The stability calculation performed by packages such as Hullform, which compares the heeled and upright centers of buoyancy, is correct but insufficient to produce a truly balanced hull. I learned this to my dismay when I designed and built a free sailing boat balanced according to this criteria alone. It was a disaster.
Digger deeper into the topic of balance I inevitably encountered the metacentric shelf notions of Admiral Turner. His methods are the subject of some controversy in Naval Architecture, not the least because he developed them first as a model yachtsman and only later applied them to full-sized boats. Perhaps the most even-handed treatment is by CA Marchaj in his book “Seaworthiness:”
"The main criticism directed against Turner’s concept of the metacentric shelf as the sole criterion of balance is that for convenience it involves only hydrostatics …
“Nevertheless, to quote K.C. Barnaby’s comment expressed during a discussion of Turner’s paper, ‘we may doubt the accuracy of some of his reasoning, but the fact remains that boats balanced on his metacentric shelf principle to turn out to be uncannily steady in their course.’ In practice, Turner’s theory has proved remarkably reliable in predicting yacht behaviour from lines plan, so the error introduced by [hydrodynamics] cannot be serious.”
(As an aside, sorting out the relation between the the static and dynamic effects would make a decent thesis topic for an aspiring graduate student.)
The description that follows is not, strictly speaking, what Turner called the “metacentric shelf”, but it is the more interesting of his ideas.
Looking at diagram 1, for a given section U-U’ is the upright waterline, H-H’ is the heeled waterline, A and B are the immersed areas and M-M’ is the heeled metacenter. This line marks the point at which the upward force of the water is concentrated. The amount of that force is equal to the combined areas A+B.
The location of M-M’ is found by fiddling around with pieces of paper as shown in diagrams 2 and 3; the line M-M’ is where the folded section shape balances on a knife edge. The degree of heel being analyzed is usually chosen to be “rail down.”
Once we have located M-M’ for a particular section we can measure the offset, or “arm” that the upward force of buoyancy exerts away from the center line of the boat, as shown in diagram 1. Multiplying this by the area A+B (measured by counting grid squares, or with a planimeter) gives a “moment,” or force exerted at a distance.
We can then plot the moments along the centerline of a the boat by doing the (tedious) job of finding the arm and area at each section, multiplying them, and plotting the result on a horizontal line. If M-M’ is to the right of the center line we plot it above and if it is to the left we plot it below. Connecting the resulting points gives us curves such as in diagram 4 and 5.
Diagram 4 is a curve for a badly unbalanced hull, in which, upon heeling, the moments of the forebody move to one side of the center line and the moments of the afterbody move to the other. The result is a yaw, or at worse a yaw and a dive at the same time. The particular curve shown is that for the big Victorian cutter “Satanita,” which was so hard to handle that in one race she ran out of control and rammed and sank a competitor. Note that since the two areas are of about the same size, this hull would be “balanced” under the looser criteria of CB movement under heel. A similar curve is exhibited by L. Francis Herreshoff’s J boat “Whirlwind,” which was notorious for its poor handling characteristics.
Diagram 5 shows the curve for a balanced hull. The moments for the forebody and afterbody are on the same side and the moments for the center of the hull are on the other. The ideal situation is when area A in the diagram equals area B, and area C equals A plus B.
It is not necessary to go through the whole hoopla to see if a design has a chance of being balanced. Simply locating M-M’ for two sections, one at about 25% LOA and one at about 75%, will give you a first-cut result. If both M-M’ lines are on the same side of the center line you’ve got a chance. If one is on one side and one on the other you’re in trouble. From then on it’s trial and error and a wastebasket full of paper scraps. It sure would be nice if somebody would provide a program that would calculate the moment of a heeled section from a table of offsets
Since Turner’s ideas were championed by cruising yacht designers such as A.A. Symonds and Harrison Butler, there is a general notion that Turner’s system inevitably produces tubby little double-enders. This is emphatically not the case. Diagram 5, in fact, is the curve for the schooner “America,” long lines, hollow entry and all. “America”'s balance was the result of trial and error in the intensely competitive environment of the New York pilot schooners. These boats raced each other for jobs through a crowded harbor, and were often sailed short-handed. They had to be both fast and handy.
In the model yachting domain, Ted Houk’s magnificent M Class design “Rip Tide”
is perfectly balanced by the criteria given above. This was intentional, as I verified by corresponding with Houk’s son. “Rip Tide” was designed in 1949, the design made the transition from free sailing to radio, and versions were still winning races and championships as late as the middle 1970’s.
It is worth noting that a balanced curve of moments is independent of both scale and section spacing; it’s the form, not the values, of the curve that counts. This means that you can take a balanced hull that was designed to one rating rule and adapt it to another without losing its balanced characteristics. I did this for the boat in the photograph, which is a “Rip Tide” modified to fit the 36 inch Restricted rules. She tracks through gusts like she was on rails.
Similar adaptations could be made to convert some of the classic (and classically well-behaved) M Class free-sailing designs to the IOM rules. Of course, to do so and claim the design as original would be the height of bad form. It would still be interesting to see how such boats would do against some of the “wedge” designs popular lately, especially in difficult venues.