new feathering prop won't allow you to get to max rpm under load, then it needs to be adjusted. If you're uncomfortable at any given speed, just pull the throttle back. But it should be able to get to max rpm.


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## jrd22 (Nov 14, 2000)

I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.


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## MedSailor (Mar 30, 2008)

jrd22 said:


> I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.


Well put. From the mechanics I've talked to, being ABLE to reach max RPM at WOT is the only real way you can tell if you have the correct amount of stress on your engine at any given speed.

How many hundreds of RPM can you fudge it and get away with? That depends on a lot of things and many unknowns. If you adjust your new prop so that you can reach max RPM at WOT and you can still hit your cruising speed at a reasonably efficient RPM then there is no downside.

At what RPM do you currently reach hull speed/cruising speed? What engine do you have?

MedSailor


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## srah1953 (Dec 15, 2012)

kellysails said:


> FYI
> 
> Yanmar 4JH5E naturally aspirated.


Hi
I note that you say your engine is a Yanmar 4JH5E and that it has a governed rpm rate of 3,400rpm. May I ask where you got this data from? The only data I can find for it from Yanmar is a maximum output rating of 53mhp at the crankshaft at 3,000rpm I'm asking because I'm trying to find a source to find out what is the maximum WOT rpm of my 3JH5E engine.
Thanks


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## Minnewaska (Feb 21, 2010)

Max rpm should be in the engine manual. You raise a very good point. My TAMD31 has about a half dozen variants, signified by a following letter. All but mine have max rpm at 3800, but mine is only 3000. I had one engine shop hear the 31 and assumed the higher rating and went down a rabbit hole trying to resolve an issue.


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## jrd22 (Nov 14, 2000)

Yep, every variation can have different ratings, our MD31A is 3500.


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## srah1953 (Dec 15, 2012)

Minnewaska said:


> Max rpm should be in the engine manual. You raise a very good point. My TAMD31 has about a half dozen variants, signified by a following letter. All but mine have max rpm at 3800, but mine is only 3000. I had one engine shop hear the 31 and assumed the higher rating and went down a rabbit hole trying to resolve an issue.


The only data that I can find in the manual is the maximum output rating of 39hp at the crankshaft at 3,000rpm. It gives a propeller speed of 1,293 rpm and a reduction ratio of 2.32. Can I multiply one by the other to get max rpm (which would make 3,000 rpm)?


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## jrd22 (Nov 14, 2000)

From what I can find 3000 is WOT.

http://us.yanmar.com/products/marine-engines/


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## MedSailor (Mar 30, 2008)

srah1953 said:


> The only data that I can find in the manual is the maximum output rating of 39hp at the crankshaft at 3,000rpm. It gives a propeller speed of 1,293 rpm and a reduction ratio of 2.32. Can I multiply one by the other to get max rpm (which would make 3,000 rpm)?


No you shouldn't be multiplying anything by your gearbox ratio. Your tachometer, if correctly calibrated, reads your CRANKSHAFT rpm, not your propeller shaft rpm. It's max RPM at the crankshaft at WOT that you're after.

My engine has at least 3 variants as well. One governed at 3000rpm, one at 36000rpm and one at 4000rpm. Mine is the 4K version.

The way my mechanical surveyor found this out for sure, was he used a sight gauge to verify that my tachometer was reading correctly as they often are not. Mine was off by 1000rpm *EDIT: this should read "of by 100 rpm at 3000rpm"*at 3000rpm. Once the engine was WELL WARMED UP to operating temperature after running under load for a while, he had me put it in neutral and advance the throttle to full throttle.  He took a reading on my tachometer, and another from his strobe sight tach thingy, and I put the throttle back to idle.

In short, if you want to know for sure where your engine is governed, then floor it in neutral once it's warmed up. Where the govener stops you is the RPM that you should be able to reach (or get near) at WOT.

MedSailor


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## Lake Superior Sailor (Aug 23, 2011)

I have a diesel YSM8 it's governed, if the prop is too high a pitch She blows black,if the prop is too low the governer kicks in and the boat is slow.So the prop that gets the speed without the black must be right.K.I.S.S.Then you just pick the throttle speed!...Dale


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