Motor mounted too high on transom?

[gatorfan6908]

So looking at the old pics of my outboard before I removed for my resto, I noticed the height seems to be higher and not sitting flush on the transom for full support. The distance from top to bottom on the transom is an even 20" and matches up w/the shaft length on the outboard. I am curious as to whether or not I should attempt to mount the motor lower on the transom whereas the cavitation plate will sit even with the keel. I feel that it is the best idea, but I can't get this bug out of my head thinking that perhaps this was done for a reason. So far throughout the resto, it looks as though the previous owner had absolutely no idea of what common sense was, so I don't think they were bright enough to mount it so high for a reason. Anyways, all input is appreciated. Thank you!!

Sorry the pics don't show "great" angles... but they were taken last august before I started the project. Mainly the two edge pieces made to sit on the transom are a good 2+ inches above the transom itself...

 

[Sea Rider]

Re: Motor mounted too high on transom?

If you assume engine sits low or high on transom it's a matter to visually check when at full plane at which lower leg height water flow is paasing by. Must have a well ballanced boat trimed so to ride paralell to water level, anticav plate included. Then you will know for sure if height is correct or how much +,- need to optimize transom height. Ideal water flow should pass under small upper deflector plate. Try to match transom height accordingly to match this condition, whether outting with a light, medium, fully loaded boat.

Check this video for clear understanding : Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Portable OB's being lighter are much friendly to move and test than heavy ones...

Happy Boating

 

[Chris1956]

Re: Motor mounted too high on transom?

OK, so you have a late 70s Merc inline motor, likely a '76 or '77 1150 or 1500. They raise 'em up like you have, to provide better performance. I would water test it under normal conditions, before changing anything. I had my '77 1500 raised up 3/4" above the transom, until the day I bought a jackplate and raised it up more.

 

[saumon]

Re: Motor mounted too high on transom?

What Chris1956 said. Engines were not made to "rest" on the top of the transom; that's simply the lowest point they can go. They were holding by the 4 bolts only and, most of the time, benefit from being mounted higher (less drag from the lower unit and increased bow lift).

BTW, did the previous owner suffer of a form of "forgot the plug" insecurity? :laugh:

 

[Chris1956]

Re: Motor mounted too high on transom?

Six 3/8" transom bolts on IL6's with PTT, plus thumbscrews. Of course that begs the question..."What happened to the thumbscrews?"

The PTT is the old style, so where are they? Did they make a model, slightly newer than 1977, that had the old PTT, but no thumbscrews, or did some PO saw them off?

 

[Georgesalmon]

Re: Motor mounted too high on transom?

mount your motor where it was in the middle holes then test run it. You can move it up and down if not running right by using the other holes. Each hull design is different that's why motors have extra hole and can be moved up and down for best performance.

 

[gatorfan6908]

Re: Motor mounted too high on transom?

Guys.... thanks for the input, as that was exactly what I was looking for. The outboard is a 1982 90hp L6. Thanks for clarifying about the height requirements... I'm glad to know it doesn't have to be all the way down. Anyways, There are actually 8 total bolts, with two of them being out of view from the pics above. The are way further down on the transom and mount the very bottom of the bracket. At this point, I think I am going to mount it as it was and just move it down a few bolts if I need to. Thanks again for all of the help!