Please, need help on prop decision

This afternoon I went out to the kankakee river for a little fishing. While out I hit a rock while idling, luckily I had the release up. When I got back to the launch I found no damage from the rock. The damage I found was from one of the two output shaft bearing cap screws missing. It had backed out and chewed through the back of the prop. So here's my dilemma. The boat has good acceleration, cruises at 23 mph with me and my year, 17-18 with two guys. Btw this is a 1992 Evinrude 9.9 converted to 15. The prop on the boat is an aluminum 9.5x10. my rpms are 6330 at full throttle, manual says 5500-7000 rpm. BUT.... It is rated 15 hp at 6250 rpm. So do I go ahead and buy a solus 9.25x10 stainless and stay at 6330 rpm? Or do I go to say a 9.25x9 to increase rpm. Or 9.25x11 to try and get a little more "speed" out of it?

Here is my boat, it isn't lite either. Front and rear decks out of aluminum, 3gal gas can, battery, trolling motor and my tackle bag and rods.

From what I am reading it seems I either need to go with a 9.25x10 stainless and have an approximate increase of 200 rpm. Or go to a 9.25x9 aluminum and increase aprox 200 rpm. I don't know the rpms when I was loaded with 2 guys but I do know I was running 18 mph. I would assume turning the motor a little faster with a lesser pitched prop would help when I have an extra guy but not really effect my speed when alone? After all, I am running below the manufactures spec for WOT RPMS. Any insight would be great!

Nobody has any input? I would like to pull the trigger on my parts today if possible.

FWIW I always pitch my props so with the typical load - even if/when that is a heavy load - the OB turns no more than 200 RPMs off the OEM rated maximum. The WORST thing for an OB motor, less not adding oil, is to lug the motor and use too steep or fast a pitch prop. Lugging can make an early death for a motor ...

I've never fooled w/ converted motors, so can't advise if you should consider 7K as the max, sorry.

Alright, thank you. If that is the case then I need an 8 pitch aluminum or a 9 pitch SS? Assuming the more efficient SS prop would yield me an extra 100-200 rpm at WOT. I know there's always the answer of "buy a few and try them" but for me it's just not practical to spend that kind of coin to "test" props. Thank you!

Also, the conversion is full. Carb and exhaust leg.

See if a 15hp comes up on that prop calculator from Turning Point Props that I posted. You can factor in the EXACT weight you carry on your boat. Another bonus of their props is they have an exchange feature, where you trade in in for another one.

They don't make smaller props like those on the 8 to 9.9hp motors, but maybe they have yours.

Well I just spoke to a boat dealer up by me. They said with my setup not to run SS as it will not really net me anything as far as performance. However, I should go from a 9.5x10 to a 9.5x9 or in solus' case a. 9.25x9. He recommended this due to the fact I am only turning 6330 rpm full throttle. If I turn a 9 pitch I should be more in the range of 6500 rpm. IE: better whole shot, and better top end performance due to being more toward the upper rpm range.

Will this dealer let you try a prop?


CMOS

As far as I know the dealer will not allow it. I went ahead and bought a 9.25x9 aluminum prop. Also, per Mr. ken at prop pros I should try to raise my motor another 1/2" or so. Currently I have my motor spaced up an inch but I get some spray out back. This could be due to the fact the boat cruises best when the motor is in the furthest IN hole.

Sounds like a good choice on the pitch. Let us know how she runs with it.


CMOS

I have searched high and low for a spec on how many RPMs on average you will increase when staying at the same prop pitch yet going from aluminum to stainless. Anyone have any idea of how this effects rpms? I do know prop design and manufacturer can very this, just looking for a round about.

Well I took a trip to Lake Michigan this afternoon after work for a little small mouth fishing. Got a little testing done with the motor as well. With the 9.25x9 prop I now run 20-21 mph with no significant increase in acceleration. WOT RPMs rose to 6600 from 6330. The boat seemed to handle better with the 10" prop. Plus I do kinda like the extra 2-3 mph. However, when at wot it does seem to have a more broad power band due to being higher rpms.

Also, I took some pics while on plane. Currently the motor is 1.5" above the transom and the anti vent plate is about even with the bottom of the hull. At this point I feel I need to move my battery up front. It seems the boat may be rear heavy when planing causing the motor to go even deeper into the water than usual. Heres a pic. From what it appears, the motor needs to go even higher.

I have made the decision to go ahead and buy a 25 hp as well. I am thinking a nice used 2 stroke Merc...

The level of the anti-vent plate looks good to me. You should be more concerned with the level of the plate with respect to the WATER surface, rather than the relationship with the hull bottom. AV plate should be even with or just slightly below the water level. Yours looks good.

Oh, the 25 HP motor - go Johnson/Evinrude.


CMOS

I am going to be working on moving the battery up front this weekend. Just need to re design my wiring system slightly.

Also may be picking up a motor. Found a pretty good deal on a Merc 25. I know, not a evinrude but should still work fine for me.