The Offshore Voyaging Reference Site

Q&A: Sailboat Stability Contradiction

Question: I recently had an interesting discussion about stability with a cruising yacht owner, and I thought this topic would be of real interest to any high latitude sailor. And I also suspect that you would have traversed this terrain long ago and have an opinion.

After the 1979 Fastnet race, the Joint Committee on Safety from Capsizing made the following recommendation: “The most significant contribution to the resistance to wave-induced capsize would be to increase the roll moment of inertia of yachts”. For a sailing yacht, adding mass at the top of the mast would increase roll inertia more than adding mass anywhere else (hulls are normally designed to support a given keel weight and depth, and adding additional weight to the keel is not recommended).

At the same time, various regulatory agencies and yacht racing bodies have firm guidelines, even rules, to ensure the highest possible Angle of Vanishing Stability (also know as Limit of Positive Stability). For a monohull, removing that same mass that we placed at the top of the mast, would have a more beneficial effect on the LPS than shifting the same amount of mass anywhere else on the boat.

Unless I am overlooking something, there’s contradictory advice here. To prepare a yacht to resist wave-induced capsize, do I favour an increase in roll inertia, or do I favour an increase in LPS? If I favour LPS, I would probably end up doing things that would diminish roll inertia (like keeping weight close to the deck).

Do you favour one approach more than the other, and why?

Answer: First off I should say that I’m no naval architect, so this question is getting pretty close to, or perhaps exceeding, the limits of my competence. Still, I will have a go and also talk a bit about our own thinking on stability as it relates to our boat and other boats we would be willing to go to sea in.

The contradiction you outline is a good example of the dangers of taking one conclusion of a long and complex report and trying to apply it to the incredibly complex and still only partially understood dynamics of sailboat capsize in breaking seas. Yes, it is true, although counter intuitive, that making the mast heavier actually increases a sailboat’s resistance to capsize—this is why dismasted boats are often repeatedly rolled in breaking waves. However, as you point out, increasing mast weight also reduces the boat’s ability to recover from a capsize or knockdown. Like almost everything around boats it’s a compromise. The naval architect designing a boat needs to balance these two conflicting requirements to come up with the safest possible, but still practical, cruising design.

For us, when balancing the two conflicting variables, we would always lean toward a higher limit of vanishing stability and even more importantly, a higher ratio of upright stability to inverted stability (within reason since a boat that is too stable can be horribly uncomfortable at sea with a very quick motion).

Our reasoning is that we believe that whether or not a boat is capsized in a given situation is far more about how she is handled than her intrinsic resistance to capsize. Any boat, no matter her design, will be rolled if caught abeam by, or broaching on, a large enough breaking wave. Conversely, a boat, even a poorly designed one, that is masterfully prepared and handled, with the right gear deployed, will probably come through the same situation upright. (See our Heavy Weather Series for some suggestions on gear and handling in heavy weather.)

However, once the boat is on her side or, worse still, upside down, no amount of skill or preparation will bring her upright; only her own intrinsic tendency to return to an upright position, and the right wave, will save the day.

Over the years, we have substantially lowered Morgan’s Cloud’s center of gravity by removing her teak decks and changing to a carbon fiber mast when our old aluminum one wore out. In addition, we always try to go to sea with our fuel and water tanks full since they are in the keel stub and have a substantial lowering effect on the boat’s center of gravity.

Here are two additional things that should be kept in mind when evaluating a boat’s stability numbers:

First, size has a positive effect on a boat’s resistance to capsize that is far more than linear. I believe I’m right in saying that no boat over 40 feet was rolled in the 79 Fastnet disaster. In other words, smaller boats, particularly those under 35’, need to have very much higher limits of positive stability than larger ones to be safe offshore. When looking for a safe smaller offshore boat, think about something like the Contessa 32, the only boat in her class that finished the 79 Fastnet.

Second, although, as far as I know, there is little real science around to prove it, I’m convinced that too deep a draft can actually make a boat more prone to capsize, not less. This is because such a keel will catch in the water preventing the boat from skidding sideways and thereby dissipating wave energy when hit. Any high performance dinghy sailor (I used to sail 505s) will tell you that the quickest way to wear the boat as a hat is to have the center-board too far down, particularly when reaching. This is probably one of the reasons that many lifting keel boats such as the OVNIs, that don’t have very high stability numbers—although, contrary to some opinions, they are respectable—have still compiled enviable track records of safe offshore voyaging. (Our guest writer Colin Speedie has done a very good post on stability as it applies to his new OVNI.)

In summary, there is a lot more to this than can be summed up in a few numbers, and no boat is safe if she does not have the right storm survival gear and a crew that know how to use it.

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