what is the right prop for my 15 horse

[57feathercraft]

hello, i got a 57 12 foot feathercraft alum boat with a 76 johnson 15 horse. i dont know exactly the top speed of it but im pretty sure its a tad over 25mph. i have one of the stock props a 9 1/2 x 10 which one of the fins is bent from hitting a rock and thats why im replacing it. i can put the engine on the third setting up so theres less of the outboard in the water and it doesnt porpose. i was looking at props and i found a 9x10 and i found some 11' pitch props. i wanna stick to alum incase i hit anything i dont want to hurt the drivetrain. whould a smaller prop with a bigger pitch be too fast for my boat?

 

[cbavier]

Re: what is the right prop for my 15 horse

Feathercraft. Do you have the owners manual. I happen to have sold a 1977 15hp Johnson shortshaft last year and the boat. I have all the owners manuals because the guy wo bought the boat forgot to take them. It's been a year and he hasn't came back. There was hardly any difference between the 1976 or 1977.
The Owners Manual says the proper Propeller is a 3 Blade 9 1/2 inch Dia x 10' Pitch Page 6 listed under Specifications.

Your Prop is going to be the same. Your Johnson is either a 15R76 or a RL76

I think the only difference between these is Long or short shaft and Either Manual start or Electric start.

My Johnson was a 15R77 Short shaft/ manual start.

 

[steelespike]

Re: what is the right prop for my 15 horse

Not an expert::
Difficult to pick props without rpm specs and speed.
If it jumps up quick with the 10" and winds up good then it should handle the 11".Just keep in mind it will have less push with a load than a 10".
I'm sure someone will be along with more specific info.

 

[57feathercraft]

Re: what is the right prop for my 15 horse

i dont think there a difference in the outboards from 1972 to the 90s. mine is the 15r76 short shaft electric start. love the eletric start, lol. the engine has 120 lbs of compression in each cylnder. i am unsure of the rpms im running. i tried figuring it out by using a tester that hooks up to the spark plug wires but theres to much of an electric field so it was getting inacurate readings. the top speed is 26-27. i just went out and rode next to my dads runabout

 

[cbavier]

Re: what is the right prop for my 15 horse

I had my 15 Hp Johnson on a Sears 12ft Aluminum boat. I had Reinforced the Transom with full with width full one inch thick Oak. I had a guy in a Bow Rider clock me side by side. He said I was running 38Mph. I beat him badly out of the hole but of course he caught and then ran away from me. I
really don't think my Boat would run 38 Mph. I think he was doing 38 when he caught me. But that's what he told me anyway after we both shut down and came back together side by side.

I do know it was FAST for a 12 footer. Only about 12-18 inches of the stern was in the water at plane.

 

[57feathercraft]

Re: what is the right prop for my 15 horse

i reinforced my transom with a 5/8 oak on both sides and a 2x1 beam and a stainless steel plate for it to mount on. stained it too and put 1/4 oak on the seats and stained it the same color. we welded up a crack on the bottom front of the boat and welded on a carbide like what a snowmobile has for the skis in the front that is about 5 inches long. turns on a dime at WOT. i also painted the outboard hunter green and the floor of the boat hunter green. polished all of the alum and found a new 76 15 horse crowl on ebay for 20 bucks. this was my project last summer and it turned out great till someone had stolen it! but i obviouslt found it . and the boat is in the same condition. the boat looks fantastic. im thinking about boring the engine over .03 with wiseco pistons to get a little more top end and that new prop. i had 4 people in it the other day at WOT and id say it went 7-10mph for about half an hour 45 mins and it had no problems didnt over heat.

 

[steelespike]

Re: what is the right prop for my 15 horse

I would say with a stock 15 26 to 27 ,actual speed, is the max in fact I would have guessed less.That motor is capable of turning some serious rpm up to 7,000.So maybe some good speed is possible.The 11" prop will really struggle with 4 people like you did before.If you raise the motor 1/4 inch at a time till just before ventilation becomes a problem you could gain some speed.Just add 1/4 inch sticks on the top of the transom.be sure the clamps still have a good bite and it pumps water.Here's a thought the 25hp up to 76
is a 22 cube motor only 85 lbs 10 more than your 15.It would have a lot more guts than your little 13 cube 15 hp. If your boat is rated for 25 it would surely scoot.I know there are some folks running 40 and 50 hp on the feather craft runabouts.

 

[57feathercraft]

Re: what is the right prop for my 15 horse

this is a 11' 6' alum boat. not a runabout. i was running next to my dads runabout to find out the top speed. my boat is rated for 7.5. i think im guna go with the 11 pitch prop. i think the engine has enough rpms for the prop. the prop that is on there now is from my dads 1972 evinrude 9.9 my johnson came with a 7 pitch prop. so if i know im going to have a bunch of people in the boat i can switch it the 7 pitch prop.

 

[steelespike]

Re: what is the right prop for my 15 horse

Ahha the plot thickens.Well you can certainly fine tune the set up and the motor can certainly handle rpms.You may want to get a tiny tach so you can be sure it isn't over reving.Also maybe buy or borrow a GPS so you can get the actual speed.Speedometers are notoriously inaccurate.I bet on that little boat you could raise the motor a lot If you could get an inch you might gain 2 mph.
Or maybe that's all more than you want,and just enjoy your boat.

 

[cbavier]

Re: what is the right prop for my 15 horse

57feathercraft did you read my post with the specs. The first post to your original question? What are you trying to do to a 11 1/2 ft Aluminum fishing boat? Make a speed boat out of it? I was just wondering. Your original question sounded like you wanted to stay within the original Specs. Now it sounds like your planning on souping it up for ____? just wondering.

 

[steelespike]

Re: what is the right prop for my 15 horse

I'm not sure if he looking for more speed or just more power.(7-10 mph with a load.Any way ,Feathercraft,I think I would be wearing a PFD when running that boat motor combination.If your boat has a flat bottom be really careful
that hard chine can be twitchy and you could end up in the drink in a flash.
Also that little boat with 4 people is probably way over capacity.Be careful and enjoy.

 

[cbavier]

Re: what is the right prop for my 15 horse

Well if he wants power. Johnson makes two props one is a speed prop and the other is a power prop both for that motor.I had the power prop.

 

[jimmbo]

Re: what is the right prop for my 15 horse

rpm range for the 77 15hp was 5500 - 6500 rpm.

There are no such things as 'speed props' or "power props' only correct and incorrect props

 

[cbavier]

Re: what is the right prop for my 15 horse

well that is what they called them and I always figured it just had to do with the pitch. Not the size in diameter.

 

[jimmbo]

Re: what is the right prop for my 15 horse

only if the engine can turn the "speed prop" at the same rpm as the "normal" prop will there be a speed increase. A lot of times engine rpms drop enough because of the greater load that the boat ends up going slower. Calling a lower pitch prop a "power prop" is not accurate either. Yes it will allow the engine to rev up faster due to the lighter load on it, however it could also allow the engine to over rev well past its peak horsepower and end up making less than rated power. Put another way, takes off quick but runs out of steam fast too, most often with a lower top speed.

That said, there are a number of high performance props out there that can make a boat go faster than the regular performace prop choices, even if the pitch is the same. Things like diamiter, rake, cup, blade shape, number of blades, vented or non-vented, over the hub exhaust, thru hub exhaust are all factors in how a prop performs.

 

[bigtwin]

Re: what is the right prop for my 15 horse

Any one who knows anything about "hopping up" a 9.9 or 15 hp OMC will tell you they respond well to changing the reed valves to fiber reeds. Do this and motor height adjustments first and then worry about the prop. I have seen 15 hp motors perform like 25 hp motors.

And remember 6500 rpm isn't the end, it's a good starting point!!!