Who knows props?

The aluminum prop on my 9.9 yamaha needs to be replaced. Its a 9.25x9 pitch. I'm thinking of going to a SS prop. Currently at WOT I'm turning 5130rpms, 16 mph at full load. The rpm range for my Yamaha is 4500-5500. I would like to see some more top speed, but I'm wondering if I will actually lose speed going down in pitch on an as prop? Or should I just go up in pitch with another aluminum? Also, using a slip calculator, it tells me I have 23% slip. That sounds high to me, anyone else?

If your prop is good and not all beat up, it's probably the right size. You mentioned it was no good so your speed is down and your RPM...well...who knows? I would try a prop the same size. Check it running light as possible. Find the best pitch using nice newish aluminum prop(s). Borrow props for testing if possible. You can always use one that is a little small. A prop with too much pitch is no good for anything. It overloads the engine. When you nail down the right pitch for your set-up, NOW you can make a better-reasoned choice in Stainless Steel, maybe only have to buy one. You can buy a whole seasons worth of gas for the cost of a stainless prop.

Prop condition is very important. Big knots in the blades and chunks missing cause resistance, slippage, and vibration. The prop is wasting gas, slowing your boat, and ruining parts. Never use a banged up prop if you can help it.

My boat does its best when I use a stainless prop that runs just over(or at)the top of the range. This is with the lightest load, trimmed up for best speed. My spare prop is aluminum, next smallest pitch. I use the spare for unknown waters and heavier loads. Once you settle on the two best props for the rig, you can stop spending money...unless you hit something.

It is a process to go through everytime an engine is mounted on a boat for the first time. Seldom do I guess right, no matter how many times I've done it before. Each boat/motor/prop/weight/trim combination is always different.

It does look like 23% slip as I think you have a LU ratio of 2.08. The "slippage" is likely due to the boat not getting fully up on plane. I think with that 42" bottom your planning surface is a bit too small to carry the weight. Assuming your RPM is correct you could try dropping down to an 8" alum prop which would (theoretically) raise your RPM to 5600 (just beyond your max) and that might be enough boost to get you over the hump. Your slippage would then decrease and you should get just a bit more speed. I would think about 18mph at 10% slip.

Remembering even if you were able to turn a 9" prop at 5500rpm and get 100% efficiency you would only be getting 22.5 mph. So I wouldn't look for this 9.9 to be a rocket ship.

I personally wouldn't spend big bucks on a SS prop to (possibly) realize those small gains.

[url=https://www.tinboats.net/forum/viewtopic.php?p=344169#p344169 said:

MDCrappie » Today, 09:12[/url]"]It does look like 23% slip as I think you have a LU ratio of 2.08. The "slippage" is likely due to the boat not getting fully up on plane. I think with that 42" bottom your planning surface is a bit too small to carry the weight. Assuming your RPM is correct you could try dropping down to an 8" alum prop which would (theoretically) raise your RPM to 5600 (just beyond your max) and that might be enough boost to get you over the hump. Your slippage would then decrease and you should get just a bit more speed. I would think about 18mph at 10% slip.

Remembering even if you were able to turn a 9" prop at 5500rpm and get 100% efficiency you would only be getting 22.5 mph. So I wouldn't look for this 9.9 to be a rocket ship.

I personally wouldn't spend big bucks on a SS prop to (possibly) realize those small gains.

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Yeah, im not looking for a rocket ship, but since I need a new prop anyway I want to get the most out of it I can. What if I were to drop down to an 8 pitch, but go with a 4 blade? I have heard the help getting on plane, but they can reduce the top speed.