Comparing fuel consumption

[boatmike]

I am trying to compare fuel consumption over a given distance for semi displacement boats like Aquastar 33 and the equivalent full planing offshore cruiser of the same waterline length. Assume say that both boats have twin Volvos of 200 HP each then the fuel consumption PER HOUR should be the same for the same RPM i guess. The semi displacement boat however will be working very hard to do say 15/16 knots while the planing boat should use considerably less fuel once up on the plane and for the same useage of power will go much faster. This means I guess less fuel usage over the same distance. But how much less? And is it most economical to go no faster than the lowest speed the planing boat gets up on the plane or is it more economical to go faster using the engines full power? All this will depend on sea state of course so the question has to be if conditions allow. Anyone got any figures?

 

[jakew009]

Assume say that both boats have twin Volvos of 200 HP each then the fuel consumption PER HOUR should be the same for the same RPM i guess.

Fuel usage is a function of the engine load, RPM is an irrelevance.

 

[Bandit]

Get in touch with the guys that did Aquastar on tour around the UK with two Aquastar 38’s on with 2 of VP 370 on with 1 x Cat 500 and ask them.

 

[Bandit]

Get in touch with the guys that did Aquastars on tour around the UK with two Aquastar 38’s on with 2 of VP 370 on with 1 x Cat 500 and ask them.

 

[boatmike]

Fuel usage is a function of the engine load, RPM is an irrelevance.

I think you missed the point Jake. Same engines same RPM under load will mean for that given load the planing boat will go faster therefore consuming less fuel per mile.

Get in touch with the guys that did Aquastars on tour around the UK with two Aquastar 38’s on with 2 of VP 370 on with 1 x Cat 500 and ask them.

I know how much fuel an Aquastar 33 consumes. I have got one! The question is how much less fuel will a fully planing boat of similar waterline length consume because for the same RPM and engine load it will go faster. In very approximate terms if 2500 RPM on 2 TMD41a gives 12-14Knots on the Aquastar and on a planing hull gives 22 knots over an hour the planing hull will consume the same fuel but go further. My question is really aimed at owners of planing boats with similar size engines. What is your most economical speed and how much fuel do you use?

 

[Portofino]

Jake’s correct the rpm and load are unconnected when considering fuel burn .
Its folly to assume thus .

 

[boatmike]

OF COURSE the load on the engine is what determines fuel consumption. I am a mechanical engineer and take that fact to be obvious. HOWEVER that is NOT the question I asked. Every vessel will be different but it is screamingly obvious that if an engine is capable of producing say 200HP at 3000 RPM the gearbox ratio and propeller design will be determined for each vessel the engine is fitted to relative to its water line length and type of hull to maximise the speed of the vessel in question . The gearbox ratio and pitch/diameter of propeller on each vessel in question will be different. If the designer has done a good job the load on each vessel will be very similar at the same RPM. What will be different is that the gearbox ratio and prop design on a displacement vessel designed for a maximum speed of 16 knots will be different to a planing hull designed to achieve 25/30 knots. Once a planing vessel gets up on the plane the hull resistance reduces. A displacement hull increases. My question relates to fuel consumption over distance travelled. At full WOT it is unlikely that a semi-displacement hull like the Aquastar 33 will ever achieve more than 16-18 knots with a total of 300HP available. At full WOT a planing hull with the same engines will go considerably faster but I have no experience of how fast in practice such a vessel can be expected to go as I have never owned one.
If you go back to my original question I asked what realistically one can expect from a modern planing hull with a similar WLL that's all. There are lots of boats out there of 33/34 ft with planing hulls. Perhaps I should simply ask what owners of such vessels achieve as a max speed and what they find it most economical to cruise at wit 300/400 HP available.

 

[JerseyDiver]

I think you missed the point Jake. Same engines same RPM under load will mean for that given load the planing boat will go faster therefore consuming less fuel per mile.

I know how much fuel an Aquastar 33 consumes. I have got one! The question is how much less fuel will a fully planing boat of similar waterline length consume because for the same RPM and engine load it will go faster. In very approximate terms if 2500 RPM on 2 TMD41a gives 12-14Knots on the Aquastar and on a planing hull gives 22 knots over an hour the planing hull will consume the same fuel but go further. My question is really aimed at owners of planing boats with similar size engines. What is your most economical speed and how much fuel do you use?

Rodman 1120 comfortable at 18/19knots slow cruise 50L per hour total for twin 260HP. I was previously moored next to an AQ33 and both hulls are quite close in length as the 1120 has the pullpit and bathing platform.

 

[Bandit]

You need to look for litres per nautical mile at cruising rpm ignore the rest.

 

[prinex]

For what is worth - I have 2x dph360 volvo on my boat (41 foot, 7 tons, Abbate Primatist G41).

At 24 kn it burns 2x40 liters an hour and 2700 rpm - that is 3.3 lt/mile and this is our crusing speed. Each engine delivers around 150 hp at this RPM (reading from the curve on the datasheet).
At 8 kn it uses 2x8 liters an hour and 1200 rpm - that is 2 lt / mile.
At 7 kn 2x4.5 liters an hour and 1100 rpm - 1.3 lt/mile and this is our "slow cruising" speed
At 4.5 kn 2x1.1 liters an hour and 600 rpm (this is with the gears just engaged)
At 0 kn 2x0.9 liters an hour - this is just the consumption in idle no gear engaged.

between 8 and 24 there is not really on option as you would be in the "take off" phase before planning and you would only put strain on the transmission / engine etc and it would not make sense at all to burn the same amount of diesel but only 1/3 of speed.

So basically the choice is 3.5 times the cruising speed at the cost of 2.5 more fuel increase. We often play "go fast or go to the restaurant" as around 3 hours slow instead of 1 fast pays dinner for 4 people at the current diesel prices.

 

[westernman]

You will find that most diesel engines will consume about 0.25 litres per hour per horse power over a wide range of their power output.

 

[boatmike]

Thanks to Jersey Diver and Prinex for some useful data. Engine consumption data is easy to come by looking at data sheets but its the hull resistance of the boat that determines how fast you go for the available HP and that data is not so easy to find.
Interesting that prinex gets about 40x2 litres when developing 150 HP each engine as that's near what I get WOT on my 2 TMD41a lumps in my Aquastar I think which are 150HP each. Might be even a bit more than that. Difference is that in my semi displacement hull won't go as fast as its shoving a whole lump of water around and has less waterline length. Never did determine consumption at 7-8 knots accurately but its probably similar to prinex too or might be a little less. I think Jersey diver is making my point though, in that his boat is far more easily driven and goes faster for less HP. That's what I am trying to get my head around. Now I am an old fart of 80 I am happy to swan around at 7 knots in my comfy seat in the Aquastar but I am toying with changing to something that would go a bit quicker and burn less fuel but I dunno if it's worth it!