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From: Alan Smith (alans@arcom.com.au)
Date: Fri 04 Aug 2000 - 01:51:04 BST
I read with interest the debates re foils an leverage. The picture at
http://www.18footer.org/images/gallery/plisson/hydro5.jpg
shows where it is possible to go. The real issue is for the class to have a
vision for the future. I would not recommend a piecemeal or adhoc approach.
In 1965 Tom Vaughan, myself and a few other "radicals" had a vision to
amalgamate the worlds 14 foot open design "skiff" classes and it is with
some pride that we view the outcome today. I note that, at the time, the
"outlandish" Australians had rules to prohibiting centre boards and rudders
increasing in thickness below the water. Atleast Tom V and Jeremy P will
smile.
There is know doubt that increasing leverage allows sails to be carried in
higher wind speeds and results in higher speed through the water. But result
higher loads in the rig and hull. As we found in Australia when we first
went to two wires, masts, centre boards, rudders and hulls all started to
break up. The materials revolution of the past four decades has made today's
boats viable.
As you try and increase performance by adding hydrofoils loads will increase
and load paths will be different, drag at low speed will increase and if you
get it right at higher speeds the drag will be less. The addition of a
horizontal foil or foils to the rudder can be exploited in a different way
and that is the vertical surface need not be so large. Rudder area and depth
dimensions are those which is necessary to achieve control in extreme
sailing conditions.
A "foiled" rudder can do the job at smaller total area and hence lower drag.
However it is probable that better all round performance improvements can be
achieved by only using the area (depth) need for the day. Hard weather,
short centre board and deep rudder and visa versa in light weather.
The short board in hard weather and at planning speeds is very significant.
At
12 knots you only need a quarter the centre board area that is necessary at
6 knots. The centre board is actually a surface terminated hydro foil and
the dynamics of the terminating surface should not be over looked. The
reduced depth and area has the following positive effects. 1 Heeling
moment is reduced. 2 Wetted surface area is reduced. 3 The water
pressure under the hull adjacent the board increases on the leeward side,
lifting the hull and increases righting moment as a result of the centre of
pressure under the hull moving to leeward. The boat has more power and will
plane at lower speeds. The down side is that the short board will stall as
you come out of a bad tack and you must pull away and gain speed before
loading the board. Hydro would have the same problem and may not even sail
in light airs!
The lecture in yacht dynamics was not my intent as I started to write, but I
trust some will find it useful. My real point is don't damage the best
dinghy class in the world with adhoc changes but do all you can to allow
development which will ensure the class is still top gun 30 years down
track.
Good sailing
alan smith
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