International 14 Mailing List
| 14 | ||
From: Ted Rogers (twrogers@earthlink.net)
Date: Tue 12 Nov 2002 - 06:24:04 GMT
Aerodynamic drag is equal to 1/2(density of fluid) x Velocity^2 x Area x
Coefficient of Drag.
(Just for Grant Bourke: That would be the projected area of one side of the
object)
So the drag at 10 knots is 4 times the drag at 5 knots. And the drag at 15
knots is 9 times that at 5 knots.
So an airplane doing 100 knots would see about 100 times as much about drag
from a round cable as a sailboat doing 10 knots.
If you're using the above formula to compare long, skinny objects like
wires and masts, it pretty well comes down to Diameter in place of Area.
Your 3" round mast, ignoring the sail behind it for a minute, and surface
roughness, has something like 24x the form drag of an 1/8" wire. Even if
the rough wire surface causes the wire to be 2x as draggy, and you have 4
full length shrouds, that's still only 1/3 the drag of the mast tube....
If you're worried about drag reduction, I'd start with looking at all the
other easy bits, such as spreaders with balls of tape on the ends, the 2:1
block at the top of the forestay, plus the junk for the jib halyard,
assorted bungies, the spin halyard block, etc. Most boats have at least a
few easy targets....
But the biggest drag reduction we can probably make without rotating masts
is to get away from round masts. We use round tubes because they're easy to
engineer and easy to filament wind. But putting a sail behind a round tube
leads to a big separation bubble on the leeward side of the main, which
both reduces lift and increases drag. Frank Bethwaite's book has an
excellent discussion of mast shape vs. mainsail flow which basically
concludes that if you're going to put a non-rotating mast in front of a
sail, a "D" shape is the best compromise, as the sharp rear corner trips
the flow before it gets too big a vortex going.
I'll leave it to some one with more time on their hands than I to go make
this work in 14s......
Ted Rogers
US 1140
At 07:21 AM 11/10/02 +1100, Peter Moor wrote:
>I seem to recall that drag increases exponentially with speed. Is that
>correct ? In other words, the drag of the external halyard and rigging
>would be 3/5 ths of FA in 5 knots of wind but substantial when flying at
>100 knots. How significant would the drag be in the average sailing
>conditions of 15 knots ?
>
>
>----------------------
>International 14 Mailing list
>To Subscribe: <mailto:14list@i14.org?subject=subscribe>
>To Unsubscribe: <mailto:14list@i14.org?subject=unsubscribe>
>
>
----------------------
International 14 Mailing list
To Subscribe: <mailto:14list@i14.org?subject=subscribe>
To Unsubscribe: <mailto:14list@i14.org?subject=unsubscribe>
This archive was generated by hypermail 2.1.4.