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post #10 of Old 03-09-2005 Thread Starter
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How heavy is too heavy II ?


I am discussing here only the importance of weight in ocean going boats.

On boats used only in coastal navigation I agree with you, for the same strength and stiffness, for me, the lighter the better and new materials and new technologies can make a light boat as strong as a heavy one, except perhaps in what regards collision. A coastal boat doesn''t need the same degree of safety stability, because if the weather goes really bad, you should not be at sea, and have plenty of time to make it to a port.

Of course, I don''t get seasick and I like the fun of going fast more than I care for comfort, otherwise I would disagree, because modern, light and fast boats rely a lot on form stability and have comparatively small ballasts (in many cases less than 30%) and heavier boats (ocean going ones) have a stability more dependent on ballast having a bigger %, some times approaching the 50%.

That means that the cruiser-racers tend to follow the movement of the wave (perpendicular to the wave hall) and the heavier ballasted cruisers tend to remain horizontal, regarding the wave hall. The implications of this are clear in motion comfort even if they don''t degrade (quite the opposite) the speed of the boat.

About the GZ (rightening arm) what you say is that a cruiser-racer has a better maximum GZ (and I agree, but also a bigger negative maximum GZ) and that a light cruiser racer, like the Swan 45, will have the same area (positive) or bigger, under the rightening moment curve. That area is the measure of the total amount of energy needed to capsize the boat; in other words, that it will require the same force to capsize both boats.

I think you are confusing things.

Letís examine the stability data of the two boats and have a look at those curves:

Well, I agree that the area under the GZ (rightening arm) curve it is about the same for the two boats, marginally better for the Swan 45.
This is not normal, it only happens because the S45 has a very exceptional 150ļ AVS , when a typical cruiser-racer normally will have an AVS between 115 and 125. I have to say that the GZ curve of the S46 is also exceptional (and that almost makes things even), having an unusually good max. rightening arm (for a pure cruising boat) of 0.85 (S45 has 0.95).

what normally happens is that the extra area that is gained in the higher and more vertical initial part of the GZ curve of a cruiser racer is lost in the descending part of the GZ curve, that is more vertical in the first case and softer (going to a higher AVS) for an ocean cruiser, thus obtaining more area that way. In the end, for well designed boats, the area under the GZ tends to be similar.

Of course, as the areas under the Rightening Arm curve are similar, the areas under the rightening moment curve (that you obtain multiplying in each point the value of GZ by the meters of the water length and then by the displacement of the boat) are very different and a lot bigger on the Swan 46, because the boat has a much bigger displacement.

I could give you exact numbers but I just don''t have the time right now (I would have to import those curves to my computer), but, taking into account the marginally bigger GZ righting arm of the S45 I would say with reasonable accuracy that the positive area under the rightening moment curve of the Swan 46 is about 53% bigger than the positive area of the Swan 45.

That means that the energy necessary to capsize the Swan 46 is more than 50% bigger than the energy necessary to capsize the Swan 45.

If , in this case it is not very important, according to my personal safety standards (the energy necessary to capsize the S45 is already a lot) , the same reasoning applies to smaller boats, and there, the importance of mass as means of generating more stability, has a lot of importance. The boat will be smaller (as the force necessary to capsize it) but the waves and the sea will be the same.

I believe that Swans are some of the best boats around and not only in luxury, but in design. Those two, as Silmaril has pointed out, are just two excellent boats for different kinds of sailing.

And even if I would believe that the S46 has a much more comfortable sea motion (I do not have any doubt that the Swan designers know exactly what they are doing), I would prefer the S45. But thatís my personal taste (very fast, sporty and safe) , and that is right now, ask me 10 years from now and it is possible that I will prefer the S46 (reasonably fast, super comfortable and super safe).

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