9.9 to 15 conversion

[MH Hawker]

I am in the process of converting a 9.9 to a 15 on my pontoon. And I am expecting to have to re prop at least one size higher in pitch. Would that be a realistic expectation, Right now I am running a 9 pitch at 5200 rpm and thinking I may have to go to a 11. motor rpm is 5000 to 6000. Of course I will wait and see what it is.

I am on a limited 9.9 lake and it will be nice to have a bit more HP when I have 6 people on the boat. I realize its not going to be a huge improvement but right now with 6 adults it drops off to about 5 mph and with 2 its around 10.

 

[steelespike]

Re: 9.9 to 15 conversion

If it's a 2 stroke depending on the year my info indicates a 15 is rated at 7,000 rpm.
As I recall the 9.9 and 15 start out with the same prop allowing the 15 to rev to its rated rpm.

 

[MH Hawker]

Re: 9.9 to 15 conversion

I realize this is a very unusual project and has a lot of factors to sort out and it would sure be a lot simpler to just change lakes and use a bigger motor, but around here the bigger lakes are full of a lot of very rude people, I started out 2 years ago with a 60 hp on them and ever time I went out I ended up having words with some one, and went to this limited lake and its been a pure pleasure ever trip and I ended up slipping my boat there. and yes I could find a 15 and rebage it but they can and do a ser number check ever now and then and I did get a new 15 hp carb just in case it dosnt work out and can put the 9.9 back on. I am not looking for much if any speed increase but a bit extra push with a load would be nice.

I hadn't looked at the rpm`s on it as a 15, very good point. its a Canadian made 1996 2 stroke mariner 9.9 ser 0G119661.

I was able to find it in my manual and its the same on a 9.9 and a 15,, 5000 to 6000 at least for this year and type of motor and the timing and reed stop gap is also the same

 

[steelespike]

Re: 9.9 to 15 conversion

Of course I was thinking a E/J motor.I 'm thinking You should stick with the prop you have till you see where you end up.
In fact i would almost put money on not making 6,000 with the 15. even with a light load.
And I'm not a gambler.
I don't have the specs on a 96 9.9/15 but are you sure the displacement is the same?
in 89 the 9.9 was 12.8 cu. in. the 15 was 16 cu. in. typically to change a 9.9 to a 15, a 50% increase in power,
would take more than a carb though some 9.9s are just choked down 15s.
I should know this ,but what size is your pontoon?
Carefully balancing the boat so its not dragging the stern or bow framing etc might gain some speed.
If you have no venting issues you might try raising the motor 2 or 3 inches.As it is your probably dragging a lot of the transom assembly with a load.
Have you thought of skinning the under side?

 

[MH Hawker]

Re: 9.9 to 15 conversion

Its a 20 foot with 2, 24 inch pontoons and I have though about under skinning but I doubt at would make a lot if any difference at under 15 mph. I have adjusted the motor height and done about ever trick I could think of. When I started out it was at 7 mph and over several try`s I was able to get it up to 12 mph

The displacement is 292 CC at 16.0 cui. Before 1995 they are different and after their the same. 1995 seams to be the break year where they just detuned the 15`s.

I wish I could post the PDF I have.

I sat down awhile ago and went over the part numbers on both motors, exhaust, midsection, block, head, piston and rings, intake and reeds,and both carbs, and only found one part different and that is the main jet. I ordered a brand new carb thinking if it didn't work out I can change it back, and I talked a bit about it in the merc section and learned that the 15 hp carb has a different venture so changing carbs was the way to go. And you are more than likely right , I not going to make any prop changes untill I get it back in the water a bit later this spring.

I spent a good amount of time setting it up including motor height and so on

These are the present numbers and speeds by gps

just me and no gear.....................12 mph @ 5400
me and gear...............................10 mph @ 5200
2 people......................................8 mph @ 5100
4 people......................................6 mph @ 4800
6 people......................................4 mph @ 4000


CYLINDER BLOCK Type
Displacement (1994 Model)
6.......................................................... 12.8 cu. in. (209cc)
8.......................................................... 12.8 cu. in. (209cc)
9.9....................................................... 12.8 cu. in. (209cc)
9.9 Sailpower.......................................... 16.0 cu. in. (262cc)
10 Sea Pro/Marathon................................ 16.0 cu. in. (262cc)
XR10/MAG10........................................... 16.0 cu. in. (262cc)
15......................................................... 16.0 cu. in. (262cc)

Displacement (1995 and Newer)
9.9......................................................... 16.0 cu. in. (262cc)
9.9 Sailpower............................................ 16.0 cu. in. (262cc)
10 Sea Pro/Marathon.................................. 16.0 cu. in. (262cc)
XR10/MAG10............................................. 16.0 cu. in. (262cc)
15........................................................... 16.0 cu. in. (262cc)
15 Sea Pro/Marathon.................................. 16.0 cu. in. (262cc)

 

[steelespike]

Re: 9.9 to 15 conversion

I wonder if rpm and speed might hold up better with a step less pitch.
Probably slower light but faster loaded.?
A skin probably wouldn't do much with a light load but could be effective loaded.
Just guessing.
Are you running a 4 blade prop? That might help a little also might handle better.

 

[steelespike]

Re: 9.9 to 15 conversion

I wonder if rpm and speed might hold up better with a step less pitch.
Probably slower light but faster loaded.?
A skin probably wouldn't do much with a light load but could be effective loaded.
Just guessing.
Are you running a 4 blade prop? That might help a little also might handle better.
I notice Solas Amita has a 10" X 7" high thrust 4 blade prop.

 

[MH Hawker]

Re: 9.9 to 15 conversion

Right now its a 9 P..3 blade, I have been considering under skinning and a 4 blade. The prop prices are very reasonable. I started to try a 7 P to raise the rpm up and I am sure that would help with a load but decided to wait and see if the hp change is going to help and of course as a pure guess I was thinking with the extra hp it may raise the rpm, but I was thinking because its a displacement and not a planing hull its more about HP especially at such low speeds.

I had considered just finding a 15 or a 25 and re marking it and I and very glad I didn't . My wife and I went off on a vacation and the DNR decided to blitz the lake and wrote 275 citations with 15 boats impounded for over HP. They did a serial number check.

So at the least it will be interesting to see how it works out. It really is amusing at times, its like watching racing snails when some one try`s to pass :twitch:

 

[steelespike]

Re: 9.9 to 15 conversion

Ah ha! theres your problem. From my limited research a 9" prop is actually a little big for a beefy aluminum utility.
If we look at your rpm numbers your barely within the rating with you and gear.
I think the 15hp will be about the same speed but use about 1.5 gph instead of about 1 gph.
I forget the formula to increase displacement hull speed but its something ridiculous like cube the hp to gain 10 mph.
And thats a normal displacement hull not a pontoon with all its water disturbance.
You've got the carb give her a try.
But I think your simply running the wrong prop.

 

[steelespike]

Re: 9.9 to 15 conversion

Ah ha! theres your problem. From my limited research a 9" prop is actually a little big for a beefy aluminum utility.
If we look at your rpm numbers your barely within the rating with you and gear.
I think the 15hp will be about the same speed but use about 1.5 gph instead of about 1 gph.
I forget the formula to increase displacement hull speed but its something ridiculous like cube the hp to gain 10 mph.
And thats a normal displacement hull not a pontoon with all its water disturbance.
You've got the carb give her a try.
But I think your simply running the wrong prop.

 

[steelespike]

Re: 9.9 to 15 conversion

Oops wrong button
A little research 12 mph @ 5400 = 47% slip.
10 mph @ 5200 = 54%
8 mph @ 5100 = 63%
6 mph @ 4800 = 70%
4 mph @ 4000 = 76%
Way off the charts slip even allowing for a small motor.
the 7" prop at 6000 30% slip = 13.9 mph
Guessing 30% , not unreasonable at 6000
The key is the rpm will hold up better as you add weight.
slip holds up better at the higher end of the rpm range
slip will go up but I'm guessing you could end up in the high single digits
with a load. Not only saves time but gas too.
Also healthier for your motor.

 

[MH Hawker]

Re: 9.9 to 15 conversion

Cool, a fuel test I ran about mid summer with just me and gear was close to that, it sucks down 2 gallons a hour at WOT @ 5400 and travels 9 miles. Used the GPS trip log on total distance.

I have two ,6 gallon tanks , one is a main and a reserve. On the last fishing trip out the main ran out and I put one gallon in it just to see what I would get out on it, thought that would be a good way to see. 29 minutes and 4.5 miles later it ran out. I had more or less figured when I put it back on the slip in 69 days , not that I am counting or any thing it will have the new carb on it and I will run a few tests and then see unless it goes over 6000 right off the bat.

The prop selector chart in my book suggests a 7 P on a 9.9 with a 900 to 2400 load and on a 15 its showing a 8 on a 800 to 2400 load, its a 2:1 LU ratio. But of course its a pontoon and that dosnt really apply.

If I need to change I will take a close look at a 7 P. And that's why I am going to try and see if the carb picks it up a bit I really doubt it will add very little if any speed on the top end, but a bit more muscle on the bottom would be nice and once all of that is done, I will be looking very close at a 4 blade prop.

I tried a 4 blade on the other motor and it helped a good bit on a low speed toon, that was on a 40 hp Nissan, WOT it lost 1 MPH but added a good bit more hole shot and moved 6 along nicely

I have the manual in a rar file if you want it send my your E mail addy in a IM.

 

[MH Hawker]

Re: 9.9 to 15 conversion

Well I got it dropped in today and its running just fine, very smooth in fact. Their was a small increase in performance. I picked up, from 8 to 9 mph and about 300 rpm`s from 5100 to about 5400 and that's not bad considering its pushing a 20 foot pontoon. So its well under the 6000 limit. I am happy with that. It should give it a but more push when I have 6 on board and thats what I was after

 

[steelespike]

Re: 9.9 to 15 conversion

delete

 

[steelespike]

Re: 9.9 to 15 conversion

We have to expect high slip numbers but that calculates to 60%.
Have to wonder if a 4 blade might lower it a little Maybe 55%Might get you up around 10 mph.
Usually as prop speed increases slip lowers.

 

[elkhunter338]

Re: 9.9 to 15 conversion

right now I have a 15hp 2 stroke johnson with a 9.25x9 4blade. It pushes my 21' cuddy cabin at 5 mph at 4600 rpms, over proped.
I am thinking of getting a 10x5 or 10x7. I bet a 10x7 4blade on your boat would be great.

 

[MH Hawker]

Re: 9.9 to 15 conversion

I have considered switching over to a 4 blade and may give it a try. I wanted to get the hp conversion done first and give it a bit to settle in and It may pick up a small amount yet today's test run was done on last years gas but it was treated. Over the winter I had the prop off I had thought it was a 9P and its a 7P. I will look into the different sizes and see what is around in a 4 blade. The next pitch lower is a 6 1/2. Pontoons are odd any way and with a small motor I really don't expect a lot of gains but it is interesting to play with.


One of the things I have noticed is around here any way their is a lot of limited hp lakes and people just except what ever it pushes the boat at and while I am slow it is a good bit faster than most of the others. MY wife says its like watching racing snails

Sorry about the mis information steelespike its a 7 P instead or a 9P that may help out the slip calculation but any suggestions on a 4 blade would sure help. Right now it has a 48-42520A12.. 9 1/4 7P on it. The good news is their is a 5,6 and a 7 available in a 4 blade and as a guess a 6 P. A quick search comes up with this in a 4 blade 6 P... 850204A12 Mercury Black Max Kicker Propeller AL Propeller

 

[steelespike]

Re: 9.9 to 15 conversion

Well the 7" did help the slip down to about 49%.If you can find a 6" pitch 4 blade its worth a try.
Should improve slip may let the motor rev a little more it should resist venting and may respond better around the dock.
Speed may hold up better with a load though when dragging all that boat through the water I wouldn't expect much.
Again it may just handle better with a load than a 3 blade.

 

[MH Hawker]

Re: 9.9 to 15 conversion

I will order one in a week or so, I fully understand its not going to be any huge improvement, but like on any other boat a lot of little things do add up. When I changed out motors to the 9.9/15 with no gear and just me top speed was 7 mph and by lifting the motor up and making several small tweaks it came up a good bit. Like this last change while its only 1 mph its still around a 10 % improvement. A 4 blade 6 P prop is only 70,00 and that's a small price and if it adds a bit more and I think it will then its fine and if it don't its still fine.
A pontoon being pushed with a 15 hp isn't ever going be fast but as long as its fast enough to have a good breeze its all good.

 

[MH Hawker]

Re: 9.9 to 15 conversion

I got a chance in yesterday to run it with 3 adults and a child on board and it did a lot better than it has in the past it was holding around 8 mph at 5300 rpm, much better. This is the normal load and the numbers are up about 2 mph and 500 rpms, its a lot more stable.