prop question

I have a 1986 evinrude 30 hp, it has a prop with a pitch of 10 x 15 on it now. I recently bought a prop from a friend with a pitch of 10.5 x 16, can I use this new prop on my outboard without worry about screwing my motor up?

I also forgot to mention the old prop is a 3 blade and the new one is 4 blade

That all depends on whether or not the prop actually fits the motor. Try fitting it. If the splines on the propshaft match the hub of the prop, it should slide on without a problem. If it does, give the prop a spin by hand to make sure it won't contact the cavitation plate. If that's all good, take it for a spin on the water. Just remember, the new prop is going to push a lot more water. You may not plane out as quickly, and you're definitely going to se a reduction in RPMs at WOT. Hope this helps!

I'm understanding that at WOT the rpm would be less. How about the surface speed of the boat? Same?

What would be advantage or not of going from three to four blade props? End of day engine can out out only so much power as is designed for a sweet spot of RPM / load / fuel / wear . . .

I find it intriguing that my 115 mercury is 4 blade CT (Command thrust) which is larger prop blades - higher thrust from stop?

And my 9.9 kicker is 4 blades - probably different pitch. Noting that the 9.9 is designated as a kicker.

Just some questions some may have answers for . . .

Thanks

Wouldn't it be an increase in rpm ?

the prop fits perfect, it is off a similar 25 hp evinrude, Im just worried about bogging down my engine, guess il just have to try it out, thanks.

Increase in pitch is decrease in rpm