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  90 hp on a 15 Mischief,anybody done this?

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Author Topic:   90 hp on a 15 Mischief,anybody done this?
BOB KEMMLER JR posted 08-18-2003 04:44 PM ET (US)   Profile for BOB KEMMLER JR  
I am really like the looks of the Red 15 mischief,and i think a 90 hp Yamaha or Nissan could really make one of these things cook.I know it's over the max 70 hp rating,but there has got to be a Whaler guy on this forum who has done this,or maybe put even bigger on the back of one of these things.I would think they are a tad heavier than the reg 15 hulls due to the cap they put on,maybe making it a bit more stable at high speed?I am seriously thinking of looking for one of these for a runabout,and maybe stepping up to a larger boat for fishing and either giving my Rage to my father or selling it.
DaveNJ posted 08-19-2003 07:33 AM ET (US)     Profile for DaveNJ  Send Email to DaveNJ     
Bob -
Relax a minute before you kill yourself. Anyone who puts a 90hp on a 15' whaler hull is asking for trouble.

Why do it ? The motor is heavy, above the recommendation, and just plane unsafe. My Dad had a 60hp on his 15' whaler and WOT was scarey. I cannot imagine a 90hp. Just because it is a Boston Whaler does not give someone license to put whatever they want on it. Did you see Bigshot's Montauk on eBay? BW hulls are unsinkable but not indestructible.
Would you put a 90hp on generic brand boat 15' long, 525lb. hull ? If you answer "No", there you have it.

Be content with your restored Rage. It is a great boat.
Enjoy it for awhile. Didn't you just finish restoring it by making it an outboard Rage ?

BOB KEMMLER JR posted 08-19-2003 08:54 AM ET (US)     Profile for BOB KEMMLER JR    
I believe the weight of the 90 nissan or yamaha is about the same as a 3cyl PT&T OMC,and i would think the extra weight of the cap would help stability.Just entertaining the thought ,thats all.Yea,the Rage is all done and i am VERY content with it for fishing and cruising around,just the hot rodder in me had me thinking about the above combo.
Bigshot posted 08-19-2003 10:42 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Saw a few 15's with 90's over the biggie. I was on one with a 115 Ocean runner...thing was sick but not a stress crack on it.
BQUICK posted 08-19-2003 10:45 AM ET (US)     Profile for BQUICK  Send Email to BQUICK     
Bob.....for a COOL look....put a 6 cyl 90 Merc tower on there (not that heavy a motor) and remember you don't have to use ALL the throttle....just occaisionally. :)


captbone posted 08-19-2003 04:21 PM ET (US)     Profile for captbone  Send Email to captbone     
The 90 yamaha will be fine and fun. You will get better numbers than the 70hp but nothing crazy.
Peter posted 08-19-2003 06:16 PM ET (US)     Profile for Peter  Send Email to Peter     
Speed predictor formula predicts a 47 mph top speed with a 90 hp motor. Anything over 40 mph on the 15' hull requires a lot of driver concentration and pretty flat conditions.
BOB KEMMLER JR posted 08-19-2003 08:16 PM ET (US)     Profile for BOB KEMMLER JR    
I thought the 15's got like 45 mph with a 70 hp powerhead rated motor? I would think the extra 20 "true" and probably more like 30 hp(since all new motors are prop rated) would hit 50+ mph.
doobee posted 08-19-2003 10:29 PM ET (US)     Profile for doobee  Send Email to doobee     
With the bucket seats, foot rest, higher sides, and wraparound grab rail, you can push this boat harder than the other 15' Whalers. I talked to a guy who claimed to have a Yamaha 70 with tuned ports, a jackplate and surface drive prop that clocked 55MPH on radar. At that speed I imagine chine walking would be a serious threat. Other feedback I have heard suggests thata 70HP has a hard time pulling a slalom skier due the the extra weight of the accessories. Also I'd recommend one of the white ones as the red had problems with fading.
montauk madness posted 08-19-2003 11:36 PM ET (US)     Profile for montauk madness  Send Email to montauk madness     
My last whaler was a 1981 15' with a 1991 90HP mercury. I really didn't have any problems with safety and could hit around 51 when it was tuned. It wasn't near as good for skiing as my montauk with the same engine however.


prj posted 08-20-2003 09:07 AM ET (US)     Profile for prj  Send Email to prj     
Bob, I think your numbers there are a bit high.

I've got a '90 70 HP Johnny, 3 cylinder and supposedly a reasonably "strong" engine. Swinging a 13x19 stainless prop, I GPS'd 42.7 mph in ideal conditions: 3-4" wind chop on an inland lake.

A newer 70 HP (we're talking prop rated for all of these) would pull 45 mph I expect.

Bigshot posted 08-20-2003 10:18 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Prj....1990 are prop rated and you should be pulling better than that. I had a 1990 70 on my 15 CC and jacked up with a 20" prop at 6k should would do over 48.
prj posted 08-20-2003 10:43 AM ET (US)     Profile for prj  Send Email to prj     
Correct re: prop rated on '90 per multiple other discussions and I had hoped for something around 45 mph, but alas...42.7.

A few thoughts:
- I have no tach so no real idea on what I'm turning RPM-wise.
- I do have a CMC manual jackplate, about 5" setback.
- Antiventilation plate measures about 1.5" above transom bottom,
perhaps this could go higher.
- The Striper model may have a couple of additional
loads in the wind resistance category, likely not
inconsequential at those speeds: Port aft pedestal seat,
bulky and tall element that can't readily be removed.
Large cooler seat, perhaps 94 qt., loaded with gear
for perhaps 60#s additional weight and large high face,
further wind resistance.
- Myself, sitting too high on the above mentioned cooler,
comfortable for travels, inefficient for top-end.

Motor compression checks at 115, 112, 108, about 6%
variance, so within acceptable range.

I take it up to speed, slowly trim her 'til blowout,
gently dip it back in and lift again to confirm,
then hold on: 42.7 mph.

Bigshot posted 08-20-2003 10:54 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Probably due to setup and being a stripper. Raise that engine up another 2". My Montauk I have jacked about 4" and could go higher.
Bigshot posted 08-20-2003 10:56 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
PS get a tach and make sure you stay under 6k redline. Plus saltwater is slightly faster than fresh I believe.
Peter posted 08-20-2003 11:52 AM ET (US)     Profile for Peter  Send Email to Peter     
My numbers are about the same as PRJ's. 1987 15' SuperSport (bottom painted) pushed by a 2003 70 Yamaha turning a stock 13 x 19 Yamaha aluminum propeller (engine mounted to transom all the way down) in salt water will get to 42 to 43 mph (GPS measured) at 6000 rpm. This is just slightly below predicted speed of 43.4 mph using 1200lbs for weight and 180 as the hull factor. Plugging 90hp and say 1300lbs for the Mischief with a heavier 90 hp motor into the formula produces 47.4 mph, that's it for stock trim.

Bigshot posted 08-20-2003 12:53 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Raise your engine up a couple holes and use a SS prop and you will see 45. The OMC 70's have more grunt than the Yamaha does. The Yamaha 90 is more like 80-85hp in my opinion. Set that 90 up right and she will do 50+
where2 posted 08-20-2003 12:55 PM ET (US)     Profile for where2  Send Email to where2     
Hole shot problems with the 70Hp engine and slalom skiers are easily fixed with a 13-1/4"x17" prop, in place of the typical 13-1/4"x19" used for speed.

Until someone comes up with a shifting prop for the smaller gearcases, this is the fix...

brisboats posted 08-20-2003 01:59 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Also look for an OMC Stinger. This 75hp ho version of the OMC 3 cylinder has a higher redline and will give any Yamaha 90 a run. They are plentiful and a real kick on a light boat.


BOB KEMMLER JR posted 08-20-2003 02:31 PM ET (US)     Profile for BOB KEMMLER JR    
I'm not sure if it's a stinger or not,but i have a 75hp 3cyl johnson on my Rage and supposedly has had some "extra work" done to it in the way of weisco pistons and boysen reed valves.My rage is a VERY heavy boat at dry weight of approx 1200-1400 lbs and it seems to move along pretty good with a full load in it.well all prop talk aside,i am still curious about these mischief boats now.I'm lucky my wife is a understanding woman lol
brisboats posted 08-21-2003 02:18 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Bob, that 75hp sounds like the perfect motor to try on the mischief.


captbone posted 08-21-2003 04:46 PM ET (US)     Profile for captbone  Send Email to captbone     

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