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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Meet the Author

John

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

7 comments… add one
  • Pete Gallienne Sep 30, 2010, 12:34 pm

    A little point that alot of people don’t realise is the big wave syndrome. I have heard many sailors say that they fear very big waves when in fact the smaller ones can be more dangerous. A friend from the Wolfson Institute said that the most dangerous wave to any boat is the wave that is equal in height to the beam of the boat this being I suppose because it can set up a motion that gets the boat rolling until she goes completely over. Just a thought.

    • John Oct 1, 2010, 6:46 am

      Hi Pete,

      That’s a really good point, thank you. I too have heard of the Wolfson study. Do you you know if the results are available on-line anywhere?

  • Pete Gallienne Oct 1, 2010, 7:51 am

    Hi John I am sorry I dont know if they have a web site. I would think that you would have to pay for any information from them as they are a commercial enterprise. Regards Pete

  • John Rushworth Mar 3, 2013, 4:42 am

    John. You ask about the Wolfson study. I think the one you refer to can be found here http://www.wumtia.soton.ac.uk/about-us/published-papers Deakin, B., “Methods of Assessing the Safety of Cruising Yachts in Terms of their Stability”, The Seaworthy Cruising Yacht Conference, RINA, November, 1991 (PDF, 1.09MB) It is a PDF http://www.wumtia.soton.ac.uk/sites/default/files/SCYC1991BD.pdf

    I recently had cause to locate and read this as it relates to the STOPS numeral. A stability rating the RYA use. You won’t find the formula on the web as the surveyors keep it for themselves to calculate ratings. A boat like mine (Victoria 800/Frances 26) didn’t have a full set of stability calculations/curve, so when coding a yacht for commercial use under the MCAs MGN280 they have to have a way of assessing stability. Hence the STOPS numeral. Mine came back with a figure of STOPS 1, 2 & 3 of 34. 2 is up to 60 miles offshore which is the safety standard I am coding to. The RYA stability data for yachts with STOPS calculations is here http://www.rya.org.uk/newsevents/news/Pages/stabilitylist.aspx

    I looked through that and found that my 26 footer had as good, if not better than, STOPS data than more modern designs, like a Bavaria 35. I put this down to modern beamy yachts having high form stability compared to my narrower, 50% ballasted boat. which brings me to the thrust of your post. My Chuck Paine designed Victoria seems tender to start with and stiffens up rapidly and this I believe relates to the roll moment of inertia, so to my mind this would spill wind rapidly, then stiffen whilst still having a high value limit of positive stability. The Deakin paper and research came out of the Fastnet disaster. Information on STOPS is at Page 9, 6.5 in the document.

    • John Mar 3, 2013, 2:59 pm

      Hi John,

      Thanks very much for the link. I shall put some time aside to read it through.

  • Paul Mar 13, 2017, 6:46 pm

    Ever regretted the carbon mast’s effect on Morgans Cloud’s Angular Acceleration?

    Your link to Marsh Marine’s “Dynamic Stability of a Monohull in a Beam Sea”* was fascinating (once I got over the maths.) Especially the bit about going weightless over the top of a wave and down the other side. Apparently, as the boat is falling, the righting moment of the ballast vanishes due to it becoming effectively weightless. The heeling moment of the sails remains in force and the yacht needs some other means of resisting it if it is not to capsize before reaching the bottom of the wave when the ballast becomes weighty again. Marsh suggests a heavy rig helps by slowing the rate of heal (angular acceleration). I appreciate this is all a matter of compromise and optimisation, and ideally one would be going over lengthwise rather then beam-on, but I’d be interested to know if you’ve ever run “over the top” in survival conditions since installing the carbon mast and what your thoughts were during the decent.

    *http://marine.marsh-design.com/content/dynamic-stability-monohull-beam-sea

    • John Mar 14, 2017, 8:44 am

      Hi Paul,

      No, we have not been in survival conditions since installing the Carbon mast, but given our boat’s size—capsize resistance goes up exponentially with size—and general seaworthiness, I would not worry about the slight change to dynamic stability of said mast. Also, the carbon mast means the boat will right more quickly from a knock down, so, as you say, there are trade offs.

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