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On Saturday 25 September 2004 9:56 am in rec.boats ZanderU wrote:
The number is determined mainly by marketing issues. In general it is the speed at which the boat was designed to operate in a continuous cruise with reasonable comfort and fuel economy. The tank range is usually specified at this speed. The word "reasonable" here is rather subjective and will be interpreted by the brochure writers according to the market they are targeting and/or competitor figures. ... This sounds right since (for displacement hulls) I consistently see cruise speed as just under hull speed. I would be much more interested in the speed that would maximize range. That would be quite a bit lower than cruise speed then! This is where you have to do some simple sea trials. Assume that you always want to arrive with 20% fuel in reserve for contingencies. If you want a more conservative contingency adjust the numbers below. Take the maximum design load (which you will usually need when leaving on a long crossing), with the weight of full fuel tanks. Fuel up to 20% and add ballast to get to the weight at which you would arrive (max - 80%fuel). Now do trials at various speeds and plot the fuel consumption. Repeat with 60% and 100% fuel, to get halfway and departure consumptions. With any luck you now have 3 lines on the graph which will give you the optimum consumption speed at 3 loads. These should be about the same but slightly worse at full load. If you have things like aircon and stabilisers you need to make sure that you include these in the test, this may mean adding in the generator consumption if your generator needs to be running. Keep these graphs and compare them with actual daily fuel usage - this will tell you when something is going wrong such as fouling or wear. In practice you will find that heavy seas and weather will also have an effect. -- My real address is crn (at) netunix (dot) com WARNING all messages containing attachments or html will be silently deleted. Send only plain text. |
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