135hp 3.0 mercrusier w/ Alpha 1 drive prop sizing

sdrgac

Recruit
Joined
Nov 29, 2012
Messages
3
Any help would be appreciated....

I have a 2006 Fisher 24' Pontoon that needs to be re-propped. I bought the boat used and it had a bad prop that was not original and I replaced it after talking to a local boat mechanic but it isn't right as I cant get WOT up to 4800. Motor was tuned 3 years ago.

What I have now is a 15 x 17 Prop. Engine WOT is around 3800 with a GPS speed of 18 mph fully loaded (6 adults, coolers, and gear) in calm conditions. Pontoons are clean and polished with little to no drag. Altitude is around 5000 feet.

I do occasionally pull a tube with a couple of younger teenagers behind so I would like to get a good speed to power ratio. Any help is appreciated.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Altitude and excesive load is killing the combo performance.. Try a 3 blade alum or ss prop in the 13-15 pitch range to pull wot revs higher towards max wot rpm range.

Right now engine is being lugged running 1 K under what should be (4800) to rev happily as loaded.

Happy Boating
 

ThomW

Chief Petty Officer
Joined
Aug 8, 2016
Messages
615
As a general rule, every inch/degree you drop the pitch of a prop you will gain about 200 rpm's. That is a very general guideline, and there are many variables--engine running condition, load, water conditions, altitude...

Hypothetically, if you went from a 17 pitch to a 13 pitch--4 inches/degrees--you should gain around 800 rpm's.

Hope this helps a bit.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,306
my guess is the local "mechanic" knows nothing about proping 'toons.

you are way over-propped. drop down to a 13 pitch
 

sdrgac

Recruit
Joined
Nov 29, 2012
Messages
3
Thank you all for the input. i have ordered a new 15x13 prop. have a great day. What speed should i see with a 13' pitch?
 
Last edited:

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Impossible to tell, prop must be water tested as usually loaded on flat calm water cond. Report max wot rpm gained with a 13 prop to see if dials + 200 rpm per 1 pitch decrease.

Happy Boating
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Yeah way over propped...I’d be tempted to get a dealer to let you try a 15” high 5.
 
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