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mercruiser 4.3L alpha one speed prop

I see they make a sleeve for the stainless prop. I am hoping to test a 17p stainless before buying. All the research I did indicates I will gain approx 300 rpms and not lose much speed.
 
My 305 V-8 Mercruiser runs 4,500 rpm and 50 mph with a 21 pitch. With my "water skiing prop" 19 pitch, it revs way over 5,000 (don't keep it there) and makes about 40-45 mph (if I keep it there).

Your experience may differ.

Jeff
 
just an update. I'm getting closer to having this dialed in. took the boat out today with an aluminum 14.5x17p. lost 2.5mph but the motor ran at 4400-4500wot. the old prop was an aluminum 14x19p. I think if I get a stainless steel prop 14x17p I'll get some speed back and stay in the 4400-4800wot range. any input?
 
As I said in an earlier reply I believe that if an aluminum prop say 17 pitch is the correct prop for your boat and you go to a stainless steel you need to go down one pitch number to get the same wide open rpm.

so in your case a 15 pitch would be the correct one. But if the marina is willing to let you try the 17 pitch then do so!!

Again I am no expert in prop theory but that is my understanding.
 
I tried a 14x17p aluminum prop and it seemed to be right. WOT rpms were 4600, top speed was 34mph with four kids and two adults. I think a 14"x17p stainless will be the right prop. The marina said I would lose about 100rpms with a stainless and gain approx 2-3mph. That should be about perfect.
 
How did it all work out in the end ? I guess that the 19P prop you had was overloading the engine at WOT... and the 17P is better matched.
For reference: I have a Mercruiser 4.3MPI Alpha One Gen 2 with a 15 1/2 inch 17P stainless prop on a 21 foot Fibreglass Cuddy Cabin boat. I can manage 30 knots at 4000rpm and 32 knots (37mph) at 4400rpm with just me in the boat and a full fuel and water load (just tested this and measured speeds from 3000-4400 rpm).
This matches the theoretical curve really well ... using pitch (17 +1 for cupping), 1.81 gear ratio, 10% slip. And it matches Couches formula quite well for a heavy planing boat (mine is 2000kg)
 
daveboater said:
How did it all work out in the end ? I guess that the 19P prop you had was overloading the engine at WOT... and the 17P is better matched.
For reference: I have a Mercruiser 4.3MPI Alpha One Gen 2 with a 15 1/2 inch 17P stainless prop on a 21 foot Fibreglass Cuddy Cabin boat. I can manage 30 knots at 4000rpm and 32 knots (37mph) at 4400rpm with just me in the boat and a full fuel and water load (just tested this and measured speeds from 3000-4400 rpm).
This matches the theoretical curve really well ... using pitch (17 +1 for cupping), 1.81 gear ratio, 10% slip. And it matches Couches formula quite well for a heavy planing boat (mine is 2000kg)
Click to expand...


Thanks for the reply. The 17p stainless was a great find for the boat. Wot was around 4350 (a little under recommended) and top speed was 37mph. I added a four barrel over the winter so I am excited to take it out. I think that will get me in the wot range and push the boat around 40mph.
 
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