Cruising at 5-7 Knots in a Power Boat

[twistedtree]

Assuming you are operating below displacement speed and running a diesel, the dominant factors in fuel consumption will be your speed and the boat's displacement. How many engines and their full power HP rating is mostly a distraction. It takes a certain HP to move a boat of a given weight at a given speed - that part is physics. Generating that HP takes fuel, and engines convert fuel to HP at a pretty consistent rate. A big engine putting out 50HP burns just about the same as a smaller engine putting out 50HP. And twins putting out 50HP combined will burn about the same fuel as a single putting out 50HP. Not EXACTLY the same, but all the engine choices are 2nd and 3rd order contributors to fuel economy.

[Cheechako]

I dont know, someone must have data on a popular trawler type with one vs two engines. I do think there is a space in the power speed that your statement would be correct, but not convinced running two 150 hp diesels at 8 knots will be the same fuel consumption than one. Maybe I'm wrong though. I know it's not true for planing boats as I've done the testing extensively. But that is different.

[David M]

A modern diesel engine producing 250 horsepower will burn roughly 6 gallons per hour. If your boat cruises at 7 knots at 250 horsepower then you are getting roughly around 1.16 miles per gallon.

To do an accurate calculation of fuel mileage you would need to know what horsepower will produce what speed. You will also need to know the engines specific fuel consumption which is measured in horsepower produced divided by the fuel burn rate.

The specific fuel consumption is available from the engine manufacturer. The horsepower required to reach X amount of speed might be available from the boats naval architect or its builder.

Originally Posted by larrylwill

Can anyone tell me what kind of mileage one could get with a 35-30ft motor cruiser with a single engine in the 250hp range, cruising at 5-7 knots? Mostly flat seas. Any kind of power boat with a decent living space. Like a Cris Craft or something like it. Using it as a displacement.

Anybody ever bough one with a blown motor and put in a small diesel just for cruising?

The estimate that it takes 250 horsepower to push a 35 foot trawler 7 knots is not correct. The correct number is going to be much much less than 250 horsepower. My best guesstimate is that it is going to be somewhere in the neighborhood of 100 horsepower.

[TeddyDiver]

The power needed is round 25 to 40hp so anything over 80hp is a huge overkill. And big engines and twins do use much more fuel, even running them idle without any output consumes more than smaller ones at half speed. Just more internal resistance.. try to crank one by hand if you have any doubts

[twistedtree]

Two engines will burn more than one, I agree. And a smaller engine will generate 50HP burning less fuel than a large engine generating the same output.

What I'm saying is that these differences, though real, are much less significant than displacement and speed in determining overall fuel consumption.

Here's a real example from twin Cummins QSC 500s in a 30 ton boat. All figures are the sum total for both engines:

Idle in neutral, 0.6gpm - let's call this the parasitic load

1000 RPM, 7.8 kts, 3.6 gpm, 2.17 nmpg. At this low speed the parasitic load is about as bad as it gets percentage wise and comes in at 16%. Dropping one engine would save 8%.

1600 RPM, 11.4 kts, 16.7 gpm, 0.68 nmpg. Now the parasitic load is only 3%, and dropping one engine would only save 1.5%

At higher RPMs it just gets smaller.

Now I've made a simplifying assumption, i.e. that the parasitic load at idle is the same as the parasitic load at 1000 RPM and at 1600 RPM, and this isn't correct, but I think it's close enough to make the point.

[AnchorageGuy]

Our Ford Lehman 120 burns 0.6 GPH at 1350 RPMs, 1.2 at 1450, 1.4 at 1550, 1.6 at 1650 2.2 at 1750, this is our normal cruising range at about 7 knots, 2.4 at 1800. But at 1950 it jumps up to just over 3 GPH. Those are not estimates or guesses, they are actual usage. Chuck

[twistedtree]

Originally Posted by David M

A modern diesel engine producing 250 horsepower will burn roughly 6 gallons per hour.

I think that number is off by a factor of 2? I've seen a variety of references to 0.06 gal/hr/hp. That would put 250hp burning about 15 gph. I checked the specs for my Cummins QSC (a modern common rail diesel which in theory is the state of the art) and it comes in at about 0.05 gal/hp/hr, or 12.5 gpm when producing 250 hp.

[Wavewacker]

What about a 32' Bolger Tenesse, or Wyoming, or the variations with a 15 to 25 hp outboard?

[u4ea32]

Originally Posted by twistedtree

I think that number is off by a factor of 2? I've seen a variety of references to 0.06 gal/hr/hp. That would put 250hp burning about 15 gph. I checked the specs for my Cummins QSC (a modern common rail diesel which in theory is the state of the art) and it comes in at about 0.05 gal/hp/hr, or 12.5 gpm when producing 250 hp.

Exactly what I was going to say.

Rule of thumb:
1 gph in a diesel engine makes 20 HP.
1 gph in a gas engine makes 10 HP.

Power required to move a boat at displacement speeds is primarily a function of waterline length and displacement, and its an exponential curve: a little more speed requires a LOT more power. Try rowing your dinghy and you'll see that it takes almost no effort to get going, but it takes quite a lot more to go faster, and even a world class athlete won't be able to plane.

If you want to cruise around using very little money, as it sounds you do, you must adjust your expectations to go slow: assume your cruise speed is the square root of the waterline length: a 36 footer at 6 knots.

Since the power is so non-linear, a small decrease in speed can easily halve the power requirement, so a gas engine at 5.5 knots probably will get the same MPG as a diesel at 6 knots.

Gas engines have advantages: quieter, cheaper, happy to run at very low power loadings forever.

Diesel engines have advantages in this use: a tiny bit faster for the same mileage, and extremely long life (decades) if its a low powered diesel (say 30 or 40HP for that hypothetical 36 footer).

Gas engines have disadvantages when not brand new from the factory: they don't run well without very good electricals, and these degrade rapidly especially in salt water. Also, everything that not the block, crank, and pistons deteriorates quickly, including exhaust manifolds, valve springs, valves, carbs, wiring, ignition, oil pan, couplers, fuel filters, ...

Diesel engine disadvantages include smell (a lot of people get sea sick smelling diesel fumes or exhaust) and noise. They are also sensitive to aging exhaust issues. Diesel is far more sensitive to junk in the fuel, so really good fuel filtering is REQUIRED.