Reply To: "Stoop to her canvas"

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#11475
Frank Scott
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    The wind and sea conditions that you report are scarcely extreme, so it would seem that the Spencer had a serious design problem.

    It is interesting to see that another of J-L Barrallier’s designs, HMS Newcastle, a double-banked frigate (keel: 1813, complete: 1814), that was specially built to counter the large American frigates, was also prone to excessive pitching in a head sea (see Robert Gardiner, Frigates of the Napoleonic Wars (London, 2012), 55.)

    The British had started to reduce tumblehome in their ships by the mid 18th century, and there are many draughts available to show how British ships whose designs were ‘copied’ from the French in fact had their tumblehome significantly reduced. Wall-sided ships were easier to build, and became quite common in Britain during the Napoleonic wars.

    The following extract from the draft of my book should help explain rolling and pitching in a brief manner. They were written with the advice of Colin Mudie, RDI, the well-known naval architect (designer of several successful sail training ships):

    Rolling
    The roll period of a sailing ship should be slow and soft, with eight to ten seconds being a reasonable rate. This allows the crew to work aloft in safety, and avoids unduly inhibiting the airflow across the sails. The inertia of the weight of the rig helps to damp the motion, but the key lies in the form of the waterplane around the load waterline.
    Positive roll damping is usually achieved by designing the hull form to lift bodily at the extent of each roll. Basically this varies the rolling period, and thus prevents any build up of synchronous rolling. If done with discretion some of the roll energy itself can be absorbed in the conversion from kinetic to potential, and back again. Synchronous rolling is a particular problem for very stiff vessels, and is excited by sea conditions where the wave pattern encounter rate comes close to the ship’s period of roll (or a harmonic thereof). The classic scenario is to be becalmed with a long ocean swell, in which circumstances stiff vessels can roll so violently that at the very least they risk damage to the rig and injury to the crew, and may roll their spars out.

    Pitching
    The rate of pitching is an important design factor, and the hull form should be planned to avoid any build up in pitch energy. This is usually achieved by a markedly different fore-body to aft-body, so that the pitching periods are different. Vessels with apparently similar ends normally have their pitch centres aft of amidships, thus providing the necessary differences in fore-body to aft-body.

    Note:
    The brig TS Astrid was a notoriously stiff sailing vessel, with a very short roll rate. In 1993 when becalmed on passage from Grand Canary to Barbados her rolling motion soon became very violent, and built up so much that she rolled gunwales under. Although keen to avoid motoring, this was clearly unacceptable, and I had to start the engine. Once we began to move under power the violent rolling ceased almost immediately.