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

wrote:
Roger Long:

One of these days I'd like to look into the derivation of hull speed.
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?
What happens when an object exceeds hull speed?
Is there any way to "fool the water" into acting as if the boat is
longer than it is?

Thanks

David OHara

I would think that if you remove the water from in front of the boat
and then replace it after the boat has gone by, there would be no
wake, or lost energy caused by the passage of the boat. Then, the
only restriction to speed would be skin friction. Energy left behind
a boat in the form of waves, or a wake, represents energy put into
propulsion which is inefficiently employed, causing a disturbance in
the water instead of increasing the boat speed. In normal hull
shapes, the full weight of water displaced by the hull must be moved
out of the way of the boat, and then it must return to the space
evacuated by the passage of the boat. That water will be disturbed,
containing eddies of water, the energy required for the formation of
waves and eddies being lost to the purpose at hand, propulsion.

I once envisioned a submarine built like a stove pipe, with a
venturi and prop inside a narrowing of the central tube. It should
produce no wake. Any such boat would never need to climb a wave, it
would rather always be going downhill, into the space where the
water was vacuumed out.

In such a sub, I also envisioned a ducted rim-driven propellor with
no hub. I wonder if there could be some advantage to this?

Terry K