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  90hp Yamaha on a 15 what size prop???

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Author Topic:   90hp Yamaha on a 15 what size prop???
pimpinawhaler posted 01-17-2004 05:15 PM ET (US)   Profile for pimpinawhaler   Send Email to pimpinawhaler  
I just bought a 2004 90hp yamaha I would like to know what the best prop would be for top end. I dont need much out the hole power but would like to have a high top end speed so what prop side do I use Thanks Taylor
Moe posted 01-17-2004 07:41 PM ET (US)     Profile for Moe  Send Email to Moe     
Using the resources in the continuousWave Reference section:

With a hull factor of 180, the performance formula says 90HP will move 1100 lbs at 51.5 mph.

With 2.0:1 gearing, the prop calculator says that to reach 5500 rpm at 51.5 mph, with the 6% slip typical at WOT, would require a 21" pitch propellor.

These calculators, and 6% slip, seem pretty accurate for those using the 70HP (2.33:1) on the 15' getting 43 mph at 5900 rpm.


Perry posted 01-17-2004 08:14 PM ET (US)     Profile for Perry  Send Email to Perry     
2004 Yamaha 90 2 stroke or 4 stroke? I hope it is a 2 stroke because a 4 stroke 90 will be HEAVY on that 15 footer.
lhg posted 01-17-2004 08:24 PM ET (US)     Profile for lhg    
What does your handle mean?
pimpinawhaler posted 01-17-2004 11:26 PM ET (US)     Profile for pimpinawhaler  Send Email to pimpinawhaler     
the motor is a 2 stroke
Peter posted 01-18-2004 06:33 AM ET (US)     Profile for Peter  Send Email to Peter     
Just to add a piece of data to Moe's post here -- in the 70 HP speed reference the propeller is an aluminum 13 x 19.
Moe posted 01-18-2004 10:33 AM ET (US)     Profile for Moe  Send Email to Moe     
Thanks, Peter, I meant to include that.


Peter posted 01-18-2004 10:59 AM ET (US)     Profile for Peter  Send Email to Peter     
No problem Moe. Question for you: have you estimated what the hull factor is for the 150 based on your boat's performance?
Moe posted 01-18-2004 11:36 AM ET (US)     Profile for Moe  Send Email to Moe     
Peter, I responded via email so as not to hijack this thread.


pimpinawhaler posted 01-18-2004 08:08 PM ET (US)     Profile for pimpinawhaler  Send Email to pimpinawhaler     
thanks you guys .....Taylor
Clark Roberts posted 01-19-2004 08:15 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
If boat is classic 15'3" hull then hull factor (K) of 200 might be appropriate since there will be virtually NO hull in the water; If boat is a new 150 then K=180 may be best. Only some accurate testing will determine and remember to keep K constant (no pun intended) for engine comparisons and predictions. The classic 15 can obtain some truely amazing performance figures when set-up (height on transom, prop pitch, trim out, load balance, etc) is optomized and an engine that actually produces 90hp will be a handfull!Happy Whalin'... Clark... Spruce Creek Navy
Bigshot posted 01-21-2004 03:18 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I had the 17" on my 17' and she would hit 54-5500. Normally I would say a 21" on a 15' but that 90hp is more like an 80hp in all reality so somewhere between a 19 & 21" is where you need to be. I would go with a 19" and you can double cupp it to make a 20" or repitch to a 21". It is easier to add pitch then to remove it. I HIGHLY recommend a manula jackplate.
Peter posted 01-21-2004 06:19 PM ET (US)     Profile for Peter  Send Email to Peter     
I agree with Bigs' suggestion on starting with the 19 inch pitch. I come up with a predicted top speed of about 48.5 with standard mounting (~80HP and ~1200 lbs). A 19 inch stainless prop with an inch's worth of cupping should just about do it. If you get fancy with a jack plate and optimize the motor's height then that motor might be able to turn a 21. However, I imagine that will be one squirly boat. Hold on tight. ;)
Bigshot posted 01-22-2004 11:16 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
My 17 did about 41.5 so 48+ sounds about right being 300+ lighter. Scary thing is my 70hp Johnson would do that on a good day. I used to swing a 20" SS at 5900 and best speed was 48.6 on the GPS but that could have been with a current etc. My boat would hit 46-47 any given day though. I may have had a really good 70 as well being I did have bottom paint.

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