Clockwise from top left: graph showing the overall limits of length, sail area and displacement within the 5.5 Metre Rule; graph showing the relationship of sail area, length and displacement within the model A-Class Rule, the rule from which the 5.5 Metre Rule evolved; theoretical and actual wave profiles for three 12 Metres at 9kt. Model 4 (top) is an Ian Howlett take-off of Ben Lexcen’s history-making Cup winner Australia II; model 13 (centre) is one of Hollom’s own development models and model 14 Crusader 2 – her flat aft run was widely considered (too) radical at the time but the design philosophy turned out to be similar to the 5.5 Metre Jean Genie
International A Class (1926…) Rating in inches = L + S0.5 4
+ L*S0.5 12D0.333 + Penalties = 39.37 inches
Where L is waterline length in inches plus half any excess in quarter-beam measure- ment, S is the sail area in square inches and D is displacement in cubic inches. There are maximums of draught, and quarter beam length, which vary with waterline length, and which incur penalties if exceeded and also a maximum displace- ment, which also varies with waterline length, and above which sail area ceases to increase. There is also a minimum free- board, based on waterline length, below which penalties accrue and a minimum displacement, also based on waterline length, below which sail area falls more rapidly with reducing displacement.
International 5.5 Metre (1950…) Rating in metres: 0.9*[{(L*S0.5
)/12D0.333 }+{(L+S0.5 )/4}] = 5.5
Where L is length measured at a height of 82.5mm above LWL plus the bow girth dif- ference plus one third of the stern girth differ- ence, the girth differences being measured at the ends of measured length. There are hard maximums and minimums for displacement
52 SEAHORSE
and sail area, a maximum draught and a minimum beam and freeboard plus other restrictions. (The 0.9 in the formula is only there to make it a 5.5 Metre and avoid con- fusion with the existing 6 Metre. Without it, it would be a 6.11 Metre).
The playing field The two graphs (above), showing the rela- tionship between length, sail area and dis- placement, define the playing field, the area where trades between these factors can be made. As can be seen, this is much more restrictive for the 5.5 Metre Rule than for the A Class Rule. The task of the rulemaker is to devise a
formula that allows boats to be designed to almost any area of the rule and still be competitive. But no rule is that good so the first task of the designer is to find the weaknesses, the area within the rule that produces the fastest boat, otherwise known as the sweet spot. This is not necessarily as easy as it
seems. Boats sitting in different parts of the rule may be dominant in a particular weather condition, so some judgment is required to produce a boat that will win multiple championships in different parts of the world across a variety of conditions. Designing boats to the four corners of the 5.5 Metre Rule, effectively clones of
each other so that variations in shape do not influence the result, and then running them through a reliable VPP, will find the sweet spot in various conditions. It is then up to the designer to come up with a successful judgment. The problem with the 5.5 Metre Rule is
that like most other rules it treats all upwind sail area as being equally efficient, whereas in practice it is not. As sail area reduces, because rig height remains con- stant, the aspect ratio and thus its upwind efficiency improves. It produces a smaller force for the same lift coefficient (Cl) but, because, due to the higher aspect ratio, induced drag (drag due to lift) is reduced, the force vector is angled more forward so that more of its force is available to drive the boat forward and less is pushing the boat sideways. The effect of this is that upwind the higher the aspect ratio the more each square metre of sail area is worth. Downwind it is different. Once the
apparent wind is abaft the beam any drag has a component of force working in the direction in which we are sailing and so adds driving force. A good VPP takes all of this into
account and will predict which is the best area of the rule to be in. As would be expected it predicts a short boat to be best in lighter airs and a long boat to be best in
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