prop choice and gear ratio questions...

wca_tim

Lieutenant Commander
Joined
May 28, 2007
Messages
1,708
I've done some searching and have read a number of threads on this topic already - thanks loads for the info guys!

current set-up: 1988 18.5' shallow v, new reman 4.3 that I would guess is turning somewhere from 225-250 hp? (little higher compression pistons, factory marine spec marine roller cam, Vortec heads, electric fuel pump, edelbrock carb and intake, careful tuning / jetting, relatively open exhaust, etc...). It really seems to run very well overall and has great power from off idle through wot. Currently pulling between 53 and 56 MPH (GPS) depending on wind, chop loading in boat, etc... with 23" Laser II at WOT of ca. 4800 rpm. Gear ratio is 1.84 per the side of the outdrive. Best speed is when trimmed to where can just start to see a evidence of "rooster tail" (talking a few inches here...) in the center of wake. Note that even a little below WOT, this is where it's at, and the last bit of throttle increases noise a little but no change in engine rpm or speed. Currently has a decent holeshot, pulls good size skiers right out ot the water, etc... Note that I'm not thinking that I'm going to make a huge difference, mainly playing / learning. I'm planning on getting something a little bigger (say 22-27) with a lot more engine in the next year or two and honestly bought this boat to play with and learn on before stepping up to much larger dollar mistakes. I bought it with a trashed factory 2bbl in it, had a local mechanic drop the engine in (had a borken shoulder at the time) and have done the majority of the work since then myself.

Questions:
1. Does the fact that I have more throttle left with no corresponding increase in rpm or speed, indicate that I'm coming out of the power band for the engine as it's set-up? (though about trying a step up on main jets, but spot checked plugs from wot shutdown look about right as far as I can tell - med tan, etc...).

2. If so, what prop might be best to try? Seems like many of the 25" or higher props I see out there are designed for heavier / hogher horsepower boats and have thicker blades, etc... might not be as efficient in transmitting power to the water? Any place to get good information on blade shape, etc...? Also, not sure if more or less bowlift is desirable for a little more top end in this kind of an aplication.

3. If I changed the gear ratio (would likely just change out the upper half of the outdrive - replaced the lower a month or so ago when sheared the lower drive shaft) say to 1.65 or even 1.5 and went with lower pitch propo until wot in the right place, how much of a gain might one see from that difference? I've read that around 19" is probably optimum in getting power to the water in blade speed ranges that are seen with typical I/O applications, and also have the impression that lower less reduction in gear ratio (all other things being equal) corresponds to less power loss in the drive system.

4. What if any effect might trim tabs have. I get a little porpoising now and it planes at about 16-17 mph. Note that I'm asking the question not in terms of improved handling, time to plane out, etc... but in terms of tweaking top end.

4. Any other factors that I'm not considering that I should? In the context of trying to wring as much top end out of current set-up as I can...

Thanks in advance for any thoughts you might have!
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: prop choice and gear ratio questions...

Sound's like that boat is running at it's peak, trim tab's will hold you up at lower planing speed's a increase your hole shot, other than that i see no gain's............but i will watch this thread with intrest..:D

Umm then again there is this little cutie.........

24502609_large.jpg




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__________________


Clean's the drool.........:redface: Shameless desire....
 
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