Need help finding new prop

I'm looking for a new prop to put on my 99 Johnson 9.9 on my 16ft lowe right now it runs at about 4500 rpms at wot.
I'm just looking to gain a little speed. Awhile back someone on the board told me that it likes to run at about 6000 rpms if I remember right. And from what I've read I need to decrease pitch to increase rpm 1inch of less of pitch yields 150 to 200 rpm? Does that sound right? It ran about 15mph last year, this year I added a pt-35 and can get up to 17mph. I'm going to go check my prop to see what pitch it is and I'll post it. I know your probably thinking why does that little bit of speed matter? I fish a lot of tournaments on 9.9 lakes and want to be able to get to my spots first. Alot of the other boats are just barely beating me sometimes.
Thanks
614-bass

How have you checked and verified WOT RPM on your engine once you added the jack plate?
Your mention of "about 4500rpm" is fairly vague.
Yes, they like to run around 6000RPM but.....usually decreasing pitch will result in a loss of speed.

If he decreased his pitch his rpm's should go up which might mean he keeps his same speed :?:

Ok first off we need to know exactly what prop you have on at the moment. I will take a guess and say you have a 9.5X10 on it at the moment (typical stock prop for a 9.9). So if your RPM's are correct and your GPS speed is correct and your gear ratio is 2:1 then you have a 20% prop slip which is not good at all. You want to be more in the 10-12% slip area and no more than 15%.
Reality is you will most likely not gain more than 1mph top end with changing props, and may actually lose a mph or two. What you will gain is holeshot by dropping pitch. Also it is best for your engine to run in the recommended RPM range and 4500 is to low. By going down in pitch it makes it harder to gain in top end speed as others have indicated. But it will be less strain on your engine to get to the 16-18mph and your holeshot will be way better.

(Going off my 1987 9.9hp specs)
recommended rpm 5000-6000 and 5500 is where you produce peak horsepower

Very true on both accounts but am guessing that he is guessing at his RPMs so I don't want to give information based on an assumption.
Second. Based on his assumed RPM and Johny25s gear ratio here is the result.
4500x10x.5.x.0009469=21.3MPH at theoretical pitch speed. Take that speed and pull 15% out for slippage and that equals 18.1mph.
However, Johny25s gear ratio is not correct. Correct gear ratio for that engine is 12:29 or .414.........soooo
4500x10x.414x.0009469=17.64MPH of which we pull the average of 15% slippage again and we get 14.99mph.
Now, raise his "about" RPM by around 700 to 5200RPM and we have:
5200x10x.414x.0009469=20.38mph of which we again reduce by around 15% for an end result of 17.32MPH.
This is why I asked for more information. I consider both of the other guys knowledgeable on this forum however I hesitate to provide an answer based on information that I think may be not as close as it needs to be and cause the OP to spend money where he may not need to.
We will see how he arrived at his "about 4500RPM" and go from there. There is always the possibility of an incorrect prop pitch, using knots instead of MPH for speed, tachometer not reading correctly or other answers. We know that if he claims RPM to be absolutely correct then something else is totally out of whack.

The rpm is coming from my tach and the speed is coming from my lowrance unit gps. I tried to check the prop quickly before work but didn't see anything on there. I'll look at it again tomorrow to see what I can find.

Well....what kind of tach are you using and if it is a standard tachometer then you must have an electric start engine? If so then what setting is your tach on?

My 2:1 gear ratio was merely a guess as was all of my calcs.

But if you have a gear ratio of 2.414:1 then there is a good chance he may have a 11 pitch on his motor. This would give him a 17mph GPS reading with 11-15% slip at 4500 rpm. Very plausible

If he has a 10 pitch and he is running 17mph then his slip is 6% or less which is highly unlikely and then I will have to agree with Pappy that the RPM reading does sound incorrect.

Well i looked at the prop further and still didnt see anything. Ill take it off and check when i get off work today. I took some pics if it would help you. And how do i find out what setting the tach is on? It is electric start

There should be a stamp on the outside somewhere between the blades at the base of the prop or on the prop hub like in this picture , it is hard to see but it is a 10.5X11 in the pic


And as for your Tach (if it is a tiny tach or similar) I believe it should be set at 360 degrees= 1 spark per revolution

When at idle you should read about 650-750 rpm in gear on the water and about 800-900 rpm on a fake lake (muffs) in neutral

Ok im going to put it in the water after work ill see what it reads at idle. Then ill check the prop again.

Also i dont think its a tiny tach after i googled it. Its a gauge in my console a faria.

Sorry it took so long I've been extremely busy. Checked it today and wot rpm was 4700-4750

Also at idle I know it doesn't sound right but it read 1100 but this was after running down the lake and stopped when I remembered to check.

Ok what pitch prop do you have. We need that info or we can't help. And your idle RPM is too high if it is reading 1100. If it was in gear and at 1100 then it is way high.

......better stop and check the tach setting before assuming that's what the engine is turning. Both the high and low range numbers are off.