In response to Paulo''s comments:
With regards to Paulo''s opening comments:
I did not say that I thought that it was a bad thing that this thread picked up a whole range of other topics, just that it had gone far beyond the original topic which was the suitability of a J-40 for distance cruising. It is the serendipitous wanderings that often make life more interesting and educational.
Paulo said:
"This implies that it is as easy to capsize a Bavaria 40 as to capsize the BMW Oracle (America cup)….Both boats have the same beam (4m), or the famous Star and Stripes, versus the Bavaria 36 ( both 3,6m).
Of course this is ridiculous, the Bavaria 40 weighs 8,3T and has a ballast of 2.9T with a draught of 1,65 and the Oracle weights 24 T has a ballast of 20 T and a draught over 4m, and if you look at the dimension of those sails you know that the Oracle has to have a massive righting moment."
My statement does not imply that it is as easy to capsize a Bavaria 40 as it is to capsize an America''s Cup Class or a 12 meter. My comment would imply that assuming a Bavaria 40 has the same beam as an America''s Cup Class and a 12 meter, these three are equally as likely to be rolled over by a wave that is large enough to roll any one of the three. It needs to be understood that a roll over is different from a capsize and my comments were in response to a comment on roll-overs. For fully ballasted boats, only wave action can cause a roll over, and the only significantly consistent factor in determining the likelihood of a roll over is the height of the wave relative to the beam of the boat. There are some secondary factors that come into play such as the depth of the keel which means that the America''s Cup Class and the 12 Meter are more slightly likely to be rolled over than the Bavaria due to their deeper keels.
Factors like the LPS, area under the righting moment curve (with the boat inverted and its gear in its actual position as inverted), downflooding, and the like are significant in determing whether the boat is likely to re-right and survive. There major differences in the design of these three boats which would result in the likelihood of differing re-righting and survival results.
I do agree with Dave Gerr that when it comes to resistance to wind driven knock downs and the speed of re-righting, that the area under righting moment is critical. I would strongly disagree that "Displacement is responsible in equal terms with GZ in originating the force that prevents capsizing." While righting moment results from a calculation of the displacement times the righting lever arm far, in a practical sense, far and away, weight distribution relative to buoyancy distribution, is much more critical in determining the likelihood of a knocked down than a boat''s actual displacement.
I can explain this by using the example that I generally cite to debunk the ''Capsize Screening Formula'', if you visualize two identical boats, except that one has a 1000 lbs of weight at the top of its mast, the boat with the 1000 lbs of weight at the top of its mast has a greater displacement but it is significantly more prone to being knocked down and far less likely to re-right because of its weight distribution.
While I cannot imagine that anyone would intentionally place 1000 lbs at the the top of their masts, if you look at the sources of the added displacement typically found in heavier weight cruisers, the added weight is typically found in locations that do not help stability such as heavier interior appointments, heavier deck and topside structures, heavier spars and
rigging, heavier mast mounted electronic components, rigid cockpit shelters, higher davit mounted- heavier weight dingies and motors, and the like. Cumulatively these can add a lot of weight while at the same time reducing stability greatly.
The reality of this was borne out in the STIX research project where heavier displacement cruisers were often found to gave greater drag relative to their stability. This meant that these heavier displacement cruisers ended up needing to carry proportionately larger sail area relative to their stability, which in turn made them more prone(rather than less) prone to a knock down.
Paulo said,
“The area under the positive part of a righting moment curve is a measure of the total energy required to roll a boat, while the area under the negative part of the curve is a measure of how much energy it will take to roll it back again.”
Here again, heavier displacement works against re-righting. With its greater moment of interia, and the fact that a heavier displacement boat is likely to float deeper in the water relative to its center of gravity requiring much greater energy to re-right, the heavier displacement boat is likely to require a longer period of time and more energy to re-right.
Paulo Said;
"In normal circumstances if, to capsize a boat, it is necessary the lateral force of a wave two times bigger than one capable of capsizing a lighter boat, then the first boat is (broadly speaking) a safer boat. If, to capsized a boat, it is necessary a wave three times bigger than the one needed to capsize the smaller boat, then this boat is safer than the one that is capsized by the wave two times bigger than the one that is capable of capsizing the lighter boat…... and so on."
The reality of wave driven knock-downs and roll-overs is that the forces are enormous, many times the comparatively small differences in displacement between one boat and another. You are correct in assuming that it is possible for one boat to have several times the righting moment of another (although again this is more a product of weight distribution and buoyancy distribution than actual overall weight of the two boats. For example, comparing a specific lighter weight boat with equal but deeper placed ballast than on heavier boat, the lighter boat could easily develop much greater righting moment and require a lot less force to re-right). But in studies of wave driven capsizes and roll-overs righting moment plays a very small roll as the over turning moments grossly exceed the righting moments involved as the weight of water and force of gravity acting on the boat are just so enormous. Using the current coefficients for wave impact, the impact loads of a breaking wave on a typical 40 footer is hundreds of thousands of pounds, easily overwhelming the comparatively minor 12,000 lb difference in weight between a very light and a very heavy 40 footer.
So, whether they are called fears or security margins, they represent pretty much the same thing; the choice of a boat depends on how much of a risk you are willing to take. But no matter what your fears or security margins, in an of itself, weight does nothing good for a boat. It does not make it more seaworthy, sturdy, comfortable, easier to handle, or stable, just heavier.
Respectfully,
Jeff
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