Logo
FULL SITE MENU

Prop Suggestions

mill686

mill686

Regular Contributor
I have a 1984 Boston Whaler Sport 15 CC with a 50 HP Mercury 4-cylinder.
It had a 13 pitch prop that generated 30MPH, but WOT allowed Ron's at 6000...way over operating range of 5000-5500. I have another 16 pitch prop that gets 33MPH, but Ron's at WOT only reach 5000. Hole shot is slow with this prop as well.

I was considering a 4-blade too, the Trophy Sport, as my kids like to participate in water sports (skiing/tubing). Any recommendations as to pitch, or does anyone have a similar setup to offer their finding?
 
Thanks. With my 16 only at the bottom end of the rpm range, would that in a 4-blade configuration only lower them?
 
I'm thinking 4 blades only come in even # pitches. Talk to a prop shop...a 14 pitch 4 blade should be about just right.
 
Yes, I would be thinking 14 pitch 4 blade (same diameter as current prop or at least within 1/4" - diameter also affects rpms, but is often overlooked).

You might want to have a look for a "Propulse" - it's a synthetic (plastic of some sort) "variable pitch" prop. I run one on my 140 Evinrude that allows me to vary between 15 and 21 pitch. For your motor I believe it comes in a 11 to 17 pitch model.

You can't change pitch while underway (you have to remove the prop to set it) but it would allow you to "tweak" things for particular outings - lower pitch when you have a boat load of kids you are towing around - higher pitch if you are out by yourself.

Price is comparable to a middle of the road aluminum prop, plus it's totally modular. So if you toast a blade on the rocks, you replace that single blade (used to be about 25 bucks).

West Marine used to sell them, but a websearch would give you an idea of availability.
 
If the 13 is maxing out at 6000 you ought to have a good hole shot. However, if your boat is deep V or heavy, 50 hp isn't a lot of ponies. It takes ponies for a fast hole. 13 to 16 is a 3" change which usually amounts to a 150 to 200 rpm change per inch meaning you would be loading your engine up a max calculated value of 600 rpm which would drop you down to 5400. Since you are down to 5000 something else is coming into play.

The 16 can't possibly improve your hole shot unless it is ported (holes drilled in the side of the barrel under the leading edge of the prop) as you are putting more load on the engine when it needs it the least.

You didn't mention what the two props were, but apparently the 16 is not as high performance as the 13 or the boat performance changes things when the speed changes which is the case a lot of times.

A 4 blade will drop rpms with everything else constant. I think you are supposed to drop 1" in pitch when you go from a 3 to a 4 for similar engine loading. Course dropping an inch will probably drop your speed also. Depends on the boat's performance characteristics. You have more blades in contact with the water so you have more work for the engine. For a 4 blade to work for you, adequate HP is required to make use of the added water contact. 4 blades are said to help in hole shots (big engines) and maintaining slow planing speeds.

My 2c,
Mark
 
Back
Top