CHAPTER 10 Multihull stability
Without going into too much detail, it can be said that while many monohulls of between 30ft and 50ft have an AVS of between 115° and 150°, a typical multihulled cruiser becomes negatively stable at around 90° of heel. This might not sound attractive, but it must be borne in mind that such craft enjoy huge ‘form stability’ by virtue of their beam and the buoyancy of the leeward hull. Their initial resistance to heeling (and hence knockdown by a passing wave) is therefore much higher. A graph of the righting energy required to capsize a monohull and a multihull of equivalent length reveals that up to around 65°, a catamaran requires a bigger slam to knock her over than an equivalent monohull even though she is little over half the displacement. If a multihull does suffer capsize, of course, her negative stability is total and she is unlikely to right again without outside assistance.
Survival options in open water
Open ocean storm survival for sailing craft Sailing boats caught out in survival conditions can opt to operate either passively or actively. Generally speaking, unless deploying special gear, lighter vessels with flat- floored hulls need to be actively sailed, while heavier craft with longer keels are better placed to look after themselves without help from their crews. Developments with drogues and para anchors have in part mitigated this situation. The possibility of knuckling down and carrying on with a passage in heavy weather has been considered earlier, but unless your destination lies far to leeward and you decide to run as a tactic to keep your boat end-on to the waves, to continue steering towards your goal is unlikely to be an option in survival conditions.
Keeping end-on Upright stability Catamaran
By far the easiest way for a boat to be thrown down or capsized is to be rolled beam-on by a steep or breaking wave. It follows that all survival tactics should aim to avoid this situation. The exception is lying a’hull which, as we shall see, is only an option for heavy displacement vessels in something short of ultimate conditions. Here are the accepted methods of achieving the right result: Running off
Monohull AVS
Running before a gale has two great advantages: it keeps the boat stern-on to the action, thus stopping her from falling across the waves; it also decreases the apparent wind and wave force by whatever her speed may be. A big race boat running under spinnaker in 35-knot winds may well be achieving 20 knots boat speed. The chaps on deck can almost light a cigarette without having to cup their hands, but if she broaches and loses way, their smokes will be blown clean out of their mouths. The difference is less dramatic on a cruising boat, but it remains a benefit not to be ignored.
Inverted stability
The secret of running safely is to maintain a sensible speed. As the wind rises, the boat will be reefed progressively and the time will come when a decision must be made about dropping all but one sail. It’s easy to assume that the last sail left up would be the storm jib, but in some cases, a boat will run more stably with a triple- reefed mainsail so long as the boom is heavily vanged and a preventer employed to keep it steady. A trysail might also be considered, but as with the jib, since it lacks a boom, it may gybe involuntarily from time to time, which is heartily to be avoided in gale or storm-force winds. Only experimenting can decide what is best for each boat.
MANUAL OF SEAMANSHIP | 103
Storm Survival
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