Converting motor to a long shaft?

[78 Mark Twain]

I just moved my 115 Johnson short shaft over to a new boat. The new boat has a higher transom and when on plane there is a lot of motor that is out of the water. At speed I can just see the top of the cavitation plate. The motor was still pumping out water but I am concerned that I might not get enough flow to properly cool the engine. Is there enough of the motor in the water to do the job or should I look to make the shaft longer?

 

[budgie14]

the cavitation plate should be about one inch to 2 inches below the bottom of the hull/transom,or you risk overheating the motor thru lack of water,,and losing too much power thru water cavitation at the prop,plus damage to the prop thru cavitation boiling at the tips.Sometimes its easier to get a motor with the right length shaft,unless the parts are readily available,and yr motor is still ok,and worth keeping

 

[Docksidemarineservices]

Ventilation, sucking air from the surface.

 

[78 Mark Twain]

I have a 78 Johnson 115 that has a blown powerhead and that motor has a long shaft. Would the shaft and lower end be compatible to switch around or will I need to buy a whole new set of parts?

 

[racerone]

I do not think there is such a thing as a " short shaft " ---115 HP Johnson.-----------------They are all at least 20" and considered " long shaft "-----------------Newer motors were also available as " extra long " made for a 25" transom.----------------------So the 78 parts will not help you in this case , not a chance.--------------------Measure your boat transom , measure your motors and post that information.--------" Measure twice , cut once " the carpenters used to say !!!

 

[78 Mark Twain]

I just got back from measuring my transom and my motor. I measured from the base of the motor to the lower end and came up with about 19 inches. Then i measured from the base to the cavitation plate and got about 21-21.5 inches. The bottom of the intake vents were about 4.5-5 inches below the cavitation plate. When I measured my transom I came up with 20-20.5 inches. I took some pictures but do not know how to post on here, can someone explain how to do that

 

[racerone]

There are no short shaft ( 15" ) models in a 115 HP OMC outboard .-------------You need what is called a long shaft for a 20 " transom.

 

[78 Mark Twain]

When I measured the leg on my motor I came up with 19 inches, but I could not tell exactly where it started so I must have a 20 inch then. Is this going to be ok with the 20 inch transom that I have? I do have pictures but do not know how to put them on here.. thank you for all of the advice you have given
Here is the link to the photos
http://i1093.photobucket.com/albums/i434/78MarkTwain/2011-04-07182016.jpg
http://i1093.photobucket.com/albums/i434/78MarkTwain/2011-04-07182027.jpg

 

[78 Mark Twain]

racerone,
is the motor I have going to be suitable for my boat? I have found another motor that I could use, but need to know wether or not I will need to replace mine.

 

[hondamech]

You've got a 20" engine on the back of boat with a 20" transom, that is perfect. The pictures look perfect. If you are really worried about the cooling, buy a pyrometer (they are cheap) and have someone run the boat while you check the top of the heads. If anything, judging from your pictures, the engine is a bit low on the transom. But, I am looking at pictures and not the real thing. Take a straight edge and put it on the bottom of the boat and the bottom of the anti-ventilation plate (cavitation plate ) if they more or less line up, you are spot on. That is one way to make certain the engine is the correct height.