Figuring Cruise speed??

[Boilermaker]

I have an older (1971)18ft fiberglass Modified Vee (deep bow) with a 1978 Johnson 115.>>> W.O.T. is 5500rpm with a 17" pitch prop. <br />What would be the "cruise" RPM ??? Do I just find the lowest throttle setting that will keep the boat on plane?? Some where higher then that??? And will this be the best fuel mileage??? <br /> (how the heck do you figure this stuff out, anyway???) <br /><br />Thanks <br />Jim

 

[Hooty]

Re: Figuring Cruise speed??

'kay,<br />Assuming a 1.87:1 gear ratio and at 5500rpm your calculated speed, without prop slippage is 47.35mph. A rough estimate of your fuel consumption is 11.5 gph at wfo. Reducing your engine speed to 4200rpm (approx. 35mph) will probably half your fuel consumption and be a comfortable cruise speed.<br /><br />c/6<br />Hooty

 

[rodbolt]

Re: Figuring Cruise speed??

hello<br /> yep what hooty says. that motor likes to run. try to keep it someplace between about 42-4800 and it will be happy<br /> good luck and keep posting.<br />on that big boat you will have 15-20% slip giving an actual speed of probably 35-37 mph.

 

[Boilermaker]

Re: Figuring Cruise speed??

Wow>>>15-20% slip>>>I figured (read guessed) abt 10%. And I thought that engine had a 2:1 gearing???<br />Anyway>> thanks for the info>>>I kind of figured about 4500RPM was a good speed to keep the engine happy, Although it doesn't seem to mind runnning up near peak R's.

 

[Hooty]

Re: Figuring Cruise speed??

It won't have any problems spinnin' 6000-6100rpm. It likes it up there!<br /><br />c/6<br />Hooty