Help choosing a prop

[Skidz]

Hey folks, longtime reader, first time poster.

I am trying to decide on which prop to buy, and I could use some advice.

I have a 15/42 Duracraft semi-v flat bottom and a 2000 Johnson 25, 2 cylinder o/b. The o/b WOT specs at 4500-5500 rpm, and with my normal load I am turning right at 5160-5180. The prop is a 10-13, 3 blade aluminum, which came on the engine new. It has a few nicks and scratches, but still does well, and the boat gets up on plane easily.

I would like to get a second prop, and use the original as a spare.

I understand that going down in pitch raises rpm by about 200 rpm per inch lower pitch. and I would like to get my rpm's just a bit higher. I am also looking at getting a 4 blade to replace the 3, specifically the Solas Amita #2213-101-12. It has an inch lower pitch, which I 'think' would get me in the right spot. But, I can't find much info on the effect of going from 3 to 4 blades and how it affects rpm.

I'm not looking for much more speed, maybe just a little; and the holeshot is good as it is. Although I would always appreciate more....

Do you think I should stay with a 3 blade, and maybe just lower the pitch an inch or so? Aluminum or SS, it doesn't make much difference to me, but I can appreciate the lower cost of aluminum. And, what about 3 blades versus 4? Would there be a noticable difference? What effect on rpm do I need to consider going to a four blade prop?

Thanks!
Skidz

 

[usskydvr]

Re: Help choosing a prop

For starters, you are correct on your pitch change effect on RPM.
Each 2" is about 300-400 RPM. For the same pitch going from a 3 to 4 blade
will reduce the RPM about 100, that is why most all 4 blades are in even pitch. So a 19 3 blade is about the same as an 18 4 blade.
On your applicaition I would stick with a 3 blade. Going own to an 11 would
bring up the RPM to the max and give you a bit more hole shot and about the same top end, maybe 1-2 MPH lower. Personally I would stick to the current
13 pitch.

 

[steelespike]

Re: Help choosing a prop

Before any prop change you should check the motor height.The antivent plate, just above the prop, should be about even or slightly above the bottom of the boat.
There may be holes on the mounting bracket allowing more height,or you may be able to simply raise it and bolt in place.If the motor is now low raising it could gain a couple of mph and some rpm.The 4 blade will allow more height, also improve hole shot,stay on plane easier,
Someday move up to a 1548 or 1554 it will likely plane even easier run faster with your typical load.

 

[Skidz]

Re: Help choosing a prop

Thanks for the information guys, I really appreciate the help.

Steelespike, right now the AV plate sits about 1/2 inch below the keel, with the engine sitting directly on the transom. I will fashion a piece of wood about 1/2 - 3/4" and put it on the top of the transom, bringing my AV plate up a bit and see how it goes. Like Usskydvr said, I may just stick with what I have, maybe keep the same pitch and just get a SS prop. As it stands, the performance is pretty good, but I am always looking for that bit more.

Thanks again,
Skidz

 

[steelespike]

Re: Help choosing a prop

The higher you go the less drag.A 4 blade will get you even higher.
Usual results here the Solas is very predictable.I think your on the right track with it.ss results can vary from a few tenths to a few mph.will generally improve overall performance but at triple the cost I wonder.The solas will usually give improved hole shot,stay on plane easier, resist slip and ventilation,at 1/3 the cost.