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My 10 ton ship, long-keeled, driven at about 5.5 kt has a consumption of about 9 nautical miles per UK gallon, or about 7.5 nautical miles per US gal, in flat calm.
The motor is a 35 hp diesel, when used flat out, can reach 8.5 kt. It's a while since I did that.
Work the engine hard, and you'll think there is another hole in the tank.
Get someone to help you. Choose a flat-calm day, with no tide. Pull the fuel hose off the tank fitting. Get a wee calibrated jug and fill it up with diesel.
Get your stopwatch and your calculator. Run your motor at various speeds and calculate the fuel consumption yourself.
It must be flat calm. Into a headwind it will alter. Into a chop the ship really drinks fuel. That 5.5 kt can become 1 kt into a wicked chop.
Write your figures down, and you can plot a wee graph to work out your optimum cruise.
To save fuel, run the ship slowly. It can save enormous amounts of fuel. The state of the hull surface also matters. In a wee sea trial about a decade ago, at Stonehaven Scotland, 1700 rpm with a wee thin weed growth meant a speed of 5.5 kt. Next morning, same conditions, with the weed film removed, that 1700 rpm became 6.6 kt.
That's quite a hit.
I simply cannot keep all the weed off the ship all the time though. It's nothing like the same problem in fresh water though, well, here at least.
Last edited by Rockter; 01-31-2009 at 08:15 AM.