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

[MM5AHO]

My Rival 32 consumes about 1.1 litres per hour at 4-5kn, and nearer 2 lph at 6 kn. It has a 29 HP MD2030. I think fuel consumption is related more to effort expended within a range of engine sizes, but a larger engine working less hard over a long period doing the same work as a smaller one working hard, is likely to last longer life.

 

[sailorman]

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.

So you are actually using about 20hp cruising spd in the Hansa

 

[Birdseye]

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

It isnt correct. At the same speed the power used will be the same and in an ideal world that would mean the same fuel consumption. But the world isnt ideal. Not all engines have the same efficiency and in any case a three cylinder engine will have greater pumping losses than a 2 cylinder one. But at the same speed it will likely be a bit more but no way will it be proportional.

 

[CAPTAIN FANTASTIC]

If the boat is heavy displacement, the 40hp engine will be more suitable and likely to be more economical in the long run.

 

[GrahamM376]

I uprated from 35HP to 43HP, 1300cc - 1800cc, same gearbox, same 2000 cruising revs, noticeable higher consumption, by around one l/hr. 3.5 to 4.5.

 

[Iliade]

If the lower HP motor(s) struggle to drive the boat against a wind/slop/tide they could easily consume a vast amount more fuel and time than the more powerful option running for fewer hours to achieve port...

 

[charles_reed]

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

Whilst that might be better than a guess - my boat, with a 23hp engine uses an average of 1.2litres/hr diesel. That includes a lot of motor-sailing with a self-pitching prop.

In the OPs example, without knowing hullform, prop size and pitch, boat weight and waterline length any guess would be just that.
All I would venture, is that a new engine is generally more economic than an old one.

 

[Tranona]

It isnt correct. At the same speed the power used will be the same and in an ideal world that would mean the same fuel consumption. But the world isnt ideal. Not all engines have the same efficiency and in any case a three cylinder engine will have greater pumping losses than a 2 cylinder one. But at the same speed it will likely be a bit more but no way will it be proportional.

That is not so. If you look at the specific fuel consumption figures in the HP band for cruising you will find very little difference from engine to engine. So a 30hp engine and 40hp engine will use the same fuel when 20 hp (cruising speed ) is demanded. The fuel consumption of a 2cyl 16hp engine at 10hp is exactly the same as a 3cyl 20hp. You can check on the power curves in the technical specs for Beta engines.

 

[Norman_E]

Without knowing what boat it is its impossible to say which engine will be best for your needs, but I doubt that any difference in fuel consumption will be significant, but if you have to drive the boat into a strong headwind and choppy sea you will almost certainly be glad of the bigger engine, assuming it is fitted with the correct propeller.
I am aware that some makers fitted the same prop even though they provided a choice of engines. My own boat is a Jeanneau 45.2 and was fitted with an 88hp Yanmar but the same 20 x 13 prop that the Standard 62hp engine used and the same Kanzaki gearbox. It made a big difference when I changed to a 21 inch Max Prop pitched at 15 inches. Consumption at 7 knots in normal conditions is about 4.5 litres per hour at 2,250 RPM.

 

[lw395]

Without knowing what boat it is its impossible to say which engine will be best for your needs, but I doubt that any difference in fuel consumption will be significant, but if you have to drive the boat into a strong headwind and choppy sea you will almost certainly be glad of the bigger engine....

Or some sails.....

 

[jac]

Or some sails.....

Depends how well you go to windward.

If a typical cruiser, low aspect rig & fin, wide sheeting angle, slightly baggy sales then after leeway you could be at 60 degrees to the wind even at 6 knots boatspeed your VMG ( ignoring tide) will be 3.

CHuck in some tide and you're going nowhere. That might be fine if you are sailing for a month or so.

If it's sunday lunchtime and you have work the next day then it might be a case of engine on and motorsail home.

 

[blxm]

In a perfect world there would be no difference. However, in the real world there will be many variables that are different in the two engines. Mechanical losses, gearbox, prop size to name a few. There will be additional losses in some areas and gains in others. It would be almost impossible to say which option would be better in terms of long term consumption without detailed analysis of all these various factors. However, I don't think it would be far off to assume that for practical purposes the fuel consumption would be broadly the same for speeds likley seen when crusing. If I had to guess, slightly worse for the larger engine but not to an extent to make me worry. Personally I would have a larger motor, within reason and budget! Can't think I would be likley the say ' I wish I had a smaller engine' in a tough situation.

 

[syTaika]

That is not so. If you look at the specific fuel consumption figures in the HP band for cruising you will find very little difference from engine to engine. So a 30hp engine and 40hp engine will use the same fuel when 20 hp (cruising speed ) is demanded. The fuel consumption of a 2cyl 16hp engine at 10hp is exactly the same as a 3cyl 20hp. You can check on the power curves in the technical specs for Beta engines.

Please note that the curves found in the data sheets for Beta do not tell the fuel consumption at different hp. The power curve and the fuel consumption curve are not directly linked together. The power curve shows the maximum available hp at different rpm. However the fuel consumption curve tells the consumption at different revs when the prop is optimally dimensioned. In optimum case the boat reaches the max speed at the max rpm and the prop takes the max power from the engine. At this specific point only. When the rpm is then lowered, the power demand will drop rapidly and will be much less than the maximum available power at the same rpm. This makes it a bit complicated to compare different engines.
In practise a bigger engine will have a bigger prop and thus provide a higher top speed. As a result it will run at lower rpm at the same cruising speed than a smaller engine. This means that the fuel consumption will probably not be much higher than with the smaller engine.

 

[reeac]

So you are actually using about 20hp cruising spd in the Hansa

No, the good old 20 bhp.hours per gallon of fuel rule of thumb says that he's using about 9 bhp.