Another Fuel Economy Question

[Tony B]

I am looking at several different trawlers at the moment. The Marine Trader 34 typically has a 120 HP engine with a cruising speed of 6K. A 36' Morgan West Indian has a 160HP engine with a cruising speed of 8K. Being retarded when it comes to motors, I made some simple calculations:

180HP divided but 120 HP = 1.333 larger.
8 Knots divided by 6 Knots = 1.333 Faster.
Looking at the above it would appear that the consumption ratio is the same exact as the Speed/Distance ratio.

Does this mean that if I am burning 1.333 times as much fuel and traveling 1.333 times further I am getting the same MPG from both vessels?

[DeepFrz]

I doubt it very much. I think you will find that at 8 kts you will be burning much more fuel than you will burn at 6 kts.

The Nordhavn Dreamers Group on Yahoo has covered this very extensively with cruisers with many miles under their belts sharing their experience. If you are looking at a long range cruiser you might like to join that group. Also Scott Flanders on Egret has many miles and a circumnavigation. He has published his fuel consumption figures a number of times in his blog.

[MarkSF]

One does 6000 and the other 8000? Cool!

Or did you mean knots?

The abbreviation kn is preferred by the International Hydrographic Organization (IHO)

[marshmat]

Tony,

Neither engine will be producing its rated output at cruise speed. The actual power being produced at cruise could be anywhere from 1/3 to 3/4 of maximum.

And different engines react differently to low-load running; many older design diesels are inefficient and may wear out faster if run at low load for a long time, while some modern computer-controlled diesels do very well at half throttle.

And a hull's resistance curve is usually far from linear, especially so when talking about displacement or semi-displacement monohulls.

So it's not nearly as simple as "33% more power, 33% faster". Without a resistance curve for each boat and a set of performance graphs for each engine, any generalizations are vague at best.

[MarkSF]

Originally Posted by Tony B

I am looking at several different trawlers at the moment. The Marine Trader 34 typically has a 120 HP engine with a cruising speed of 6K. A 36' Morgan West Indian has a 160HP engine with a cruising speed of 8K. Being retarded when it comes to motors, I made some simple calculations:

180HP divided but 120 HP = 1.333 larger.
8 Knots divided by 6 Knots = 1.333 Faster.
Looking at the above it would appear that the consumption ratio is the same exact as the Speed/Distance ratio.

Does this mean that if I am burning 1.333 times as much fuel and traveling 1.333 times further I am getting the same MPG from both vessels?

180hp is 50% larger than 120 for a start, while 8kn IS 33% larger than 6.

[motion30]

My 34' Mt with a 120 runs around 7knts (8.2mph) at 1700rpm burns 2.3gph

[ATC]

My 44' Thompson (weighting 37k) with a 4-53 Detroit Diesel rated at 165hp doing 1300 rpms (7 knots) burns 2.38gph.

[sigmasailor]

Fuel used against speed (in water) is a third power. Soooo, going 8 knots will use 8 (yes: eight) times the amount of fuel of going 4 knots. I'm talking about speed up to hull speed here.

Eric

[EricGrab]

The speed/length ratio of a boat is important to understanding efficient running of a full displacement boat. The formula is speed divided by the square root of the waterline length. Units in knots and feet. Example: 7 knots, 38 foot waterline equals S/L of 1.29. Turns out when you reach S/L over 1.3 the hull will make a wave longer than the waterline. When this happens the boat tries to "climb" that wave, and efficiency goes way down. Not all boats are efficient at S/L of 1.3, but it should generally be pretty close. So get your waterline length, and it will tell you an efficient speed.

[David M]

Does this mean that if I am burning 1.333 times as much fuel and traveling 1.333 times further I am getting the same MPG from both vessels?

No, in full displacement mode speed vs horsepower is not a linear function. It is closer to a cubed function.

The way to calculate miles per gallon is to get the engine manufacturers horsepower vs consumption rate vs RPM curves. You will then need to run the boat to determine what speed it gets at what RPM. Converting RPM to horsepower you then get a speed versus horsepower curve.

I have plotted these curves for my boat and it helps to have a fuel consumption meter. Many newer engines come with these. You will need a GPS and figure out the average for both directions to eliminate the effects of wind and current

[fastcat435]

Originally Posted by sigmasailor

Fuel used against speed (in water) is a third power. Soooo, going 8 knots will use 8 (yes: eight) times the amount of fuel of going 4 knots. I'm talking about speed up to hull speed here.

Eric

This shows very nicely in the graph made of the Green Motion electric propulsion system below , with a diesel engine the same applies off course