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.