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aluminum to stainless prop rpm advice

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I’m new to the forums but been reading a lot of good information but thought I might throw my situation out there and see what you guys had to say

I recently bought a new 21’ Thunderjet Luxor OB OS (aluminum hard top fishing boat) with a new F200 Yamaha Outboard which has a recommended WOT of 5000-6000 rpms.  I bought it at sea level and brought it up to 6500 ft. elevation (elevation sucks). So I have playing with props.  I am currently running an aluminum 13p x  16” prop getting the motor broke in. I installed trim tabs to help plain out.  It runs at 5800 rpm at 34 mph at WOT.  I would like to go a pitch up and get a little less rpm and help with fuel and change to a stainless prop.  I know changing a pitch should drop about 200 rpm but if I put a 14p x 15” Solas HR Titan prop (about the only 14 pitch I can find that will fit it) how much will a lil small diameter  stainless prop change the rpm. Like I said I’d like to run around 5500 rpms. Its a fishing boat that I want to get from point A to point B as fast and best fuel mileage I can. My other option is a stainless 13p x 15 5/8” Solas New Saturn prop. I seem pretty limited on prop selection at that low of pitch. Thoughts and opinions??


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The 1 inch pitch will bring you down about 200 while the 1" smaller diameter will bring you back up about the same. That prop may very well do nothing for you. Keeping it at 5800, the motor will like this. Also bringing your wot rpm down may worsen your mileage.

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As elevation goes up ? Horsepower MUST GO DOWN. Less Oxygen per cubic foot, compared to sea level.  I forget the % loss of H P for each 1,000 of elevation increase. I will do a web search for the loss % at 6500 feet.

At 3% per 1,000 feet a 100 hp motor is losing almost 20 hp. BUT....If the other % from a owners manual is used ?  - 3.5 % per 1,000 feet.

BUT  Some engines breath more poorly at those heights. 

Engine swap to a Rolls Royce V12 & about 3200 cubic inches. :)

Serious edit

Check DIRECTLY with Yamaha about fuel mixture recalibration at that height.  Might be necessary. Check before running up there & over loading the engine too severely.

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