Strengthening a Transom

I recently bought a 15' Gregor aluminum boat with a Yamaha 40hp outboard. The transom is starting to crack where the jack screws tighten down to hold the motor on. I would like to reinforce the entire transom to hold the weight and the thrust that it will make.
I was looking for advice on whether I should use a piece of .250" plate aluminum and brace the whole transom or I also have 3" x 3" angle aluminum to possibly build some type of brace that is welded flat up against the transom. I'll use the angle to add gussets to the corners with either way I do it.

 

Hey ducknuts, Sounds like you got it sorted,Make sure the .250 plate does not make the transom too wide for the motor, secondly I would make the plate just big enough for the engines saddle mounting. I assume you are welding and not riveting?

 

Thank you very much for the advice. I was thinking of using the plate to cover the entire transom plus brace the corners, do I need this much plate, or is just under the saddle enough? Would four inch long pieces of the angle welded in at the top of the transom / gunwale corner under the factory gusset be enough to hold that joint?

 

Its difficult to cunjure a picture. Personally I think 1/4 plate all over the transom is overkill. The boat was designed to take a 40 wasnt it? then I would think that plating over the cracked section and to the size of the saddle with welding would bring it back to stronger than standard.
Get hold of the outboard down by the prop and give it a good shake you should be able to see what movement you have.

 

The boat was only rated for a 25hp, I'll probably use plate the size of the saddle and brace the corners then call it good, thanks for all your help.

 

Oh well hang on a minute, thats nearly double HP perhaps some knees spreeding the load of the newly welded plate going down to the keel. Its below the plate you need to pay attention.

 

There are knees inside the boat but they are not very thick and connect about in the middle of the transom and then to the bottom about six inches from the transom, so far they seem to be holding up fine, no cracks or signs of stress. The center section of the transom and the top corner gussets were what I was more worried about.

 

The gussets at the top and transom where the motor bolt on are just starting to crack, this is why they are my main concern.