Page 298 - Practical Ship Design

256 Chapter 8

The best way of avoiding a broach is by a very quick helm response, made at the
very first sign that a broach is imminent, aimed at turning the yacht so that the wind
is dead astern. The best, indeed almost the only, method of recovering from a
broach is to completely free the sheets of all the sails so that these stream out in the
wind and cease to develop any force.
In ships, broaching is a much rarer phenomenon and the mechanism which
causes it is slightly different, the broaching of ships being caused by the waves
rather than by the wind. If, when sailing away from waves the crest of a particularly
large wave overtakes the ship and hits one quarter - rather than hitting the stem
squarely - the ship will tend to be thrown round into the direction in which the
wave is moving. This will bring the ship beam on to the sea and result in a severe
list. The fact that at the time the crest hits the stern the preceding trough will be
passing along the ship means that the draft at amidships and forward is reduced
lessening the resistance to the turning force exerted by the crest. Whilst the crest is
still on the quarter, the bow is buried and the stem is raised, the next trough then
arrives causing the waterline to drop away from the rudder reducing, or in a severe
case eliminating, its effectiveness just when it is most acutely needed.
The operational lessons to be learnt are:
(i) the need for the helmsman to take great care when steering a ship in
following waves of significant size and to be ready to turn the ship’s stem very
quickly into any larger waves approaching from either quarter;
(ii) the need for the Master to decide in good time if the waves seem to be
becoming too large for the ship to take comfortably on the stern to alter course
immediately and if necessary to heave-to.
This digression into yacht sailing and ship operation has been made because few
naval architects appear to understand what broaching is and just how dangerous it
can be. It is perhaps worth emphasising that broaching is not limited to small ships
but can be a real danger to quite large ships.
Reverting now to the steps that can be taken at the design stage to produce a ship
less likely to broach.
As a broach is caused by a wave hitting the stem quarter of the ship the effect
will be more severe if the stern presents a big target to the wave. Large flat
transoms on comparatively small ships such as fishing vessels appear to be a case
in point.
As the mechanism of a broach involves the ship’s stem being carried round any
thing that helps to resist this sort of motion is helpful, so a reasonable amount of
deadwood aft should be retained. On the other hand a bulbous bow and particularly
a ram bow may contribute to broaching by digging into the water and becoming the
fulcrum about which the ship swings. There may even be something to be said for
the preference which some older yacht designers had for having some balance
between the two ends of a yacht.