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CruisinGA 01-28-2013 06:30 PM

Jon boat O/B height
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So I have a little camo 12' jon boat that I use for frog gigging and duck hunting. It's a little too small, but the price was right.

Neighbor gave me a 9.9 Johnson, but boat is rated for 3 hp... really showing my redneck side now :D

With a little motor tuneup and trim adjustment, this sucker does 22 mph now, but the transom is showing signs of having 3x the rated hp on it.

It appears to me that the motor sits too low in the water, lots of spray around engine at WOT. Anyone have any experience with jack plates on small boats? What is the ideal height of the lower unit with respect to the bottom of the transom?

While I am beefing up the transom to handle the increase in power, I might as well get it set up for the ideal height too.

Note: picture shows my '65 Evinrude 6 hp that I used until I came into the 9.9hp.

KahunaCraft 01-28-2013 07:10 PM

That's about the top end for what you have... You might be able to adjust the manual trim on it get a little more.

Here are some similar posts on some other forums...

Good luck. I'd tie a 25ft rope with a floaty to one end and the motor to the other...just in case.

psychobilly 01-28-2013 07:14 PM

The horizontal fin on the lower unit should be level with the bottom of the skiff. Oh, and you may want to add a stiffener piece of wood on the transom now before the aluminum starts cracking under the stress of the 9.9. Don't ask me how I know this. 😄

AJones 01-28-2013 07:44 PM

Not sure if anyone makes a jackplate for small motors and jon boats. From my bass boating days, you need to run a straight edge along the bottom of the boat extending beyond the transom. The cavitation plate on the engine above the prop should be setup approximately inch above the bottom of the boat. This would be a good starting point. If the motor has adjustment holes in the mounting bracket, raise the engine a notch at a time until cavitation starts and then back down a notch. Warning, running too high will cause cavitation and overheating because of lack of water to intake holes. The shaft length on the 9.9 hp may be too much for the transom of your 12 jon boat. You may need to modify/rework the transom to mount the motor high enough.

Good luck.

boater8987 01-28-2013 10:58 PM

I have discussed proper outboard height with Mercury Marine for my 300 Verado's before. Water just touching cav plate under way at level trim close to wide open. Can be used with mostly any outboard they said. There are many varibles besides cav to keel placement. Transom setback, bottom design, speed, etc. This one is still a little deep.

Here you go this is about correct.

TRBenj 01-29-2013 11:03 AM

Lots of opinions on the exact placement of the cavitation plate relative to the hull... some say slightly above, some say slightly below. I found that going perfectly level yielded pretty darn good performance (much better than several inches below, as youre describing). I found one of these T.H. Mini Jackers on ebay and its a nice piece. I use it for the 15hp Johnson on my 12' aluminum boat.

Sodar 01-29-2013 11:09 AM

Bob's has mini adjustable jack plates, too...

cbryan70 01-29-2013 11:13 AM

depends on shaft length. is it a 15 or 20 inch shaft on the motor? How tall is the transom now?

boater8987 01-29-2013 11:58 PM

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Quote: much better than several inches below, as youre describing.

If your refering to my post. I did not describe several inches below. I gave the expert opinion of the manufacturer that designs, engineers, builds and has to warranty their motors. Not my opinion their expert advise. I am not an expert by any means and not trying to P.O. or step on any toes. No harm no foul. They stated many varibles. Here is a example of a set back transom step hull / note cav plate several inches above keel. Water travels up off bottom to set back motors allowing less drag more speed. As a motor gets closer mounted to a flat transom boat it needs to be lower than this aplication, thus their statement about water touching on cav plate under way is a safe position. Could very well end up level with keel depending on varibles. That is their way of getting it close to correct. I realise jon boat is a considerbly different application with only one motor, but their procedure is a proven method. And as AJones stated, another varible, motor too high water starvation occurs, then motor meltdown and or prop blow out (loss of speed and steering) Also stated setback jack plate is a good way to be able to safely move motor up and can create less drag and more speed gives more distance and height from transom so water has more time to roll upwards to feed water intake and prop. Most setback jack plates will list the setback distance and height. Then the fun begins dialing it in. I have a 12' G3 with a 10HP 1956 Johnson to play with. Good fun.

CruisinGA 01-30-2013 07:56 AM

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boater8987- I think TRbenj was referring to what I was describing when he said "several inches below transom"

Regardless, some good info in this thread.
I'm not sure about the jack plates- don't really want to move the o/b back because the boat already sits low in the back if there is only one person in it. I think I will just look at O/B height with regards to the bottom of the transom, and if I have to just raise it vertically.

See pic, I guess this is a 20" shaft motor. Measured my transom and it is 15"

See other pic with red circle- I have always set the motor all the way down on the transom, just noticed this shoulder- is it meant to be set on the transom if you want to raise the motor?

I couldn't set the motor on the boat yesterday, but knowing the transom is 15" and looking at the motor with the tape measure, it looks like I may already be close.

Next step after setting transom height is to find a carb off a Johnson 15hp.... apparently the 9.9 and 15 of this vintage are exactly the same outboard except for the carb.... :D

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