Acoording to ISO 12217-2
event when a boat reaches a heel angle sufficient to immerse the masthead, and from which it may or may not recover without intervention
Table 5 — Required minimum angle of vanishing stability
Design category Required minimum angle of vanishing stability, φV(R)
A (offshore) φV(R) = (130 - 0,002m) but always ≥ 100°
B (nearshore) φV(R) = (130 - 0,005m) but always ≥ 95°
m is mass of boat in kg
Now, lets hit a mono with a big gust of wind
in sheltered water
. The boat heels over, and several things happen:
1. The righting moment increases, typically up to a maximum at 75 degrees or so, and does not go to zero until the angle of vanishing stability--say 120 degrees, when the mast
is well into the water
2. The forces on the sails
decrease, going to zero at about 90 degrees, or before the mast
hits the water.
3. The boat develops severe weather helm
, which rounds it into the wind
, further decreasing the pressure on the sails
All of this happens without human intervention, and the boat finds another stable point (with a bunch of flapping from the sails and an increased heel level. The reality is that a mono can't get into a whole bunch of trouble without wave action (unless you are a gung-ho racer
with a spinnaker
up, which is another chapter).
Now lets look at a catamaran
in a similar situation. When the gust hits the boat starts to heel, and several things happen:
1. Instead of increasing like the mono, the cat righting moment starts to drop after one hull
lifts out of the water, or at about 10 degrees of heel.
2. The pressure on the sails drops like the mono, except that once the hull
starts to lift
, the wind pressure on the wide structure on the boat starts to make an increasing contribution to tipping it over.
3. The cat hull shape does not produce much weather helm
as heel increases, which means that the boat will not round up as severely as the mono, and an autopilot
may be able to keep course.
Once you start to fly a hull, the catamaran
is in an unstable situation. Unless there is intervention in the form of releasing at least the mainsheet (a sharp turn downwind may help, but is not as effective), the next stable position for the cat will be inverted. Even releasing the sheets
does not work once the heel angle exceeds 50-60 degrees, because the wind pressure on the hull and bridgedeck is enough to push the boat over.
The catamaran is blessed with significantly greater initial stability than the mono, which provides for the possibility of improved performance by carrying more sail. However, that increased performance comes with a downside risk. If you increase the sail area to displacement
on a cat, it lowers the gust required to capsize
Here is what ISO-12217-2 says about cats :
7.5 Stability information
Since sailing multihull boats and form-stable monohulls may capsize, information as listed in a) to d) below shall be:
— displayed in a durable manner inside the boat close to the main companionway or navigation station, and
— provided in the owner’s manual (see Annex F).
NOTE “A durable manner” means a rigid plate or flexible label affixed to the craft in such a way that it can only be
removed by the use of tools.
a) The stability hazards to which these boats are vulnerable, including the risk of capsize in roll and/or pitch,
particularly in breaking seas.
b) The apparent (i.e. relative) wind speed (expressed in knots or metres per second) at which the area of
any practical combination of sails should be reduced when sailing in calm water in the minimum operating
condition, taking account of the hazardous effects of gusts. Additional information relevant to the maximum
load condition can also be provided if desired.
This information can be calculated using Annex G (which includes a margin for gusts), or alternatively be
derived from sailing trials. The method of determination shall be stated.
NOTE For form-stable monohulls, refer to 6.5.2 e).
If derived from sailing trials, the wind strength quoted in the owner’s manual shall correspond to a wind
speed of not greater than 70 % of that required to
1) lift the windward hull of catamarans out of the water, or