jack plate question

[bountyhunters]

DOES it weaken the transum any ? looking at puting a 25inch shaft on a lund pro v 1775 .or should I look for a 20inch shaft ?

[REW]

A jack plate does not weaken a transom.
A jack plate simply mounts to the transom in a matter that is identical to the way that your motor is mounted.

Having said that, a jack plate - -- does stress a transom more -- than simply hanging a motor on the transom.

From basic static and dynamic theory, the force on the transom is a function of the push from the propellor times the length of the lever arm. In the case of a non jack plate mounted motor this lever arm is in the relm of 6-10 inches. Basically the set back of the engine from the transom to the centerline of the prop drive shaft.

Now, when you add a jack plate, you are adding an additional lever arm in the set back distance of the jack plate.
For example if the set back is 2 inches, there will be minimal increase in thrust.

However, if you are mounting a jack plate with a 10 or 12 inch setback, you are getting a pretty long lever arm.
Remember, the entire weight of the motor - in the case of a 150 hp 4 stroke, you are looking at 600 lbs just sitting there at the end of this long lever arm.

So, if in doubt - check with the manufacturer, and find if any jack plate or a jack plate of a specific setback might change the warrenty on the boat.

On the water experience has proven that some boats simply don't fare very well with the addition of a jack plate -- i.e. the transom breaks out of the boat - in some models with some engines.

Be safe, check with the manufacturer. If in doubt - either don't add the jack plate, or add reinforcements to the transom.

If you look at many of the bass boats with jack plate capabilities have 4-5 inch thick transoms, with massive reinforcements in the transom area.

Take care
REW