Only 4500 -4800 RPMS- 90 HP Johnson on 20'SF

[Fr. Frank]

Smaller diameter will give you more RPMs. I think you'll need to go to a 15" pitch, though.

[Bushwacker]

In addition to a 13.75 x 15P prop, I also tried 13.5 x 17P and 13.25 x 19P aluminum props on my old 90hp-at-the-prop "115", (1975 vintage) on my Seafari. It turned the 17P up to about 5200; 19P only turned about 4900-5000. Best prop was 13.75 x 15P, which turned 5500-5600 in either stainless or aluminum versions. If the blades are badly chewed up on the props your testing, you've lost a bunch of efficiency and may be getting misleading results. I know several folks running 115's on 19-20' hulls and they're all running 15" pitch props! There's probably a pretty strong message somewhere in all that data!

[Fr. Frank]

The gear ratio makes a huge difference. I used to run a 15"P on my current Seafari when I had the 120 Chrysler. HP at the prop was about 95-100, and I turned 5200 rpms. The gear ratio on my Chrysler was 1.98, and I had a top speed of about 28-29 mph, and cruised at 23-24 mph.

Now that I have an '06 Mercury 90 hp Optimax, I have about 100-105 hp at the prop, my gear ratio is 2.33, I turn that same 5200 rpms, running a 12.75"x20"P SS 4-blade, with a top end of 32 mph, and cruise at 4500 rpms at 25-26 mph.

I was running a 3 blade Apollo SS, in 13"x21", with a top end of 34 mph at 5100 rpms, but it would fall off plane below 21 mph.

[Texascoastalfish]

Thanks again Boys for the great Data. I will try the 13.75 X 17 since I bought it. ( 15 bucks on ebay), and Ill get a 13.75 X 15 ( new). We will see what the prop and lifting engine will produce. Thanks again

[Texascoastalfish]

I took her out and ran the 13.75 X 17, and got 4900 RPMS at WOT and 31 MPH. Looking at the clearance between the bottom of Cav plate and the tips of the prop we are looking at 1/4" ,I also have another 13.25 X 17, and a bid on a 13.75 X 15. I'm 1" above keel with motor now. I'll try the smaller diameter 17 P first, then the 15 P 13.75. trying to get above 5200. Im 400 short now. I haven't checked compression yet, I am running with a Stingray Hydrofoil. Sounds like the 15 P is how I'll have to go being at minimum HP for that boat and keep the 17 P for spare. I run shallow alot in this Bay system and seem to nick the props often. Thanks in advance again!

http://i358.photobucket.com/albums/o...g?t=1231862144

http://i358.photobucket.com/albums/o...g?t=1231862304

[skipjake04]

I second what everyone is saying. but if you want call Mike @ Mike Hood propellers 713-649-7508. He is on Telephone rd., just north of Airport. There is not another better around here, I feel. It's pretty impressive to watch him in action.

[Texascoastalfish]

Update. I should have checked compression from the get-go.
After not getting above 4900 RPMS with still a 17P prop and motor 1" above keel, I checked compression and I found bottom portside cylinder 70 PSI all the rest had 120. Pulled head and found no visual damage, bore seemed smooth , no gouging, I did find a very small piece of aluminum in the head dome. Decarbed Cylinder 2X with Seafoam, no change. Put some 30 wt oil in cylinder and compression jumped to 90. Bad Ring(s). In stead of pulling powerhead and rebuilding 1 or all cylinders I opted for a little more HP and got a totally rebuilt, bored over and fully warrantied 1995 112 SPL from a local marine motor shop (cash is king these days, bargains abound) Its now more like a 120 HP due to .020 boring and bubbleback exhaust Vs. Flatback exhaust. Had it rigged yesterday and I'm ready for the break in procedure. I will hopefully be in the 5500 RPM range by Sunday I stuck with the 17P prop for the higher HP. I now have 90 Johnson on Craigslist and if I cant sell it for 750 bucks I'll rebuild and put it on a skinny water skiff like an Explorer or something real shallow. Never can have TOO many boats. ( in the end maybe 1 less wife if this keeps up !)