The aluminum prop on my 9.9 needs to be replaced. 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?

You will gain nothing with a SS prop on a 9.9 hp. Don't waste your money.

You will gain nothing with a SS prop on a 9.9 hp. Don't waste your money.

I would believe that, had I not personally seen it make improvements on 3 other 9.9 outboards.

  Anytime a new prop over a used one will work better. What makes a SS prop better then a aluminum one is that it doesn't flex. A 9.9 hp doesn't have enough power to flex either prop. So either prop in the same pitch will give you the same results. You can buy special machined props for the larger motors and maybe the 9.9's too. But your talking a lot of money for very little difference. I think you would be better off to lighten the load and balance the boat better to get more speed.

An alluminum prop is lighter and could be faster.

How many rpm are you turning now? What prop is on it? How many rpm did the motor turn when the prop was good?

An alluminum prop is lighter and could be faster.

How many rpm are you turning now? What prop is on it? How many rpm did the motor turn when the prop was good?

Its a 9.25x9 aluminum now, turning 5130 rpms. This is the only rpm I have ever known it to turn, but the prop is in rough shape. The motor is a 1987 and it looks like the original prop. I cant raise the motor either, the water intake is on the bottom side of the cavitation plate and its even with the bottom of the boat now. Also, using a slip calculator, it tells me im getting 24% slip which seems high to me.

If your max recommended rpm is 5500 and you're now turning 5130, you can probably go down one inch in pitch with no problems.  I wouldn't hesitate to get a stainless prop for the increased durability and it might get you a dab of speed due to the decreased flex, but small motors don't have the torque to flex their props much.  On a 9.9 hp boat running 16 mph, we're not talking night and day differences here, so only you can decide if the added expense is worth the modest added performance.  But if you need a new prop anyway...

Yeah, I have decided not to go stainless. I was wondering though if maybe I should get a 4 blade prop, to help the boat get on plane. Im wondering if I drop a pitch and go to a 4 blade if I would lose speed, im pretty sure I wouldnt gain any.

I think if you want to get on plane faster then put a fin on the motor. The less boat you get out of the water the better off you'll be. Again your motor doesn't have the HP to make a four blade do it's magic. You'll never get a big motor performance out of a 9.9 motor.

 You'll never get a big motor performance out of a 9.9 motor.

I agree with you on that, but im not looking for big motor performance. What I am looking for is to get the best possible performance out of what I have.  Thats why im trying to figure out the best pitch prop for my setup.

All aspects of performance will improve with a SS Prop. If your current RPM is 5130 with just you and a normal load I would definitely drop down one pitch size if one is available. Most companies only offer a limited number of SS props for their small motors. Normally on the small motors, the SS prop only cost about twice what an aluminum prop and you will be wondering why you hadn't done it a long time ago. If someone says it's not worth it, they just haven't tried one. Dropping down one pitch and going to SS also, you will see a big difference in hole shot, even with a 9.9.

Do not go up any in pitch thinking you will gain more speed, most likely it will slow you down more and will definitely hurt hole shot and performance all the way around.

All aspects of performance will improve with a SS Prop. If your current RPM is 5130 with just you and a normal load I would definitely drop down one pitch size if one is available. Most companies only offer a limited number of SS props for their small motors. Normally on the small motors, the SS prop only cost about twice what an aluminum prop and you will be wondering why you hadn't done it a long time ago. If someone says it's not worth it, they just haven't tried one. Dropping down one pitch and going to SS also, you will see a big difference in hole shot, even with a 9.9.

Do not go up any in pitch thinking you will gain more speed, most likely it will slow you down more and will definitely hurt hole shot and performance all the way around.

That 5130 is at normal load with me and another person. I very rarely fish alone, so there will be 2 people in the boat 99% of the time. Thanks for the advise

You'll gain a little of both with a s/s prop. If you add in that plus the durability factor, it's probably worth it. A 4 blade would help but going from 3 to 4 blades isn't as much of an exact rpm change as going from a 3 to 3. I'm not sure they make 4 blades that small without going custom which I don't think would be worth the $$.  This info should be at least semi helpful http://www.tinboats.net/forum/viewtopic.php?f=3&t=26628

I don't know much about props that small to know what your options are, but decreasing prop diameter will also increase rpm without dropping pitch.