Tuning a 28hp Johnson

[quackaddict1]

Hey guys, a question for all of you. I have a 1995 johnson 28hp on a 16 foot crestliner deep v fishing boat. The motor currently has a 9.25x11 pitch prop. According to my tiny tach the thing hits WOT around 5200 rpm with an empty boat (me, gas can and battery). It seems to go fine and picks up to plane in a reasonable distance, but the relatively low RPM's with an empty boat worries me. We duck hunt out of this boat, and subsequently we carry a bunch of gear when we do.

The cavitation plate runs about an inch below the bottom of the hull, would raising the motor ( This would require some serious work and possibly the addition of a Jack plate ) increase my RPM's enough to not need to change pitch size of the prop? I need to replace the prop (beat it up last duck season) and was wondering what course of action would be best, messing with motor height and then picking a prop or getting a prop with less pitch ( say possibly a 10 or 9 pitch)?

 

[Bondo]

Re: Tuning a 28hp Johnson

Ayuh,... I'd try the 10" pitch...

 

[steelespike]

Re: Tuning a 28hp Johnson

You can probably have the 11 repaired.What is the speed at 5,200?
The simple way out is a 10" pitch.But you would still be dragging too much motor through the water. If you could raise it 2" you could probably get
that rpm with the 11".The 10 will give you some hole shot but you may lose a little top end.

 

[quackaddict1]

Re: Tuning a 28hp Johnson

Sleelspike, to get 2 inches of elevation I will need to look into a jack plate, the clamps are right at the top of the transom, though the motor is bolted to it as well. This is why I had thought about just going with a lower pitch. IF I could get that 11 pitch spinning at RPM I could probably get a few more MPH out of it though.

I have no idea how fast it goes, and really, I don't care. It gets us there way faster than the 7.5 HP on the 14 footer could. All I am really concerned about is longevity of the motor, and lugging all the time can't be great for em.

I could get the 11 repaired, I just thought that if a pitch change was needed, now is the time, I will likley still get a new one just to have a spare, but I will get the motor in the right spot before I do that.

 

[Sandhammaren05]

Re: Tuning a 28hp Johnson

The heavier the boat and the weaker the motor, the less you can jack up the motor. However, the cavitation plate ought to be above the bottom of the boat, and the motor ought to turn upwards of 5800 RPM or more. You'll need a cupped prop (ball peen hammer and trailer hitch ball will do if you learn how to do it). Cupping will decrease cavitation and lower RPM, so jacking up the motor and running the 11" pitch might still work. As for jack plate, buy two 1/4" aluminum plates and some 5/16" stainless bolts. Simply bolt the plates onto the transom and use a block of wood between them. A jackplate with offset, setting the motor back away from the transom, is equivalent to running the motor deeper in the water. I have a 1983 35 Johnson on a 15' Allison bass boat that I run at 19" transom height with much-cupped 14" pitch prop, 41 mph. Your 16' boat is too heavy, and the bottom's probably too hooked, to jack the motor up much more than about 17.5".

Hey guys, a question for all of you. I have a 1995 johnson 28hp on a 16 foot crestliner deep v fishing boat. The motor currently has a 9.25x11 pitch prop. According to my tiny tach the thing hits WOT around 5200 rpm with an empty boat (me, gas can and battery). It seems to go fine and picks up to plane in a reasonable distance, but the relatively low RPM's with an empty boat worries me. We duck hunt out of this boat, and subsequently we carry a bunch of gear when we do.

The cavitation plate runs about an inch below the bottom of the hull, would raising the motor ( This would require some serious work and possibly the addition of a Jack plate ) increase my RPM's enough to not need to change pitch size of the prop? I need to replace the prop (beat it up last duck season) and was wondering what course of action would be best, messing with motor height and then picking a prop or getting a prop with less pitch ( say possibly a 10 or 9 pitch)?

 

[quackaddict1]

Re: Tuning a 28hp Johnson

Thank you guys, I will work on getting something rigged up to get more height out of the motor, we fished tonight with two guys, two batteries, and a gas can and I got 5K even out of the motor, and I don't think just stopping down pitch will fix it.