Prop help at altitude

[nervouswater]

I have poured through search results and got some of the basics down, but it seems the advice varies based on the specific setup, hence my post. this is my setup:

16' 2000 Pathfinder Tiller model (a 'tunnel hull' flats boat)
2001 25hp 4-stroke Yamaha F25ELHZ
9 7/8 x 10 1/2 3-blade prop (i dont know the rake or cup???)
Run primarily @ 5000', occasionally up to 8500'
Current top speed right around 18-20 MPH (NO TACH)

So I am running a boat that had to be spec'ed for sea level so I am guessing my OEM prop is likely not really right for altitude. i am also pretty clueless on what it takes to dial in a fuel injected engine for altitude, so i thought adjusting the prop was the way to go. Finally, the prop on there now is getting pretty dinged up and it's probably time for a replacement anyway.

My question: What size prop would folks recommend? Do I need to do something to the engine instead? Both? Any advice is very appreciated.

thanks in advance,
brian

 

[steelespike]

Re: Prop help at altitude

Unfortunately speed only indicates if the boat seems to be reaching a reasonable speed.Without the rpm there is no way to evaluate the setup.
Also without a tach there is no way to know if the setup and prop are right.
A tinytach is reasonable and easy to install.It will fit any 4 stroke with the same number of cylinders.I think a Fuel injection can adjust some to altitude
but 5,000 may be asking too much.It will be running rich at altitude.
Maybe they can set it up for 5,000.Get your tach and give us the stats.
I think under ideal conditions that setup would be around 30 mph.Assuming The boats not too heavy and the tunnel doesn't cause any problems.