Johnson 300 Hp repower

[Strider]

Hi folks,

I am planning on re-powering my 1991 25 foot Hydra Sport WA this winter. It has the original 8 cyl 300 HP single Johnson on it. I have been happy with the performance of the boat with this motor. (30 mph cruise at 4500 rpm and about 39 mph at 6000) These speeds are fine for where I fish but the gas consumption is scary.

What I'm wondering is this: Does anybody know what the actual horsepower curve for this motor is? Dr. Dhadley know what HP I'm actually getting at these RPM's? I'm wondering if a new 250 Hp single would give similar performance.

I'm building a flotation bracket for the boat to gain cockpit space and am trying to decide single 250 or twin 150's. Or maybe the 140 4 stroke Suzuki's. The 300 Suzuki would be fun but I hear they'll be wayl over 20K. The Yamaha 300 HPDI still seems to be having some troubles.

Thanks,
Ken

 

[DJ]

Re: Johnson 300 Hp repower

The "bracket" changes the whole matter. You cannot compare apples and oranges.

I suspect you can get similar performance (with bracket) out of a 250.

If it were my boat, I'd be looking at Bombi' E-Tec's.

 

[Strider]

Re: Johnson 300 Hp repower

Yup those E-Tecs look awesome. Those are certainly on my list.

 

[Strider]

Re: Johnson 300 Hp repower

Anybody else have an idea what that motor is actually putting out at those RPM'S?

 

[MarkV_Deep6]

Re: Johnson 300 Hp repower

Hi

I am not trying to hijack this thread but you are the second person that has this kind of performance from this similar setup. I have very similar if not the same boat and engine setup.
On an 86 25' WA Hydra Sport and am getting 52 Mph @ 6000 rpm , cruising speeds are about 35-38 mph in the upper 4K rpm range. I got an extra 400- 500 rpm and 2mph by taking Mr. Dhadley's advice and lifting the engine on the transom. I still have one hole I could go up if I wanted.
I just really think the performance disparity points to something else amiss in the setup of these boats. I know they are heavy but the differences are not easily explainable.