Engine Transom height

[necks]

Sitting in the house today not enjoying the below zero weather for the third day outside. I found a old 1960 Outboard Fawcett paper magazine, and ran across a article on How to Set OMC Johnson’s outboards on transoms, for correct cavitation plate height. First time I ever seen a diagram like that, looks to be a lot quicker than using a ruler and easier. Anybody ever see a photo like this? Gene.

[joecb]

The instructions say “one inch or less below the transom”  however the picture shows the fellow with three fat fingers width below , that’s about 3 inches ???

The objective is to minimize the drag from submerged lower unit, but still get “clean” water to the prop.

Joe B

[outbdnut2]

Interesting way of measuring! …..that’s the first time I’ve seen 3-fingers used to measure anything except filling a glass of ice with whiskey!

Ideally, you want to have the cavitation plate below a center keel on the hull due to turbulence caused by the keel, so if there is no center keel (or anything else hanging from the center of the hull), you can usually have the plate pretty much even with the hull.

I’ve noticed Johnsons/Evinrudes form the 1950s on up that I’ve measured give you about 18 inches from transom to cavitation plate for a short shaft motor, and this allows approx 3 inches to get far enough below a center keel.

Dave

[joecb]

Dave brings up a good point about turbulence from the keel. If your boat has a pronounced keel it should be feathered off flush at the transom to minimize that turbulence ahead of the motor.

Joe B

[necks]

Been playing with Mercury’s the last few years, started getting into the 1957-58 Johnsons up to 35hp. I’ve got running and using a 1957 5.5- 10hp Johnsons. Those motors always ran deeper in the water than the Mercury’s. I always set the cavitation level with bottom of boat, if not a little bit above, on the Merc’s, never had any problems. Always installed Johnson motor’s on fishing boat deep enough so that 1 inchx5inch plate above the cavitation plate was in the water. ( I believe that is the cover for the water jacket which is about 1 inch x 5.) I would think or wonder if that depth would cover that plate or whatever that plate is called. I’m thinking that 3 finger picture would get it done. I got a couple of 1970’s Johnson’s 4hp and 15hp that don’t have that plate. I wonder how long OMC ran that set up, and what problems they had with it if any? I’m ready for Spring, winters are getting too long. Gene.

[Author]

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