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Motor shaft length

boatboylll

boatboylll

New member
I have 79 predictor ive just finished restoring. I am working on finalizing plans for a motor. Is anyone running a 25 in shaft? Im set up for dual cable steering hot foot and have a bobs 6inch jackplate. Can I run a 25 inch shaft? How will handling be affected?
 
It'll be way too long, the transom is designed for a 20". The prop will be too far down in the water, especially on a light hull like a Predictor where you want the prop right around the surface.
 
We tried a 2.5L 175 25" Mariner on my Predictor, along with a 4" setback jackplate. By using the highest mounting holes on the engine along with raising the jackplate all the way it will work, it just isn't ideal. (Similarly I have the same engine mounted on my Eluder, it isn't perfect, but it works a lot better than on the Predictor!)

Part of the problem with the Predictor is the engine weighed so much the stern sat really low, and it was already low with the previous 20" 2.0L 150 that was on it (even with the rub rail.) So low the slightest wave would send water into the hole that was cut in the stern for the steering cables. The 2.5 weighed more, and the long shaft weighs more. On the Eluder the rear of the boat sits higher when at rest (rub rail is a couple of inches out of the water.)

Another problem was we couldn't get it to stop porpoising at water ski speeds, you had to get it above 35 or so before it would stop.

I think if you had to run a 25" shaft (because that's what you have right now,) you have have to drop down to a 90-125 or less HP motor, basically a 3 or 4 cyl outboard.

In this vid you can see how low the back of the Predictor sits with the 150, it was even lower with the 175:

th_VID_20110716_155712.jpg

In this pic you can see how much higher the back of the Eluder is:
2012-09-15_16-11-11_460.jpg
 
No way in hell. Those transoms really need a 15 inch mid section to put the shaft where it needs to be. Ive got a 1996 20 inch Mercury 150 on my Enticer ( same lower hull as yours ) and Ive got to jack the motor 1/2 the way up on the jackplate and put it on the motor on its bottom mounting holes to get the prop shaft 4 inches under the pad. Its a good 6-7 inches higher than if I just bolted it up to the transom. I'm too cheap to buy a 15" midsection. 25"? No way.
 
wired said:
No way in hell. Those transoms really need a 15 inch mid section to put the shaft where it needs to be. Ive got a 1996 20 inch Mercury 150 on my Enticer ( same lower hull as yours ) and Ive got to jack the motor 1/2 the way up on the jackplate and put it on the motor on its bottom mounting holes to get the prop shaft 4 inches under the pad. Its a good 6-7 inches higher than if I just bolted it up to the transom. I'm too cheap to buy a 15" midsection. 25"? No way.
Click to expand...

It depends on what you are going to do with it. On most boats, when the say the transom is 20", thats what it is. On a Cmate, they are closer to 23". If you are not looking to squeeze every last tenth of a mph out of it, it will work just fine. You would be surprized to see how many 20" boats are out there running 25" motors on a jack plate!
 
wired said:
No way in hell. Those transoms really need a 15 inch mid section to put the shaft where it needs to be. Ive got a 1996 20 inch Mercury 150 on my Enticer ( same lower hull as yours ) and Ive got to jack the motor 1/2 the way up on the jackplate and put it on the motor on its bottom mounting holes to get the prop shaft 4 inches under the pad. Its a good 6-7 inches higher than if I just bolted it up to the transom. I'm too cheap to buy a 15" midsection. 25"? No way.
Click to expand...

I also disagree with this, on my Predictor when we had the 20" motor on it we could raise the motor high enough with the jackplate that the prop lost it's bite with the water at the highest setting, plus at the lowest setting the powerhead was practically in the water.
 
Shaft Length

Thank you guys for your input looks like im going with a 140 looper 20 in shaft anyway. Thanks for all the info. Trying to get it going for the Toys for Tots Poker Run on the St Johns in November.
 
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