JAXAshby wrote:


but it sounds good, because it -sounds- like hard science. just like in one of
the only three books ever written about sailing states.

Jax, we need a good laugh. Why don't you explain hull speed for us?

DSK

Why don't you explain hull speed for us?

DSK

it doesn't exist in any fashion as a scientific "limit", so there is nothing to
explain, scientifically.

psychologically, on the other hand, want easy answers to complex issues so will
believe most anything handed to them.

On Sat, 17 Jan 2004 17:30:05 -0500, 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.

D/L refers to the displacement/length ratio, which can affect whether the
boat is truly limited by it's wave train. A long skinny hull can go faster
relative to it's theoretical hull speed, that's how catamarans and Navy
destroyers reach higher speeds.

Hope this helps.

I like that explanation - simple yet complete.

Nice.

Later,

Tom
S. Woodstock, CT
----------
"My rod and my reel - they comfort me."

St. Pete, 12 Lb. Test

I like that explanation - simple yet complete.

also wrong, but what the hey it is simple and complete.

On 17 Jan 2004 23:17:07 GMT, (JAXAshby) wrote:

I like that explanation - simple yet complete.

also wrong, but what the hey it is simple and complete.

Where exactly is it wrong?

Later,

Tom
S. Woodstock, CT
----------
"My rod and my reel - they comfort me."

St. Pete, 12 Lb. Test

also wrong, but what the hey it is simple and complete.

Where exactly is it wrong?


among other things, there is no reason whatsoever that a boat MUST go over top
of a wave rather then through it, and a sin function does NOT explode at zero
degrees.

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

On Sun, 18 Jan 2004 08:54:44 -0800, "Lloyd Sumpter"
wrote:

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

=============================================

Lloyd, what is your water line length (LWL) ?

On Sun, 18 Jan 2004 11:57:33 +0000, Wayne. wrote:

On Sun, 18 Jan 2004 08:54:44 -0800, "Lloyd Sumpter" wrote:

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

=============================================

Lloyd, what is your water line length (LWL) ?

30 ft - with a "fudge factor" of 1.35 gives hull speed of 7.4 knots. Easy to see
5, she will do 6 on a 10-knot breeze, 7 is a good reach, 7.5 is reaching on 15
knots, 8 is close-reaching on 25 knots or so...

Lloyd

and will only add
that it is theoretical

it is not even theoretical. It is and always was just scientific sounding
gibberish.