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Old 09 April 2007, 12:58   #11
Country: UK - Scotland
Town: Ardfern
Boat name: Moon Raker
Make: Humber Destroyer
Length: 5m +
Engine: Honda BF 90 D
MMSI: 235035994
Join Date: May 2006
Posts: 694
Originally Posted by BruceB View Post
AFAIK the only differences between a Yam 60 and 70 hp are the carbs and the exhaust. Presumably yours is much the same as the Yam
Yes, it's a Yamaha powerhead.

Originally Posted by Hightower
Kohaku which is a 5.3m RIB with a Merc F90 (same as yours) did 2-1.5nm/l at cruising (4000rpm) on a cruise where WOT was used most of the time she was drinking it at 1nm/l.
Interesting, same sized boat and engine. I seem to get an average of 1.7nm/l at 3250-3500rpm. Guess it would be 1.5 at 4000.
Originally Posted by jwalker
You're going to need x amount of power to sustain y amount of speed and, for a given efficiency, you'll use z amount of fuel. So it's going to be down to using the engine at its most efficient speed. Just reducing its maximum possible power is not likely to do it unless, by doing this, you also improve efficiency
Yes, as I thought. Guess I was wondering whether running at higher revs would improve efficiency. I'm thinking not.
Originally Posted by wavelength
I think you have the answer in your own hands-just dont drive flat out! We have a 90carbed merc 4 stroke on a 5.5 Ocean Pro. More economical than our Suzuki 90 injection 4 stroke and have never had the situation where the fuel "just disappears" at any speed. Need to drive a 2 stroke V4 to see fuel vanish
I think you're right. You've made me feel so much better - I had the illusion that the modern injected engines were much more economical. Guess 'disappearing fuel' is all in the mind. I'm still getting used to engine power after many years using 'free' (apparently) wind. You've reminded me. I did have a part share in a Glastron speedboat in the late 60's, with a big Evinrude on the back. Buried it as an unpleasant memory, I think.

Thanks all for thoughts, ideas and advice. I'll leave things as they are, keep the throttle arm short, lie back and think of the poor chaps in the Falklands.

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