The Nelson Story (c/o Motorboat Monthly)
When the late Commander Peter Thorneycroft first put ideas on paper for his idea boat, it was to have an all-weather capability to enable him to commute from his home at Bembridge on the Isle of Wight to the family firm of Vosper Thornycroft at Portsmouth. The waters of the East Solent are often stormy, particularly during the dark days of winter, with a short steep chop that taxes the stoutest of craft. But the Commander’s boat not only had to handle this, it had to do so without slowing down.
Thus was born the semi-displacement hull form that was become known and respected by pilots, police forces and other marine operators who have to go to sea regardless of the weather.
The essence of the design is a deep forefoot and fine forward sections to cut through head seas, rapidly levelling out to give an almost flat hull from midships back to the transom, necessary to give lift at high speed, and prevent the stern from digging in too much. A tight rounded bulge rather than a hard chine softens the ride, while a knuckle running the full length of the boats keeps the bow wave and spray down as much as possible. A pronounced deep keel adds directional stability when running down the waves and protects the propellers from floating debris, or the bottom should the boat run aground.
Semi-displacement boats can be driven easily up to 18-20 knots, but above this they require proportionately greater power input than a pure planing hull. The benefit comes in their ability to keep up their top speed in sea conditions that would force planing boats to slow down. Even more significantly, when they do have to slow down, they have no planing threshold to cross.
A pure planing hull will have a transition period that occurs between 10 and 15 knots, and can only comfortably travel either above or below this speed. When conditions worsen, and speed is forced down from 20 knots or more to 16 knots, it will be in the situation where backing off for an extra large wave will drop it off the plane, leaving the skipper to have to apply more power to get it back over again. This constant rise and fall in speed means that most people opt for much slower speeds in bad conditions. The semi-displacement hull on the other hand can be set to run at any intermediate speed depending on the waves.
The Nelson name originated from the first yard to build the boats. Keith Nelson and Co were based near Thornycroft’s home at Bembridge and derived their name from the proprietors, Keith Butts and Arthur Nelson Compton. The designs quickly became known as Keith, Nelson boats, later shortened to Nelson. On the design side, Thornycroft set up his own company, TT Boats, in 1961, and while Keith, Nelson were to succumb to the vagaries of the marine market, TT continued after the death of its founder in 1987, and is now in the capable hands of Arthur Mansell, one of the Commander’s protégés.
The family stretches from the diminutive 18-footer up to a 70ft model, and is continually being modified, usually with the aim of providing greater internal volume, while still retaining the characteristic head-sea performance.
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