Force exerted by outboard on transom

On Trailer boats the most damage to the transom is probably from going on rough roads with the motors weight being carried on the transom or the trim device. The heavier the motor, the lighter the boat, the smaller the trailer tires and the heavier the trailer springs can really contribute to this. What I have done to remedy this is run a removable brace that supports the motors weight. It runs from the motor to the trailer so that the transom doesn't carry the weight of the motor when towing.

Dave T

 

We took some sea trials few years back where we had 3D forces and moments recorded between transom and 70...350 hp outboards with special in house build "bracket". Accelerations were recorded near transom and near CG. Seatrials showed that accelerations played most significant role especially when a boat gets airborned or smashes trough waves. In rough rides accelerations near transom are almost the same as near CG and even more in worst landings (7...10 g's at most).
So just calculate accelerations or limit it due human tolerance, multiply by mass of outboard and calculate vertical force and moment around transom edge. Remember there is also deceleration along x-axis playing a role. Remember force and moment from Trust... and that all previously mentioned doesn't happen nececcerily at the same time