I know you like your Vanguard and are used to how she behaves. Any of us, if we own a boat for a while and sail her enough, become comfortable with their boat's iteosyncracies. Its just human nature. But based on my experience with both of these boats, the case of the Vanguard vs Tartan 30 comparason is a classic example of why these surrogate formulas ( Capsize Screen Formula and the Motion Comfort Index) are inadequate in providing useful information about the relative behavior of boats of very different designs.
In this case, the Tartan 30 has a longer waterline, (especially important when considered relative to length) which is important for pitching motion comfort and tracking ( also helpful with roll motion but less so than pitch). The Tartan has a higher ballast ratio carried lower in a deeper keel, and offers better hull modeling from a roll comfort, stability, and dampening standpoint, and yet the formulas would seem to suggest that the Vanguard would be more stable and comfortable. Sailing both, I can assure you that the formulas are wrong on this. If you spend time comparing the Vanguard you must admit that the Vanguard rolls through wider angles than most boats of that era, (and later eras as well) and that the same hull shape that causes the Vanguard to stiffen up at 15-20 degrees of heel also results in a bit of a jaring motion at the edges of the roll.
But to explain why generically I say that the Capsize Screen Formula and the Motion Comfort Index are useless....
First of all both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas had limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which are the major factors that actually control motion comfort or the likelihood of capsize.
I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.
And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, a very heavy rig, heavy deck hardware,and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a short length of the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies