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Author Topic:   Question about Prop. / (Pitch and Dia.)
Buckda posted 04-17-2003 08:03 PM ET (US)   Profile for Buckda   Send Email to Buckda  
I recently purchased a new propeller for my 15' Sport GLS (70 HP Evinrude).

The serial number of the propeller on the engine indicated that it was a 17" Diameter, 15" pitch. It was quite scuffed up and slightly chewed.

So...I purchased a Michigan Wheel Aluminum prop to replace the old one. That propeller is a 17" Diameter, and 19" pitch.

I installed the propeller on Saturday.

My question is this.

The new propeller looks "smaller". I did not have time to measure for proof, but could it be because of the more aggressive pitch, less blade was facing me? Perhaps the silver tips of the chewed older propeller reflected more light and gave the appearance of more surface area?



Dick posted 04-17-2003 08:39 PM ET (US)     Profile for Dick  Send Email to Dick     
When buying a prop pitch is the important factor. The same diameter prop may be available in several pitches so the diamter means nothing.
If you have been happy with the performance of a 15 pitch and you switch to a 19 pitch you are not going to be happy camper.
With the 15 pitch what RPM did you get at WOT? You want to keep the top RPM in the upper end of the manufacturers recomended.


Buckda posted 04-17-2003 08:47 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Hm...doesn't answer the question...but to answer yours:

The 15" pitch prop allowed me to "bury the needle" on the tach - 6000 RPM - so I'm not sure how high it let's the engine rev, but I know that my recommended range is 5000 - 5800 RPM per the owners I wanted to get that under control. Was looking for higher top-end speed. I figure I will lose about 400-600 RPM, but gain some when I raise the engine a slot or two to put me in the mid - to high side of the recommended RPMs.

I have not yet "Water tested" the new wheel, but don't think I will be unhappy, but time might prove me wrong - we go on the shakedown spring camping trip on May 16 at Mullet Lake.

Dick posted 04-17-2003 09:01 PM ET (US)     Profile for Dick  Send Email to Dick     
A water test is the only sure way to tell. If you plan on raising the engine do it before you make a test run with the new prop.


lhg posted 04-17-2003 09:08 PM ET (US)     Profile for lhg    
Within a given series of props, diameter often decreases, slightly, as pitch increases. As Dick indicates, a prop is only purchased by pitch, or style of application. The diameter # is irrelevant, and is up to the prop designers.
Sal DiMercurio posted 04-17-2003 10:55 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Sorry, but your prop is not a 17 diameter.
They don't make a 17" diameter for any outboard that i'm aware of.
You might be reading it backwards, cause a 17 dia would hit the antivent plate.
Are you sure it's not a 15 x 17 ?
Even if thats the size [ 15 x 17 ], it's the wrong prop for a 70 hp engine.
The largest dia for a 70 hp is 13.25
A 17 x 15 on a 70 hp wouldn't get 6,000 rpms, be lucky to get 3,000.
The first number is the dia, the second is pitch.
The proper prop for that setup should be around a 13.25 x 17 or 19.
mustang7nh posted 04-18-2003 08:14 AM ET (US)     Profile for mustang7nh  Send Email to mustang7nh     
I had a new 15Sport in 1987 with 70hp Johnson. I don't remember the prop but it definitely blew past 6 grand on the tach. It was quick as hell right out of the box. I think it would do 6500 rpm. It never blew up and most other sports couldn't keep up. I had an aluminum prop and had I kept it would have gone to a SS and up in pitch to preserve some engine life. I think some of the setup problem may be due to the fact that those boats are so light that when you load them up with several people and gear a smaller prop may be advantageous. But I was usually just me and a friend and we probably only weighed 150 each back then.
where2 posted 04-18-2003 01:10 PM ET (US)     Profile for where2  Send Email to where2     
13-1/4"x19"SS cupped is the prop you want for max top end. However, hole shot with a slalom skier in tow is lousy.

Buckda posted 04-18-2003 01:24 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Thanks guys.

I may be mistaken on the Diameter - because it definitely spins without coming any closer to the anti-ventilation plate than the old one - but I am certain that I have bought a 19" pitch prop (and it still "looks" smaller).

I usually have only one other person in the boat with me - sometimes we're loaded with camping gear, but I don't use it to ski.

So, you guys are suggesting that when I know I am going to have a fully-loaded boat, I should switch to the old prop, and when I'm going to be goofing around or looking for max top-end, I should go with the new one?

Sal DiMercurio posted 04-18-2003 05:10 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Buck, in order for us to give you the correct size prop, we first must know the recommended maximum rpms the engine is rated for, & the correct size prop you "now" have on the engine, & what is the max rpms she's twisting now.
Not knowing what your using the boat for leaves us up in the air.
We can give you the answer when you give us our answers, but not until then.
Even though I feel a 13.25 x 17 or 19 might be the right one, I can't be sure until you come straight with some answers.
Bigshot posted 04-21-2003 09:55 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
13x19 is the right prop. If you break 6k have it double cupped but I doubt you will. She willd o fine on the holeshot, knock your butt outa the boat if not careful.
Buckda posted 04-21-2003 01:31 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Bigshot - it was your advice that led to my purchase of the new prop - and I believe you when you say it is the correct set-up.

I totally respect your opinion and your knowledge and appreciate your input. To recap excerpts from my earlier posts:
Engine: 70 HP Evinrude
Recommended Max RPM's - 5000 - 5800 according to Clymer's manual
Old prop "Buried" the tach - at 6,000 RPM' there is no way for me to tell exactly how high the old prop let it rev.

Boat is in another state, so it will be take a while for me to confirm old and new prop dia - as I said, I was probably mistaken about diameter....but pitch definitely was 15 on old prop, and 19 on new.
I don't use the boat for skiing...mainly for touring, fishing and camping.

Thanks for your patience.


Buckda posted 05-05-2003 11:05 AM ET (US)     Profile for Buckda  Send Email to Buckda's the update.

The original prop was 13.75 X 15
The new prop is 13 X 19

New max RPM with engine trimmed 5600 RPM...runs at 5400 when engine fully "down".

Trimming the engine results in a bit of chine walk...especially in light chop into a stiff breeze (on the St. Joseph river).

Have to say...she "feels" faster...but I haven't run the boat since early January, and didn't bring the GPS to verify.

I'm happy...thanks for the advice Bigshot.

Regarding the original question...the prop looks smaller because it 3/4 of an inch.

Bigshot posted 05-09-2003 04:15 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Raise that engine up a hole or 2 and you will get closer to 6k(redline) then she will really sing. trim the snot out of it until she breaks loose at WOT and you will get more than 5600, then bring it down till she grabs. That is the funnest Whaler with a 70 on it and she should do over 45 with that prop. Mine did a tad over 48 with a 20"SS prop and jacke to the hilt.....enjoy!
Buckda posted 05-28-2003 08:06 PM ET (US)     Profile for Buckda  Send Email to Buckda     
GPS WOT: 42.3 MPH (5600 rpm)
375 Lbs of "guys" aboard
20 Lbs for the cooler
18 lbs for anchor
15 gallons of gas

Speed calculated from average of 3 runs on smooth water (4-5 inch wind ripples).

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