BOATS AND THEIR BITS


Note: There are also inflatable boats and RIBs (Rigid Inflatable Boats) but the prospect of larger fish hooks and any size gaff being waved about near to air-filled rubber tubes is somewhat risky, let’s only note their existence in passing.


HULL SPEED


On page 10 we touched on the fact that the top speed of a displacement boat is limited by the length of its hull at the waterline. To understand why, we should imagine a boat moving forward as the throttle is progressively opened. First a small bow wave appears and another at the stern (Fig 1:8). As speed builds, a second wave appears forward and starts to move aft (Fig 1:9). Faster still and that second wave has moved all the way aft and is now reinforcing the original stern wave. The hull finds itself supported at the bow by the bow wave and at the stern by the stern wave, with a whopping great trough in between. The boat is now at its ‘hull speed’ − effectively as fast as she can go.


1


Fig 1:7


Fig 1:8 Low speed


Fig 1:9 Mid speed


What we’re seeing is a relationship between forward motion and wavelength − the faster the speed, the greater will be the distance between crests, which is the reason why that second wave moved aft. If the boat attempts to go faster still, the stern wave will move away aft, clear of the transom, and the stern will sink into the trough (Fig 1:10). The boat will then literally be struggling to power uphill − clearly a hopeless task.


Fig 1:10 Maximum hull speed


A Portuguese fishing boat at hull speed. It can go no faster.


RYA Seamanship for Sea Anglers 9


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