On Sat, 17 Jan 2004 17:30:05 +0000, DSK wrote:

Lou Dempster wrote:

Could someone please tell me what a knotical mile is in comparison to the
standard mile. Also what does D/L limited speed: 10.08 kt. and Disp. Hull
Speed: 5.90 kt. mean.
I am just fooling around with a hull designer and would appreciate any info.

Well, other have answered the nautical mile question, so I'll try the other two.
Displacement hull speed is the highest speed a given hull can reach when limited
to it's wave train. What this means is that the boat is fully supported by
displacement of water (as opposed to planing, where the boat is supported by
hydrodynamic force of water going by under the hull) and so therefore it has to
push water out of the way, making waves. A less obvious point is that the water
has to come back together again as the boat passes.... in traditional hull
design, the key to designing a fast boat was always said to be to minimize the
stern wave.

Displacement hull speed is determined by waterline length. The further apart the
bow & stern waves are, the faster the connected wave train can move across the
surface. At 5.9 knots, the crests of the bow and stern wave are about 18' 4"
apart, so I bet that is the waterline length of the hull you are designing.


I'll agree that this is a great explanation of "hull speed" and will only add
that it is theoretical. "Displacement" boats often exceed "hull speed" in
certain conditions without planing.

But hull speed is an excellent "mark" to do calculations with. If you're doing
around hull speed with, say, a certain engine and prop combination, don't expect
to go much faster if you you add more hp, etc. If you're nowhere near hull
speed, doubling hp may close to double your speed. But as you approach hull
speed, it takes more and more hp to go any faster. (For a sailboat, replace hp
with "sail area, wind speed, etc.")

Lloyd Sumpter
"Far Cove" Catalina 36 - hull speed 7.5kn, has seen 10