CHAPTER 2 Water speed / ground speed
Unless modified by a linked function, satellite navigation systems read out speed over the ground (SOG), which is the resultant of speed through the water modified by the effects of tide or current. Water speed, or ‘boat speed’ is the basic building block on which all performance calculations should be based. It is measured by the log which, if not correctly calibrated, will render much of this chapter superfluous.
Modifying factors to hull speed – Sail
Multihulls and modern race boats
Very narrow hulls of light displacement and shallow body draught, such as fast, racing multihulls, increase the value of the multiplying constant (normally around 1.3) by virtue of being exceptionally easily driven. Other fast sailing designs do the same with wide, light, shallow hulls operating on the edge of the displacement mode for much of the time.
Many sailing craft have an ‘overhang’ aft which lies above the waterline at rest, but which is immersed as the stern wave builds with increasing velocity. This hidden extra length is factored into a boat’s true hull speed, which accounts for some boats being faster than might at first sight seem likely. Similar effects are achieved by long overhangs at bow and stern when a boat heels to the wind.
Extra length gained by stern wave
Planing
Racing dinghies are able to exceed their hull speeds by rising up onto their own bow waves and planing. Once this state has been achieved, speed is limited only by sail area, stability and the skill of the jockey.
Surfing
The highest speeds experienced by most displacement sailing yachts come when travelling downwind in waves. If pushed sufficiently hard a wave picks up the stern and the boat careers down the face of the wave like a surfboard. She then ceases to be a true displacement hull and can exceed the theoretical maximum. Light displacement yachts surf more readily than heavy, but any vessel can achieve it if driven hard enough under the right circumstances.
A planing dinghy MANUAL OF SEAMANSHIP | 13
Speed in theory and practice
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 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172
