On 2008-09-23 14:45:05 -0400, "Roger Long" said:

OK. 30 foot waterline at 5 knots would be a speed length ratio of .91.

My boat should be fairly similar in characteristics, heavy but with a
modified keel. Look at the graph towards the middle of this page:

http://home.maine.rr.com/rlma/StriderFuelSystem.htm

Waterline length of my boat is 26 feet so equivelent speed would be
4.64 knots. She reaches that speed at 1560 RPM with 4 POB. Horsepower
at that RPM is only 4.2.
Displacement is 13,500 pounds or 6.03 Long Tons so .70 HP/ton. Your
displacement is 18,000 or 8.03 tons so you need 5.59 HP to go 5 knots.
.056 gallons per HP per hour is typical for most diesels so you would
burn .31 GPH. This doesn't sound like much but you are talking about
a pretty slow speed.

At an economical speed length ratio of 1.15, typical cruising speed for
most sailboats, 6.3 knots for your boat, you would be burning a bit
over twice that. At a speed length ratio of 1.29, flat out, you would
be burning about 4 times that.

Also depends upon the efficiency of the prop, hull & keel shape, wind,
wave action, patience, etc.

I suspect your estimate is not far off, as our 2GM20F has a minimum
burn of about .26 gph, which I measured from about 3 knots (1200 rpm)
through about 4.5 knots (1800). At our normal cruise of 5-5.5 (2200)
it's about .33 gph. 3200 (6.5-7.4 knots), we burn a bit over 1 gph.

[Yes, our waterline is only 22.5 and we shouldn't to better than 6.65,
but those are multiply-checked speeds.] [and yes, the multi-day runs at
3 and 4 knots were excruciating.]

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/