Taking stock


America’s Cup 37 is long gone and dusted so no time to lose preparing for AC38. First a quick assessment of where we are now and how we got here… Dave Hollom offers analysis


Now America’s Cup 37 is over and we finally have a Protocol and a venue it is probably an opportune moment to look at some of the technical innovations of AC37 and some innovations that might be used in AC38. It is also probably about time we tidied up some of the nomenclature associ- ated with these AC75 foiling monohulls; especially the use of the words Skeg and Bustle, which annoyingly seem to be used interchangeably to describe the expansion of volume on the bottom of these boats that stretches for most if not all of their length. The Equipment Rules of Sailing (ERS)


define a skeg as ‘a fin attached immediately in front of a rudder’; a fin as ‘a fixed hull appendage primarily used to affect leeway or directional control’; and a rudder as ‘a


48 SEAHORSE


moveable hull appendage primarily used to affect steerage’. This object, variously described as a skeg or a bustle, stretches, near enough, the length of an AC75. But in most cases it could still be described as being immediately in front of the rudder. Trouble is on some of the AC75s, notably Britannia and Patriot, there is a big gap between the rudder and this object – so per- haps a skeg in some cases but not in others. Unfortunately there appears to be no


definition of a Bustle in the ERS. The term seems to have originated to describe the expansion of volume in the stern sections, in front of the rudder, on Metre boats. Originally the term described an article


of ladies’ clothing designed to enhance the shape of their bum, so it fitted well in the description of what was happening to the stern sections of Metre boats. However, we now seem to be using the term to describe this expansion of volume along the length of these boats, not just at the stern, so I think that, in this instance, it is not a good term. Using the analogy of women’s cloth- ing, I suppose outsize would fit the descrip- tion. So perhaps the term Keelson would be better? Trouble is a keelson, although not in the ERS, is defined by the dictionary as a longitudinal internal structure on top of the keel, so the term keelson doesn’t quite fit the bill either. I think I will stick with Skeg for the purpose of this article! To me the most important purpose of our AC75 ‘skeg’ is to restrict or prevent air


from passing from the high-pressure side to the low-pressure side of the boat and so increase the effective span of the rig and thus its efficiency. Effective span then becomes the one variable that the design team might be able to influence. If the bottom of a rig had no gap between it and the sea, its effective span would be doubled; and because lift-induced drag varies as (1/span)2


induced drag would reduce by


four times compared to if the bottom of a rig is some distance from the sea surface or other endplate. Any gap between the bottom of the rig and the endplate will reduce this effective increase in span… Important to note too is that the size of gap that almost totally destroys this effect is small. On conventional craft getting the


bottom of the rig flush with the deck is good enough as the hull fills the gap down to the water. However, on flying machines the hull is normally well clear of the water, leaving a gap between the hull and water surface for air to escape. In a Moth this is partly solved by the


daggerboard filling some of this gap. How- ever, where the Moth suffers more is in not being able to seal the sail right down to the deck along its entire foot. (Doing so makes the crew crossing of the boat difficult, if not impossible). If this could be done, along with a new daggerboard design to better fill the gap beneath the hull, then Moths would sail very much faster. On an AC75, however, the foil arms are


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150