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Any boat when it's moving produces several clifferent wave patterns. But, producing waves involves expending energy, so the smaller the waves you can make, the better. At low speeds, the drag is almost all friction. At 'hull speed' and beyond, it's wave-making drag that dominates.
As a boat starts moving forward, a bow wave appears with a shallow trough and a secondary wave forming behind it. There's also a stem wave, but we can ignore this for the moment. As boat speed increases, so does the wavelength between bow wave and secondary. When the speed (in knots) is equal to about 1.34 times the square root of the waterline length (in feet), the wavelength and the boat's waterline length will be equal, and the secondary wave will be at the stem. The boat is now at its 'hull speed'.
The hull is now supported fore and aft by the crests of its own waves, with the beamiest, most buoyant part of the hull partially suspended over the deepened trough. The situation is made worse because the secondary wave and the stem wave are now in coincidence and will reinforce each other! If hull speed continues to increase, the secondary wave will move clear astern and the after parts of the boat will start 'squatting' into the trough ahead of it, so the boat fmds itself in the ridiculous position of trying to climb the slope of its own bow wave - impossible for a displacement type.
The flattened stems of those early racing motor boats may have been intended mainly for stability; but as engine powers increased it became apparent that they also helped resist squatting. As the bows started to rise, the flattened after sections pressed down on the water instead of sliding through it. The water, of course, pushed back, just as it does on a water ski or a skimming pebble. The bows of the boat may have been supported by buoyancy, but hydrodynamic forces were lifting the stern: it was planing! It was to be at least another half century before the phrase 'semi-displacement' started to catch on, but that's exactly what these boats were. By the time the First World War was over,. engines had improved to such an extent that the planing principle could be extended to Cover a whole boat, rather than just its stern. The advantage is that once the whole boat is planing, you're not actually making bigger waves by going faster. Wave-making drag reaches its maximum at just over hull speed, then stops increasing. Other forms of drag become more significant, but at least they don't increase as dramatically as wave-making drag.
Up to and including the Second World War, most planing motor boats were designed with planing ability as the main consideration. That's to say, they had almost flat bottoms at the stern, and very shallow vee-shaped sections forward. As late as 1950, the deadrise (the angle at which a boat's bottom panels slope upwards from the horizontal) of a typical planing hull was just 3° at the transom, and perhaps 13° amidships.
It almost goes without saying that such a flat-bottomed hull is likely to bounce and slam uncomfortably in even the smallest of waves, and that in moderate conditions you're likely to have to reduce speed if only to keep your teeth intact! The solution is almost equally obvious: instead of making the boat wide and flat, make li deeper, and give li a fll1er, more knife-like stem. They're much less 'efficient' in terms of their planing ability, but the deeper vees we see nowadays are very much more comfortable than their predecessors. At about the same time as deep vees were starting to catch on, a new word entered the motor-boating vocabulary. Legend has it that the fIrst 'Nelson' was a 23 foot roundbilged launch that went wrong in the building process. It was badly hogged before it even hit the water, but turned out to be capable of far higher speeds than any of the unhogged boats from the same yard. From that small, unpromising - and probably apocryphal - beginning grew the mighty reputation of semidisplacement boats in general, and the 'Nelson' designs in Particular. Look at their sections, though, and you'll see a remarkable similarity between the Landguard Nelson 33 of 1994 and the racing launch of 1904: there's the same deep, narrow forefoot, semi-circular mid-section, and flat-bottomed stern. They're certainly not the same boat, nor were they designed with the same use in mind, but both are the shape they are because it suits their intended purpose.
That's the thing about hull shapes: you need to look beyond superfIcial details such as age or colour, or what other people might think, and
decide whether it really suits your purpose. |
Boat Handeling 