International 14 Mailing List
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From: Alan Smith (alans@arcom.com.au)
Date: Wed 13 Nov 2002 - 02:21:18 GMT
The following brief comment may assist
When the forces acting in the horizontal plane on a sailing boat are
considered i.e. the "lift" (force at right angles to the direction of
flow)of centre board and rudder, the drag on all the wet components, the
lift of the sails and the drag of all the components in the air stream, it
can be shown that ATAN(drag water/lift water) plus ATAN(drag air/lift air)
equals the apparent wind angle.
In the case of a 14 on the wind in a maximum full sail breeze the apparent
wind angle is in the order of 25 degrees and this therefore equates to a
total lift to drag ratio of about 2.1. From the crew weight and position and
vertical distance between the centre of pressures of the centre board and
sail we can calculate the sail lift to be about 160 pounds force. The total
drag is therefore about 70 pounds. The majority of this 70 pounds is drag
induced by the generation of the lift forces and hull drag. Total windage
drag a small but real factor. At 18 knots a 4mm 6m long halyard in free
stream contributes about 0.2 pounds drag (if the halyard is held hard
against the mast its drag could be negligible or it could "trip" the follow
and cause a considerable increase in the drag of the mainsail).The crew
if they keep shoulder to shoulder may keep their windage down to about
4 pounds, if the crew is well separated their drag may exceed double this
figure.
It may be of interest that the 12 meter Dame Pattie (damn pity) tried
rotating aerofoil plastic mouldings on her standing rigging. It was not
possible to stop the plastic section from oscillating (fluttering) violently
and consequently it was a disaster (one of many for DM)
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