Re: Designing boat plans
There's a rule of thumb calculation for determining the motor size for a boat based on length, the USCG publishes it. BUT... it's only for determining the max motor size based on a rough estimate of boat size, and it assumes that the designer of the boat did a good job in the first place.
There's a reason that good marine architects (boat designers) make a lot of money... it's not a simple thing to design a safe boat that performs well, especially if it's not made in a factory.
To give you a parallel example: If you saw someone post in a forum that they had downloaded free skyscraper plans from the Internet and wanted to make a few changes, would you ever want to step in that building?
To design a boat that is safe, you need to do a lot of math. For example, if you look at the scantlings for just the transom piece, you have to first get the look and shape you want, and decide on materials (metal or glass or wood). Then you need to figure out how much torsion (twisting) stress the sides of the hull place on the transom, how much force the engine places on it pushing forward, take into account force from waves in a following sea, determine how to transfer force from the transom to longitudinal members (stringers), determine how much the rest of the hull will place force on the stringers that's transferred back to the transom, and every other major force that affects that component.
Then using the known strength of the material you are working with (glass, resin, epoxy, metal) you figure out the thickness (and in the case of glass the layup) you need to handle the forces involved, with a good margin of safety.
If anything changes in the boat that changes one of the numbers for force applied to the transom (change in stringers, or larger engine, or lengthened boat) you really should re-do all those calculations to be sure your boat isn't going to fall apart in use.
It's easy and okay to make minor changes, like deck layout or shear height, but major changes may affect the integrity of the design, and be a safety problem. If you make major mods to a boat hull without involving a marine architect, be prepared to lose your shirt if someone sues you after an accident.
So, to post all of the above in short format: If you want to make more than minor mods to a boat and you don't know there's a colossal amount of engineering involved in making it safe, hire a marine architect to do the work for you.
There's a lot more to it than basic math and a few formulas. It's a whole engineering discipline.
Erik
