Question about props and RPM

ok not to bring up an old topic but I have a question that I cannot seem to be sure of. I think I need to go down a pitch on my prop. I am running 5450 RPM at WOT with just me in the boat at around 18.5mph and around 5200 with another person in the boat. I have a 1987 Johnson 9.9 with a 15hp carb and it has the stock 10" pitch prop on it and I know the 9.9 and 15 came with the exact same prop as OEM. The RPM range for a 15 is 5500 to 6500 and for a 9.9 is 4500 to 5500 i believe. My question is this.....I need to get my RPMs up solid into the range for a 15 and I am thinking that going to a 9" pitch SS prop would do the trick BUT would the extra efficiency of the SS prop put more load on the motor and lower the RPMs because it would have less slip and actually move more water? I always hear that a SS prop gives you a little more RPMs compared to an aluminum one because of the thinner blades but the whole less slip thing got me wondering and second guessing myself. I just would hate to spend the money on a prop and it be the wrong one and I do not know anyone that I can borrow one to test. I know I am probably way over thinking this but it has been bugging me all day.

Ok I have the exact same 1987 9.9 johnson you have, but I have converted mine to a 15hp. First off the RPM range for the 9.9 is 5000-6000 rpm with peak HP being at 5500. The 15hp RPM range is 5500-7000 with peak being 6250. Now that being said, if you haven't converted your motor to the 15hp then I will advise you not to go over the 6000rpm mark.

Ok next thing is the SS prop. A lot of people will tell you not to put an SS prop on a small motor because you will not gain anything. And that is complete rubbish! An SS prop will out perform an aluminum hands down. There are some high end aluminums that do perform close to the SS props though. Rule of thumb with and SS prop is you can go one pitch higher than the same aluminum and stay close to the same RPM range.....give or take 50-75 rpm.

So if you went to a 10 pitch stainless expect your RPM's to go up about 150 putting you about 5600 with just you in the boat. Now if you went to a 9 pitch SS you will be in the 5750 range. But remember when you go down in pitch your speed will usually go down also, but your holeshot will increase.

I say turn her into a 15hp get a 10 pitch stainless and you could probably hit 6100-6300 rpm and do about 21-22mph theoretically with that 2.41:1 gearcase.

Oh and with the speed, prop pitch, RPM, and gearcase ratio you have a 14% slip currently. My guess is that a stainless could drop this to 12% or less due to the better cupping and design that stainless usually has. I bought a Solas stainless prop for my 30 hp johnson and was very happy with the price ($175 shipped) and the performance gains. I have yet to buy an SS prop for my 9.9/15 johny but if I ever start using it again on a regular basis I will purchase one for sure.

thanks Jonny for the input, I have converted the motor to a 15 about 3 weeks ago and gained several 100 RPM (about 500 to be exact) and about 3mph with the conversion just figured it would be a little more, which i am happy about, just not happy that the RPM's are at the low end of the range, thought i would gain more. Now i am 6'7" 225lbs and my boat is a 1432 alumacraft and i have a series 27 batt up front, the motor is on a minijacker about 1" above the bottom with a doel fin on the motor, like you said i was expecting to be in the low 20's on speed with just me in it. Wondering if its just the narrow hull and my big a$$ sitting in the back putting load on the motor, while driving it, it just feels like it should have another "gear" to go into and something is holding it back.

Yeah a 15hp will only do so much. And the narrow hull will not help this situation either. You are surely not getting enough RPM for the motor to produce the 15hp. I had a very similar issue after doing my mod to my motor. I went from the boat not even being able to get on plane after putting a custom floor in my boat with the 9.9 to planing and doing about 16-17mph after the 15hp mod. Trust me, at the time I was very happy that the motor finally had the balls to plane my son and I but I wanted more out of it. Fact is I should have just got a bigger motor (which I did the next year).

I have a 2002 Lowe 1467t and I found that the limits for my 15hp was about 900-950lbs total weight (that's the boat, motor, gear, gas tank, persons, battery, flooring....etc) for the motor to be able to plane. I usually ran with about 750-800lbs and that is where I got the 16-17mph. Fact is you are not going to get anymore speed out of the motor with the weight and boat you are pushing. Really you are getting pretty solid numbers right now for a 15 considering what you are pushing. A stainless 10 pitch may get you 1-1.5mph gain but if you go down in pitch you will start losing top speed. Every inch of pitch you drop you are going to lose about 1mph even though you will gain about 200rpm and your holeshot will be better.

I ended up buying a 1988 25hp this spring, converted into a 30hp and now I can hit 29-30mph with my son and I in the boat. After I put the 25/30 on I realized that the 15 never really had enough power to completely get the bow down and the boat on a full plane. The 15 always had the bow pointing in a slight upward direction, but the 30 sits the bow flat down and the boat skims the water now.

I am not sure what the HP rating of your boat is but if you really want to get into the 20+ mph area I would suggest saving your money and getting a bigger motor. My boat is rated for a 25 but like I said I am running a 30 badged as a 25 now and have no issues.

If you want another 1mph get a stainless 10. But if you want more speed get a bigger motor and save your money, because you are getting about all you can out of that 15.

Oh and if that hydrofoil will usually slow a boat down. It will help you plane at a lower speed but will typically cause drag and cut down on top speed if the motor isn't at the perfect height

ok you said something that has me thinking now.....I have had several glass boats in my life with motors from 115 to 225hp. With those boats you always start with the motor all the way in, get the boat on a plane, then start to trim them out until you find the sweet spot, which normally would have the bow up slightly. Obviously I cannot have a power trim system on this little boat but I have been operating it in a similar fashion with the motor pin out enough to have the bow slightly up (ussually run either hole 2 or 3 counting from the transom, hole 3 I slow down a bit but makes me feel safer in rougher water, have not tried hole 1 yet)...are flat bottom boats supposed to run "flatter" on the water than other hull styled boats, if so, maybe i need to go to hole 1 and ,because like you said, it does seem like the stern is not "skimming" on the water and is a ways down into the water actually even though I am hitting 18+mph, I just attributed this to my 225lbs and 6 gal tank back there. Is it posible to go 18mph and not be on a plane?

Well you can try hole number one but I seriously doubt the motor has the power to skim you no matter what position it is in. I tried every pin position with my 15 and the best performance was hole 2 or 3 I believe. And yes having the trim option of the power tilt motors allows you to dial in your motor a lot better.

My 30hp will skim my boat in any of the 4 pin slots. Top speed will change between slots but the bow always comes down due to the motor having plenty of power for the boat. Mine is a 14' V-hull by the way. Fact is the 15 just doesn't have the power to do this with the amount of weight you have in your boat, which is the same problem I had. I am not a light man either.....210 lbs. Add that to the motor in the back with you and possibly the fuel cell the motor just doesn't have the HP to push the boat hard enough to get over that peak to get the bow all the way down.

jaxjonboat,
you said your 9.9 has the 15hp carb, i read that you also need to change the exhaust to complete the upgrade....maybe it's part of the reason for the low rpm gain.

I must have missed that....... But yes the tuned exhaust tube will give you about 1.5hp gain which will increase RPM. I guess I was just under the assumption that he had did that mod also I still do not think you will achieve 20mph though. A 100-150 RPM gain would probably be likely but that would only get you to about 19mph

you should not have to change the exhaust tubes on a late production '87, the part numbers are the same for a 9.9 or a 15, they both use the tuned exhaust tube unlike the earlier years. I guess they did this to save money. Mine also already had the reed valve spacers. I think i am going to try the SS 10" pitch and just run with it, i very seldom have anyone else in the boat with me anyways, if later I start to bring someone i will purchase a smaller prop, like your said just with the hull shape i think i am close to the limit of this little motor and I am getting more and more comfy with it performance the more i learn.

Have you actually got inside and seen if you have the round tuned exhaust? If I remember correct it wasn't until 1989-90 that the 9.9 got the 15hp tuned exhaust. My 87' had the square one in it. But I did have the reed spacer already in mine.

Ordered a solas saturn SS prop today, 9.25x9in., stock alum prop is 9.5x10. After the wife and I went out yesterday and my new REAL made in USA tiny tach(my 3yr old son took the Chinese knock tiny tach that I had on it apart and destroyed it haha) confirmed my RPM was too low. With she and I the best I could get was 5300 on the 3rd pin out and about 17mph. I am hoping this will get me in the upper 5000's if not right at 6000 with just me in it, if it does my speed should remain the same or very close according to my math and I will be happy I guess, knowing that is all she has to offer. At least I might pick a little on the hole shot. Will report next weekend on results, hopefully I will have the electric start conversion installed by then too.

by the way the new SS prop, bushing, spacer, nut and cotter pin delivered to the door was $133.00, not to bad I think from iboats.com

Was really wanting 20mph out of this but my hopes are diminishing and reality setting in...I hate reality lol.

Today was the first test run on the new SS prop and all i can say is wow!!! Had some unexpected results....WAY less vibration, which is surprising because the old prop looked fine. Ok on to the performance part.....I went down one pitch to a 9" from a 10". My rpm's went to a hair over 6200!!!! was not expecting that!!! previous best with old prop was 5450. Hole shot is improved noticeably and you could feel the boat lift at about 16mph during acceleration, never did that before or if it did was not noticeable. Bottom line....this was probably single best add on improvement I have done to date, granted I was a little lighter today than normal. Hit 20.2 mph which was my goal...Just can't get over how much smoother the motor ran at the higher RPM's that was a nice surprise for sure..

I used the navionics app on my android phone for the speed, by the way what a great app!!!!!

SS props make a world of difference to virtually any size motor. Some like to say that SS props are worthless on smaller motors (under 40hp) but I know from first hand experience that SS props will improve performance. Glad you hit your 20 mark =D>

I also can tell on my 30hp that the SS runs far smoother than any of the aluminum props I have ever run. Almost like aluminums are out of balance or something? An SS is like driving a big ole caddy down the freeway.....nice and smooth

Jonny, your not fibbin, that was the FIRST thing i noticed.......SMOOOOOOOOTH and hardly ANY vibration in the handle, oddly enough the motor seemed to run better too. Once I got past 5500 the motor WOKE up so to speak and was just hummin, what a beautiful sound.....This one single addtion has FAR exceeded my expectations and I have told all my friends with aluminum props they need to go SS!!!!! they would not regret it.

I am a little surprised at how much the RPM's went up. Would have fig 300-400rpm increase by dropping a pitch and going to SS but not the 700+ that you got. You may have been able to turn an SS 10" at 6000 and possibly hit 21-22mph? but the 9" will be better when you get a load in the boat for sure.