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Happy new year to all!!

I am going to hang the 1985 Evinrude 90 back on my 1981 17 Custom this weekend. I have added a 6" set back manual jack plate. My question is; how high should I mount the motor on the jack plate? All the way down? 2 holes up?

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The general guideline is for every 3" of setback, the motor needs to be raised 1".

Assuming the baseline is the anti-cavitation plate is 1" above the bottom of the hull, with 6 inches of setback, the motor should be at least 3" above the hull.

Warning: Adding the weight of the bracket and moving the boat's center of gravity back 6" will greatly contribute to the boat wanting to porpoise on plane.

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Phil thanks for this info. Very useful.

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Thanks Phil!! That is exactly the info I was looking for. The boat will be quite heavy with 3 batteries, 20 gallons of fuel, a 20 gallon live well, and front and rear casting platforms. I hope I can shift weight around to negate the porpoising.

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You should consider a stainless steel prop that has the right properties for your application.

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The prop makes a difference in the height you will have to experiment a little. NMy 15' has a 5" set back. With my OMC SST prop I run 3.25". With my Stilletto SS prop I run 4" above the keel.

My boat initially had a the motor mounted all the way down with blind holes. My boat with the jack plate is less likely to porpoise than with the motor all the way down. The only my 15' really porpoises is when I have the motor trimed out and I forget trim the motor back down.

Obviously you have a different hull than I do, so your result may vary.

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Nobody seems to have answered the initial question, which is where to mount the engine on the jackplate.

I have the same hull, and since these have the older shallow transom configuration, it is most likely the jackplate itself is mounted a little higher than normal on the transom in order to get nuts on the bottom bolts. At least that was the predicament I was in. So I mounted the engine all the way down, ON THE JACKPLATE, in the top set of bolt holes.

Most jackplates have 4 to 5 inches of vertical adjustment, so you have plenty of height to work with if needed.

To find the optimum height FOR A GIVEN PROPELLER (even aluminum), raise the jackplate up in 1/2" increments, test for slippage, etc. When it happens, then drop it down 1/4" and you will be at optimum height.

As I indicated, different props require different running heights.

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The jack plate only had the lower red mounting holes so I set it up in a Bridgeport and added the green holes. The weather and cattle chores has kept me from mounting the engine so far. Maybe this weekend. Fishing time will be here soon.