Outboard Engine Mounting Height?

[4link1955]

I have an 18' Sunbird (1985) with a 135 evinrude on it and I was wanting to play around with engine mounting heights. Is there a rule of thumb to go by? I have the aftermarket Tilt & Trim unit on my boat so therefore it sets the motor back off the transom about 6". I want to be able to get the most speed out of it I can. Any sugestions will be greatly appreciated.<br />Thanks:<br />Jason

 

[oldboat1]

Re: Outboard Engine Mounting Height?

The plate above the prop (think the proper name is the ventilation plate -- but usually referred to as the cavitation plate) should be even with the bottom of the boat, or up to an inch (approx.)below the bottom.

 

[Joe S.]

Re: Outboard Engine Mounting Height?

For best all around performance (i.e. less ventilation around turns) place the cavitation plate even with the bottom of the transom. For best top speed place about 1/2 and inch above the bottom of transom. I gained about 1.5mph on top end by raising mine up plus got rid of top speed porposing.

 

[grandx]

Re: Outboard Engine Mounting Height?

To add to Oldboat1's, even with the bottom of boat +/- 1" is the standard anwer. I don't know what your after market t&t looks like, or how it sets engine back, but if you really have 6" of motor set back from standard position, you may be able to lift it even higer (cleaner water further back), the less motor in the water = less drag, but don't lift water pick up out of water when running or you may burn it up. Otherwise there is an optimal position somewhere.

 

[oldboat1]

Re: Outboard Engine Mounting Height?

grandx is right on the tolerance(s) -- plus or minus an inch up or down. Thanks.<br /><br />In thinking about this a little more, though, I'm not sure that the rule-of-thumb applies with a 6" setback. I'm inclined to say that it would apply -- that water flow six inches back from the bottom of the transom at planing speed would still require the plate to be about even with the bottom (plus or minus). That is, the plate would be the same depth six or eight inches back (with the boat on plane) than if the motor was mounted in the normal postion without the setback.<br /><br />just don't know about the setback issue -- getting into high performance areas.

 

[Dhadley]

Re: Outboard Engine Mounting Height?

The X dimension (engine height) is determined by the style of propeller you are going to use on a given hull design. Generally speaking, you can run a standard stainless prop (like an SST) higher than an aluminum prop. An OMC Raker will run higher. A Stiletto 3 blade will run higher yet. A 4 blade will allow you to raise the motor again. The larger the diameter, the higher you can run the motor. Watch the water pressure as you experiment. <br /><br />As a rule of thumb -- for every 5" you go back you can go up 1". Keep in mind your transom is at an angle so as you go back the motor naturally goes down slightly. <br /><br />Just as an example, my V6 on the Aquasport is up about 2" (plate vs bottom) running a 3 blade Stiletto. About a 400 dollar prop. The V6 on my Hydrostream is up about 8" using a 4 blade Mazco. About a 700 dollar prop.<br /><br />Good luck!