Prop for 15 hp Johnson

This is my first boat and motor. I've already learned much from this forum, thanks. My boat is a 1436 Lowe Jon. The motor is a 1984 15 hp Johnson. I use it for river fishing primarily. With a friend and trolling motor and battery my weight is at about the max for this boat--610#. Also I added a Mac's River Runner prop protector. The increased drag from the prop protector is quite noticeable, but considering where I use the boat seems a fair trade. Top speed with this setup is about 15 mph. At this speed the motor seemed like it was lugging so I added a tach to check. The rpm's were 5K give or take. The prop I'm running is the standard 9.5 x 10. Do you think it would be worth the wile to go to an 8, or even a 7. Would it be easier on the motor? Do you think I'd gain some speed? Thanks


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I'm pretty sure your minus about 500 RPM, I'd want to drop down 1" in pitch at least and gain 200 RPM. I assume the same rations go for small eng. as they do larger ones. each 1" gives you a gain of 200 RPM thereabouts. You may want to drop 2" but talk with a prop guy first.

If you don't know one I can get you a contact from another forum (bandofboaters).

I'd appreciate that--prop guy connection, thanks.


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My service manual says rpm's should be between 5500 & 6500. So I'd like to gain at least 500.


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ktoelke54 said:

I'd appreciate that--prop guy connection, thanks.


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Reach out to Ken2 2nd post on the thread, IMO he's one of the best and his advise is of course free.

https://www.bandofboaters.com/forum/technical-forums/meet-the-experts/330-ken-reeves-prop-gods

That's a good load for such a small boat but I would assume you would get a little more out of a 15hp on there. It's apples to oranges but I have a 1648 riveted flat bottom typically loaded with 2 smallish guys at maybe 320-340 lbs total, trolling motor, battery, fuel, tackle, cooler, fishing seats and a floor. Probably around 700 lbs total including the motor. Hull is around 300lb according to Alumacraft.

With a 1992 Johnson 15hp and a 10" prop it runs 6250 rpms at 20 mph. Same motor with a 8" prop runs 6800 rpms and about 18.5 mph. With a 1982 Evinrude 9.9hp and 10" prop it runs about 5000rpms at 15mph with this load.

Now I only have it loaded about 60% of the max rating plus a larger planing surface so I don't have to get much lift to get on plane but the overall setup is heavier than yours counting the hull. I would think we would have more comparable numbers though still. Given you are reaching 5000rpms it's fairly safe to assume the engine is in good running condition. One thing you may want to check, is to see if the engine actually has a 15hp carburetor if you don't already know for sure. Some could have easily swapped it out for a 9.9hp one at some point.

My overall weight including the hull is about 800#, before I added the prop protector I was hitting almost 20 fully loaded. The prop protector really slowed the boat, I think part of the problem is it dropped me out of the engines power-band. I haven't much experience with two stroke boat motors, but my experience with some dirt bikes of this era. I expect my motor has a somewhat narrow power-band.

I'm pretty sure I have a 15hp carb, as before I added the prop protector (I didn't have the tach at this point) the engine sounded and felt like it was in its power band.

At this point I think I'll try an 8 pitch, I'd be pleased to gain even a couple mph, putting the loaded top speed at about 17mph. But mostly I think it'd be easier on this old motor if I were able to get her back up in the 6K zone.


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Lots of guys here will refer you to this detailed prop calculator:

https://turningpointpropellers.com/

Small motors have much more extreme variations in RPM's with subtle changes in the boat & can be much more difficult to nail down consistent performance. I'm in the same boat as you are struggling with my current 1436 setup to find a prop that works for it.

ktoelke54 said:

My overall weight including the hull is about 800#, before I added the prop protector I was hitting almost 20 fully loaded. The prop protector really slowed the boat, I think part of the problem is it dropped me out of the engines power-band. I haven't much experience with two stroke boat motors, but my experience with some dirt bikes of this era. I expect my motor has a somewhat narrow power-band.

I'm pretty sure I have a 15hp carb, as before I added the prop protector (I didn't have the tach at this point) the engine sounded and felt like it was in its power band.

At this point I think I'll try an 8 pitch, I'd be pleased to gain even a couple mph, putting the loaded top speed at about 17mph. But mostly I think it'd be easier on this old motor if I were able to get her back up in the 6K zone.


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Wow, that is a big loss from the prop guard. But 20mph with that load sounds like you must have the correct carb. Do you recall what your rpms were then? From the numbers I posted above...my 15hp runs 20mph at 6250 rpms which is about 18% prop slip. I'd imagine you were in similar rpms.

An 8 pitch will definitely help get your rpms up and reduce prop slip, but I wouldn't count on it giving you any more speed. Typically you have to really be over-propped and lugging the engine to see speed gains when downsizing in pitch. You are 500 rpms under the WOT range for a 15hp but I would say you aren't lugging it bad enough to see a speed increase.

I'm interested in knowing what guard you have on which is causing this. Would you mind posting what it is?

wmk0002 said:

ktoelke54 said:

My overall weight including the hull is about 800#, before I added the prop protector I was hitting almost 20 fully loaded. The prop protector really slowed the boat, I think part of the problem is it dropped me out of the engines power-band. I haven't much experience with two stroke boat motors, but my experience with some dirt bikes of this era. I expect my motor has a somewhat narrow power-band.

I'm pretty sure I have a 15hp carb, as before I added the prop protector (I didn't have the tach at this point) the engine sounded and felt like it was in its power band.

At this point I think I'll try an 8 pitch, I'd be pleased to gain even a couple mph, putting the loaded top speed at about 17mph. But mostly I think it'd be easier on this old motor if I were able to get her back up in the 6K zone.


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Wow, that is a big loss from the prop guard. But 20mph with that load sounds like you must have the correct carb. Do you recall what your rpms were then? From the numbers I posted above...my 15hp runs 20mph at 6250 rpms which is about 18% prop slip. I'd imagine you were in similar rpms.

An 8 pitch will definitely help get your rpms up and reduce prop slip, but I wouldn't count on it giving you any more speed. Typically you have to really be over-propped and lugging the engine to see speed gains when downsizing in pitch. You are 500 rpms under the WOT range for a 15hp but I would say you aren't lugging it bad enough to see a speed increase.

I'm interested in knowing what guard you have on which is causing this. Would you mind posting what it is?

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Don't know the rpm's prior to the prop guard as I didn't install the tach until after. I think I lost about 4 mph to the prop protector, it's a Mac's River Runner. Bought it from Cabelas, I read the reviews prior to purchase so wasn't shocked at the speed loss. Many of the reviewers posted similar losses. It only makes sense if your running where prop damage is likely, which i am. Thanks for your input, i won't get my hopes to high on the speed increase with the #8 prop but I think I'll give it a go.


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Update: I got the 8 pitch prop (replaced the 9.25 x 10) out on the water today and was both surprised and pleased. My RPM's increased from 4900 to 6400. This put the 15hp Evinrude at the top of the power band (5500-6500 recommend) The motor just ran better, and the speed (16.5) increased between 2-3 mph. This was with a fully loaded 1436 with a prop protector on the skeg--which adds a lot of drag.





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ktoelke54 said:

Update: I got the 8 pitch prop (replaced the 9.25 x 10) out on the water today and was both surprised and pleased. My RPM's increased from 4900 to 6400. This put the 15hp Evinrude at the top of the power band (5500-6500 recommend) The motor just ran better, and the speed (16.5) increased between 2-3 mph. This was with a fully loaded 1436 with a prop protector on the skeg--which adds a lot of drag.





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Nice! Was that a 3 blade prop you purchased?

wmk0002 said:

ktoelke54 said:

Update: I got the 8 pitch prop (replaced the 9.25 x 10) out on the water today and was both surprised and pleased. My RPM's increased from 4900 to 6400. This put the 15hp Evinrude at the top of the power band (5500-6500 recommend) The motor just ran better, and the speed (16.5) increased between 2-3 mph. This was with a fully loaded 1436 with a prop protector on the skeg--which adds a lot of drag.





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Nice! Was that a 3 blade prop you purchased?

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Yes, a 3 blade aluminum Solas.


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ktoelke54 said:

My overall weight including the hull is about 800#, before I added the prop protector I was hitting almost 20 fully loaded. The prop protector really slowed the boat, I think part of the problem is it dropped me out of the engines power-band. I haven't much experience with two stroke boat motors, but my experience with some dirt bikes of this era. I expect my motor has a somewhat narrow power-band.

I'm pretty sure I have a 15hp carb, as before I added the prop protector (I didn't have the tach at this point) the engine sounded and felt like it was in its power band.

At this point I think I'll try an 8 pitch, I'd be pleased to gain even a couple mph, putting the loaded top speed at about 17mph. But mostly I think it'd be easier on this old motor if I were able to get her back up in the 6K zone.


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Did you ever reach out to Ken2 in the link I posted, just wondering what advice he gave.

Crazyboat said:

ktoelke54 said:

My overall weight including the hull is about 800#, before I added the prop protector I was hitting almost 20 fully loaded. The prop protector really slowed the boat, I think part of the problem is it dropped me out of the engines power-band. I haven't much experience with two stroke boat motors, but my experience with some dirt bikes of this era. I expect my motor has a somewhat narrow power-band.

I'm pretty sure I have a 15hp carb, as before I added the prop protector (I didn't have the tach at this point) the engine sounded and felt like it was in its power band.

At this point I think I'll try an 8 pitch, I'd be pleased to gain even a couple mph, putting the loaded top speed at about 17mph. But mostly I think it'd be easier on this old motor if I were able to get her back up in the 6K zone.


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Click to expand...

Did you ever reach out to Ken2 in the link I posted, just wondering what advice he gave.

Click to expand...

Yes I did reach Ken2 and he was very helpful. He said that I would most likely experience more than ten % increase for each inch pitch. That an 8 would be as far as he'd go and I might like a 9 better. I started with an 8, my rpm' fully loaded went from 5000 to 6400. I'm going to give the 9 a try when things thaw out up here.

Thanks for the connecting me with Ken2, his advice was right on!


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