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Want to see how much you know? Want to show everyone else h
much you know? The first three people to submit the correct ans
to the following question will win a Westlawn tee shirt and cap, a
will also receive a Know It All certificate. The answer and winners
be published in the next issue of
The Masthead.
Who Will Be The June 2010
Know It All Winner?
Email your answer to:
title of “Know It All,” with all the rights and privileges pertaining
thereto. Westlawn tee shirts, caps, and—of course—distinguished
Know It All certificates are on the way to each of our brainiacs.
The complete answer to the March 2010 Know It All question is
as follows:
Of Cycloids and Gravity:
Researchers studying waves and ship motion (particularly Wil-
liam Froude) discovered that—without other influence, like shoal
waters or interfering wave patterns—ocean waves follow a form
known as “cycloidal.” Also called “gravity waves,” the individual
particles making up each wave followed the path of a single
point on the rim of a circular ring rolling along a flat surface.
Naturally, there’re are all manner of combinations of these roll-
ing patterns, and many cycloids will interact with each other, but
it was a very useful discovery.
Any regular motion can be reduced to mathematical expression.
In our case, for seawater on planet Earth, we learned the length
(L) between wave crests is equal to 5.12 times the wave’s period
(P) squared. (The period is the time it takes for two crests to pass
the same point.) We also found the speed (S) of the wave crests
is equal to 3.03 times the period. A little algebraic manipulation
gives the classic hull-speed formula as shown in the sidebar.
In reality, boats do go faster than hull speed. Such vessels lift up
and skim over their bow wave—they can plane. To do this, lots of
power is needed along with a properly formed hull. Alternately,
light and/or slender hulls can knife through the bow wave, with-
out having to climb up it and so exceed classic hull speed.
For fairly heavy boats that must push a lot of water out of the
way, the classic hull-speed formula works (at least reasonably
accurately), and so it has been accepted as “gospel.” According
to gospel, no non-planing boat can go faster than this hull speed.
We now know that this isn’t really so. We’ve discovered that hull
speed—at least classic hull speed—isn’t truly accurate and the
1.34 constant isn’t constant, but actually varies as a function of
displacement/length ratio. This is covered in detail on pages 12
and 13 of the
June 2008 issue of
The Masthead
.
Where:
L = wavelength
P = period
S = speed
2
2
2
2
2
2
2
2
L = 5.12 x P
and
S = 3.03 x P
or
S
P =
3.03
So, substituting . . .
S
L = 5.12
3.03
S
L = 5.12 x
3.03
S
L = 5.12 x
9.1809
L = 0.55768 x S
L
S =
0.55768
S = 1.7931 x L
S = 1.7931 x L
S = 1.34 x L , The classic hull speed formula
You have a well-constructed, fiberglass-composite, 46-foot sailing catamaran,
Two Tranquil
. Beam overall is 26 feet and
displacement is 6 tons.
Tranquil
’s cutter rig has a sail area, with 100% fore-triangle, of 1,100 square feet. The mast and
rigging are rugged and properly stayed. Chainplates are strong and correctly aligned. Sail-handling gear is well thought
and sized appropriately. The center of effort of
Tranquil
’s sailplan is 29.4 feet above the DWL.
With regard to stability and sail-carrying power, is
Two Tranquil
suited to offshore cruising and why or why not?