BoatBanter.com - View Single Post - Effective speed much less than theoretical hull speed.

wrote:
One of these days I'd like to look into the derivation of hull speed.

Check out the work of William Froude.

Can you suggest a very basic explanation (a source thereof). I'm just
curious as to when it applies. Like:

Does it apply to non-rigid hulls (hulls that might flex in the middle)
Does it apply to totally submerged objects?
Does it apply to towed objects, like dinghies?

Yes, yes, & yes.
Usually making hulls non-rigid makes them slower for given power. There
are some interesting studies of marine mammals that can swim faster than
science suggests they should, and apparently they can change their shape
to promote laminar flow and possibly reduce wave pressure areas.

For totally submerged objects, the rules are a bit different and they
don't have the same limits as displacement-supported objects on the surface.

What happens when an object exceeds hull speed?

That depends a lot of the hull & the conditions. Some "objects" will
plane, ie will transition from being supported by displacement to being
supported by the dynamic lift of water flowing under the hull. In other
cases, the cross section density is high enough that wave resistance is
less (ie very narrow hulls).

Is there any way to "fool the water" into acting as if the boat is
longer than it is?

Sure, that's what bow bulbs are all about.

Fresh Breezes- Doug King