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Author Topic:   90-HP on Over-Powered 15-Super Sport Limited
15whalerSSL posted 03-29-2004 08:04 PM ET (US)   Profile for 15whalerSSL   Send Email to 15whalerSSL  
I just bought a 90hp 2 stroke yamaha and its on a 15 whaler super sport limited wonderful motor, the prop is a 13X17 the motor turns 6100RPMS and doesnt seem to go all that fast what size prop should I try to get some better performance and get my rpms down...Thanks alot Taylor
jimh posted 03-29-2004 09:05 PM ET (US)     Profile for jimh  Send Email to jimh     
A 15-foot hull with a 90-HP is probably going to be very fast. Your motor may even be able to swing a 21-inch propeller. Be careful with the speed that you can get. You are over the rated horsepower for that hull.
Sal DiMercurio posted 03-29-2004 09:11 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
First of all, what is the maximum rpms the manfacturer says that engine should be run at ?
If you change your prop to a 13 x 19 s/s, the engine will run between 5600 & 5700 rpms.
Don't expect much more speed.
Wait a minute, I just re-read your post.
A 90 hp on a 15 sport, your over powered aren't you ?
That 90 hp should be able to spin a 21p prop in a heartbeat on your boat.
By any chance, is your engine mounted all the way down as low as it will go on the transom ?
If so, it needs to be raised at least 1 set of holes & possibly 2 sets.
By doing this, your engine will gain rpms, thus allowing you to twist a 21p prop.
Sal DiMercurio posted 03-29-2004 09:14 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jim, sorry about saying the same thing, your post wasen't up when I was typing.
screamin whaler posted 03-31-2004 07:27 PM ET (US)     Profile for screamin whaler  Send Email to screamin whaler     
The first thing that I would check is your set transom height. my sisters boyfriend runs a 90 merc on a 15 classic. His cavitation plate is set about 1" above the bottom of the hull. I am trying to remember what prop he runs. Pretty sure it is a 13x21. With that setup he runs 68.7 on GPS with a minimum load (himself, 12 gallons of fuel and a life jacket). The thing just absolutely rips!!! It still freaks me out and that is pretty hard to do. Best of luck.
SS17 posted 03-31-2004 07:59 PM ET (US)     Profile for SS17    
What rpm is he slinging a 21 to get 68 gps? Would have to be at least 6500 with a 1.81 ratio, it it's a newer motor with 2.33 gears, thats, what, 8400 rpm by my calcs.
LHG posted 04-01-2004 12:52 PM ET (US)     Profile for LHG    
For a 2.33 Merc 90 2-stroke to be running 68 mph, it would have to be running a stern lifting Merc cleaver racing prop, probably about 32" pitch and jacked way up with racing water intakes. The highest pitch Laser II made, 26" pitch, only gives 55 mph with this engine according to Merc's prop charts. This would be about the right prop for the rig, and maybe could be used on the Yamaha 90 also if it will turn it. At only about 82-85 HP, there is no way a small cube Yamaha 90 will run with a Merc 90 on a light hull. I wonder how one would push a Montauk 170?

It would be interesting to see how a 90 HP on a Sport 15 plugs into the prop selector.

Ector posted 04-01-2004 01:07 PM ET (US)     Profile for Ector  Send Email to Ector     
On my 15 classic hull i had a 75 john (3cyl); it was really fast (and furious) and i had a 19 prop over it.
I hadn't a gps that tima ... but i can tell when i punched it it glued me to the seat...
so your boat with a 90 has to be a monster....(but i think a 17 prop is too small)
Peter posted 04-01-2004 09:11 PM ET (US)     Profile for Peter  Send Email to Peter     
I suspect that the 68.7 MPH might really be 48.7 MPH, which makes alot more sense. To go 68.7 MPH in a 15 foot Whaler would be (1) scary and (2) require 175 HP, unless of course, its a motor made by Mercury, in which case we know that the laws of physics and thermodynamics don't necessarily apply. ;)
15whalerSSL posted 04-04-2004 08:30 PM ET (US)     Profile for 15whalerSSL  Send Email to 15whalerSSL     
I think i an goin to try a 21 Thanks
Bigshot posted 04-05-2004 03:05 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You need to run whatever it takes to get you at 5500 or UNDER. that 90 won't last long at 6100. The 90 is light but don't expect too much out of it being she is more like a 80+hp engine. I would say about 48mph on that rig. you will need a jackplate and a low water pickup if you want more. My OMC 70hp fetched me 48 swinging a 20" SSt prop at 5950rpms jacked to the hilt.
southbaywhaler posted 04-05-2004 10:33 PM ET (US)     Profile for southbaywhaler  Send Email to southbaywhaler     
68.7 MPH sounds unreal for a 15'. I just got back from a bass fishing trip and we were on a 16'11" ranger with a yamaha 150 that topped out at 54 MPH. That felt freakin' fast and I wanted to put on a helmet.
Toad2001 posted 04-06-2004 02:54 PM ET (US)     Profile for Toad2001  Send Email to Toad2001     
Maybe he was going downstream or with the tide and got a 20 MPH push.
I've had my trawler going 14.7 knots on the GPS too...
Normal cruising speed is 7.5 Kn.
BOB KEMMLER JR posted 02-26-2006 12:46 AM ET (US)     Profile for BOB KEMMLER JR    
I know it's a old thread,but i just repowered with a classic 90 myself.Looking to get an easy 50+mph out of it,but was curious if this motor would turn a 25P prop on a 15 Mischief?I'd go higher if the bottom end didn't suffer too badly
sraab928 posted 02-26-2006 07:42 AM ET (US)     Profile for sraab928  Send Email to sraab928     
Hey Bob,

Gave up on the 115 for now huh? I was really wanting to see that in action.

I wish I would have known you were looking for a prop - I have a 22 pitch quicksilver ss -15 spline - if it doesnt sell on ebay you are welcome to it. - Its the prop of that Yamaha 70 I have on your old rage. Cant wait for spring!

Talk to you soon

runpasthefence posted 02-26-2006 07:51 AM ET (US)     Profile for runpasthefence  Send Email to runpasthefence     
Congrats on the new motor! I made the same purchase about 10 months ago, and I absolutely love it.

As for your prop situation. You should not be turning any more than 5500 rpms. Any more than that requires further engine lubrication and can cause damage etc etc.
I think a 21 is too much, especially with the added weight of the SSL. I use a Yamaha model stainless 19" pitch on my 15'. I think it is the perfect prop for this boat. My calculations are at WOT
-5500 rpms (exactly Yamaha recommendations)
-48.5 mph (heavily loaded boat)
- less than 1% prop slippage (0.01978947368421058 to be exact)

If you're game (and it seems more and more of us are) there is some more speed hidden in that motor/boat combo. One suggestion has been to add a manual/hydraulic jack plate for optimum motor height and performance. I think this is a great idea, in fact I just took delivery of a brand new hydraulic this week. It is obvious the higher the engine, the more aggressive the prop rake, however, whether or not the raised engine height would allow for a higher pitched prop is true...I am not sure. That is why I am not buying a new prop until all of my new goodies are bolted on and I can collect some new rpm/mph data.
Bottom line-
No further modification= 19" stainless black Yamaha series prop
Further modification= further testing, and share!


Teak Oil posted 02-26-2006 09:51 AM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Bob there is no way you are going to be able to swing a 25" prop with a 90, MAYBE a 22" Laser will be the most you can swing.

Maybe you could try a 20" Laser and a 22" and see how they perform

BOB KEMMLER JR posted 02-26-2006 11:06 AM ET (US)     Profile for BOB KEMMLER JR    
Teak-Remember this 90 has the 2.33 lower unit ratio and not the 2.1 thats more common on the older mercs.Thats why i thought it could turn a higher pitch prop like that.According to Merc guru LHG,this motor is closer to 100 hp than a 90.

Scott-yea the inline is just giving me little fits here and there,mainly in the tilt and trim and possibly has a bearing seal issue.So i figured i'd just hang the 90 on there so it will be a reliable toy and no problems when i want to just take it out to blast around the 10k islands.

For right now i just got a 21p aluminum to break the motor in on and give me some kind of idea as to where i might end up with in the pitch range.I have access to a free 24p chopper,so i'll give that a go for sure.I'll be mounting this in either the middle or second to last set of holes for starters.

Going to keep the inline on a stand and go through it completely as time allows.I might take it out for a spin with the inline before i take it off,so i'll let you guys know how it goes.

skiprobinson posted 03-08-2006 08:57 PM ET (US)     Profile for skiprobinson  Send Email to skiprobinson     
I have a 15' CC with a 3 cyl Yamaha 85 HP. I ran the boat over the Christmas holidays and at WOT I was running about 42 MPH ( on the humminbird) with a 15 pitch prop. I bought a new stainless 19 pitch from iBoats and I am DYING for a weekend to get the boat in the water with a GOS and see what she can do. I dont think that a 90HP would be that much faster and I do weigh in at 275 lbs.

My .02 worth.

BOB KEMMLER JR posted 03-08-2006 09:09 PM ET (US)     Profile for BOB KEMMLER JR    
Sounds like you should be able to get more out of it than 42 mph Skip.Guys are doing that and a little faster with 70 omc's.

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