Dinghy prop - No more 8-3/8 diameters?

[livendive]

I have a 9.5' inflatable with a 5 hp Mercury 2 stroke. The existing prop is 8-3/8 by 7 pitch, and it'll plane me (230 lbs) and my smaller dog (50 lbs) with no issues, however can't quite get me and my wife (125 lbs) up on top. The prop is pretty dinged up, and the last time I tried to replace it locally, I ended up with a 7.8" by 7 pitch that would only just barely get me on a plane, by myself, if I really worked at it (crawl forward, then back). I boat on the Columbia River, where typical currents this time of year can exceed 5 knots, and even in low flow season can get to 4 knots in the narrower sections, so planing at around ~13 mph is much preferred to plowing, which can barely move me upstream at all (guessing ~6-7 mph). Resorting to online shopping, I first went to Mercury's prop selector page, which, for my setup (assuming 5000 rpm and 12 knots) recommends 8-3/8" by 8 or 9 pitch. The problem is, I can't find anyone who sells such a thing. I ordered one, and got a 7.8", which I know from past experience, doesn't work. I've called a couple other shops that were advertising 8-3/8", but also reported I'd get 7.8", unless I was willing to drop to a 6 pitch. I'm not sure which year Merc changed their spec, but I'm discouraged about the prospect of finding the size I want in stock.

So what's my best option here...
7.8 x 7 (which I know sucks)
7.8 x 8
8.4 x 6

Or is there another manufacturer (besides quicksilver/mercury) that can get me where I want to be? Or any way to squeeze a tad more oomph out of my 5 hp 2 stroke? Otherwise, I guess I'm shopping for a beater 9.9 hp, at which point I won't care about the prop.

 

[steelespike]

Re: Dinghy prop - No more 8-3/8 diameters?

While diameter can affect rpm; usually if any affect decreasing diameter increases rpm.
Are you sure something other than the prop isn't affecting there motor.
It is normal sometimes for diameter to decrease when pitch increases and vice versa.
Increasing pitch will lessen power.If you still have the original prop you may be able to have it repaired at a prop shop.
Not usually cost effective on a small prop but in your case it may solve your problem.
I'm surprised that it struggles.Is it a 3 blade?Did you try different trim settings?
Are you sure the inflation pressure of the boat is right?
If you need to change props logic says the 6" will give you more power and could solve the planing problem when the wife is along.