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post #14 of Old 01-25-2010
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Two quick points here, the article on the Pearson Vanguard by Tom Dove was based on his single sail on a Vanguard in 8 to 12 knots of wind on the Chesapeake.

Obviously, Tom never owned a Vanguard and, never tried to deal with one in winds over 20 knots, or sailed one in a short chop, or on a delivery in the Atlantic, or sailed one of these in light conditions especially in a left over ocean swell. I have literally hundreds of hours on Vanguards, boats with new sails and great crews and boats that were beat to death. If the person writing the American Beauty review had actually spent some time with these boats, his review would not talk so positively about the seaworthiness or motion comfort of these boats. If he sailed one and then the next day sailed a Coaster in tough going, his review would not be all that glowing.

I also want to make a critical point about the inaccuracy of the quote that says,
"Considering these Pearson boats have a DL ratio over 350 and capsize value around 1.7 (compared to 1.94 on a Tartan 30 or 1.81 on venerable Bristol 30, 1.76 on Bristol 29) there is hardly any boats that can match them for off shore!

Well, numbers are numbers and math is math.

according to the calculator below bristol 29 has a motion comfort of 28 vs. 34 of Vanguard..Alberg 35 pretty much did set the standard on these numbers back in the day and is just an amazing boat!"

I would like to point out that neither the capsize screen formula or the motion comfort index tells you a thing about either the likelihood of a capsize or the motion comfort of the boat in question. I have included an explanation of why I say this below.

But also, even L/D is misleading in the case of these older CCA era boats because they had such a short waterline length relative to their overall length. If you calculate the L/D the normal way, it is true that the 32'4" Vanguard (with a 22'-10 LWL 1'-5" shorter than the Tartan 30's 24'-3" lwl) appears to have a L/d of 431 vs the Tartan 30 at 274.

But if you compare these boats by D/length on deck, (which highlights the shortcoming of using LWL in a comparason between a normal LOA/LWL and the extreme short waterline lengths of the CCA era) the Vanguard comes only slightly heavier at 154 vs 146. But the real significance to the behavior of the boat is in how that weight was distributed. In the case of the Tartan 30 the vertical center of gravity was much lower than the Vanguard due to its deeper keel and better ballast ratio.

During the period that we owned the Vanguard, ours was scary it was so tender. We spoke to Phil Rhodes about the problem. Mr. Rhodes acknowledged the problem and explained that the boats were supposed to have 400 lbs of 'trim' ballast but that they as built they exceeded the estimated weight so that they were already sitting on their lines but where under ballasted. We added quite a bit of additional ballast which helped some but made the boat even more of a roller than she had been.

Its seems that as soon as someone posts a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. And no sooner than poster questions the seaworthiness of some boat, that someone cites the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.
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 have 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 really are the major factors that 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 1000 pound weight at the top of the mast. (Yes, I know that no one would install a 1000 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, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, 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 single point in 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.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

There is a convenient calculator at>>

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.

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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay

Last edited by Jeff_H; 01-26-2010 at 08:13 AM.
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