Sail size vs wind

Sail size vs wind

Post by Bill Cri » Sun, 09 Sep 1990 03:59:34

A while ago there was athread on sail size, vs windspeed, and
I mentioned a wind speed chart that had forces listed on it

It is from Essentials of Seamanship by John Watney, Chartwell books.

The tables gives Beaufort ratings, descriptions, US Weather terms,
and "Approximate pressure in lb per ft**2".
There is no explaination for this, but it seems to be pressure
or force on a properly trimmed sail? At least I did rough calculations
for how much wind I can hold down in a close hauled Laser (Math is
easier because heeling moment, and righting moment are at right
angles, and big centerboard is dominanat lateral resistance).
The table seems reasonable. The forces also follow a square law
which is what I would expect.

Here They are

Wind Speed in knots  Pressure lb/ft**2
  1-3                  0.01 - 0.03
  4-6                  0.06 - 0.2
  7-10                 0.2 - 0.4
  11-16                0.4 - 1.0
  17-21                1.0 - 2.0
  22-27                2.0 - 3.0
  28-33                3.0 - 4.0
  34-40                4.0 - 6.0
  41-47                6.0 - 9.0
  48-55                9.0 - 12.0
  56-65                12.0 - 16.0
  >65                  >16.0

If anyone can figure reasonable numbers/angles for sailboard rigs,
and see if they fit?
I figure:I weigh 170lbs, and IF I rake mast 45degrees to windward,
and then have 70% of my wieght on the rig, that gives 170lb sail
force. for a 65sqft sail (6.2M) that gives 2.6 lb/ft**2 which means I'll
be in this position at about 22- 27knots. Seems reasonable given that
by time you are raked to 45degrees, you would be at least fully powered up
and more likely substantially overpowered? Also, I would expect a
fully battened RAF type sail to provide more focce than a sailboat sail,
so that would reduce the 65sqft sail  fully powered  or overpowered
windspeed to say 17-23
knots? Sounds reasonable to me.
Note this would also give a 170*0.70= 120 lb horizontal drive force
either forward if on a reach, or mainly sideways if beating(ever wonder
why your skeg tends to spin out!).

What to do with this info?? I don't know?? If you know your weight, and wind
speed you COULD calculate how big a sail to take?

Lets see? 40 knots = 6 lb/sqft therefore I need a 170/6=28sqft sail.
I guess I'll rig the 3.0 and live with the extra power, or take the
2.6 and survive to tell about it;-)

What do you think? How big a correction do we need for RAF rigs vs
stiff mast soft sailboat sails?

Bill Crick "life's a reach, and then you gybe" <- stolen from someone!