Question on Transom

[nathanhooper]

So, is their a weight to distance ratio, or something to that effect, that you have to take into consideration when determining if you can use a Jack plate on a boats transom?

I have a 15' Trophy bass boat, very sound transom, and I am considering putting a jack plate on it to allow me to use my short shaft Merc 500 on it. The motor weighs approx 170-200 lbs. I need to drop the motor 5" or so, I am not going to try to run this boat and motor too hard, just a temp fix. I will need to clear the back of the transom to allow the steering of the motor. Not sure yet what this distance would have to be.

Is there a distance, from the transom, that I need to make sure I do not go over in mounting that much weight back their? My boat is rated for up to 90hp, and like I said, the transom is still real solid. However I understand that when you put more weight backward on the transom it will cause more torque on it.

I hope this question is coming out alright. Thanks guys. Ask any questions if they will help make sense of what I am asking, if that makes any sense. lol.

 

[Bondo]

Re: Question on Transom

Ayuh,... It's either Trial,+ Error, or get a Longer shaft motor...

 

[Frank Acampora]

Re: Question on Transom

Let's get a little theory here:

Consider the engine thrust moment as below the hull and not acting on the hull parallel to the bottom. Consider the last foot or so of the hull as a pivot point.

The further back you set the engine, the greater effect of thrust moment to lift the bow at speed and the greater effect to sink the stern at rest. Generally, lifting the bow gets more hull out of the water and allows decreased drag, thus higher speed---until you get to the blowover point.

So yes, there is a setback and weight relationship with a jackplate. What is it? I don't know.

 

[5150abf]

Re: Question on Transom

I have seen GIL brackets almost 3' long so you can go back as far as you want, the problem will be, as Frank pointed out, the farther you move the engine the farther you move its weight and with a 15' boat I wouldn't think you would have much to play with.

Maybe move a battery up front or something to counter weight the boat.