Overpropping and Engine Strain

[robbieg]

Been reading some articles suggesting slight overpropping to acheive hull speed at lower than max engine revs. For example on an engine that will rev up to 3600 rpm you prop it so that it maxs at 3200 and so at 2600rpm you will get the same boat speed as would need 3000 rpm on an engine propped to max at 3600rpm. Lower rpm means lower fuel consumption and quiter cruising with the trade off of not having max hp available in a head sea etc

All fair enough but I thought overpropping meant putting strain on the engine-from what these articles suggest however the engine only starts to suffer if you try to get her to max revs which she can't do and starts throwing out black smoke etc. Provide you run the engine a couple of hundred rpm below this point she isn't under any undue strain so shouldn't affect engine life etc.

Anyone know whether this is right or does carrying too large a prop put a strain on the engine accross the complete rev range? Interested because it seems I may be slightly overpropped-although by accident rather than design!

 

[greenalien]

I always understood that boats should be propped for max hull speed at peak power, not peak revs - peak power is usually around 10 - 15% below peak revs, although you would have to consult your engine manufacturer for exact figures. This is certainly the best justification for fitting a tacho to a diesel engine!

 

[Tranona]

You will not save any fuel - consumption is a function of power used. Minor "overpropping" is not an issue - for example my new prop maxes at 3400 out of 3600. However a prop which restricts your revs to say 3000 means that you are overloading the engine and will never be able to use full power.

So, get it as near right as you can - at cruising speed you should be using 75% of maximum power, which in a typical small auxilliary is around 2600 rpm (out of 3600) - bit lower if you have one of the newer Volvos, for example which max at 3200. on my 2030 I get 5.6 knots at 2500 and just over 7 at 3400 - almost spot on.

 

[Deleted member 36384]

Over Propping Experience - Advice Is Dont Do It

My yacht is over propped for the reason that you state: lower rpm for higher speed. I understand it was done for so called doldrums crossing.

Anyway, whatever the reason, it is not very good as far as I am concerned. I suffer from significant cavitation at what would be normal cruising rpm in forward and in reverse.

When I compare to a sister yacht, I can say that I suffer from significantly more prop walk in reverse and vibration.

The vibration issue was dealt with by fitting an Aquadrive (thrust CV type bearing). The Aquadrive was fitted as a result of a sheared coupling. When I bought the yacht I was familiar with that model so was not expecting such issues with the propellor. I delivered her in boisterous conditions and found she punched into big waves (1.5 - 2 m), high wind (F8) at 7.5 kts with no loss of speed, which surprised me. It was only later when i was down below that I detected the significant cavitation noise (imagine hundreds of steel ball bearings being thrown against the hull). Anyway, I cut back on the throttle to 1500 rpm and she cruised along at 6 kts in the same condition.

She was being berthed into a Force 8 at my home port on delivery and I got blown off the pontoon a few times. My final plan was to go in with a bit more speed and use the reverse thrust to halt the boat and kick her in while my crew got a line ashore. I powered up reverse thrust and sheared the coupling bolts. It all ended well, just, but cost me a new prop shaft and addition of a Halyard Aquadrive, amongst other things.

I still have the large propellor and it will be changed in the future. I cant run my engine at optimum power (Perkins 4236).

So my experience is that over propping is not a good idea and can cause many other issues. Ultimately the coupling failure was due to a loose engine mount, which was down to the vibration caused by the cavitation.

The sister yacht motors very well with none of the issues I experience except the prop walk, but on a much smaller level.

 

[GrahamM376]

When fitting new engine/gearbox and prop, I was advised by Thornycroft and Featherstream to prop it to max BHP revs, not torque.

 

[TQA]

You will not save any fuel - consumption is a function of power used. .

Not the case.

A diesel engine will be at it's most efficient rpm in terms of turning fuel burnt into output power at the point of MAXIMUM TORQUE not maximum power which is at a higher RPM.

This is basic engineering theory and has been confirmed by experiment over and over again.

Google The brake specific fuel consumption curve for more info if required.

So it is most economical to run an engine at the point of maximum torque. There are some cases where outside factors not related to the engine might change this but they would almost always be where an engine of disproportinate size was fitted.

I used to teach this stuff.

 

[greenalien]

it is most economical to run an engine at the point of maximum torque

but as this may be significantly below the point of maximum power, the prop should be matched to max power; you can then run at max torque revs for best economy, but this won't be max available speed.

 

[TQA]

So in practice I believe that most sailboats are operating with a prop size that does not allow max RPM to be reached in flat water.

The prop size being between that which only allows it to run at best torque RPM and one that allows max rpm to be attained.

IF we were talking racing powerboats where fuel consumption is unimportant then you prop for attaining the rpm where max power is generated. N.B. This would still be below max RPM on almost all engines.

 

[Cuan]

I had a problem with a prop that was too small, even though it was the specified size for the engine and gearbox, and the prop cavitated at cruising revs ( no tacho, so when the engine 'sounded right').
The professional advice I got was to fit a bigger prop due to the underwater profile of my long keeler, as the water would be disturbed at the point it gets to the prop.
The new prop has improved speed, handling etc. I cant really say what it has done for fuel consumption as we haven't needed to motor for more than 4-5 hours at a time since the new prop. Previous consumption was around 1.2lph, and I have no reason to believe that this has changed much.
I just think that its worth considering all of the variables when looking at props, and this includes what you are expecting it to push through the water, and how the water will be flowing when it gets to the prop; the underwater profile.

 

[pappaecho]

I re-engined an Evasion 32 with a perkins 4108, and a 2:1 gearbox to a 15 x15 prop, which
maxed at about 8 knots.
A 35 hp beta 1305 was definitely held back not being able to achieve much over 2000 rpm. What was worst was 3.2 knots on tickover! Re propped with a 15 x 9 which was great as tickover gave 1.2 knots, and could get max revs of 3600. However the engine used considerably more fuel to achieve 5 knots cruising. On balance I would suggest a 15 x 11, would be a better balance .

 

[sailorman]

I re-engined an Evasion 32 with a perkins 4108, and a 2:1 gearbox to a 15 x15 prop, which
maxed at about 8 knots.
A 35 hp beta 1305 was definitely held back not being able to achieve much over 2000 rpm. What was worst was 3.2 knots on tickover! Re propped with a 15 x 9 which was great as tickover gave 1.2 knots, and could get max revs of 3600. However the engine used considerably more fuel to achieve 5 knots cruising. On balance I would suggest a 15 x 11, would be a better balance .

i could only ever get 7.5kts ( clean bottom ) with my 4108 on a 31 ft waterline @ 2900rpm
the new Nanni 4150 that is 200cc less & rated @ 37.5 hp @ 3000rpm ( propped to give 2950 rpm ) gives me 7.5 kts as before

 

[Tranona]

So in practice I believe that most sailboats are operating with a prop size that does not allow max RPM to be reached in flat water.

The prop size being between that which only allows it to run at best torque RPM and one that allows max rpm to be attained.

That is simply not true. While the specific fuel consumption may be at its lowest at the point of max torque, torque in itself is largely irrrelevant in a saiboat where the engine is at effectively constant speed. It is power that drives the boat and the amount of power (energy) used that determines fuel consumption.

Props are chosen to achieve max speed at close to maximum revs which also on most engines is the point of maximum power output. Just looked at the power curves of the Beta engines up to 38hp and every single one achieves its maximum power at maximium revs.

In terms of transmitting the power to forward motion a fixed pitch propeller as most of us use is only efficient at maximium revs, but the power band of the engine is such that it can be effective across a wide range of outputs. A prop that is too coarse will have excess thrust at low revs making the boat difficult ot control at low speeds and prevent the engine from producing its full power. One that is too fine will allow the engine to reach full revs but not provide enough thrust to reach maximum potential speed.

All production boats are normally propped to achieve max hull speed at close to maximum revs/power (assuming the engine has enough power to achieve that speed).

 

[Deleted member 36384]

.... A prop that is too coarse will have excess thrust at low revs making the boat difficult ot control at low speeds and prevent the engine from producing its full power....

From my own experience I would add that to the other disadvantages I noted in my post above. At idle rpm, in flat water, I achieve 3.5 - 4.0 kts, a complete PIA in close quarter situations.

 

[Lakesailor]

Why prop a boat to achieve max speed at maximum revs if the motor has enough power at lower revs to achieve max speed?
That means the motor is spinning faster, with less load than the power output could cope with.
Not a good way to run an engine.
Keep the revs down if possible. Longer life. More efficient.

 

[Tranona]

Why prop a boat to achieve max speed at maximum revs if the motor has enough power at lower revs to achieve max speed?
That means the motor is spinning faster, with less load than the power output could cope with.
Not a good way to run an engine.
Keep the revs down if possible. Longer life. More efficient.

No. Diesels are intended to run at their designed RPM and power. As noted overpowered and overpropped boats are a PITA at low speeds. You do not save fuel as you still need the same amount of power and therefore consume the same amount of fuel for a given boat speed.

 

[Red Admiral]

overpropping

My own boat had been deliberately over-propped for better fuel consumption. I asked Norris of Isleworth to reduce the pitch by 2 inches to the correct pitch and the result was a smoother, quieter engine with cooler running temperatures. The boat went faster and used less fuel. QED

 

[Lakesailor]

No. Diesels are intended to run at their designed RPM and power. As noted overpowered and overpropped boats are a PITA at low speeds. You do not save fuel as you still need the same amount of power and therefore consume the same amount of fuel for a given boat speed.

I think you have misunderstood what I said.

the motor has enough power at lower revs to achieve max speed?

I was referring to propping a boat to achieve max speed at max engine speed, which is pointless and wasteful.
That is apart from the point that motors each have a sweet spot at which they will run more smoothly and economically. Higher revs can introduce harmonic imbalances which cause rougher running and more wear. Things like valve overlaps and gas scavenging work best in given rev ranges.
It's rare that a motor is at it's best at full revs.

 

[Tranona]

There is no need to run the engine at full revs. If it is propped correctly, as I explained earlier you can achieve an appropriate cruising speed at somewhere around 70-75% of max power - which on most engines is the "sweet point"

It is myth that you reduce fuel consumption by running at lower revs. If you need 20 hp to drive the boat at cruising speed, as in my boat, then you will use exactly the same amount of fuel if you are running a 30 hp motor at 2500 or drawing 20 hp at 2100 if you had the optional 40 hp engine.

If you look at the specific fuel consumption curves of a typical small diesel, you will find the grammes per KW/H is virtually flat over the middle range, meaning that the marginal increase in fuel is linear to the marginal increase in HP (or KW) used. It only starts to climb when you get to the last 20% of the rev range.

In practical terms you can show this by looking at the %power increase required to move from crusing speed to top speed - usually the power required is anything up to 50% higher (in %age terms) than the %age increase in speed.

The only advantage of overpowering a boat is that you can achieve a higher cruising speed for cruising revs - but you use more fuel doing it. You cannot go significantly faster in a displacement boat as even with the extra power you do not have enough to overcome the wave resistance.

Running a diesel at lower revs has minimal effect on life - indeed it could do the opposite as running cool and not putting load on the engine can lead to bore glazing. One of the reasons for longer life with fresh water cooling is that you can run the engine hotter than with raw water cooling. Mechanically small diesels will run up to around 8000 hours - 80 years of a typical yachtie's usage! My 2030 has done 3500 hours, mostly cruising at 2500 revs - is as good as new, burns no oil and uses about 2.3l fuel an hour. Perfectly matched to the boat.

 

[greenalien]

the motor has enough power at lower revs to achieve max speed

If that is the case, then the engine is too big for the boat. Most diesels work best (i.e. most efficiently) under a full load at maximum power (which is usually about 10% less than maximum revs, but will vary from engine to engine). If the engine is too big, it will never be running at its most efficient, and will also be more, rather than less, likely to wear out sooner, as it just isn't properly loaded; there is also the probability of running it too fast, which will cavitate the prop, causing very inefficient running and damaging the prop.

Engine and prop both need to be correctly matched to the hull length and displacement, and if the engine is the wrong size, no amount of tinkering with different props will be satisfactory.

 

[GrahamM376]

Why prop a boat to achieve max speed at maximum revs if the motor has enough power at lower revs to achieve max speed?
That means the motor is spinning faster, with less load than the power output could cope with.
Not a good way to run an engine.
Keep the revs down if possible. Longer life. More efficient.

Our new engine came with a warning "minimum full load rpm 1,500". Motior sailing at low revs is the worst thing you can do to a diesel, glazes the bores, gums up the rings and shortens its life.