On Mon, 28 Sep 2009 09:14:57 -0700 (PDT), Roger Long
wrote:

Several months ago, I made this statement during a discussion of
stability here. The minor flame war that resulted made it impractical
to defend the proposition and it wasn't much fun anyway.

Buoyancy may be imaginary but experience has demonstrated that it's
better to be on a boat with it than without.

"Roger Long" wrote in message
...
Several months ago, I made this statement during a discussion of
stability here. The minor flame war that resulted made it impractical
to defend the proposition and it wasn't much fun anyway.

While I was away, I had occasion to put together a web site section on
the subject of stability and made the subject line the title of the
first chapter. If anyone is interested in the technical aspects of
what floats their boat with a minimum of math, click:

http://www.rogerlongboats.com/Stability.htm

My son's former physics teacher reviewed the buoyancy section and
pronounced it "Beautifully explained" so it's had some minimum of
vetting aside from being basically a written version of a guest
lecture I used to present to college students.

The last chapter is a brief introduction to the endless foolishness in
the Coast Guard stability regulations for sailboats. This recently
got me an email from a retired inspector saying basically, "Thank
goodness someone finally said something!"

Enjoy




Way too simplistic, I'm afraid. You seem to attribute buoyancy to gravity
alone. WRONG!

Let's create a system that has 1/100 Earth gravity. Let's place a 1/100
Earth gravity sphere half full of water and half full of air in outer space.
Let's pressurize this sphere to one atmosphere. Let's float a boat in the
water. The boat that weighs a ton only weighs 1/100 ton in this sphere but
the atmospheric pressure is Earth normal. Therefore the pressure upwards on
the boat's hull would be the same as on earth (due to atmospheric pressure)
according to your ill-conceived theory. The boat that is already floating
100 times higher than on earth due to its being 1/100 the weight (mass)
would be pushed even higher. If your theory were correct this would not be
the case.

I rest my case.

Wilbur Hubbard

On Sep 29, 2:14*am, Roger Long wrote:

I'd be glad if you'd remove the apostrophe from "in all it's varying
amounts " on your Buoyancy.htm.

Cheers

Bil

On the capsize issue, it seems to me that this should be addressed the same way
that the FAA did for load limits (G factors).

Differentiate by heeling moments - ie sail area and wind strength.

Rather than trying to say that a particular layout is "safe", determine the
allowable wind strength for different sail arrangements.

Top sails, sky sails and lighter kites for lighter conditions and strip down
for higher (Or Rated) winds.

On Sep 29, 11:04*pm, cavelamb wrote:
On the capsize issue, it seems to me that this should be addressed the same way
that the FAA did for load limits (G factors).

Differentiate by heeling moments - ie sail area and wind strength.

Rather than trying to say that a particular layout is "safe", determine the
allowable wind strength for different sail arrangements.

Top sails, sky sails and lighter kites for lighter conditions and strip down
for higher (Or Rated) winds.

That actually doesn't work. The real issue, which has never been
addressed properly by regulation, is unexpected wind increases and the
rate of vessel response. Having the proper sail set for a specific
wind velocity isn't going to make you safe if there is a squall
bearing down that you are not prepared for. Current USCG regulations
for sailing passenger vessels will let you have a vessel that can be
capsized by that squall by reducing the sail plan to an unusably small
area for normal conditions. At the same time, the will prevent the
certification, in many cases, of a vessel that would be knocked down
by the squall but recover without flooding. It's crazy. Be sure to
read the last section of the stability site.

Sail area is only one of several factors effecting heeling moment.
Siimple course changes that create luffing cause dramatic reductions
in heeling moment as any sailor knows. So does easing sheets and
reefing.

Every properly operated sailboat on the wind will have its sailplan
and trim adjusted to bring it to a normal heel angle. There is then
some larger angle at which bad things can happen such as water
entering open hatches or capsizing in the case of the wide flat
vessels the USCG rules favor. It is the margin between those two
angles, and its relationship to the possible speed of response to
sudden wind increases that is the critical factor. This is
independent of the total sail area. Sailing at 10 degree heel with a
large sail plan in a 15 knot wind that suddenly doubles in strength is
very much the same as sailing at the same angle with a sail reduced by
reefing or striking individual sails in a 30 knot wind that suddenly
doubles in strength. Proper regulation might require sail reductions
at some specifice heel angle as SOP and would insure sufficient margin
between this angle and the angle of downflooding or capsizing. This
is independent of the maximum total sail that might be carried. It's
a matter of hull design, not sail plan. USCG rules let you operate,
and indeed even promote the operation, of dangerous hulls with total
sail area restrictions that are irrelevant to actual safety in the
winds where accidents are most likely to occur.

All of this is very statistical and you can only get a handle on what
is required by population analysis of a large number of vessels that
draws a line between the ones that have capsized and those that have
not. One of the few intelligent things the Coast Guard did in the
history of sail regulation was recognize this fact and attempt such an
analysis. Unfortunately, they couldn't find data on any capsizes so
they just threw a lot of absurd dynamic theory at the problem and came
up with a mess that plagues large sailing vessel regulation to this
day. I conducted a much larger population study in the early 1980's
for the USCG and ASTA joint task force on sailing school vessel
regulations. We had data on vessels that had capsized by that time so
we were able to justify some improvements in the regulation of
educational vessels but it was no help to the commercial fleet that
still operates under the original crazy criteria. We still had to
work within the existing USCG methods though so sailing school vessels
still have the flawed emphasis on total sail area.

I took all the data I had at hand as a result of this project and
performed another population analysis that evaluated only hull
characteristics and put no restrictions on total sail that could be
carried or even measured sail plan at all. This created the same,
actually better, distinction between successful vessels and casualties
than the application of the USCG methods. A draft of this paper sent
to a friend and fellow researcher in England led to many of the
principles being incorporated in their much more rational set of
regulations for large sailing vessels. I got too busy though trying
to make a living, keep my business going, and start being a father to
ever finish up the paper for publication.

--
Roger Long

Roger Long wrote:
On Sep 29, 11:04 pm, cavelamb wrote:
On the capsize issue, it seems to me that this should be addressed the same way
that the FAA did for load limits (G factors).

Differentiate by heeling moments - ie sail area and wind strength.

Rather than trying to say that a particular layout is "safe", determine the
allowable wind strength for different sail arrangements.

Top sails, sky sails and lighter kites for lighter conditions and strip down
for higher (Or Rated) winds.

That actually doesn't work. The real issue, which has never been
addressed properly by regulation, is unexpected wind increases and the
rate of vessel response. Having the proper sail set for a specific
wind velocity isn't going to make you safe if there is a squall
bearing down that you are not prepared for. Current USCG regulations
for sailing passenger vessels will let you have a vessel that can be
capsized by that squall by reducing the sail plan to an unusably small
area for normal conditions. At the same time, the will prevent the
certification, in many cases, of a vessel that would be knocked down
by the squall but recover without flooding. It's crazy. Be sure to
read the last section of the stability site.


I think I get your point now.

FAR 25, which covers load factors and gust loading in commercial aircraft
doesn't have to deal with flooding and the resultant stability changes!