I guess thread has been boarded by pirates and hijacked already, so

This can’t be a rigid rule, surely. Would a ULCC 1500 ft. long therefore have a hull speed of almost 52 knots?
And does the difference between displacement and semi-displacement hulls have any bearing on what Tranquilis is saying about destroyers exceeding hull speed?

I picked the formula I did simply because it appears on every on-line calculator I found. Someone who actually understands marine architecture will have to address the possible variables. (For example, I am a bit surprised that the hull speed takes no acount of beam. A supertanker has very nearly twice the beam of a large carrier, and I (not knowing the physics of marine architecture) would have thought that that would have a serious effect, yet no reference I have seen addresses it at all.)

(Additionally, all the U.S. 1,000+ foot carriers have a “rated” speed of 33 knots, yet I have heard persistent rumors that when they open them up, they exceed 40 knots–right in the range of the 42 knots that their hull speed would indicate.)

That formula is a very simplified “rule-of- thumb” applicable mainly to small boats with very standard hulls (and the coefficient ranges from 1.2 to 1.4 depending on the boat type etc). I know I have a more complete formula somewhere but I cannot remember where and it is too late for me to search the Net. It is easy to see the simplified formula is aplicable only to certain narrow ranges because it is not applicable to catamarans due to the hulls being very narrow for their length. The destroyers have a length/beam ratio of about 10 whereas my boat it’s closer to 3 and that makes a huge difference.

We should also realise the concept of hull speed is in itself a simplification even when calculated with the more complex formula. If you plot power/speed it is not like there’s a precise point, rather the curve gets steeper. The way I understood “hull speed” was defined was that speed which when exceeded would cause the boat to plane (surf).

I’m not saying it is not possible for a destroyer to exceed hull speed but I’d like to see this confirmed by some authority rather than by our sloppy calculations.