Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

[Trawler man]

Hello,
I have a 10.5' Avon RIB (inflatable,glass hull)thats carrying about 550lbs(people and gear) it is rated for a 15hp and up to 1200lbs.
We have a 1975 Johnson 15 that runs great but will not plane us off.I can get a 2002 yamahha 4 stroke 15 but I have received different opinions about this helping anything.I have also considered a yamaha 25 hp 2 stroke(one of the smaller 25's I have found)
So would the yamaha 15 help anything ? could I use a larger prop on the 4 stroke and help ? Can you get more power out of the yamaha 15 ?
How much would trim tabs and or a fin on the motor help ?
Thanks

 

[JB]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

15HP is 15hp, except that pre-mid-80s engines are rated at the crank and later engines at the prop. Your '75 Johnny probably has 12-13hp at the prop.

Assuming that your Johnny is putting out full power I doubt that you would notice any difference in performance going to a heavier 4 stroke 15hp engine.

I think the first steps are to ensure that your Johnny is making full power and is correctly propped. It should plane your RIB pretty easily.

 

[Solittle]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

I'm with JB. I'll bet your 75 needs some TLC. I have a 75 9.9 & it gets the wife & I up on a plane quite nicely in our 13' alumnium skiff

 

[cobra 3.0]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

I have a 1997 15hp 2 stroke Evinrude and it will plane 3 people each weighing 200 lbs and all our fishing gear very, very easily in a 12 ft fiberglass boat and also on a 13.5 ft aluminum.

Something is definitely wrong with your set up/ motor.

 

[tashasdaddy]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

agree with all, i have a '55 15 hp rude, that planes my 14' and runs out at 20mph with 2 200 pounders plus gear.

 

[steelespike]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

I think at least part of the problem is that little 10 footer doesn't have much surface to help it plane that load.
Give the motor a compression check and see what the pitch of the prop is.You don't want a bigger prop.You may want a prop with a little less pitch to help you get on plane also try trimming in all the way.If it doesn't seem to trim in enough you may have to make or buy wedges to go between the clamp and the transom to give a little more in trim.

 

[Trawler man]

Re: Old johnson 15 vs Yamaha 15 4 stroke ...heavy small boat question

I lean more towards what steelspike is saying,I think the length is a big factor.I will see what prop I have and I think the motor trimmed all the way down still has a couple degrees on it so wedges may help also.