What is the most efficient cruising speed?

[Peter & Judy]

Thanks for the interesting data. I have experimented with my boat on every trip as I work out fuel consumption on every cruise. I do it the old fashioned way, by looking at the fuel tanks and my distance records on the GPS, so my numbers are less accurate than yours are. I tend to run a fairly heavy boat, with two kayaks on top. Most often I am loaded with supplies for the average two week journeys that we go on. I have found that there are about two speeds that we feel comfortable travelling at. For cruising I tend to travel at about 2000 RPM which has me moving at about 5.5 knots. When I need to or want to pick up the speed, I go on plane at about 4400 RPM, which gets me about 12 -14 knots depending on conditions. This is with my 90 HP Honda. These are speeds I like to travel at. I also like to give 'Gretzky" my Honda 9.9 HP Kicker a workout at least every other day and will spend an hour or more travelling on it at WOT which will move me at near 5 Knots. Fuel consumption is really low with the kicker. For me a normal fuel load is about 200 litres with my two tanks and my 45 litre slip tank. I'm Canadian, so we no longer use miles or gallons, and our gallons were bigger than your gallons. My current range estimate based on travelling with 90 HP at 2000 RPM 75% of the time, 25% on plane and also using the kicker for an hour or so every other day. I now use the 'easy math' that if I have 200 litres of fuel, I should be able to travel about 200 NM.

[Tom Hruby]

Yes there will always be variability between boats, motors etc. We can however try to normalize to variables that are universal. David Gerr in his Propeller Handbook attempted to do that using his knowledge and empirical data. Using his equation I calculate that it takes 2 shaft horsepower (1500 watts) to move 1000 lbs of boat at hull speed, and there was not much difference at this speed between displacement and planing hulls. Based on the data I have seen this number is +- 15% of what folks have found. The power needed at lower speeds is reduced much faster than the speed (a hyperbolic curve).

So, based on this equation it should take 6000 watts (8 shaft hp) to move a 4000 lb 22ft C-Dory at hull speed and 2000 watts at 5mph. This comes out to be 400 watt hours per mile or the equivalent of about 1.6 Oz of gasoline per mile (a gallon of gas holds 33,000 watt hours of energy). Any higher rates of consumption are a result of the ineffiencies of the gas motor. I have found that Gerr's equation does provide a reasonable value of the actual electrical energy I have needed to power the four electric boats I have had over the last 26 years .

[robhwa]

CDfolks; for the sake of international and nautical folks...here is a reinterpretation of my data, including the estimates for the 90. I only did two serious estimates based on using an external tank, warming things up, moving with no/low tides or significant wind, and placing the external tank on an accurate scale. I used 6.07 lb/gallon as density for gasoline.

Again, remember that this is:

1) just my boat, relatively lightly loaded, just me and no gas in the main tanks
2) I tried to keep under low wind and tide
3) I expect less mpg when boat is well loaded

I'd be thrilled to see similar data from other folks. If you have it, you can use your multifunction display to adjust rpm/speed, etc. You really don't need to know exactly.

[T.R. Bauer]

I have logged an hour or two on my boat and found your numbers to be perfectly reasonable. Thanks for sharing them. As somebody noted up in the thread, 17-18 mph does seem to be the best cruising speed, in regards to efficiency, for my boat too. I have an idea why....maybe because of a sweet spot in cam grind, conservative rpms, and a light throttle setting resulting in a more efficient curve. Pure speculation on my part.....