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A not-too-technical comment on today’s A-Class

April 10, 2010

A short piece by John Osborn.

A not-too-technical comment on today’s A-Class. (John’s words not mine)

THE INTERNATIONAL A-CLASS

A Weighty Problem for the Designer.

Back in the 50’s and 60’s most winning A-Class yachts weighed between 52 and 62 pounds. Now they are around 30 to 35 pounds or, since we re-wrote the rule, around 15 kg. The question teasing the designers today is, “How far can we go in this direction?” The scope of the A-Class rule is very wide. In fact a Micro Magic or a Footy could be given a rating. Intuition tells us that they might not be very successful. Light displacement has many attractions off the water, (you can work it out) but we must maintain a competitive performance or there is no point. In a fresh wind many new boats will rise into a semi-planing position, but the yacht needs to be good in light winds as well. A design which emphasises planing ability may be compromised for displacement sailing.

At the upper end of the wind speed range, and going to windward, the length of the yacht is the dominant controller of speed. The yacht produces a wave form in the water surface with it’s trough around the bulky part of the hull. The yacht “sinks” into the trough and extends the length of hull in contact with the water. Consequently the effective length of the hull may be much longer than the measured length of the waterline. Designers experiment with hull shapes to maximise the difference.

In light winds and mostly going downwind, the dominant factor is sail area. Perhaps more correctly it is the “power to weight” ratio; sail area per kg. The designer’s job is to balance the conflicting requirements. Length and sail area increase the rating. Displacement reduces it. It’s a remarkably clever rule. When it was drawn up in 1922 nobody could have envisaged the yachts which compete today, and yet the rule serves us as well now as it did then. Yachtsmen are notoriously conservative. It has taken fifty years to halve the displacement. How long will it take to halve it again, and is that what we really want? Will we find that there is a practical minimum for a competitive boat?

John Osborn.

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