Spare/second prop choice?

[eavega]

So, I have found what I believe is the correct prop and setup for my rig and the way I want to run it. I have a 1976 yarcraft Yankee 15' tri-hull bowrider with a 50 HP motor. I get it running at 6K RPM WOT loaded with about 700 Lbs (boat capacity is 825) at 26 MPH (by GPS) using a 13p 3-blade aluminum Solas propeller. I did need to install smart-tabs in order to get the boat to plane reliably. According to a prop-slip calculator, I am looking at about a 7% slip on that setup, which is acceptable (Theoretical top speed is 28 MPH). I am now looking to have a second/spare prop. I'll mostly be using the boat for fishing, with loaded weight being at most the 700 Lbs I tested with, usually less (two adult fishermen, plus maybe a kid or two, second battery, anchor, maybe a trolling motor, bait, and cooler). My secondary use would be either just cruising or pulling a single-rider tube with just a driver and a spotter in the boat.

I originally had a 17p prop on the motor, but could not get the motor to rev above 4800 RPM wit this prop on, even with the smart tabs. I really can't explain why I broke the 200 RPM per inch of pitch rule where stepping down 4 inches resulted in a 1200 RPM gain (300 RPM per inch) to put me spot-on in the operating range of my motor. The only explanation I have is that perhaps there was some invisible defect with the 17p prop? The prop had no visible dings or defects, but that's not to say that it may not have been somewhat fatigued since it looks like the original prop that was put on that motor in 1972.

Anyway, I digress. My question is; for my second prop should I just stick with a 13p aluminum prop, step up to a 15p prop for possibly a little better performance when carrying a lighter load (i.e. tubing), or maybe go with a four-blade or stainless prop? Thanks in advance for suggestions and comments.

Rgds

 

[metriccrescentwrench]

Re: Spare/second prop choice?

I really can't explain why I broke the 200 RPM per inch of pitch rule where stepping down 4 inches resulted in a 1200 RPM gain (300 RPM per inch) to put me spot-on in the operating range of my motor. The only explanation I have is that perhaps there was some invisible defect with the 17p prop?

That's not a RULE, more of a scientific wild a$# guess. You decreased the pitch of your prop by nearly one fourth (4/17), which would have about the same effect as decreasing your gear ratio by that much. Your prop pitch decreased by about one fourth, your rpm's increased by about one fourth, seems about right.

IMHO, I doubt if you'll see much more top end with a 15 pitch prop. You might see if you can borrow one just to see, but I'd bet you'll be happier with the improved hole shot of the 13.