VP2030 most efficient engine revs

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Hi
Having a discussion on the boat ?
Apart from sailing, what’s the most efficient engine revs for a VP2030. The yacht is a Dufour 36 Classic 2004

I say 2000 revs gives us approx 3.75 knots. Crew wants to up to 2750 but I feel that’s to high.
Then the engineer fixing the the engine said it will rev to 3500 quite happily.
Thoughts please
 
I’ve had mine for over twenty years. The Volvo instructions give max revs minus 400 as the cruising revs, which in my case would be 3000. I have only used those revels for short bursts but I once used 2700 for 24 hrs, with the occasional stop for oil and filter checks. For passage-making I commonly use 2500 but seldom less than 2200, since the engine runs more smoothly over 2000 in my set-up. This is on an HR34.
 
My attitude is that if you are running the engine it's to get somewhere. With a similar size/weight boat with a similar power Yanmar engine I regard 2,600 to 2,800 rpm a good cruising rev range, giving about 6 to 6.5 knots. If you want absolute maximum fuel efficiency lower revs are probably better, but not necessarily any better for the engine and possibly worse: diesels like fairly hard work.
 
General rule is 65-70% maximum revs for long term cruising. On my 2030 which was in a charter boat (Bavaria 37) that did a lot of hours of motoring we asked charterers to stick to 2400 (big notice on the steering console!). If the boat is propped correctly this should give you a speed of approx 5.5 knots in flat water. That engine did 3500 hours with no problem and under 2l an hour.

3.75 knots at 2000 is a bit on the low side speedwise. suggest you do a series of controlled runs at 200 rpm increments and plot speed against revs. You should be able to get to at least 3400 and 7.6 knots or so, but suspect it will run right to the governor at 3600.

No harm in using higher revs - the important thing is to load the engine as much as you can at any given revs. Volvo require a new engine to be propped such that hull speed is reached at just below maximum revs and this ensures that the 2400-2600 range gives a comfortable cruising speed. Actual revs you use may vary depending on the "feel" of the engine - in mine 2400 was good and on my later D1 30 which has a lower maximum it was 2100. Also good idea every so often to give the engine a run up to maximum for a few minutes.

Not really a question of "efficiency". The biggest problem with yacht engines is they do not get enough hard work because they often run lightly loaded for short periods followed by long periods idle. The usual manifestation of this is coked up exhaust elbows from incomplete combustion and exhaust temperature too low.
 
Tranona said:
General rule is 65-70% maximum revs for long term cruising. On my 2030 which was in a charter boat (Bavaria 37) that did a lot of hours of motoring we asked charterers to stick to 2400 (big notice on the steering console!). If the boat is propped correctly this should give you a speed of approx 5.5 knots in flat water. That engine did 3500 hours with no problem and under 2l an hour.

3.75 knots at 2000 is a bit on the low side speedwise. suggest you do a series of controlled runs at 200 rpm increments and plot speed against revs. You should be able to get to at least 3400 and 7.6 knots or so, but suspect it will run right to the governor at 3600.

No harm in using higher revs - the important thing is to load the engine as much as you can at any given revs. Volvo require a new engine to be propped such that hull speed is reached at just below maximum revs and this ensures that the 2400-2600 range gives a comfortable cruising speed. Actual revs you use may vary depending on the "feel" of the engine - in mine 2400 was good and on my later D1 30 which has a lower maximum it was 2100. Also good idea every so often to give the engine a run up to maximum for a few minutes.

Not really a question of "efficiency". The biggest problem with yacht engines is they do not get enough hard work because they often run lightly loaded for short periods followed by long periods idle. The usual manifestation of this is coked up exhaust elbows from incomplete combustion and exhaust temperature too low.
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This could be a problem for those that do a lot of motor-sailing at low revs. I agree that the speed sounds low. I don’t get the 6 kn that some people claim with 2000 rpm but from 2200 upwards the folding prop seems to bite better and I get roughly 6 knots plus 0.1 for every 100 rpm over 2000, ie 6.5 kn at 2500, with a clean bottom in flat deep water. I get the impression that my 2-blade folder is more efficient the faster I go, so that I don’t gain as much in efficiency by going slower than I would expect.
 
There is no answer, or the answer is too complex and spoils the 'sailing'

It depends on the prop and the yacht.

and what you mean by 'efficiency'

The answer is do what Tranona suggests, (which is something any act owner should do - its easy) - plot, revs vs speed vs fuel usage in flat water and you will have one answer.

but plot revs, speed and fuel effiecny with one knot of boat speed from, well trimmed, sails and you will have another answer

and a very different answer if you want to reach point'A' with a 20 knot head wind.


On a twin engined cat its more effective, in terms of litres per Nm to use only one engine (unless we have a winning lottery ticket :) )

There is no one right answer.

Jonathan.
 
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As I understand it, the engine is at its most efficient fully-loaded at the maximum torque speed specified by the manual. However, in a boat it will almost never be fully loaded at that speed, and the most efficient engine speed at lower loads are likely to be different. (The 'throttle' on a boat diesel sets the revs, not the fuel supplied, so it's difficult, without extensive bench tests using a brake, to identify the most 'efficient' engine speed at all loadings.)

Meanwhile, the faster the boat goes, the greater work the engine has to do, and the increase in loading is is not linear. So as you increase engine speed, you'll be using more fuel (especially so as you approach 'hull speed'), even if you haven't yet reached the 'most efficient' engine speed for the loading. (I suspect the maximum Nm per litre of fuel is at a very low speed.)

Throw in a few other factors and it's too complex to know with certainty, and also depends what you mean by 'efficiency' (e.g. are you including how quickly you get your destination, how much you enjoy the intervening period. etc.), as Neeves and others have suggested above.

In real life it's a matter of choosing a subjective acceptable balance of progress, noise/vibration, and fuel consumption that's within the maximum continuous engine speed specified by the manufacturer (it's highly unlikely to be outside that range).

Keeping the engine serviced, keeping the boat's bottom clean, and adjusting your 'cruising' speed (and perhaps heading) to give the boat's smoothest ride through the waves' frequency, size and shape will also help 'efficiency'.

Given that fuel costs for a sailing boat are likely to be a very small proportion of its overall costs, and we're doing this for fun (supposedly!), in my view worrying too much about the exact most 'efficient' speed/revs is not an efficient use of one's sailing thoughts!

p.s. I forgot to mention that the prop efficiency varies both with revs and with speed through the water.
 
You really do need to decide what efficiency you mean - most efficient way to travel at 6knts or the least litres used per mile? Agree with the above comments and without over analysing it I would do some runs in still water at different RPM. I did this and found that a bit below hull speed a small increase in revs gave quite a big speed increase. As hull speed is approached, the revs go up more and more for smaller gains. It doesn't need to be precise - you want comparative information. You then need the fuel consumption at different revs for your engine (manufacturers data) and calculate the miles travelled per litre or gallon at the different speeds. For my boat there is a "sweet spot" at about 5-6 knts where I make very little wake and am well below hull speed of about 7 knts. At this speed the engine is at 14-1600rpm versus a max of 2700 (not your engine). Diesels are usually most fuel efficient at the revs where maximum torque is developed. The reserve deals with waves, wind, a dirty bottom and the occasional sprint for tide.
 
Supertramp said:
You then need the fuel consumption at different revs for your engine (manufacturers data) and calculate the miles travelled per litre or gallon at the different speeds.
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I agree with the rest of your post, but that won't work. The manufacturer's data sheet will not tell you how much your engine will consume at those revs in your boat

The figures manufactures provide for fuel consumption at different revs is for that required to achieve the engine's maximum potential at those particular revs. As I said above, most of the time in a boat your engine will not be putting out the maximum potentially available power at a given revs.

The boat's engine control lever (unlike a car accelerator pedal) sets the engine revs, not the amount of power provided or fuel used. The engine's governor provides only the amount of fuel required to reach and then maintain those revs.

The fuel consumed will be much more closely related to the load on the engine (principally the boat speed in this case) than to the engine revs.

To take an extreme example, if you run your engine at max revs in neutral it will consume only a small fraction of the diesel required to provide the engine's rated maximum power at those revs, which is what is indicated in manufacturer's specs.

For another way of looking at it, my own boat happens to be fitted with a 36hp engine, as opposed to the standard 20hp, so it is rather 'oversized'. It's fuel consumption will be much more similar to the 20hp version of the same boat (a little higher due to additional cylinder, friction and capacity, weight, etc.), than to the same 36hp engine fitted to a much larger boat.

The manufacturers specs say my engine will consume IIRC 8l/h at maximum revs, but that figure is what is required to produce the full rated 36hp. I have never had anything like that rate of consumption, because it takes less than 36hp for my boat to maintain hull speed. The engine is almost certainly never required to produce 36hp. I expect it will accelerate to max speed much faster than the same boat with the standard engine, but consume only marginally more than the 20hp (due to the inefficiencies mentioned) once it reaches that speed, as it will only be producing exactly the same hp output as a 20hp (or any other capacity) powering an identical boat to that speed.

You may be surprised I agree with you it's not worth over-analysing it, but it's interesting (at least to me ? ) to consider what's going on, and in any case, if you are going to analyse it at all it needs to be based on how engines and oats really work, and what manufacturers' specs are really telling you.
 
You are right. It's a bit like cruise control in a car and watching consumption rise on an incline. I simply used the example to show a simple way to get near to an optimum speed/load balance on a given boat. If you can readily measure the real data then you get a more accurate answer. I think the key is to find the maximum speed before starting to drag a big wave out the stern as hull speed approaches.

If the optimum is minimum journey time without destroying the engine then a whole different set of questions arise.

For me, and given the price of diesel, most miles per litre does it. Without a way to display instantaneous consumption I guess the consumption effect from wake and the speed increase from another 1000 rpm. It must be about right as my 60hp gives me 2l per hour at 5.5knts which looks like the going rate for that speed.

The OP is probably best settling on a speed below hull speed unless his crew are very impatient!
 
Getting back to the OP. He should be able to get just under 6 knots at 2500 if it is correctly propped and 7.5 knots at full power - pretty much the same as most boats of that size/type. If not then he needs to look at the prop size and type.

I have found it very useful over the years to use a calculator like Vicprop to try out different engine/gear ratio/propellers for a particular boat. Very straightforward with most modern boats as their props run in clear water and there is usually not a restriction on size through tip clearance - more so on saildrives. The HP increments most engine ranges have of 10,20,30 and 40 (I know there are some in between) fit well for most boats in the 25-40'tange although of course there are some that don't fit neatly often because of displacement.

I used this approach a lot when working out what power train would be best for my GH given the constraints of the prop aperture and the long, wide keel that masks the prop. Ideally it needs a 17" prop. The hp requirement for hull speed is 26hp and for 6 knots 16hp. The final choice was between a Beta 25 or 30, but the former would not get hull speed and even with the biggest reduction ratio would only swing a 16". Perhaps more important the 16hp was at 2800 rpm rather than 2300 of the 30. Given there were no problems with physical size and the price difference was around 10% I went for the 30.

The factor that makes the difference is displacement at 5.3 tonnes. A similar size boat with a lighter displacement such as a Fulmar would be fine with a 25, and I believe this is the most common replacement engine for the original 20hp.
 
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