Fuel Consumption Puzzle

[Bowman Sailor]

I repowered my sailboat when I bought her 15 years ago. At that time I selected a low reduction ratio and a two-blade prop. I've never been satisfied with her performance when powering into head seas. So, two years ago I replaced my transmission with one with a higher reduction ratio so I could spin a three-blade prop and have the engine running higher on the power curve. The performance results both before and after the transmission and prop change are summarized below:

Vessel: Bowman 46 fin keel sloop, 30,000 lb. displacement, 32 ft waterline
Engine: Perkins 4-108M
First Transmission: HBW-150, 0 to 1,034 hours
Replacement transmission: ZF-12M transmission = 1,034 - 1,315 hours

The original setup with the lower reduction ratio and two-blade prop gave the following results:

• Hurth HBW-150 with 1.88:1 reduction
• 18X12 two-blade prop
• Cruise: 6.8 knots @ 2,000 RPM
• Max RPM = 2,250
• Fuel consumption @ 6.8 knots = 0.85 gph
• Poor performance into head seas

Here are the results obtained since changing to the higher reduction ratio and three blade prop:

• ZF-12M, (HBW-125), 2.63:1 reduction
• 18X11 three-blade prop
• Cruise: 6.3 knots @ 2,700 rpm
• Max RPM = 3,200
• Fuel consumption @ 6.4 knots = 1.2 gph

In addition, I've been working through a number of issues that I believe are related to the higher engine speeds and loads since changing the reduction gear and prop:

• significant increase in oil leaks, (front & rear crank seals, valve and lifter covers, etc)
• Engine over heats if pushed beyond 2,700 RPM, (changed both pumps, cleaned heat exchanger, cleaned water injection nipple, replaced all flexible and exhaust hoses)
• lots of noise and increased engine vibration
• I'm frustrated about burning more fuel to go slower

The boat does, however, have a lot more punch when motoring through head seas. So, why do I burn more fuel to go slower? I figured that I would be producing the same horsepower to go 6.8 knots regardless of the engine speed, since the engine should be doing about the same amount of work.

Any Ideas?

Steve