15 Hp Johnson won't plane 12 foot boat.

[Kawaiikeno]

Don't be too sure that the idle jet ( slow speed needle) only affects Idle
I have a great running 1979 15hp #15904E that requires you to open the idle jet about 1/4-1/2 turn from best trolling setting to best high speed
Mike

 

[saltchuckmatt]

Re: 15 Hp Johnson won't plane 12 foot boat.

A 15 HP motor + 6 gal. tank + 250 lB person all in the back is a lot of weight in a 12 ft. boat. Too much to balance the boat. I only weight 140 and with a 9.5 and 3 gal tank I have to use ballast in the front of my shallow 12' boat if I want it to plane quickly. A couple others made similar comparisons but also mentioned a passenger who generally sits in the center or front and that helps to balance the boat or larger heavier boats. My brother had a 12 with a 15 and had the same problem when he was alone. Remember that small boats are much more sensitive to weight distribution than larger ones.It may be one of the other problems mentioned or a combination of, but I think your biggest problem is the weight distribution.

I believe this could be the culprit too. That 15 should go like hell on a 12 foot boat. Unless it's some tank fiberglass boat. I have a 12 ft alum boat with a 8 hp. When alone I have to shift my wait WAY forward until I get on a plane. Once on a plane, goes like hell. If I stay in the back and it doesn't plane engine never reaches high rpm's. Otherwise, wrong prop, long shaft on a short shaft boat (20inch outboard on a 15inch transom) sure me boats don't plane well and if so that motor won't spin up until it planes.

 

[dwco5051]

I have a 12' Meyers aluminum boat that I normally use on electric only lakes. A couple of years ago the oldest boy got a 5hp Mercury and I bolted it on to try it. I found that a 5 gallon drywall bucket would fit between the front seat and bow and after some experimenting filling it half full of water would balance it out and it would almost pop up on plane. Don't know what the top speed was but it was fast enough to be a hand full if the water was a little rough. Put an old battery, cement block, or something with a little weight as far forward as you can get it and try it out.