Catamaran science.^

[DownWest]

One point not mentioned so far? Is that if a mono capsizes, it is likely to come up again. A cat, once over stays there..

 

[srm]

One point not mentioned so far? Is that if a mono capsizes, it is likely to come up again. A cat, once over stays there..

I think you will find that some modern wide beam monos are relatively stable when inverted, due to form stability. They will need another big wave to bring them back into positive stability. Once past the angle of vanishing stability any vessel will remain stable once inverted. If sails are still set they will act as a roll damper helping keep it there.

Also, look at the down flooding angles that can both reduce the righting moments and sink them.

As the saying goes - all boats are compromises.

 

[B27]

Going right back to the original question:
According to tank testing results of scale models (Oceanography and Seamanship by Willian G VanDorn 1974)

"No vessel (tested) capsized or pitch-poled in single none breaking waves.
All vessels tested occasionally capsized or pitch-poled in single breaking waves when the wave height approximated their beam or length overall, respectively.
In capsizing a vessel was characteristically caught in the curl of the horizontally breaking vortex, becoming, as it were part and parcel of the breaker."
(All the models were monohulls).

So on the face of it likelyhood of capsize is a function of breaking wave height =/< than the beam of the vessel. So a catamaran should be less likely to capsize in a given sea state than a monohull of similar length. However, as we discussed hull form and sail configuration will also have an influence, especially on a catamaran.

In my post #21 my catamaran survived a breaking wave with a height that was probably near its beam by sliding sideways in the breaking water until the wave passed underneath. A monohull with deep keel or catamaran with board down may well have tripped over its keel. The keel/board would be held in dense water as the hull was subject to the impact of the breaking crest.

The average RTIR seems to be good for a couple of inverted cats?

From watching 'dinghy' cats go over, it's often a case of hitting a wave at speed, rather than being passive in the water and having a wave invert the cat.
The cat is simply quite light and fast enough to go out of control if the bow buries or whatever. I have done similar in a fair range of monohull dinghies.
The centre of gravity is a few feet above the water, if you stop the bow by stuffing it in a wave, the rig is keen to keep going, which can spin the boat into a broach, lift the rudder out of the water or a few other variations of 'splat'. This only requires a steep wave tall enough to lose the bow in.

 

[srm]

The average RTIR seems to be good for a couple of inverted cats?

From watching 'dinghy' cats go over, it's often a case of hitting a wave at speed, rather than being passive in the water and having a wave invert the cat.
The cat is simply quite light and fast enough to go out of control if the bow buries or whatever. I have done similar in a fair range of monohull dinghies.
The centre of gravity is a few feet above the water, if you stop the bow by stuffing it in a wave, the rig is keen to keep going, which can spin the boat into a broach, lift the rudder out of the water or a few other variations of 'splat'. This only requires a steep wave tall enough to lose the bow in.

Yes,
Now you mention it: similar to the description I heard, some 20 years or more back, from the crew of a production performance cruising cat that capsized in fairly moderate conditions under spinnaker.(they were not racing). Apparently, it was a fairly gentle experience to start with.

 

[DownWest]

I think you will find that some modern wide beam monos are relatively stable when inverted, due to form stability. They will need another big wave to bring them back into positive stability. Look at the angles of vanishing stability to see how large an angle they may remain stable once inverted.

Also, look at the down flooding angles that can both reduce the righting moments and sink them.

As the saying goes all boats are compromises.

Yes, quite aware of that, I was just looking for my copy of Marchaze (spelling!) on the '79 Fastnet.

 

[Chiara’s slave]

Yes,
Now you mention it: similar to the description I heard, some 20 years or more back, from the crew of a production performance cruising cat that capsized in fairly moderate conditions under spinnaker.(they were not racing). Apparently, it was a fairly gentle experience to start with.

The classic multihull capsize. The Gunboat tyat went over in this years RTI conformed to this. Carrying too much sail downwind. You’re fine when you’re flying along, but if the helmsman sticks it in the back of a wave, the boat will slow from teens or more down to maybe 5 knots. The apparent wind increases by 10 knots. You were comfortable in 18 knots apparent, but find out the hard way that 28 knots apparent is too much for your sail plan.