In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/