More powerful engine option - will it use more fuel when cruising same speed?

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Ric

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I am looking at the market for a particular sailing yacht that is offered with either 29hp or 40hp engine options. The hull speed of the boat is about 7knots. Will the cruising range of the boat under power be significantly less with the more powerful engine, if cruising at the same speed?

I am thinking that as the power required to drive the boat through the water is the same, the more powerful engine will deliver the power at lower rpm so will be operating at a different thermodynamic efficiency. But most diesels have very similar specific fuel consumptions and efficiency so I think the difference will not be very large.

Of course the more powerful engine will use more fuel if used to drive the boat at higher speed or into a heavier sea, but that is an entirely different scenario.
 
Generally no it should be the same if using the same power within reason. At low revs the bigger engine may use marginally more but if the smaller engine has to be run near its maximum to get hull speed it may be the other way around. I doubt if the additional weight of the larger engine will be significant so generally I would go large.
 
I think that you will find very little difference in consumption over that power range. I've recently re-engined from 17 to 25 hp. My, admittedly rough, checks show that there is perhaps a small increase in fuel consumption l/hr but I'm motoring slightly faster anyway, since the smaller engine was under-powered, so "mpg" is very similar.
Inefficiencies in prop selection would probably have more effect.
 
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A few days ago when my boat was being surveyed the surveyor (who is also a working marine engineer) asked me how much fuel she uses at cruising speed (about 5 knots)

I said that i had no idea and he said that as a rule of thumb if the boat has a 15 hp engine it will use approx 1.5 litres per hour and if it has a 30 hp engine it will use approx 3 litres per hour

If you use this theory you would assume the 29 hp will use 2.9 litres per hour and the 40 hp will use approx 4 litres per hour

Others on the forum may be able to say if this is correct or not

Regards Don
 
The larger engine will probably have a higher smooth quiet comfortable speed, so you will drive the boat faster, using a bit more fuel.
Also the bigger engine will use more fuel holding its set speed against waves etc, while the small one will be knocked back.
If there is no wind and you are organised to know the most economical speed and disciplined enough to stick to it, the range will likely be much the same.
In detail, it depends on the prop and gearing, as well as the characteristics of the two engines.
If both engines have fixed props, the big one will slow the boat more under sail.... So you'll motor more....

It wouldn't be a deciding factor for me, provided the smaller engine was adequate.
 
dom said:
He's not !!

It's consumed hp that matters.
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Which probably means that he is correct under certain circumstances. I think that if the two engines are both being run at normal maximum cruising revs, so say roughly 80% of maximum, the 10% rule is probably close to reality. Of course, at these revs, and with correct propping, the larger engined boat will be going faster.

Richard
 
RichardS said:
Which probably means that he is correct under certain circumstances. I think that if the two engines are both being run at normal maximum cruising revs, so say roughly 80% of maximum, the 10% rule is probably close to reality. Of course, at these revs, and with correct propping, the larger engined boat will be going faster.

Richard
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Not in my experience... talking to others about "practical" consumption (i.e. what their boat consumes at a similar cruising speed to us) our car tech Ford seems to be a bit more economical than Yanmar/Volvo (no Beta/Nanni to compare with)
 
Don107 said:
A few days ago when my boat was being surveyed the surveyor (who is also a working marine engineer) asked me how much fuel she uses at cruising speed (about 5 knots)

I said that i had no idea and he said that as a rule of thumb if the boat has a 15 hp engine it will use approx 1.5 litres per hour and if it has a 30 hp engine it will use approx 3 litres per hour

If you use this theory you would assume the 29 hp will use 2.9 litres per hour and the 40 hp will use approx 4 litres per hour

Others on the forum may be able to say if this is correct or not

Regards Don
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I hope my twin 165hp boat doesn't read this! I know it's taking things to extreme, but on the Thames at 5mph it uses about a gallon an hour. This is similar to my 50hp Broom (Albeit I often only run one engine)

It's speed and hull which make the difference IMHO :encouragement:
 
I doubt if the difference in consumption for a given speed will be enough to make an economic difference but there is a small downside to having a larger engine, which is that in light use and when motor-sailing it may not be working hard enough to get up to temperature quickly or run at a power which is best for it.
 
Don107 said:
A few days ago when my boat was being surveyed the surveyor (who is also a working marine engineer) asked me how much fuel she uses at cruising speed (about 5 knots)

I said that i had no idea and he said that as a rule of thumb if the boat has a 15 hp engine it will use approx 1.5 litres per hour and if it has a 30 hp engine it will use approx 3 litres per hour

If you use this theory you would assume the 29 hp will use 2.9 litres per hour and the 40 hp will use approx 4 litres per hour

Others on the forum may be able to say if this is correct or not

Regards Don
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This assumption is broadly correct BUT twill depend on HP being used @ cruising spd, the 29 HP may be utilizing say 25 HP & the 40 horse will do the same (ie 25 hp), so in theory both engines will consume the same its at full rpm the consumption will differ
 
Always get the bigger engine if you're a cruiser, choose the smaller one if you intend to race.

To push the boat forward at a constant speed against wind and water resistance you need a certain amount of energy - this is a function of the hull and underwater profile plus the above water profile. This will remain the same regardless of which engine is fitted.

That energy comes from the fuel in the tank, the engine, gearbox and prop just converts fuel into thrust, and a higher hp engine can convert more fuel into more thrust than a smaller one - but at a steady speed, side-by-side they are both doing the same amount of work and should be consuming the same amount of fuel, assuming the conversion efficiency of both drivetrains is pretty much equal (bigger engine might have bigger alternator and an extra cylinder which will result in more operational losses). Dependent on engine power, gearbox and prop size this will just show up as fewer revs on a bigger engine to achieve the same boat speed.

The advantage of a bigger engine is that there is more power in reserve when factors like strong headwinds or large waves come into play. You can't do much about max hull speed, but you can punch through waves or motor into a headwind better with a larger engine.

My boat was originally offered with a choice of 19hp or 29hp engine, I'm glad it was ordered with the 29hp engine.

The comment about a larger engine not really being under load and running cold could be misleading, it is very dependent on the boat in question, the 40hp might be ideal and the "option" of 29hp may be an underpowered lump for cruiser/racers to get them in and out of the marina. That's certainly what the 19hp option on my boat was there for.
 
I checked consumption on my 30" Jeanneau ( approx. 4 tons ) with a 18 Hp Yanmar and always used 1.5 - 1.7 ltr per hour at 2300 rpm giving me 5 kn.
Now I have a 29 hp Volvo on my 34" Hanse (approx 7 tons ) and are using about 2.0 ltr/hr at 2100 rpm giving me 6 kn.
 
Fendant said:
I checked consumption on my 30" Jeanneau ( approx. 4 tons ) with a 18 Hp Yanmar and always used 1.5 - 1.7 ltr per hour at 2300 rpm giving me 5 kn.
Now I have a 29 hp Volvo on my 34" Hanse (approx 7 tons ) and are using about 2.0 ltr/hr at 2100 rpm giving me 6 kn.
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Just curious - how do you tell how many litres/hour you're using?
 
Go for the larger option - I doubt it makes much difference to consumption but it depends on revs and waves etc surely as others have said and if worried carry more spare cans? Bavaria fitted 19hp as standard on many 34s and while not underpowered and having good range I think the 29hp version is regarded as more saleable by brokers. I would have thought the same thought might prevail on a resale of a 29hp compared to a 40hp. It really depends on added cost of buying the larger one though
 
Easy... Note engine hours in log when you fill up... when you fill up next, note engine hours in log, note number of litres filled... litres divided by hours difference = lph... My cruising rpm is about 1600, unless it's bang on the nose in a chop when it goes up to 2000
 
ashtead said:
Go for the larger option - I doubt it makes much difference to consumption but it depends on revs and waves etc surely as others have said and if worried carry more spare cans? Bavaria fitted 19hp as standard on many 34s and while not underpowered and having good range I think the 29hp version is regarded as more saleable by brokers. I would have thought the same thought might prevail on a resale of a 29hp compared to a 40hp. It really depends on added cost of buying the larger one though
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I would second this.

The diesel consumed will be virtually identical.
The servicing costs (unless ridiculous difference between the two engines) will be the same.

So your running costs will virtually the same.

There is an argument re getting up to temperature but again with the sort of HP difference we are talking here, I'm not sure it makes a difference. The Larger lump may be at 1800 rpm with the smaller one at 2200 but it's hardly comparing tick over to WOT.

Finally at some stage, you may really need more HP, possibly when fighting larger waves, strong headwinds etc. It's easy to reduce the power output from the large engine, reduce the throttle. It's impossible to make the small engine suddenly give you the extra 11hp.
 
as Gladys said, note engine hours when you fill up 100% and divide. Enter data into a spreadsheet and average over several years. My tank sensor did never work, so i had an engine hour sticker at the nav station telling me when I reached the 40% level in the tank.
 
johnalison said:
I doubt if the difference in consumption for a given speed will be enough to make an economic difference but there is a small downside to having a larger engine, which is that in light use and when motor-sailing it may not be working hard enough to get up to temperature quickly or run at a power which is best for it.
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That was certainly the case in my old boat a Bavaria 38. The first owner had specified a Volvo D2 55. It was just to big for the boat and anything above 2,000rpm just caused the stern to dig in and did not make us go any faster. I believe the standard engine was 40HP which I think would have been more than ample.
Of course whether it applies in the case of the O.P. does depend on the specific yacht, but if possible I would ask owners of the model concerned for their views on the size of engine that was suitable for it.
 
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