OMC/BRP Made Al. Prop vs Al. Aftermarkets for 9.9/15HP Motor

[scout-j-m]

I recently purchased a 1992 Johnson 15hp. It came with an OMC 8" aluminum prop. Maybe saying it is a BRP prop is a better description as it is identical to the current style of Johnson/Evinrude props they sell here on iboats and I have no clue if thats what the OEM ones looked like back in 1992.

Anyways, the motor is on a 1648 Alumacraft flatbottom (~300 lb hull, 35 hp max rated) with 6 gallon fuel tank, group 29 battery, trolling motor, floor, fishing gear, and two 170 lb men. I'd consider it a relatively light load for the hull. With the weight distributed just right and the motor trimmed so that the anit-vent plate was parallel to the hull bottom it ran at WOT at 6800 rpms and 19 mph. According to a prop calculator that is only about 11% prop slip which I would imagine is excellent for a riveted flatbottom hull with such a small motor as compared to the hull rating.

The motor has a WOT range of 5500-7000 rpms I believe so the 8" prop seems like it will be a good one to hang on to and pair with my load. But I haven't ran the motor with me alone in the boat so I would like to get a second prop in maybe a 10" pitch for those situations. Possibly even a 9" 4 blade as it may be a good compromise for running with either 1 or 2 people and not having to change props.

I still have an old 3 blade, AL, 10" pitch prop from my 1982 9.9hp Evinrude I need to try before I make any decision, but my point of all of this and my question is to see if anyone knows the difference between the current Johnson/Evinrude/BRP props being sold compared to the after market brands like Solas, Turning Point, Michigan Wheel, etc in terms of cupping and blade size particularly for these smaller props for these little 9.9/15hp motors like I have?

 

[steelespike]

It is very likely todays props would do better than those from the good old days.

 

[Sea Rider]

With you alone wot rpm should increase to near or around 7 K, if exceeds, just throttle less while watching tach. No need to change prop when boating alone. Just for the record, post max rpm achieved.

Happy Boating

 

[scout-j-m]

It is very likely todays props would do better than those from the good old days.

I assumed so too. I'm still primarily curious between the difference in the new model OMC props versus the Solas ones in design because the Solas Amita specifically mentions in the prop description that it has larger blades, better suited for today's 4 stroke motors. I know typically 4 strokes have better "bottom end" power whereas 2 strokes are better on the top end (at least for dirt bikes) so I thought if they indeed have larger blades to suit a 4 stroke that it may be a poor choice for a 2 stroke. It is the cheapest one available which is why I would love for it to work.

 

[scout-j-m]

With you alone wot rpm should increase to near or around 7 K, if exceeds, just throttle less while watching tach. No need to change prop when boating alone. Just for the record, post max rpm achieved.

Happy Boating

Yes, I am certain it would hit close to 7k if not more. With the same load my 9.9hp increases 500-600 rpms with just me in the boat with a 10" prop (I know, not an apples to apples comparison by any means). I would really like to get a little more speed out of it with just me in it though. Would propping it to be at 6500 rpms WOT be too strenuous on the motor? That would be 2/3 of the way up the WOT recommended range, plus the previous and later model 15hp motors are rated only up to 6500 rpms so I wasn't sure if the 7000rpm max OMC originally recommended still stood true.

 

[Sea Rider]

You?re at near max wot range as currently loaded, why do you think need the Solas prop? If yours is a 2 stroke OB saty with that prop. Won't get much more out of a underpowered OB.

Happy Boating

 

[flyingscott]

Here is the solas I run on my 1977 15 hp and is going to go to my 89 15 hp. On my 1977 model the performance was identical to the factory prop. I am very happy with it. It is also a strong prop as my boat was totaled last week and both motors lower units were stuck in the ground like lawn darts and the prop did not bend.

 

[scout-j-m]

Here is the solas I run on my 1977 15 hp and is going to go to my 89 15 hp. On my 1977 model the performance was identical to the factory prop. I am very happy with it. It is also a strong prop as my boat was totaled last week and both motors lower units were stuck in the ground like lawn darts and the prop did not bend.

Ouch, glad it survived. So is it also a 10" pitch? Does it perform identical to the OEM one every aspect?

 

[scout-j-m]

You?re at near max wot range as currently loaded, why do you think need the Solas prop? If yours is a 2 stroke OB saty with that prop. Won't get much more out of a underpowered OB.

Happy Boating

I was just thinking since it had a 1500 rpm WOT range, instead of the typical 1000 rpm, that it may be ok to do so.

I'm also curious why the range was bumped up to 7000 rpms over the span of a just few years in the late 80's to early 90's. I tried researching to see if there was a TSB (If OB manf even do those) that recommended limiting it to 6500 rpms at a later date but had no luck.

I'll try the 10" pitch one I have before or if I even decide to do make a change though. Ideally I'd prefer to have a single prop that will perform adequately with 1 or 2 people in the boat and then keep a spare that is for an extra person or extra heavy load of gear, so either a 8" or 7" pitch.

 

[flyingscott]

Yes that is a 10" pitch. Honestly I could not tell the difference between the props. Both my props are 10" pitch, the factory one has the more curved ears and the solas which has the more standard ears. Mine runs on a 14' starcraft seafarer runs good and the solas is cheaper than factory. Both my 15s the 77 and the 89 are rated to 6500 rpm

 

[Sea Rider]

Better be 101% shure if that OB runs full wot at 7 K or will blow anything inside crankcase. Evi likes playing with such uneven min-max wot range on their year engines. Used to have a 96-15HP rated for 5-6K. If that OB screamed at 6K can't possible imagine at 7K...

Happy Boating

 

[scout-j-m]

Thanks again guys.

I took it to the lake this morning by myself to try out the 8" pitch prop with just me in the boat. With just me it ran 6930 rpms and speed fluctuated from 19.5 mph up to 21.5...the wind was stronger than usual so there was a bit of a chop and had a tailwind or headwind at all times with the lake being so skinny. My previous numbers were from really smooth water and only a slight breeze.

I meant to put on my 10 pitch but forgot my toolbox at home. I haven't yet gotten this boat fully setup so I am missing things like a spare prop wrench as of now.

 

[Sea Rider]

With a 10 pitch expect to lose best hole shot, 600-800 wot rpm and top speed. Post numbers just for the record. Props should be tested on flat no wind water cond..

Happy Boating

 

[scout-j-m]

I tested out the 10" prop yesterday. Conditions were unfortunately similar to the last test with stronger wind and light chop. With just me, the WOT rpms were just above 6500 and speed was consistent at 21.0-21.5 with occasional bumps up to 22 or so. I didn't really notice a difference in holeshot either and it was still humming pretty good at WOT. Given that I am still 2/3 of the way up in the WOT range I may stick with the 10" prop for solo fishing trips and keep the 8" for when more people are aboard. It will require a little more testing before I decide though.

 

[scout-j-m]

Figured I would bump this post to add some new results.

I still have both the 8 and 10 pitch props I referenced above. I also bought an aluminum 4-blade 9" pitch Solas which I have now tried. The boat is still 95% the same as well as the load and weight distribution. I am getting about 20.5 mph at 6250 rpms with me alone. With another person around 19.5 mph at 6150 rpms. And with my twin 4 year old boys and another person right at 19 mph at 6000 rpms.

The 4 blade doesn't seem to offer a noticeable holeshot over the 3 blade 8 pitch. Both produce similar speed but the 4 blade is turning 800 less rpms and is right around the rating for 15 hp. I'm not sure if that makes it more or less fuel efficient though. So far to me, it appears the 4 blade 9" pitch is not a good option because 1) adding a 3rd adult or more gear would probably drop the rpms to the very bottom of the WOT range 2) despite providing additional holeshot over the 10" pitch 3 blade, holeshot is not really an issue for this hull since it drafts so shallow and planes so easily.

I believe I will stick with a 3 blade 8 and 10" pitch prop with the 10" being the standard one and reserving the 8" for a larger load. I am now curious though if I would gain anything from a stainless 10" pitch prop. I'm sure any gain would be minimal, however, it is worth noting that my motor's cavitation plate is about 1-1.5" under the bottom of the hull and I do plan to fabricate a home made jack plate to correct that and also create 4-5" of setback. You may be thinking this is a lot of work for nothing on a motor this small but it is fun. And I'm actually not wasting money throwing it at props....the 10" one I have been referencing has a spun hub that works fine 90% of the time but will begin to slip after long WOT runs so it will need replaced or repaired anyways. On top of that I have three OMC 9.9/15 hp motors that will need props to go with them when I sell them so plan to pair the 4 blade with one of those prior to listing. On top of that my local 10hp restricted lake just bumped the limit to 25hp so I hope to repower soon too and another reason why I am going ahead with a jackplate.

 

[Sea Rider]

Have you checked what's going on at wot at back transom when at plane ? Check at which lower leg portion is water flow passing by with engine trimmed to 90° and deck weight evenly distributed.

Are there water splashes up lower leg/over transom, what's the OB behaviour on tight close fast turns, does prop aerates ? Test on flat calm water cond to confirm if the OB sits at the sweet transom height spot. If so, no need to add a jack plate, nor set back the OB.

What find really strange is that Evinrude offers differente min-max wot rpm ranges for their 9.9/15 HP OB's with same 250 CC powerheads through the years of manufacture.

Happy Boating

 

[scout-j-m]

Have you checked what's going on at wot at back transom when at plane ? Check at which lower leg portion is water flow passing by with engine trimmed to 90° and deck weight evenly distributed.

Are there water splashes up lower leg/over transom, what's the OB behaviour on tight close fast turns, does prop aerates ? Test on flat calm water cond to confirm if the OB sits at the sweet transom height spot. If so, no need to add a jack plate, nor set back the OB.

What find really strange is that Evinrude offers differente min-max wot rpm ranges for their 9.9/15 HP OB's with same 250 CC powerheads through the years of manufacture.

Happy Boating

Water is flowing up to the second plate (at the top of the lower unit) above the anti-vent plate. I don't get water over the transom though aside from some spray from the transducer. No aeration in any maneuvers and seems to handle fine on tight turns.

 

[Sea Rider]

Water is flowing up to the second plate (at the top of the lower unit) above the anti-vent plate. I don't get water over the transom though aside from some spray from the transducer. No aeration in any maneuvers and seems to handle fine on tight turns.

If water flow at speed is skimming right unde small upper plate, then you're at the sweet transom height spot, way to go to handle most water scenarios with top prop thrust.

Happy Boating

 

[scout-j-m]

If water flow at speed is skimming right unde small upper plate, then you're at the sweet transom height spot, way to go to handle most water scenarios with top prop thrust.

Happy Boating

OK, I didn't realize that. I thought that meant I was getting extra drag.

 

[jimmbo]

You are getting extra drag. The ideal height will vary depending on speed, water conditions, and choice of prop. Some prop designs can handle running partially submerged better than others. I’ve had some engines mounted so the entire anti-ventilation plate(the one right above the prop) was visible above the water when looking over the transom.
Your speed range is low enough that the extra drag is nothing to be concerned about, but any excessive spray can be reduced by raising the engine so the AV plate is even with the keel(I am assuming you have a tin boat, with a pronounced keel about an inch or so below the bottom of the hull). If there is no keel or it tapers off a foot or two prior to the transom, then raise the engine until it is even with the hull bottom. Report back the results