how should the dimensions of the ships be decided
As I posted a question earlier, I want to make a trash collector boat/ ship. The thing is, how should the dimensions for the same be selected. I have searched it on google, but required a precise answer.
Well, there are a lot of factors that go into this. Some of them--but certainly not all--would be things like; cargo weight & volume, drivetrain weight, crew capacity, passenger capacity... all the things you'd need to determine the ship's displacement weight.
Then there are all the hydrodynamic performance factors that will drive the ship dimensions; length, side depth, draft, etc.. So, there's a lot to take into consideration but, there are probably plenty of tables, diagrams and empirical design formulas to work from. Just need to find a source for this.
I'd imagine a lot of this could be pulled from something like the IHS Fairplay database, looking at similar types of ships.
By the way, this is a really neat project someone has obviously put a lot of time and effort into regarding parametric ship design:
You can see that like many things, it's an iterative process and if nothing else, it helps one understand the logic ladder behind the design process. It's much the same as with everything else in the engineering world. Always lots of variables... some readily apparent, others not so much.