A22 design Joakim Rodebäck
I still take a peek now and then on the A22 Klubben site.(Yes I know it is called S22 now). I saw about two months ago the Farr design, but only now, do I have the inspiration to comment. Usable for your site and discussion of our mutual friends in Sweden.I can only comment on it in general on the shape and the keel, as I do not know the length and draft.
Hull Shape:
The shape is more or less what I wanted to do. I would also choose for low windspeeds in calm water to have a very short forward bow-overhang. See 1964 Olin Stephens' Constellation 12M design, in which the bow was shortened.( Hoytt: The Twelve Meter Challenges for the America's Cup). Only I would follow Thompson Design in placing a small knuckle above the waterline, lacking the resources to do Computer Flow Design for a longer forward overhang. I will mail you my incomplete concept in AutoCad if you want it for publication.
The Farr boat however seems very wide and squat, suggesting a heavy emphasis on stability by crew-weight and form-stability, at the expense of high wetted surface and form-drag. This would only become useful in stronger winds 9-11 knots and above, strange because there you would go for more length L and longer overhangs. (Old argument: short and wide, or long and narrow) However, Farr Design has some powerful software in CFD and enough experts to get a good result.
Since the Tiger (1982?), no bustle is allowed in the class. In this they conform. But the 1967 adaptation of Constellation, which I saw in 2003 in the Herreshoff Museum on Rhodes' Island, shows how instead of a bustle you can also use a skeg, which is within the Square Meter Rules. (Hoytt in the Twelve Meter Challenges, explains that a skeg or bustle maintains laminar flow in the afterbody for longer), Constellatin being almost as fast as the 1967 Intrepid with bustle, skeg and a separate rudder.But do not despair; the S22 is a grown and mature class, the old boats will still get a good chance to beat the new Farr Design boat in most conditions outside its optimum.
Keel Shape:
Apparently Farr Design has gone for a large and deep trapezoidal keel. This would suggest more lift, meaning pointing higher. Especially the practically straight leading edge points to having high lift combined with relative low induced drag. as is the tip chord, leading to an approximation of elliptical loading of the foil.( Marchaj; Aero-Hydrodynamic Principles of Sailing). But this at the expense of wetted keel-area.
Keel Shape:
Apparently Farr Design has gone for a large and deep trapezoidal keel. This would suggest more lift, meaning pointing higher. Especially the practically straight leading edge points to having high lift combined with relative low induced drag. as is the tip chord, leading to an approximation of elliptical loading of the foil.( Marchaj; Aero-Hydrodynamic Principles of Sailing). But this at the expense of wetted keel-area.
The wide and deep keeltip, will give more volume and place weight low down, creating more stability again. But also more lift near the keeltip, increasing the size of the vortex and induced drag. But being better than traditional keels, having a larger aspect ratio.
Rudder Placement:
One problem which may arise is course-stability and turning with the imagined rudder. The same problem which haunted the Square-Metres in the 1920s and the 5.5 Metres in the 1960s, at a time in which they also had a short long-keel. They had to steer the boat with sail-handling. The design of a long keel with good handling capabilities is no longer an expertise with our new designers. I will wonder how the new design handles. I myself had to resort to Skene's Elements of Yacht Design, to get an understanding of this aspect, and also discussed some details with Olin Stephens, being alive and a designer who had designed fixed keels and split appendages. And it is he who recommended the 1940 edition of Skene's .(Available again now from the Mystic Seaport).The Tiger shows a way, which I will adapt. The rudder-axis going straight down, to give maximum leverage and control to the rudder, placed as far as possible aft.
Keel Draft and Resistance:
Baader's The Sailing Yacht has some figures on 30 Sq.M. yachts. From it I distill that induced drag from the keel is responsible for some 32% of the total drag of the yacht. Square Metres are renowned of high pointing capability. So I conclude and have calculated that the surface area of the keels is more than enough, the best way to increase performance is to increase span (is draft) with a suitable tip, reducing the considerable vortex which exists around the traditional keels.Van Oossanen (1984) in assisting in developing the winged-keel (Australia 1984) states that the wingtip is particularly effective if placed over the last 40% chord of the keel.The 22 Sq.M has too much keel-area already, so below the keel measurement line, we can use a single tip going deeper and increasing span, and aspect ratio, and reducing induced drag. This is what I adopt a swept aft tip. iving lower wetted surface area than the Farr design.
Rudder Shape and Outline:
Skene's warns us that the upper part of the rudder, close to the hull is in non-laminar flow and gives little lift. We should reduce the rudder-chord as much as possible in that location, or even let it start somewhat lower on the keel.Then in about the middle of the rudder we should have the rudder's maximum chord for maximum effectiveness, in clean flow. Rudder chord to be about 30-40% of keel-chord.At the lowest point the rudder-chord must also be reduced to reduce drag.Marchaj confirms that lift of an airfoil must be reduced towards the tip, to reduce the vortex at the tip and induced drag.(The Farr design rudder axis is not in the picture, I wonder how they solve this tricky choice.)Following Skene's instructions we get the classic semi-elliptical outline of the rudder. This is what I will adopt in my design.Furthermore 1964 Constellation had a heart-shaped end-surface at the top of the rudderblade to reduce leakage of the high-pressure side to the low-pressure side of the keel. This is also a feature I will adopt.
So far my immediate comments. Any other comments of my friends in Sweden and elsewhere?
Greetings
Roland de Rooy
from Amsterdam, the Netherland's. "
Roland de Rooy flankerad av Martin Carlsson och Allan Förberg i Sandhamn 2003Egen kommentar: Inga dumma tankar, synd/kul att Roland trodde det Farr som designat. Jag återkommer med lite kommentarer när jag hunnit tänka lite själv....
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