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  Propeller for a 15 Classic with 90-HP

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Author Topic:   Propeller for a 15 Classic with 90-HP
edwardschnog posted 04-12-2007 08:45 AM ET (US)   Profile for edwardschnog   Send Email to edwardschnog  
Hello to all of you...
I was wondering who can give me some advice on which prop to use, to get the highest possible speed out of a 15 classic (Alert) whaler with a 90HP 2-stroke Yamaha engine.
Data of the 90HP Yamaha engine:
RPM range: 5000-5500
Shaft length: 20 inch
Gear ratio: 13:26 (2)
Weight: 261 lb (119 kg)

According to this diagram the ideal prop pitch should be 22".

Diameter-Pitch-RPM----Gear/ratio---Slip--Mph
13,25----15----5250----2------------6%---35,0
13,25----16----5250----2------------6%---37,4
13,25----17----5250----2------------6%---39,7
13,25----18----5250----2------------6%---42,0
13,25----19----5250----2------------6%---44,4
13,25----20----5250----2------------6%---46,7
13,25----21----5250----2------------6%---49,0
13,25----22----5250----2------------6%---51,4
13,25----23----5250----2------------6%---53,7
13,25----24----5250----2------------6%---56,1

Can anyone with practical experience inform me what the best prop for this boat/engine combo is and how to setup the engine to the transom to get the highest possible speed and less chine walking !!

Thanks,
Edward

runpasthefence posted 04-12-2007 09:08 AM ET (US)     Profile for runpasthefence  Send Email to runpasthefence     
http://continuouswave.com/ubb/Forum4/HTML/005164.html

A Yamaha 90 will not spin a 22" pitch prop on a 15' Alert.
Your diagram would imply that a 24" pitch prop would be the ideal prop if you seek top speed. However, I don't believe you'll be able to plane the boat with anything above a 21", maybe a 22" but I doubt it. Even if you can, you will have sacrificed hole shot considerably.

There are several ways to raise your motor on the transom of your Alert. One is to simply mount the motor in the bottom-most set of holes. Another is to use a manual or hydraulic jack plate/setback plate. These not only raise your engine, they also set the engine back as to create a greater fulcrum effect when trimming th boat.
There are a ton of different factors to consider when it comes to chine walking. Higher engine height helps, however, once engine height reaches a certain level you need higher "raked" props to maintain "bite" on the water.

Because the 15' hull's lifting strakes do not extend to the transom of the boat, trim angle has much influence on chine walking as well. If you trim the boat onto the lifting strakes you will eliminate chine walking at WOT. However, you will lose speed as you increase wetted surface. You may also want to invest in hydraulic or dual cable steering as steering input also has much influence on chine walking.
Bottom line is, your 15' has a rounded bottom. Some fast boats have had round bottoms in the past, and they can be driven at high speeds without chine walking. You just need to set it up properly to do so. It also takes a considerable amount of driver skill.

Tom W Clark posted 04-12-2007 10:00 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Edward,

Your calculations are sound, but it is always best to use some baseline data as a point of departure when trying to improve the performance of an outboard powered boat through the use of different propeller(s).

What propeller are you using now? What speed are you seeing with it?

edwardschnog posted 04-12-2007 11:44 AM ET (US)     Profile for edwardschnog  Send Email to edwardschnog     
Dear All,

Thanks for the sent info. This will really help me alot.
I still do not have the engine, I am thinking which one to buy, a 70hp or an 90hp. However I am leaning a bit more towards the 90Hp. The reason I said that the 22" pitch looks like the best fit, is because alot of people mention that they reach GPS clocked speeds of 52mph. So this would feedback on a 22" when looking into my sent diagram. But it's good to know that a 22" will not workout for a 90Hp engine on a 15 Alert. So most probably I should order a 21" Pro Series prop from Yamaha. See their website: http://www.yamaha-motor.com/outboard/accessories/acscitemdetail/5/406/1349/8534/all/1/3972/0/detail.aspx

Has anyone ever tried the Pro series Yamaha prop?
Or does anyone have a suggestion for the best performance props available on the market today?


The Judge posted 04-12-2007 02:59 PM ET (US)     Profile for The Judge  Send Email to The Judge     
Do a search and you will find that most are running the 19".
Tom W Clark posted 04-13-2007 09:40 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I suspect the Yamaha Pro Series is made for Yamaha by Precision Propeller, Inc. the maker of the Stiletto and Turbo brands. If so, the 21" Pro Series will perform like a 22" or 23" prop form most other manufacurers. i do not think the 90 HP Yamaha will be able to turn it that high on the slightly heavier Alert 15.
Fishcop posted 04-13-2007 11:15 AM ET (US)     Profile for Fishcop  Send Email to Fishcop     
Get the 90hp and a Stiletto Advantage 1 19p. I have the 90hp Yamaha on my 15' with an 18gal tank and it goes faster than you would ever want to go. Lots of power for pulling water toys and people.

I put a doel fin on and it lost 2mph, but does not get too bad now with the chine walking.

Tom W. Clark can get you the prop, once you have the engine.

Oh yeah, use a lanyard for the kill switch!

http://img.photobucket.com/albums/v219/Waha/100_0928.jpg

http://img.photobucket.com/albums/v219/Waha/100_0933.jpg

http://img.photobucket.com/albums/v219/Waha/100_0929.jpg

Fishcop posted 04-13-2007 11:20 AM ET (US)     Profile for Fishcop  Send Email to Fishcop     
Sorry,

Third hold down from the top, you should expect upper 40's. It will feel much faster!

Good luck,

Andy

edwardschnog posted 04-13-2007 10:17 PM ET (US)     Profile for edwardschnog  Send Email to edwardschnog     
Nice pictures Mr. fishcop !

If I decide to buy a 90Hp for my Alert15 I would certainly go for the Yamaha Pro Series 13.5x19 prop. I really appreciate all the support I got from this forum.
Tom you said "slightly heavier Alert 15.". How much heavier do you think the Alert 15 is compared to the other 15 classics? Is the additional weight due to more fiberglass used on these boats or because of more phenolic used in the hull? Do you think that the transom of an Alert 15 is stronger than that of a 15 classic?


Cheers,
Edward

an86carrera posted 04-13-2007 10:58 PM ET (US)     Profile for an86carrera  Send Email to an86carrera     
I would guess that the Alert is a bit heavier But, 90hp on a 15' will be great.
Len
lastmango posted 04-17-2007 06:16 PM ET (US)     Profile for lastmango  Send Email to lastmango     
Edward, Isaw where you were looking for info on props for the 90HP setup on your 15'. I left you some info over on the discusion[1982 sport 15 46-MPH] thread.

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