finding the right prop

[eggsuckindog]

slip- is a calculation reagarding the efficency of the prop, you plug in your criteria - prop pitch - RPM- speed - LU gear ratio and the calculator throws a number up - 15% and under is good 20 or above you need to try something

[Bushwacker]

Originally Posted by GodsReel

will you fellows explain slip? Thanks

Think of the prop as working like a wood screw, where a prop with a 17" pitch travels 17" for each revolution if there was NO slip. Add gear ratio, engine rpm and throw some constants into the equation to convert minutes to hrs and inches to miles and you can come up with a theoretical mph (with NO slip) for a given rpm. The % difference between the theoretical and actual mph at the same rpm is slip.

[shine]

latest prop is big 15.25" by 17 pitch 3 blade from mercury. basically same as as Yamaha SW series II

42 mph at 5900, makes for 12% slip. Windy day, good sized fishing load (3 guys, gear, 80 lb ice, half full boat well, 50 gallons gas.

I'm pretty happy, did get a little venting in the sloppy conditions of Saturday. I could probably stand to lower the engine by one notch.

I think Im happy for now, might try to find an 18 one day

[Bushwacker]

Originally Posted by shine

. . . I could probably stand to lower the engine by one notch.

Before you lower motor, check where the AV (anti-ventilation) plate is when you're running up on plane, trimmed out where you normally run. Top of plate should be out of the water! Min planing speed is also a good thing to check when comparing props because bracketed boats tend to be a little stern heavy and often won't stay on plane below about 20 mph or more. A good stern lifting prop will help it stay on plane at a lower speed, like 12-13 mph, and makes the boat less throttle sensitive in a following sea.

[76Red18]

Hey Shine those numbers look pretty good. If you have a prop shop nearby, have them add more cup to the trailing edges so the blades get a little better grip and that cavitation should disappear. Might even be able to raise the motor another notch as Denny stated.

[shine]

Originally Posted by 76Red18

Hey Shine those numbers look pretty good. If you have a prop shop nearby, have them add more cup to the trailing edges so the blades get a little better grip and that cavitation should disappear. Might even be able to raise the motor another notch as Denny stated.

might just do that! I want to get the fuel meter hooked up first so I have some better data. Of course, before that I have to get lights working, put in some LED's, install washdown, install spring locks on hatches, etc .......

[GodsReel]

what is my slip with these numbers 17 pitch, yamaha four stroke 225 LU gear?,44mph, 6000 rpms? Three blade prop.

[McGillicuddy]

Originally Posted by GodsReel

what is my slip with these numbers 17 pitch, yamaha four stroke 225 LU gear?,44mph, 6000 rpms? Three blade prop.

-2% in other words the prop slip computation calls bs on your numbers.

In theory the fastest your data would allow is 43 mph. I don't recall what Yammie youre running but you might double check your gear ratio. Seems they were 2:1 for a long time.

There are a host of prop slip calculators on line just plug in the numbers and brag

Here you go The actual calculations are above the calculator if you want to practice old school math:
http://go-fast.com/Prop_Slip_Calculator.htm

[shine]

without doing the math myself, the merc prop calculator says you have 7% slip with those numbers..

http://www.mercuryracing.com/propell...calculator.php

[McGillicuddy]

oops, I guess misinterpreted the listed 225 to be 2.25 gear ratio as opposed to being hp.