How to make a catamaran immune to capsize?

This is a hypothetical exercise, so if the question offends your sensibilities then simply don't respond.

Let's ignore capsize due to heeling moment caused by sails, pretend that the boat is adrift with no sails up or without any active control mechanism, perhaps in the middle of a hurricane with large, steep breaking waves. Let's also give ourselves design freedom. Minimum accommodations, storage, and comforts, not necessarily in the hulls. Whatever mast(s), board, rudder design and layout suitable to the goal. Of a reasonable size, say, less than 50'.

What are the factors involved in the capsize of catamarans?

Considering the factors involved, how would you mitigate the effect of each factor?

 

Beam on to large breaking waves.

Good seamanship.

 

Thanks for your contribution... I have read and watched many accounts of people who were put at the mercy of the sea with failed rudders, sail control, engines. So, I thought it may be interesting and useful to explore what *design elements* would make a catamaran more likely to survive such a circumstance without capsizing.

 

bolt it down to the earth.... or afloat weigh it down with water...

 

Add keel and rudder foils, but don't go fast, so they act as sea anchors vertically. Make it semi-submersible. Ballast the windward hull to keep wind from getting under it, unless wind is opposing wave direction. Have a long rope lowered between the hulls, carrying a heavy weight, as done thousands of years ago.

 

John Shuttleworth covered that in a talk available on his web site, complete with equations: Considerations for Seaworthiness https://shuttleworthdesign.com/NESTalk.html
The equations are:

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Terms of these equations ordered by what makes capsize more or less likely, and what you can control, with my comments in brackets.

What contributes to capsize:

• Ei = energy transferred during impact. (Can be controlled by limiting impact moment and increasing roll moments of inertia)

• Mi = impact moment (some of the terms contributing to impact moment, in the second equation, can be controlled by either the designer or the skipper)

• A = area of hull superstructure
• Cd= coefficient of drag of hull superstructure.

• r = impact moment lever.
• vi = velocity of impact (mostly out of your control, except to the extent that you can let the boat skid sideways)

What resists capsize:

• Ir = roll moment of inertia of the boat
• Ia = inertia of water entrained by the hull (known as added mass)

What is out of your control:

• t = time
• d = density of seawater

If you design from scratch to resist wave-induced capsize, minimise the area and drag to sideways impact of the weather hull, maximise the rotational inertia, and fiddle with the impact moment lever by controlling the direction of the force of impact. The easiest way to do so, without having to deploy devices like a Jordan series drogue to increase the inertia of entrained water, would be a very wide Pacific proa (because rotational inertia increases with the square of the distance of mass from the point of rotation) with an ama that barely floats (small area for its mass) or is even dense enough when flooded to be submerged at rest, to that it is below where breaking crests impact. It can't have any impact drag if the impact passes by above it. Wind drag on the rig and leeward hull would still prevent windward capsize. A lowered Bruce foil or L-shaped foil to windward would increase entrained mass, but resists sideways movement and needs to be assessed for how it changes the impact moment.

If you have a catamaran, choose a hull shape that deflects impacting water up and over the boat, rather than deflecting down and lifting the windward hull (the force of the impact is a vector, and its direction determines the impact moment lever). Install mass that is fixed to the boat in the hulls, not the bridgedeck. Moveable mass goes into the windward hull.

So a catamaran designed to minimise the effects of wave impact might have hulls with very low freeboard for much of their length (to reduce impact drag), rising up at the ends for longitudinal stability. Where you need accommodation in the centre, the outside of each hull would have tumblehome above the waterline to deflect breaking crests up and over the boat. The hulls can flare out on the inside, because the lee hull being lifted by an impact is not a problem. Or have high freeboard and outside tumblehome across the whole length. But keep the hulls wide and shallow so the boat can slip sideways, reducing impact velocity a bit.

Disclaimer: I haven't tested any of this, I am not a naval architect, but my advice is absolutely free and worth every penny you pay for it.

 

Deep hullswidely spaced, short mast, size matters!

 

Drogue

 

Of course, the drogue considers keeping the boat not abeam, but in many storm states; seas are confused and not always with the wind.

In Richard Woods well documented emergency in the Pacific, I believe they had a drogue out (from memory), but the reason they ditched the ship was the confused seas often hit them abeam. Sorry I did not consider mentioning earlier.

 

No boat is immune to capsizing. Good seamanship includes being prepared for the worst.

 

I believe Rory survived a cyclone in a Wharram Tiki 21. Low freeboard, low windage. The boat can be swept in breakers towing a drogue/warps. Watertight hatches. It only takes one good knockdown in a storm to have put me off traditional compaionway sliding hatch and washboards, the amount of water that can come in when submerged was an eye-opener.