Rig strength: sailing loads vs capsize

For a beach cat or dinghy, a capsize is no big deal; right the boat and go on sailing. When a larger sailing vessel capsizes, often the rig goes to pieces. Why is that?

Is the reason that loads during capsize are larger than during sailing, independent of size, and that rigs are only designed for those loads if capsize is part of normal operation? Or is there some size-dependent factor that makes it more difficult to design for loads seen during capsize in larger boats? If so, what is that size-dependent factor?

One possibility is this: displacement scales with the third power of linear dimensions, stability with the fourth. For a multihull, that would also apply when upside down. As surface waves roll the boat and move the rig through the water, sail area scales with the second power. Then account for lever arm which increases the load, and it increases the speed with which the rig moves through the water, and the load is proportional to the square of the speed. Then load may scale with the fifth power of size. If the relevant aspects of strength don't scale up as fast, rigs become more vulnerable to capsize as vessel size increases.

I suppose for a ballasted monohull, there is the energy needed to make the boat capsize, which may also scale with a different power than strength, but I don't know how.

 

I can think of three big reasons for the difference in your examples.

Multihulls have much higher initial and ultimate stability, so their rigs are designed to a higher load in upright, normal operation, and because they stay vertical in operation, there is less performance incentive to minimize weight aloft.

Larger sailing monohulls are more negatively impacted by weight aloft (~every gram aloft requires a kilo in the keel) and because they heel in a gust, they are designed lighter to their lower max righting moment in air. So the monohull rig strength is a lower percentage of the boat weight.

Your point about size scaling has some merit, but it applies to both vessels. When a large multihull goes over, there is less weight (so less momentum) relative to rig strength, and once over it is stable turtled. In large waves, water velocity gradient can still break-up the rig. In a large mono the momentum of the boat is high relative to the strength of the rig, and the event doesn't end with the crash. The boat is likely going to right with sails full of water and wave energy so it is likely to come up sans rig.

 

Multi hulls are rarely self righting. Once flipped it takes a crane to fix.

Monos can be self-righting. But as the mast passes thru the water several times as mush strain is encountered than when air is passing thru the rigging.

 

The rig on a cruising multi is designed to fold rather then let the craft capsize...

 

While the answer is fairly simple, the actual causes of why the rig collapses are more complex. The simple factor is that water is ~800 times more dense than air. Even allowing a FoS of 5, a velocity in water 1/40th the wind speed at knockdown will overload the rig. The more complex factors are shroud tension, mast compression, and wave orbitals. Note that except for wave orbitals, the first two are not concerned with sail area. Sorry Skyak, well designed multihulls have much less ultimate stability than well designed monohulls, its the nature of the beast as ultimate stability is measured by roll angle, not righting energy ( see this post Canting Keel Monos vs Multihulls https://www.boatdesign.net/threads/canting-keel-monos-vs-multihulls.13511/page-15#post-115354 ). In quiescent water, most well designed monohulls will recover with all sails up. In MORA, it was a requirement that mast and all normal sail be placed in the water and the boat shown to rise; but no speed, minimal waves. Shrouds and masts can fail when in the water because they are subjected to side loads 40 to 100 times greater than they have in air, regardless of wind velocity. It is these out of column loads that cause the loss of the mast, either by shroud failure or section failure. Just looking at what is left of the rig tells which item failed first. Finally, there are wave orbitals to consider. Significant wave particle speed in a nominal SS3 can approach 4 ft/sec: ~15 psf on the sail area in the water. Additionally there is the wave surface to consider in relationship to the hull and rig. The rig in the water gains a huge amount of hydrodynamic and added mass. Depending on the phasing and the wave surface, the rig/hull intersection could see a load of 1 or more G's times the mass of the hull.
This is not conjecture. I spent a considerable amount of my professional career designing, analyzing, maintaining, and operating through the water rigging systems to be used at sea. We had wind limits, we had wave limits, we had crane alarms, and fendering systems; pulling something up through the air/water interface is fraught with issues.

 

Are you sure? That's the first time I heard that, and load-dependent sheet releases would seem to do the same thing, possibly more reliably, and with less risk of rig failure. I especially don't understand how this approach could be compatible with having a safety factor for fatigue.

Fair enough, but that would apply to small boats, too. I have not worried about a beach cat rig even when I pitchpoled so fast that my memory only recorded me leaning out beyond the rear beam one moment, and being in the water the next. The only time I broke a mast was when the water was too shallow to let the boat turtle, and I was too slow to right it. But breaking masts when or after capsizing seems to be pretty normal for large boats. That suggests some scaling factor to me.

 

You can go through ISO12215-10 standard for sailing rigs of catamarans.
In brief: the rig of a multihull is dimensioned based on demihull 'starts to fly' condition. There are safety factors above that, but they are at the end defined by the manufacturer of the rig.
From my experience, rig of a cruising catamaran designed for 'survive the capsize' will have monstrous mast section and rigging dimensions. But it is OK for small beach cats.

 

Perhaps, but it could also be a manufacturing condition; i.e. there is a minimum/maximum wall thickness for the mast extrusions. You could check the cube-square law against the slender column theory and will likely find that the critical buckling pressure is 1/4th for the 2x size system because Pc= pi^2*E*I/4*L^2 where pi^2E is basically a constant. So you could increase I, but at what cost for weight aloft?