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.

--
Roger Long