Back to boat stability:
Ten years ago I had already an interest in boat design and particularly in boat stability and seaworthiness. I had read already a lot about stability and had left behind the nonsense of the capsize ratio and had a good understanding of the static stability. Had already seen a lot of GZ curves (arm length curve) and understood the importance of weight on the RM curve ( you obtain a RM curve multiplying each point of the GZ curve by the boat displacement).
The area behind the positive part of the GZ curve represents the energy needed to capsize a boat and because you obtain the RM curve multiplying the GZ curve by the boat displacement, weight is an important factor in static stability.
I was trying to find some answers about that here, a lot of years ago:
How heavy is too heavy II ?
At that time I was asking myself what the right weight should an offshore boat have and made a lot of comparisons with the RM stability curve of the boat I had with stability curves of boats that I would like to have (bigger boats) to establish a minimum AVS and a minimum RM area, regarding what I wanted for my next boat.
And then my attention was focused in two facts:
1 - The OVNI 43 (and 435) had a lousy static stability curve with a relatively low AVS but it was justly considered as one of the best offshore and voyage boat, a seaworthy boat that sailed everywhere without any problem.
That was the boat that Jimmy Cornell had chosen for himself after several circumnavigations in heavy boats, and a boat that deserved from him the biggest praises in what regards seaworthiness: A light aluminum centerboarder, the type of boat he still consider as ideal for offshore voyaging and circumnavigation.
2 - Add the minis, an incredible small and light boat (26ft and about 1T) obviously with a small RM curve (big GZ curve but small RM curve because the boat is very light), a boat that needs a small amount of energy to be capsized and that had crossed the Atlantic racing , year after year, sometimes with bad weather, in huge number (each race has about 70 boats), without any significant problem. One of them has even circumnavigated non-stop.
So, there is something wrong with basing the stability and the seaworthiness in what regards stability only on the Static stability.
Obviously the Dynamic stability counts for much in the capacity a boat has to resist capsizing and contrary to a very popular opinion I don’t think it has nothing to do with the roll moment of inertia, or the boat mass. If it had, the Miniclass racers or the light OVNI (with little inertia) would be boats that would be easily capsized.
It has all to do with the way a boat dissipates the energy of a breaking wave:
if the boat transform all that energy in a rolling movement (tripping on the keel) the chances are that the boat will capsize. If the boat can dissipate the energy of the wave in a kinetic movement (sliding laterally) the chances are that the boat can resist capsizing.
A heavy boat needs objectively much more energy to be capsized but if most of the wave energy is transformed in a rolling movement tank testing shows that it does not matter much. Any breaking wave over 1/3 of the LOA of the boat can capsize it (no matter the weight) if the boat does not transform the wave energy in kinetic movement.
The energy of a breaking wave is so big that to have a bit more or a bit less RM does not matter much.
So, in what regards dynamic stability what counts most is all that facilitates the dissipation of the wave energy trough kinetic movement (instead of a rolling movement):
Low mass, small underwater appendices (keel and rudder), low freeboard, beam and of course a proportionally high RM (that will not be big in absolute value because the boat has a low mass).
As you probably have already noticed this description suits well in a Mini class racer, or a 40class racer or in an Open 60, that are boats whose hull shape has been developed for transatlantic and circumnavigation solo racing, where ease of use and stability are paramount.
And because NAs are not stupid this tendency is also a marked tendency on modern cruising boats, I mean small appendices, low mass and beam even if low freeboard is hard to get giving the needs of interior space.
Dynamic stability is very important but that does not mean that Static stability is meaningless. I would say that there are still some very important points to consider:
The RM force the boat is making to right itself up at 90º is probably the most important, others are the AVS point, the downflooding point and the proportion between the positive and negative part of the curve.
The Max RM is also important but that is not an absolute value and it is proportional to the weight of the boat and the sail area it needs to sail. I would say that a GZ max value gives a better picture here, becuase it is a comparable value with boats with diferent mass, if the LOA is not very different.
However Static stability it is not definitively the only thing to look at when considering the implications of stability on boat seaworthiness even if it is a lot more meaningful than the old capsize ratio.
These are basically my thoughts about boat stability. I am sure that I know more than some years back, but probably I will know more in the future, so please don't consider this as a finnished subject
.
It is not closed for me and even less for the ones that know more than me and that design sailingboats. Regarding those, you can bet they will use what they know for building better and faster boats, so you have just to look at what they are doing, especially in what concerns offshore racing where stability is not only fundamental to safety but also to carry more sail and to have faster boats.
With time that knowledge will be used to make better, faster and safer cruising sailboats.
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I like both a lot.
