Tands hit one big uncertainty in wind pressure calculations.... how does one
account for hull ,superstructure and flybridge shapes in the pressure
formulas....maybe the formula is an average for power and for sail boats which
would be good enough for me...but I still don't know how to reasonably
approximate the surface area of my boat use in the formula...especially the
pointed hull shape...
Rob Brueckner
Hatteras YF

Hi Rob,

You don't need the surface area (which is extremely difficult to calculate,
you need the projected area. You only need to estimate the area within the
outline of the boat. This can be made up of simplified geometric shapes
(rectangle and triangles mostly for a boat). Take a look at your boat, bow
head on. On my trawler it would be best represented by a small rectangle
(house) sitting a larger rectangle (hull). If this isn't clear I could
e-mail a sketch off list.

Regards,

--Jim

"...but I still don't know how to reasonably
approximate the surface area of my boat use in the formula...especially the
pointed hull shape...
Rob Brueckner
Hatteras YF"

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'Lo All,

Just some observations:

1. At Homestead, FL, after Hurricane Andrew, several ROUND objects,
   including a tall round water tower sitting on a round pipe whose
   diameter was about 1/4 or less of the diameter of the tank, and a
   round plastic playground set at a (?Burger King?) fast food
   restaurant, were standing as if nothing had happened. All of the
   structures around them were extensively damaged, most to the extent
   of having been blown completely away, except the foundations.

2. Few boats will lay quietly to the wind without some "hunting",
   which, of course, exposes a greater cross section than just a bow-on
   aspect. During the last few hurricanes which blew through the Panama
   City, FL area, I watched many boats at anchor, observing their
   behavior. As I recall, only one old shrimp boat actually stayed
   fairly well streamlined to the wind. All of the others hunted to some
   extent, some violently. But even that shrimp boat was occasionally
   almost broadside to the wind, as gusts came from various directions.
   This turned the boat so that is was more broadside to the main force
   of the wind. We like to think that high winds come steadily from one
   direction, but at least in the confines of bayous such as this one,
   they do not. Often the gusts come from up to about 90 degrees or more
   from the main direction of the wind.

3. Even in a hurricane, the wind speed is not constant. As strong
   gusts strike a boat, the boat reacts, according to its mass, as well
   as wind direction and force and wave forces. The effect is to stretch
   the anchor rode, which then contracts. These constant changes are the
   greatest contributor to rode failure in my opinion. That is to say,
   chafe. I have seen well protected lines worn completely through,
   after wearing the chafe protection through (and, often folks neglect
   to properly protect their lines at all).

4. Hurricane holes (places where folks anchor their boats during
   hurricanes) usually have a limited fetch, so that the waves cannot
   build to extreme heights. In our bayou, I have never seen more than
   about 3.5' or so wave heights at the height of hurricane force winds,
   and then only when the storm surge (water pushed into the bayou by
   the storm) is about 4' or so over the spit of land which helps
   protect this bayou. The fetch is then about doubled.

From this, I conclude that the shape of the object has a great deal
to do with the overall wind load exerted. Also, a boat is: 1. not
round, and 2. will not stay streamlined bow-on, so
calculations/guestimations of wind load must take these factors into
account. Not being a mathematician nor a statistician, I would not
presume to be able to calculate what the actual wind loading on a
boat would be under all of the varying conditions a boat is subjected
to. Add to that, the wear on the line, especially if it is a used
line, and the task of deciding what size line is best becomes almost
impossible from an empirical standpoint. One just has to use
experience, hopefully gained at others' expense, to determine what
should probably be best/adequate. Bigger is not always best, as it is
the stretch in a rode that provides much of the protection to a boat
in a storm. No stretch, as in an all chain rode, and a boat can be
violently jerked to destruction. I have not seen that happen, but
have been told of it by several old-timers, who adamantly feel that
for a storm, a chain and nylon rode is far better than either alone.

During the last hurricane that came through here, I had about 25'
more line out on one of my bow anchors than usual because the storm
surge was predicted to be greater than usual. My little 45# CQR has
about 15' of chain. The anchor line was almost cut in two about 5'
from the chain, where the line laid across something on the bottom
and was cut. Two of the three strands were cut completely in two and
the last one was partially cut. Old timers have told me to make sure
that the nylon was not laying on the bottom when the anchor rode was
taught, as it would cut, especially if laying on oysters. This is
just a sand bottom, and I didn't think too much of it, but will in
the future. For what it may be worth, my anchor rodes are 3/4" nylon
with 15-20' chain on the anchors. For storms, I can put out a
Fortress 55, a 35# Danforth, a 45# and 60# CQR, plus tie to trees and pilings.

Take care and be safe.

Wayne
M/V Celestial
Albin43 Sundeck