Why is it that longer ships can travel faster than smaller ships?

Question

wckeller · Answer

I'm not entirely sure, however, I would assume it has something to do with a mass to thrust ratio. Larger vessels typically have much more powerful engines than smaller ships. 
Even though smaller ships are lighter, they are not capable of using larger engines due to their size. All of this is an opinion so take what I say with a grain of salt.

irishboy123 · Answer

if that is generally true, why do they use a speed boat rather than an aircraft carrier or oil tanker every time they go for a new world record for speed on water?!

wckeller · Answer

So I just did a little research about your question and found out that larger ships such as an aircraft carrier have a max speed around 30 knots, meanwhile your average speedboat can reach speeds of up to 80 knots. Cruise ships also have a relatively low speed of around 24 knots. So my first assumption about the thrust to mass ratio was incorrect. The smaller boats are much faster than larger ones. Maybe the larger engines just can't keep up with the weight they have to push?  They aren't as efficient as smaller motors? I'm not sure, but I can undoubtedly tell you that my first response was incorrect.

irishboy123 · Answer

Nah mate

i think it was right

you just did it the wrong way round