Prop Pitch Mystery

[BobG]

OK, I boat at a LOT of different elevations from about 3,500 feet above sea level to about 10,000 feet above sea level. Colorado is amazing that way. Of course, at 10,000 feet, my 115 is only putting out about 90 horsepower. I have to choose my pitch, based on the elevation of the lake.

Now, here's the mystery: I do a lot of my boating along the Front Range - about 5,000 feet elevation. If I run a 13" prop, I'm banging off the rev limiter at 6,200 rpm. If I go up to a 15" prop, I'm at 4,800 RPM trimmed all the way down, and if I trim it up, I can get that up to about 5,300 RPM, but the speed is the same - 23 MPH. So all the trim is really doing is increasing the slip. That's just the way it plays on my boat.

My prop guy just sold me a 14" pitch prop. It's a Mercury Black Max. I've never seen a 14" pitch offering anywhere, including Mercury's web site, but this guy carries them in aluminum AND in stainless. We'll soon see where that 14" pitch prop puts me at WOT!

So much for the "each inch of pitch gives you a 200 RPM change" theory!

[Bamaman]

I received a 14 pitch prop on my F150 when new 7/12, and it was pushing 6300 rpm's. It was a Yamaha M series stainless steel prop.

I replaced it with a Reliance 15 pitch, and it's great--right at 6000 rpms.

[rbiederwolf]

I found on my boat when I had a 115 Merc that each pitch gave me closer to 400 rpm difference.

[Shopguy]

1200 rpm in 2" of pitch is a lot of diff. Are they the same brand of props? rake and diameter must be different too.

Props are like shoes, the same number size on two diff brand of shoes don't always fit right. But, 1200 RPM?! what's up with that Bob.? And always check rpm tilt down.

I really need to drop from 17 to 15. Most aluminum props listed are 15x15, so I'll try it. I usually have a full load and can only pull about 5g's with the 17p.


D

[BobG]

1200 rpm in 2" of pitch is a lot of diff. Are they the same brand of props? rake and diameter must be different too.

In fact, my first two were the same - Solas 13.75 X 15 and 13.75 X 13. I exchanged the 15 for a second 13. Then I received a 13.87 X 15 Merc for my birthday - it behaved about the same as the 13.75 X 15 Solas. The 15 will be fine for when I'm at "low elevation" - 3,500 to 4,500 feet. Then the 14 from 4,500 to about 7,500, and the 13 above 7,500.

[BobG]

Update:

Started with the 14" pitch Mercury prop this weekend, on Blue Mesa Reservoir - elevation 7,500 feet.

Topped out at about 5,400 RPM, and 21 MPH. Looks like the 14 is just a bit too much at 7,500 feet.
Swapped on the 13" Solas, and saw 6,200 RPM and 24 MPH.

We'll see how the 14" pitch does at lower elevation this weekend. We'll be at Pueblo Lake State Park, elevation 4,880.
If I ever get to McConaughy in Nebraska, the 15" pitch Mercury should be just the ticket.

[Bryden24shp]

Slip numbers are coming in at 29% for the 14 and 23% for the 13.
Holy Crap!
I come up with a Merc Black Max Extra Cup, part #77340CP1 14 diameter X 13 pitch. Should get you into the 12 to 18% range, which is totally acceptable for your set up.

http://www.ebay.com/itm/Mercury-High-Th ... 0549312558

May be worth a shot!

[kryptonite]

Slip numbers are coming in at 29% for the 14 and 23% for the 13.
Holy Crap!
I come up with a Merc Black Max Extra Cup, part #77340CP1 14 diameter X 13 pitch. Should get you into the 12 to 18% range, which is totally acceptable for your set up.

http://www.ebay.com/itm/Mercury-High-Th ... 0549312558

May be worth a shot!

Yes, go with the extra cupped prop, they work much better on a pontoon.

I tried my cheap backup solas 13p on my buddies tritoon with a 115. It had terrible slip.