The largest scale ship that I've seen was 14 feet long. It wasn't done but had all of the running gear installed. He had it powered by 2 power seat motors from a Volare' to give you an idea of how long ago it was when I saw it. The owner was giving a ride to a 98 pound boy to see how much more ballast that he'd need. For long run times and if you plan on using something like Minn-Kota's for power then I'd look into deep-cycle batteries. They're heavy at around 70-80 pounds and you would get a lot of run time out of them plus enough to run accessories.