Transom height vs shaft length

[BrayD]

My apologies if this has been beaten to death. I keep reading that short shaft boats have a 15" tall transom and ideal running conditions place the motor plate essentially level with the bottom of the boat.

My short shaft boat measures very close to 15" at the transom. I have a couple short shaft motors in the shop and they both measure far longer than 15" from the transom top to the motor plate.

Why the discrepancy? This is a '57 18hp Johnson, but there's a '96 Mercury sitting next to it that measures very similar.

 

[airshot]

That 15" is not an exact number, I have saw from 14" to 16" depending on how it is measured. Most transoms are on an angle, but the proper measurement should be true vertical, not measured on the angle. Way back then it was considered the cav plate was supposed to be an inch or two below the keel/ bottom of the boat. Today we have rethought this and find we get better performance if the cav plate is right at the bottom and we also have props designed to run closer to the surface of the water than what we had many years back. On small outboards these are not considered performance motors so the depth of the cav olate is not as critical. Tolerances do and will vary. On serious performance boats/ motors they have adjustable mounts to fine tune that height for optimal performance.

 

[BrayD]

That all makes good sense. I was just surprised to see it being multiple inches longer than the transom height. I could do some work that would allow me to play with the motor height relative to the bottom of the boat, but I swap motors often so I'm not sure I want to put forth the effort to build around one particular shaft length.

If Johnson decided the cav plate should be that low back in '57, that's good enough for me (on this boat).

 

[mbullen]

It's worth noting that the 50s is also when transom height standards were just being agreed upon among the manufacturers.
One can always count on there being a range of inches in the mix.
The finicky folks would use small battens or stir sticks to stack under the transom clamp to dial in performance.

 

[Bailey Boat]

AND...... they also make transom plates that allow the mounting of the motor at any height your heart desires...

https://www.homedepot.com/p/T-H-Mar...to-35HP-Wooden-Block-Included-MJ1DP/315623572
AND..... I happen to have one for sale!!

Had to use another link to get it right.....

 

[flatheadsteve]

The general rule is 15"= short shaft 20"= long shaft and 25"=xl shaft. The transom height could fall anywhere between those listed numbers as manufacturers vary it a bit. My xpress has a 22" transom and wears a long shaft motor. I could probably get away with an xl shaft If I chose since it's on a jack plate.

 

[LDUBS]

That all makes good sense. I was just surprised to see it being multiple inches longer than the transom height. I could do some work that would allow me to play with the motor height relative to the bottom of the boat, but I swap motors often so I'm not sure I want to put forth the effort to build around one particular shaft length.

If Johnson decided the cav plate should be that low back in '57, that's good enough for me (on this boat).

Sometimes leaving well enough alone is the best option. If you enjoy the boat that is all that matters.

 

[JL8Jeff]

My short shaft jet outboard sits 2-1/2" above the transom to tuck properly into the jet tunnel. I've just run most of my boats the way they were set up when I got them so I never paid close attention. With this jet tunnel boat, the previous owner had a prop motor on it which I sold when I got the jet. I had to make multiple adjustments to get the height for best performance and least amount of spray back. I also had to add transom wedges to get rid of porpoising. With a clamp on motor, I'm not sure I would want it mounted too high, but I've never had a clamp on so I might just be over cautious.