Evinrude 15 prop selection, have all the tech info. Just need advise!

[Swilkins614]

Well it's time for a new prop. One of my prop shaft bearing bolts backed out and chewed into the back of my stock 9.5x10 prop. So here my question. RPMs are 6330 at WOT, manufactures spec is 5500-7000k rpm. Peak power is at 6250. I am in the peak hp area but when I buy my new prop should I drop a pitch to bring my rpms a little higher? With myself and my gear the boat does 22-23 mph. With two guys it runs 17-18 mph (not sure on loaded rpms because I just installed the tach and ran it by myself) . The boat accelerates ok with two people, and when I'm alone it takes of like a bat out of he//. I would prefer not to loose my unloaded cruising speed. So... I am trying to decide wether I buy a 9.25x10 stainless (due to the theoretical increase in rpms due to being more efficient) or stick with a cheap aluminum and remain at 9.25x10 or possibly drop to a 9 pitch and go up to approx 2530 rpm at full throttle. Thank you in advance!

 

[Swilkins614]

Ops meant 6530 at full throttle with 9 pitch

 

[Sea Rider]

How old is that horse ? If that?s a Evinrude rebadged Tohatsu no way that engine revs at 7 K wot rpm, no matter if being a 2 or 4 stroke engine. At the most 6 K if 4 strokes and 5800 if 2 strokes. Stay with factory delivered prop, is a top performer. Which type of boat and size have you installed it on? If at 6330 K already and going for one less pitch probably your wot rpm will rise around 500-600 rpm more and top speed should decrease, and that's with you alone and assume lightly loaded boat.

If having a rocket hole shot, would stay there and compansate high rpm with less throttle and tach or by adding more weight if liking going wot. Before venturing going blindly buying a spare prop and throwing money into the blue, check factory stated wot min-max rpm numbers..

Happy Boating

 

[Swilkins614]

This is a 1991 Evinrude 9.9 conversion to 15. I own the manual for the motor, in the manual it states 5500-7000 RPM WOT is spec for the 15hp with max hp being achieved at 6250. The boat it is on is a 1988 Lowe Rover Jon 1440. I have front and rear casting decks made out of aluminum, 3 gal gas tank, battery, front mount trolling motor plus my tackle bag and rods.

 

[Swilkins614]

Here's a video of me cruising the river yesterday. I was going 21-22 mph into the wind with about 1' waves.

 

[Sea Rider]

What mods have you made to the engine to bump it to a 15 from a standard 9.9. Seems OMC used to like playing with CC and wot rpm numbers. My previous 96 Evi being a 250 CC would only run 5-6 K max wot numbers. Anyway if already slightly passing sweet 6250 rpm which tops 15 HP why do you want more rpm.

If you dial one less prop pitch to rise max wot rpm to or around 7 K, could benefit with a slight better hole shot, but will drop top best speed as achieved before. It's useless to sacrifice better top end speed against hole shot when already you have a rocket hole shot, but that's me. Can't have both factors doing their homework at same time as one opposes the other.

Happy Boating

 

[Swilkins614]

It has a carb swap and exhaust swap. 15 hp have "tuned" exhaust. When I began looking into props I was under the impression 1" down would yield a 1-200 rpm increase. If it does end up giving me a 5-600 rpm increase and a degrease in top end speed I will likely be selling the 9". So any one needing one, watch the sales section! Lol also, currently the motor is shimmed up 1" and the vent plate is just about even with the bottom of the boat. Per Ken at Proprops I am going to try and raise it another .5" and see how the 9" performs. If it doesn't give me what I want then I will likely throw my old damaged 10" back on to see how the in increase in height pans out. At that point I'll just order another 10".

I am not disappointed with how the boat performs, it runs great and takes off great. However when I got to reading about not running your motor at peak or close to peak rpms and how it decreases the life and efficiency of the motor. Then I starting thinking about re proping. Especially after I damaged mine yesterday.

 

[steelespike]

The Evinrude/Johnson,9.9/15 is kind of a cool motor Basic dimensions are the same and the base prop is the same.
The 15 just turns more rpm. They have a load friendly gear ratio.2.42. So while about 24 is possible with the stock prop
A numerically lower ratio might produce more speed.
A cupped 10" stainless prop may actually load the motor more,Not like an increase in pitch but some.
If the motor can operate within its rated rpm range for most activities I wouldn't worry about the max rpm if the performance is good.
You don't went to prop so low that it starts to drop out of rpm range as you add weight.
If you go to a 9" prop you "should" gain rpm,hole shot and load power but top end "may" suffer.

 

[Swilkins614]

I should be receiving the prop this week some time. When I get it in and install plus do my water pump I will report back on my findings!

 

[Sea Rider]

It's a waist of time and effort playing with different engine heights to dial best engine water performance. If you post a nice lower leg side pic showing both small upper plate and lower anticav plate, will send a short cut procedure once pic is posted to dial sweet engine/transom height.

Was 1991 reed valve assembly same for both 9.9 & 15 HP engines ? On same model 96 + engines having 250 CC, besides a carb swap was also necessary to swap 15 HP reed valve assembly due to having slight wider orifice intakes to play better with more fuel intake provided by larger 15 HP carb.

Happy Boating

 

[Swilkins614]

I'll take a pic shortly and post it. I only say I installed an exhaust manifold so people believe me. On the 1991 motors the reeds, reed stops, and exhaust are all exactly the same. I verified this by looking at part numbers for both motors. It was pre 1990? Or 1989 where you has to change the reeds, exhaust esc. Everyone wants to believe they are different motors when in fact my year is a 9.9/15 with a bigger carb for 15 hp. Heading out now to take a pic of the boat

 

[Swilkins614]

Here is the motor with an extra .5" spacer. During my previous runs I was running the motor at 1" raised. Aprox .5" below hull. With the spacer you can see it is about level to slightly above depending on tilt angle.

 

[Swilkins614]

I should also note. This boat prefers to be in the fully tilted down position. All other locations the bow rose too much and the a$$ end would squat in the water. I could only get about 19-20 out of it until I tilted all the way down and I ran 23

 

[steelespike]

Thats a nice looking Jon.
Jons make a nice stable fishing platform and the flat bottom helps it get up on the water.
While your speed is respectable the 4 or 5 keels of a Jon aren't conducive to speed.
Your numbers calculate to 7% slip. A very respectable number that may be little suspicious
as usual slip is in the low double digits.

 

[Swilkins614]

Steelespike, thank you very much. It is my first boat of my own and for myself, it is absolutely perfect. I also love the way I don't have to worry about beating it up either! I am right there with you when you question my speed.

I didn't realize I was only getting 7% slip. I actually measured my speed off of my phone GPS. Out of curiosity I looked up how accurate the phone GPS' are and to my surprise they are said to be just as accurate as say a Garmin. Here is a screen shot from that day. When I get all my parts install and run the motor again I will take a video with my actual Garmin GPS.

 

[Sea Rider]

How do you drive that combo, aft transom hand tiller steered, tiller steer while sitting on aft pedestal seat, small center console at middle boat's length ? Just a balance reference issue... That boat must be really light to achieve that fantastic speed with just a 15 HP OB.

Happy Boating

 

[Swilkins614]

When fishing by myself I sit in the rear seat closest to the transom, no pedestal, just seat in mounting hole. Also, it is tiller steered no console. I believe the weight distribution is just so that the boat rides perfect. 3 gal gas, battery plus myself in the rear. Aluminum casting decks up front with trolling motor and tackle bag up there as well while under way. The tilt position made the most difference for me. Most don't run fully tilted down as they feel (and reality) if you tilt up the front comes out of the water as allows for faster speeds. The way I see it is I have a 15 hp OB. No speed demon... I want all the thrust I can moving me forward, not necessarily spending energy trying to push the bow up and forward.

 

[Sea Rider]

The purpose to sit engine at the sweet transom height reflects the need to make the most out of that portable engine, on larger engines not an issue, thus having more HP will compensate much better any tail water drag as opposed to less HP engine in which you need to take all those stated ponies out for top engine and boat performance.

Let's rig the Jon boat as if were your first boating day :

(1) Float boat alone and trim engine to be perpendicular to water level (90 deg) usually second, third hole out transom.

(2) If alone move fuel tank and any other heavy objects forward, if not, seat one passenger up front to have a well distributed deck weight.

(3) Go for a wot spin on flat calm no windy water condistions, being spot on trimmed, anticav plate and boat's hull should ride parallel to water level provided that deck has well distributed weight.

(4) Visually check by pulling your head out transom if with any back or over transom water splash. If engine produces a neat flat middle wake you're ok, if with over transom splashes, engine sits too low, water flow at speed is passing over small upper water deflector plate (yellow line) and hitting round middle leg portion. Will need to raise engine accodingly with wooden shims till water flow passes at speed slightly underneath small water deflector.





4 This is the sweet engine/transom height, once dialed there's no need to move trim angle from initial set position, engine will deliver its top thrust and water flow will be nicely cut at the sharp edge formed between small water deflector plate and lower anticav plate producing least tail drag while achieving top hole shot and close turns with no prop aereation.

Have no experince whatsoever with Jon boats but these principles can be applied to any boat as well, try them out. If you manage to sit engine at the sweet height, any prop pich you test will work top. Can go for the 7K max wot mark if you want.

It there's a point that you don't fully understand, let me know.

PD : The only way to determine at which lower tail height water flow is passing by when boat is on plane is pulling your head, this is not a assume, guess not even asking at the forums issue, must be checked while at trial.

Happy Boating

 

[Swilkins614]

Thank you sir. I will certainly try those things next time! I received all my parts today. Installed new waterpump kit, all new LU seals as well as the new prop. Everything went pretty smooth too. Only about two hrs labor.
At first glance the new prop has much larger blades. I am hoping this doesn't effect my performance too much.

Also, I found a very small section in my manual that states WOT RPMs should be centered between min and max WOT RPM. 6250 is the in between mark. Looks like I may be selling this prop after all and going factory size in stainless or alum.

 

[steelespike]

The old prop looks like it may be a weed resistant design.
I think the new one will out perform the old prop if they are the same pitch.