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Dear Prop experts, how is diameter related to pitch and speed?

O

overdrivegear

Member
Hello everyone,

I have an 84 Bayliner Capri with the little aq125a that runs like a champ. Right now, I'm running a Piranha 14x19 cupped prop getting 4100 RPM at WOT and about 32 MPH. It works great, awesome fuel economy, smooth etc, but the RPM is a little low according to the manufacturer. I know I should be more in the 4400 range for WOT. I also have an aluminum (non-cupped) 15x17 that I put on to pull tubes and skiiers. That prop gets me up to 4500 PRM no prob.

What effect will there be going from a 14x19 cupped Piranha to a 15x19 cupped Piranha? I understand how pitch and slip works, but I don't quite understand how pitch and speed is related to diameter. Would going to a larger diameter with the same pitch bring my RPM down even more?

I'm just trying to squeeze another couple MPH out of my boat.
 
Think of the diameter and pitch like you would a car's ring and pinion. A bigger prop will be harder to spin. So to get the WOT rpms up you want a smaller prop. I'd do the 15x17 all the time.
 
Greater diameter, pitch and even cupping will slow rpms. Basically anything that makes the prop harder to turn in the water will slow the engine down. Stick with whatever keeps your engine running where it ought to be. Disconnect the speedo and just have fun? :)
 
I agree with James.

Any prop(s) that you try, will be completely dependant on whether or not the correct WOT RPM can be reached.... and not only reached, but reached for the anticipated load.
Load = both weight and pulling load... I.E., passengers/gear and whether pulling tubers/skiers or not!

Don't cheat the system.
Keep your WOT RPM within specs, or you will risk over-burdening the engine.

This may mean two props.... one for each usage.
Two birds one stone in that you'll have a back-up/get me home prop should you damage one.
 
overdrivegear said:
So changing diameter has the same effect as re-pitching? In other words, going from a 14x19 to a 13x21 will theoretically give you a zero net gain?
Click to expand...

If only it were this simple. Fact is, prop selection is more art than science. The best thing would be to have a dozen or so props, a stopwatch and a perfectly calibrated tachometer. You'd try each prop over a measured mile and use the stopwatch to determine the best speed. But all that is pointless unless you are racing. Just pick one that's in the ballpark and go have fun.
 
Props are as easy as they are complicated!
Staying within one brand and type, the basic rule is as follows:
Change pitch 1" and the revs change about 200.
Change the dia 1" and the revs change about 100.
Add a cup to a non-cupped prop and the rpm changes 1-200.
To obtain maximum 'push' at lower rpm and when pulling skiers/tubes, max cruising economy -add dia (increase blade area) and reduce pitch.
To get max top end speed, reduce dia and max out pitch.
To increase 'grip' when trimming, turning etc, add cup.
If going with pitch above 19", SST is recommended as the alu props may have too much blade flexing.
 
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