Bryan Glover wrote:
....
will no doubt cry foul, because someone with a commercial interest in
this subject dares make a post.


I dunno, your post does seem too blatantly commercial to me.

Worst Case Breaking Wave Strike
I have chosen the case of the Winston Churchill in the 1998 Sydney
Hobart race as an example of a worst case breaking wave. The Churchill
was a classic wooden sloop of 25 tons displacement and 55 ft. LOA. Of
the experienced crew of 9, two perished in the accident.

From "Fatal Storm' by Mundle. "A sea came out of nowhere", said
Stanley, " I could feel it from where I was in the aft coach house. It
picked the boat up and rolled it down its face - 25 tons of boat- into
the trough at a 45 degree angle. It was like hitting a brick wall when
we hit the bottom". A crewman below reports that a sudden motion of
the ship picked him up and threw him 7 ft. He observed that 8 ft of
the heavy timber bulwark and planking had been torn off near the
leeward shrouds, and the ribs were exposed. . The boat filled rapidly
and sank in a matter of minutes.

This is an unusual type of accident. Although there are records of
many storm casualties, I am aware of no documented instance of a well
found yacht of the size and reputation of the Churchill and crewed by
an ample group of expert sailors, suffering such catastrophic
structural damage that it sank in a matter of minutes. How could this
possibly happen?

A couple of ways, none of them provable. IMHO part of why the Churchill
sank was repeated stressing & flexing of the hull, and partial failure
of her fastenings. Also that the extreme stress on the rig helped open
up the hull as she went down that wave. Once there was any opening at
all, the hydraulic force of the water ripped big chunk of planking off.

Basically, in these kinds of conditions, the boat is getting dropped on
her side every wave. Or partly on her deck. Picture this, picking up
your boat 30 feet or more in the air, then tilting it a bit past 90 and
dropping it. Bad, eh? Not as bad as doing this same thing for hundreds
of reps, interstpersed with having trainloads of water dropped on the
boat from height.


Considering that Navy cruisers have been sunk in typhoons, and
battleships taken severe damage, it's a little much IMHO to expect a
small sailboat to withstand anything the ocean can throw. *No* tactics
or special gear or design type is going to help. It's a case of simply
don't be there when it happens.


History shows that the probability of a yacht being capsized and
damaged by a large breaking wave is strongly influenced by the
displacement of the vessel. Yachts under 35 ft. have a poor history
while yachts over 50 ft are rarely capsized and damaged.


I think there's more than this factor at work here, although youo're
right about bigger = better. Bigger boats are also faster and tend to
get less exposure to the extreme conditions, and they are able to take
active measures to avoid the worst long after the smaller boats have
switched to total survival mode.



To understand this phenomenon we must consider the concept of energy.
A moving car or boat has energy. This form of energy is called kinetic
energy. Kinetic energy is measured in foot-pounds. Kinetic energy can
be calculated by the formula KE=1/2 (w/g) times (v squared). Where w
is the weight of the car or boat, g is the acceleration of gravity
(32.2 ft/sec) and v is the velocity in ft./sec.

Thus a 3,000 lb. weight traveling at 30 mph (44 ft./sec.) would have a
kinetic energy of 90,000 foot pounds.

To put this in perspective, this is roughly equivalent to stopping ten
rounds from a .50 cal machine gun. Destructive, nyet?

... Now...and this is very important
to our understanding of the Churchill disaster...if the moving vehicle
strikes an object, the kinetic energy determines the severity of the
collision and the extent of the damage.

In addition to energy due to motion, a vehicle can possess energy due
to height. This type of energy, also measured in foot pounds, is
calculated simply as the height times the weight. A 3000 lb car
hoisted to a height of 50 ft. would have 150,000 foot pounds of
energy. If dropped from 50 ft to a solid surface, the car would
dissipate this energy in damage. If the car was compressed by 2 ft.
the average force during the impact would be 75,000 pounds. If it
landed on its top and compressed four feet (because it was softer) the
average force would be 37,500 lbs.

That's not quite right, the force due to deceleration isn't linear.
Comparing a braking distance of 2' to 4', the energy is less than
half... about a third would be closer...


Rich Hampel wrote:
It would be interesting to get the scantlings of the Churchill to
reconstruct a forensic analysis of the forces that were involved.
Without knowing the exact structural schedule, the dissipation of
impact is always a 'guess' ... but some value of 'e' between zero and
one. It would be fairly easy to back-calculate such values and arrive
at least the momentum value. Especially if the boat was constructed to
a 'Lloyds scantling rule' the calcs. would be fairly easy.

Yes, but it would be impossible to determine exactly how much of the
destruction was due to pure force and how much was due to even slightly
degraded structural integrity. Hidden rot, bad fastenings, possibly some
modification or addition that affected the structure... plank-built
wooden boats are really complicated and every single piece is supported
by every single other piece.

The Churchill was said to be very well maintained and recently rebuilt,
but we have no way of knowing her actual, true, material condition at
the time of the accident.

Personally, I don't think it would matter. As I said earlier, it's my
belief that *no* structure built by man can stand up to the ocean at
it's worst.

Fresh Breezes- Doug King