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post #2514 of Old 04-10-2012 Thread Starter
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Re: Interesting Sailboats

Very interesting photo of the boat that won the last Transat for 40class racing boats, the boat that G1000 posted while I was doing this post.

Looking at this photo we can see clearly how this hull "works" and the advantages of this type of transom/hull design:

Marked by the water, at black, we can see the more usual hull sailing position that is not very different in shape to the one of a more classical boat, offering little drag.

When the boat goes to the heeling max position (like it is now) it is offered to the water a completely flat horizontal surface that provides a huge resistance to more heel, adding a lot to RM. On a classical boat at this heeling angle the surface offered is a curved one that will not offer any considerable resistance to heeling.

On bigger hell angles we are not sailing anymore, at least in a effective way, but trying to bring the boat up and on these big heeling angles form stability counts not for much. A class40 or a more classical boat will work the same way and it will count on ballast and draft (low CG) to right the boat up.

Some has a idea that a open boat has a huge form stability but a bad reserve stability or a big inverted stability. That is not true. The boat has a huge form stability a very good reserve stability and the proportion between positive stability and the inverted stability is very good by any standards.

Here the stability curve of a Pogo class40: Big AVS, big relation between the positive part of the curve (positive stability) and the negative part (inverted stability) and an overall massive static stability for a 40ft.

Compare that with the typical stability curve of an half-tonner, a popular old offshore racing boat that many consider very seaworthy:

It is also a GZ curve but in Ft while the one from the Pogo is in m. To roughly convert divide the values of the Half-tonner curve by 3.

Both boats have a not very different displacement (3.500kg to about 4000/4500g for a 40class boat).

To join to that very good Pogo static stability the boat has an even better dynamic stability related with its low mass, beam and small underwater area that makes him able to dissipate the energy of a breaking wave with kinetic movement (lateral and rotating on a vertical axis) other than a rotational movement.

These two characteristics make this light boat unusually seaworthy and capable of racing on the sea is really mean, in high latitudes.

Especially for Eric I will post the stability curve of his new boat, the cruising version of a Pogo Class40, the Pogo 12.50:

First an advertency, the Pogo Class40 curve is a GZ curve (length of arm) and this one is a RM curve. To pass from this one to the first one you have to divide each value by the light weight of the Pogo 12.50.

As you can see both curves are not very different and your boat has also a good AVS but most of all a massif righting moment at 90 of heel and a very good relation between the positive stability and the negative stability.

With this information you learn that with the keel up the boat has an AVS of about 100 and you have plenty of positive stability. This means that in settle weather and light winds you can perfectly sail the boat safely in shallow waters. In fact the positive stability of the boat with the keel up is bigger than some of the boats that have capsized on the 1979 Fastnet.



Last edited by PCP; 04-10-2012 at 12:22 PM.
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