Originally Posted by PCP
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 90º 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 100º 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.
I said this already before: there’s very little to look forward to at the end of a nice sailing holiday, except catching up with this excellent thread. Thanks again guys, for a few hours of wonderful video’s and very interesting opnions.
Paulo, if had known you were in Combrit two weeks ago (Pogo 50 pictures), I would certainly have picked you up to welcome you aboard our TriMen. then afloat in Ste-Marine to work out the last details. I very much regret we missed this opportunity to meet you in person and may be even have a spin with the boat together…
Last week we sailed the boat over to Nieuwpoort, which was a cold but nice and very valuable experience. I will be happy to discuss this in more detail later, but the bottom line is: the boat dislikes pointing or sailing dead downwind, keeping up the speed is the issue and then the VMG is always very correct. It is quite a different way of sailing compared to more traditional designs.
I mentioned before the statement of an experienced class 40 sailor: it’s just like a big 470 dinghy. I’ve been sailing a 470 for almost 30 years and could not agree more. “Sail the boat under the mast” and first try to build up the apparent wind. Then you get exhilarating sailing everywhere between a close and a broad reach.
Thanks a lot again for your thorough stability analysis of these “open 40” type of designs. Our first experience shows you are once again right on top.
The initial (form) stability is as spectacular as the 4m50 wide (and honestly quite disgraceful) beam. Even with myself and my two basketball centre players of sons on the same side, the boat hardly moves.
Under sail, more than 20° of heel only slows the boat down. But before you get there, you have already enjoyed the enormous power of both the hull (form stability) and the 3m deep, leaded keel (weight stability).
Between l’Aber Wrach (North Brittany) and Cowes we kept all the sail (full main + solent) up in 25 knts on a broad reach. With nice, long, 3m high waves and gusts up to 35 knts the average speed was around 14 knts with some wonderful long and thrilling surfs up to 21 knts, without ever feeling out of control.
So our first experience after 450 NM with the 12.50 is: WYSIWYG.
A big 470 with visually basic, but functional and in fact quite comfortable accommodation for our crew of 6.
It is very reassuring to know that this is also a very safe design, thanks a lot Paulo!
I wouldn’t really dare to sail it with the keel up, not because of stability but because this makes the boat behave like a lightweight long-keeler. This is indeed a very unnatural and inefficient configuration, as we experienced when manoeuvring in windy conditions. Long live the bow thruster!