dumb about x = y2 =? building a boat

[kenguy]

Hi all. The reason why i joined this site is because i am dumb about x + y= z or something like that.
I never had much in the way of education and my Maths went out of the window because i had a nasty teacher who always called me a cheat.
You see i have this trick brain that can multiply and divide effortlessly and come up with the right answer, but i could never put down on paper how i worked it out in my head. So i often got caned for cheating despite my close neighbours in the class getting it wrong.

So though i greatly admire people who can do the x + y bit it goes completely over my head. But ask me a multiplication or division question and i somehow know the answer. Well not somehow, for i apply basic logic and use the figures closest to the sum to multiply and deduct or add accordingly.
My teacher of course wanted it put on paper in his strict format.
In this capacity my brain works out the answer quicker than a person on a calculator.

So when i saw a question about boatbuilding on this site and the formula for length + width etc, i could not understand it.

I need help working out whether the boat i am designing will be safe and float without breaking up. Is there one of you admired bright sparks that can assist me in laymans, or dumb language. Like the height can only be a third of the length, or the width must be half or one quarter of the length.

My boat will be 15metres long, 5 metres wide and the cabin will be 2 metres high, the width of the boat, 5mts, and 2metres high.
it is intended to be built and used in the typhoon free shallow waters of the Philippines.

Is there a specialist here willing to help me in this?
I also need to know about the timber sizes for the ribs and keel.
I can e-mail the drawn plan if need be.
Thanks
Ken