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Springtime Discovery

Don Silsbe

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The basic problem here is that the transom is week in bending and the knee to the keel is not a good solution.  I consider a knee on such boats to be the wrong way to go about it.  It does not work against the forces properly that a transom gets from an outboard.  Heresy, I think not.  Analise the forces on the transom when an outboard is thrusting the boat.  An outboard generates a force forward at the lower or propeller end and the same force is aft on the upper end of the transom and forward at the keel.  A weak plywood transom like yours is opposite to what is needed to counteract these forces and the knee is useless.  None of the outboard boats I have built in 60 or so years have had a knee and none have ever experienced a problem like yours or too many boats built from plans that specify such transom support.  The keel of your boat is quite able to resist the forward force of an outboard without help and the weak (in bending) plywood upper transom is just what you don't need.  The best method is to have a non-bending (lateral) upper transom that transfers the force to side decks, sides or adequate sheer structure.  A strong board on the forward face of the upper transom that is adequately secured to decks and sides strong enough to take the force is the best method to cure your problem.  


I think the transom to keel knee is a left over from older boat construction and is not the most useful way for new boat and especially those with outboard power even though many designers specify them.


There are several means of resisting transom deformation by transferring the transom twisting outboard forces to the boat structure.  Strong and well secured knees between a stronger upper transom and the sheer structure can take care of thrust from small outboards in a way that resists this force. on a boat without side decks.  Larger outboards with large such forces need more structure between the upper transom and the boat side structure.

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