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Johnson 25 hp Conversion

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Mr. Mark FTE

New member
I’d really appreciate some advice on my 2002 25 hp Johnson long shaft - should I convert it to a short shaft or try something else?
I have a 16 ft. Aluminum boat with a 15” transom and I have this log shaft, which is no- kidding pristine. Almost looks like it was never used and it runs great. I fabricated a pretty good jack plate which allowed me raise it up to 6”, but it’s still pushing water like a plow. The cavitation plate is flush with the keel and it seems that it should be fine, but it’s not. I figure either I’m missing something or I need to reconsider converting it to a short shaft. I hesitate the conversion as it’s such a good motor as-is, and I really don’t want to mess with it as I’ve never done a conversion before. Does anyone have any ideas?
Thanks!
 
Thank you Racer! That’s one of my fears. Model J25RSNF. Worst case I could leave it alone and get by. I can still do 16 mph, but just not optimal and starts splashing water back into the boat.
 
The model # indicates it left the factory as a short shaft.-----Does it have a 5" spacer between exhaust and lower unit ??
 
If you don't want to modify the motor you could get a mini-jack plate and use the motor as-is. The plate provides 3" of rise and it has multiple bolt holes for mounting, so you should be able to raise it up to a full 5" to match the 15" transom on your boat. Shop around and you can find one for under $150.

TH Marine Mini-Jacker Jack Plate
 
kevinj said:
If you don't want to modify the motor you could get a mini-jack plate and use the motor as-is. The plate provides 3" of rise and it has multiple bolt holes for mounting, so you should be able to raise it up to a full 5" to match the 15" transom on your boat. Shop around and you can find one for under $150.

TH Marine Mini-Jacker Jack Plate
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Thank you Kevin. I did build an adjustable jack plate and I have actually raised it 6” so that the cavitation plate is flush with the bottom of the keel. This is the mystery - I’ve theoretically raised it enough but it still doesn’t seem right.
 
Easy to covert this back to a short shaft.----Remove spacer , find shorter driveshaft , water tube spacer ?-, shift rod spacer?----I can't touch / see your motor, but easy job here.
 
racerone said:
Easy to covert this back to a short shaft.----Remove spacer , find shorter driveshaft , water tube spacer ?-, shift rod spacer?----I can't touch / see your motor, but easy job here.
Click to expand...
Racer,
Thank you very much for your help! I expect I will start looking for parts and how to videos. :)
 
Mr. Mark FTE said:
Thank you Kevin. I did build an adjustable jack plate and I have actually raised it 6” so that the cavitation plate is flush with the bottom of the keel. This is the mystery - I’ve theoretically raised it enough but it still doesn’t seem right.
Click to expand...
When using your homemade jack plate and you're on plane, where is the cavitation plate in relation to the water? Typically, the more setback you have, the higher you can run the motor so maybe if your motor is several inches back of the transom it's still allowing the cavitation plate to run too low.
 
kevinj said:
When using your homemade jack plate and you're on plane, where is the cavitation plate in relation to the water? Typically, the more setback you have, the higher you can run the motor so maybe if your motor is several inches back of the transom it's still allowing the cavitation plate to run too low.
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Good question as I unfortunately didn’t look to see. I looked around with the motor going flat out and just got water in my face. I just went out just now and measured the setback. It’s exactly 5.5” from back surface of the transom to the back surface of the jack plate. Are you thinking then that a smaller setback could help?
 
Mr. Mark FTE said:
Good question as I unfortunately didn’t look to see. I looked around with the motor going flat out and just got water in my face. I just went out just now and measured the setback. It’s exactly 5.5” from back surface of the transom to the back surface of the jack plate. Are you thinking then that a smaller setback could help?
Click to expand...
Yes, at 5.5" setback you could easily run the motor 1" higher or more. The water "rises" as it passes the transom so to speak, so more setback translates to a higher setting for your cavitation plate. You likely got splashed because the midsection above the plate was ploughing the water.
 
kevinj said:
Yes, at 5.5" setback you could easily run the motor 1" higher or more. The water "rises" as it passes the transom so to speak, so more setback translates to a higher setting for your cavitation plate. You likely got splashed because the midsection above the plate was ploughing the water.
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Thank you Kevin! I had never considered this, but it makes sense that of course the water would be rushing back ups to fill the void created by the passing boat. I will see if I have enough space to extend the slots.
:)
 
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