Seem like the right way to go ?

RCAnderson

Seaman
Joined
Oct 5, 2011
Messages
68
Howdy all, I need to get approx 300 more rpm out of my engine to meet the WOT specs for my engine.

Engine - Mercruiser 888 (ford 302)
Leg - MC-1 with I believe a 1.65 to 1 ratio (someone who knows for sure please correct me if I"m wrong)
Current WOT - 3650 rpm verified with digital tach
Design WOT - 3800-4000 rpm
Max boat speed - difficult to determine because of the currents around here, but say 30 mph
Boat - older Zeta, 24 feet, weight is HEAVY (5\8 thick fiberglass, no foam) deep V hull, I suspect about 6500 lbs
Current Prop - 16x16 3 blade aluminum

Talking with one of the prop shop guys here, he's going to re-pitch my prop to 14" pitch. I have a spare prop that came with the boat, and is a 14.25 x 17.

What will the boats performance be like if I go ahead and re-pitch to 14" ? I suspect better hole shot, and my cruise speed\max speed will be different, and my fuel usage will change as well, but by how much. I don't really want to be cruising at only 20 mph (current cruise is about 26-27 mph @ 3300 rpm). What speed should I expect with 14 pitch running the engine at approx 3400 ?

Thanks
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
Re: Seem like the right way to go ?

assuming your a re-pitching the 16x16 - my guess is your top speed will be 29 mph and your speed at 3400 will be about 24.... this is just based on the prop calculators. I know when I went from 17 to 15 i lost 1 mph - but my boat is happier for it. I gained 500 rpm at least but the prop i had was really old. Typically you gain 300-400.... so you should be right around 4000
 

RCAnderson

Seaman
Joined
Oct 5, 2011
Messages
68
Re: Seem like the right way to go ?

Yes, re-pitching my 16 x 16. I am wondering about overall fuel consumption. If the engine is working less hard at cruise, even though it may be at a slightly higher rpm, will I be using more fuel ? Maybe the throttle position will not have to be open as much to maintain cruise speed\rpm, and so maybe a decrease in gallons per hour ? I'm not sure, but I think I may try it.

RCA
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
Re: Seem like the right way to go ?

i think I'm doing a little better on gas when I went from 17p/4500 rpm to 15p/5000 rpm.... but it might all be in my head. Boat's stink on fuel economy as you know.
 
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