2 props....

[bcguy]

Boat is 36' 12 tons... 50 hp diesel

Prop guy says 17" diameter x 9" pitch
I have the same prop (Campbell Sailor) 15" diameter x 9" pitch

If I use the 15" diameter prop will there be any negative effect it will have on my engine...

.... thanks

[atoll]

1"daim=2" of pitch so your 15" prop will be 4" under pitched,assuming the calculations are correct.
try this for the correct calculation

Vicprop - Propeller Calculator

[bcguy]

Thanks Atoll... I do have the correct prop which is 17x9 purchased at a prop specialty outfit. I should have stated the 15" is a spare and my question was whether or not using the 2" smaller diameter prop (correct one is 17") would somehow do damage to the engine...

[atoll]

shouldn't damage the engine,except that the engine in theory would need to turn aprox 600 rpm faster to creat the same thrust/speed through the water.

ie 17"@3000rpm+ 6knots or 15"@3600rpm=6knots

though you may get more slip and possible cavitation with the smaller prop and higher fuel consuption.