The weekly prop question

"Got back from my maiden voyag

"Got back from my maiden voyage yesterday. '86 Bayliner Capri 16' (the short one), AQ131D - 275. I'm also at about 6000ft elevation and never plan on running it lower.

Anyway, the boat has a 15x15 on it. It actually pops up on the plane pretty good but WOT with me and the dog was 4200 and more like 4000 with the wife and kids in the boat. I also murdered my 40 year old muscles trying to get up on a slalom ski about six times.

So I did some homework and have a basic understanding of how pitch and diameter affect WOT and low end torque. I can't remember my recommended WOT RPM but it's about 5000 right? So I would want something like a 15x11? (200 rpm per degree pitch?) I know I could also go smaller diameter but am a little unclear as to why one would change diameter as opposed to why one would just change pitch.

Oh, and I've read some of El Pesc's posts about cupping. What exactly does that do? Is it for torque or top speed?

Any recommendations or explanations welcome."

"Need to go the other way Bob.

"Need to go the other way Bob. More pitch, i.e. 15x15 equals 14x17 in pitch. You would need a 14x19 or a 14x21 to get up to wot (~4800-5100). Lowering the pitch lowers wot.

Cupping is useful for a better hole-shot, i.e. torque.

HTH

Ron"

"Actually, increasing pitch lo

"Actually, increasing pitch lowers WOT RPM. If you are doing only 4200 RPM you should try a 14x13 or 14x14, that should bring your RPM to about 4800. You can also try a 15 x 11 as you suggested, but don't expect the boat to go too fast.

Cupping makes the prop "bite" better in the water and prevents ventilation, so with a cupped prop there is less slip than with an uncupped prop. Therefore, cupping has the same effect as adding some more pitch to your prop at higher end RPM."