Re: Twin Mercury 90's
Normally my motto is: "Too much horsepower is never enough." But in this case, I think you are biting off a bit more than you can chew. This is the kind of modification that you put many hours of planning into and then, many hours of work.
You will be making a "pocket rocket" which will be illegal in some states. Boats are not like autos where you can build your engine as hot as you want and still drive it on the street. Additionally, you must be careful where and when you use the boat so as to not become a dirt surfer or buy the granite headboard. And, you certainly don't want to spend the rest of your life paying off a lawsuit judgement.
Also note that a single 180-200 will probably give far better performance with far less headaches in handling and mounting.
However, if you have your heart set on it, here's how to strengthen the transom:
The transom does not stand alone; it must be integrated with the hull and act as a complete assembly to absorb thrust forces.
Notice the top cap in the first photo. It is 2 X 6 aluminum channel with the 2 inch top cap 1/4 inch thick and the 6 inch internal flange 3/16 inch thick. The outside aluminum plate is .080 hard aluminum. The assembly keeps the transom from bending or flexing.
The third photo shows another 14 foot flat bottom boat with stainless steel angle gussets in the corner of the splashwell and .080 hard aluminum fishplates attached to the sidewalls. This assembly ties the transom into the splashwell and transfers loads through the splashwell into the rear of the hull.
This boat had the transom raised 5 inches so the last photo shows a larger external plate to reinforce the built-up transom and tie it into the rear of the hull.
The second photo shows the boat at speed (55 with a 90, 60 with a 125. It was probably rated for a 35 or 40 when built.). It does NOT show all the other modifications to strengthen the hull and bottom so as to absorb forces these speeds generate. I run this boat for 1 or 2 hours and glue it together for at least 8.
