Sailing small ships in the following sea

Far more attention is paid to the behaviour of ships sailing against the waves than with them, although the latter are a bigger threat to safety. Grim (1951) demon- strated, both theoretically and by model tests, that even if a ship has adequate static stability she can be capsized if the waves strike her astern. There need be no syn- chronism between the ship's natural period and the period of encounter. In any case, a ship cannot be capsized through synchronism alone. As Grim has demonstrated by model tests, the greatest danger arises when the speed of the waves is equal to that of the ship, or when the waves overtake the ship so slowly that an almost static situation is created and the ship lies on the crest. In this position the lever arms and the degree of static stability can be reduced to such an extent that the ship capsizes even without the action of dynamic forces. The stability is reduced when the ship is travelling on the crest of a wave, as compared with calm water, but is increased when she is in the trough of a wave.

If the ratio between the length of the wave and the length of the ship is higher than 1, then there is the additional danger that, owing to the contrary orbital motions of the water, she will veer across the crest and the trough, i.e. get out of control, and be rolled over by the sea, especially in breaking seas.

A further possibility of capsizing in seas running obliquely astern has been pointed out by Kempf (1938).

This danger arises when the ship rolls heavily, to which synchronism may be a contributory factor, and is struck by a big wave on the windward side when inclined far to the leeside. In this case the centre of buoyancy is moved to the windward side, so that a static capsizing moment is created. If, in this situation, the kinetic energy of the wave is added and the leeside rail dips under water, there is an immediate danger of capsizing.

It is well worth noting Grim's finding that synchronism is present not only in the case of synchronism between the ship's natural rolling period and the period of encounter. Grim found several points of synchronism where, on occasion, very large rolling angles were measured. These points arise when the ratio between the period of the change of the stability tj and the rolling period t of the ship is 0.5 ; 1 ; 1 .5 ; etc. The largest angles were measured at tg/t=0.5. The rolling angles become smaller with increasing period ratios.

Another great danger arises when a big wave slowly overtakes the ship and floods the deck along its entire length. The danger is increased if the doors of the deck structures are open. Several small German vessels, including a trawler, were lost* in this way in recent years.

Owing to the extra weight of the water above the centre of gravity and the free surfaces on deck, such a diminution of stability takes place that the resistance of the ship to the dynamic attacks of the sea is almost, if not completely, exhausted.

It must be expressly pointed out that the dangers referred to can become acute only if the waves pass the ship slowly, i.e. when they come from astern. The speed of the ship must be reduced to allow quicker passing of the waves so that the danger cannot develop.