Jack plate for short shaft motor on a long shaft transom?

[orbanp]

Hello Everyone,

I have a 14' Lowe alu boat (1467WTSN) with a long shaft 25HP 2-stroke Mercury motor on it.
I also have another short shaft motor that I would like to try/play with.
I would not want to cut down the transom permanently.
Is a removable lowering jack plate a viable proposition? That would set back the engine by about 6" or so. Would that upset the balance of the boat? The short shaft 25HP motor is about 110lbs. Both of them are tiller control.
Any comments, advice?

Thanks, Peter

 

[kfa4303]

Re: Jack plate for short shaft motor on a long shaft transom?

Hi orbanp. Sure, you can use a jack plate to adjust the height of your motor as you like. Generally speaking, you want the cavitation plate, found directly about the prop, to be level, or slightly higher than the keel (bottom) of the boat. Here's a link to one that would work for your needs. You could buy one, but as you can see it's really just 4 pieces of heavy gauge aluminum,which you could probably get a sheet metal shop to make for you for much less than it would cost to buy a new one, and it would be custom made for your boat/motor combo. While adding a jack plate does offset the motor from the transom, which effectively lengthens the boat and affects its performance as such, the difference in you case will be nominal. You may also be able to find a larger version of the adjustable kicker motor mount in the pic below too, but they're usually built for smaller HP motors below 10 hp, but there may be a bigger version. You may want/need to add a tiller extension of some sort to your motor like one I made out of thin-walled 1 1/4" PVC for my 20 hp. Hope that helps.

http://pensacola.craigslist.org/boa/2789117371.html

 

[Bondo]

Re: Jack plate for short shaft motor on a long shaft transom?

Hello Everyone,

I have a 14' Lowe alu boat (1467WTSN) with a long shaft 25HP 2-stroke Mercury motor on it.
I also have another short shaft motor that I would like to try/play with.
I would not want to cut down the transom permanently.
Is a removable lowering jack plate a viable proposition? That would set back the engine by about 6" or so. Would that upset the balance of the boat? The short shaft 25HP motor is about 110lbs. Both of them are tiller control.
Any comments, advice?

Thanks, Peter

Ayuh,.... You'll no doubt have an interference issue between the motor, 'n the transom...

The tiller, 'n cowling will hit the hull...

 

[JB]

Re: Jack plate for short shaft motor on a long shaft transom?

What Bond-o said. ^^ +1

 

[orbanp]

Re: Jack plate for short shaft motor on a long shaft transom?

Ayuh,.... You'll no doubt have an interference issue between the motor, 'n the transom...

The tiller, 'n cowling will hit the hull...

Interesting, and very valid comments, never thought about that!
I guess I would have to check all the details!

In any event, this motor is similar to what I have:
<http://www.oddjobmotors.com/whirlwind.htm>

Mine is a bit newer, it is from '72.
They went from cross flow to loop charged scavenging, that is the reason for the HP increase (bore and stroke remained the same).
The price also went up to 400 rubles, according to the owner's book that I have.

Here is an outline of the engine. Assuming the drawing is to scale (more or less it looks like it is), there is about 2 1/4" distance between where the clamps would hold the motor (top of transom) and the bottom edge of the power head. That is not the 5" that is the difference between a long and short shaft engine.
From the same diagram, the distance between the top of the transom and the cav plate is about 16.9".
So it might or might not work, depending on the actual transom height of the boat.

Thanks, Peter

 

[Bondo]

Re: Jack plate for short shaft motor on a long shaft transom?

Ayuh,... You might be able to pull it off, if ya used, say 2' of set-back....

That way the motor will clear the transom, 'n you'll be able to tilt it as well...

 

[kfa4303]

Re: Jack plate for short shaft motor on a long shaft transom?

I agree. Here's a pic of a fancy schmancy set up, but it's the same idea. the platform sets the motors far enough back from the transom and at just the right height, that they can trim/tilt and steer through their entire range of motion. I'm sure there's a cheaper/easier way to do though.

 

[NYBo]

Re: Jack plate for short shaft motor on a long shaft transom?

So it might or might not work, depending on the actual transom height of the boat.

That pretty much sums it up. All the rigamarole required to get a motor with too short a shaft to work properly on such a small boat doesn't make a lot of sense, IMO. The only exception would be if a kit to make the leg longer exists, which is highly doubtful for this motor.

 

[Jeep Man]

Re: Jack plate for short shaft motor on a long shaft transom?

A good friend of mine is struggling with the same issue, albeit with a larger motor. I suggested early on that the best way was to get a shaft extension. He didn't want to pay the cost. He has since tried a few "jack plate" ideas with poor results. To date, he has spent more than what the extension would have cost. Do it right or don't do it at all.

 

[orbanp]

Re: Jack plate for short shaft motor on a long shaft transom?

Thanks for everyone for the comments and advise!
In the end, it looks like a futile effort to use a jack plate, and most probably not worth pursuing it.
I will measure the transom height though...

Thanks, Peter