what size prop should I get?

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dsujen

Petty Officer 1st Class
Joined
Oct 16, 2004
Messages
252
I have a 1987 j90tlcur on a 1969 starcraft 16' open aluminum boat. The prop size that is on there is 13 3/4 x 15. The boat wildly accelerates and gets on plane in about 2 seconds. I have no tachometer or plans on getting one. What size prop should I put on so I don't over rev the engine and get more top speed. I estimate the top speed to be about 35 mph now.
 
gm280

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,590
So many questions on here about the size prop. First unless you know exactly what your WOT RPMs are presently, I can't begin to tell you what prop to try, and more then likely others would be able to either. And that is because without knowing if you are presently over the RPM limit, how could anybody tell. So the MPH are basically useless especially a guesstimation of speed... You will have to tach your engine with a tachometer and post those numbers on here for any serious help... JMHO!
 
S

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
I agree ,your motor needs to operate within it's rated rpm range.Too low pitch will result in that amazing hole shot but wildly over rev and of course you can't tell how much or when it is. Too much pitch will result in poor performance and the motor will lug using more fuel and eventually causing premature repair problems.
Your even guessing your wot speed.
Open aluminum boat doesn't tell us much. My open aluminum boat weighs 375lb dry weight.
All up weight is 1,000 lbs. with a 200 lb driver.
Is your boat 4 seats with tiller steering,no live well, steering,multi batteries, casting decks etc etc.
Regardless of the boat size and weight only one or 2 props from each type would work accurately.
An induction tach will cost $40 or 50 dollars be sure to consult with the dealer about your motor.
A gps for speed will get us accurate mph. Some cells have a gps app.
If the boat is a plain open boat I would start with a 21" prop GPS speed will give us a very rough idea of the rpm.
You need 4 numbers to estimate rpm;gear ratio,prop pitch,speed, and slip. Slip is tricky and a key number And we would be guessing slip.
With a gps,2.00 gear ratio and prop size we would have 3 of the 4 numbers. normal; slip could be from 7% to about 15%thus its key relevance
I would guess about 12% slip.
The calculator is only as accurate as the numbers input.
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
As already said, but I'll add my 2 cents worth... No tacho, all bets are off. And even a dash mounted tacho is only a rough indication, you need to put a proper, calibrated, shop tacho on it, just for the test... What you do is load the boat as you would normally, hook up the shop tacho and take it for a WOT run with normal trim. See what the revs are and make a decision based on those revs.... No guessing, no estimates... As Steelespike said, get it wrong and you WILL damage the engine. Revs too high, a leg pops out of bed, revs too low and pistons disappear down the exhaust system.... When I was a dealer doing engine repairs, I saw plenty of both... Do it right, get it right, or it will cost you an engine....

Probably not what you wanted to hear, but it is what you need to hear... Sorry...

Chris...........
 
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Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,496
there is a sticky at the top of the prop forum that says "Read First". please read it first.

Also, it would help to know exactly which 16' 1968 starcraft you have because according to the list of 16', outboard models, you are over powered with your 90 hp

http://www.fiberglassics.com/library/File:Starcraftbb005.jpg
 
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