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post #123 of Old 04-24-2013
Brent Swain
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Re: Bob Perry's take on Wolfenzee's dream boat

Originally Posted by PCP View Post
Brent, I don't pretend to know that much but some of those boats are designed by some of the best NA around. They utilize CFD to study the hulls keels and rudders and I am sure that they are like that because it is the way they work better.

Regarding the canting of the keel what is the point to have a 25 angle on a boat that sails with 15/17 of heel? it seems to me that the better efficiency would be obtained with a vertical foil in what regards sailing heel.

Regarding the keels having a small attachment you are probably imagining that each one is fixed to the hull. In the RM case (and in many other boats with skinny keels) they are both connected to the same steel structure inside the boat that distributes the efforts by the hull.

Regarding beamy hulls reducing ultimate stability I don't know what you mean. That depends basically to how lower is the CG and normally those boats have considerable drafts a god B/D and bulbed keels. That gives them a good AVS and a very good reserve stability at 90.

Yes, when inverted they have a big inverted stability but again how easily they return to its feet depends on the CG. It is convenient not to forgot that to the big inverted stability corresponds a much bigger positive stability, much bigger than in a narrow boat. In the end, besides a good AVS and a good reserve stability what is important is the proportion between the positive and inverted stability (a much bigger positive stability).

Regarding that thing about the anchor being wrapped about the keel, those boats are popular in France, there are hundreds of them and I never heard complaints about that neither anybody referring that as a problem with that kind of keels.


Make yourself a model, a flat piece of wood with keels like these will do. Then tie a line to the bow take it between the keels, then to one side. Picture a boat in this position in a strong beam wind, and see if laying broadside to the wind and swell, with the line under the full load of the wind and sea abeam, will allow you to get the rode off the keel. I have no doubt that tank tests show this type of keel to sail well, but tank tests dont ask the question of how you get a fouled anchor rode off one when you are abeam to a strong wind and sea on a dark night. Numbers and math dont answer that question, and a lot more practical questions about actually using the boat.
My first boat was a Pipe Dream designed by Francis Kinny and tank tested inthe Davidson lab in New York, the tank where all the US Americas cup boats were tested. She had a short keel with rudder attached. With zero sailing expereince, I looked at the rudder and thought " Maybe a rudder on a skeg six feet further aft would be better." Then I thought " Gee, I have no expereince so I beter do what the designer drew." Off the wind the boat was almost uncontrolable in strong winds,an absoluite abortion of a design screwup, so when I got to New Zealand, I replaced the rudder with a separate rudder six feet further aft on a skeg, just as I had originally thought, with zero experience.. The improvement was huge. That completely blew my confidence in the infalibility of world reknown "Experts."
Einstien said "Wisdom doesn't come from studying , Wisdom comes form showing up for life."
Very few boats sail in strong winds at less than 15 degrees of heel.
Angus Primrose, while crossing the Atlantic in one of his very beamy designs was capsized, and was dismayed at how long she took to right herself. He later did some calculations and found that even a slight reduction in beam made a huge improvement in ultimate stability.
Again, boats staying capsized was never a problem, before excessive beam became the norm.
A beach ball with a high CG and a tiny ballast ratio has a very high ultimate stability. A wide raft with a very high ballast ratio and low CG will have a very poor ultimate stabilty. Thousands of pounds of buoyancy in a wheelhouse will be the equivalent of adding far more weight in the keel, in the inverted position, when it comes to ultimate stability. The more the midships section resembles a beach ball (Trunk cabin with high camber) the higher it's ultimate stability. The more it resembles a raft, the lower it's ultimate stability. The calculations they use dont take this into account, making them dead wrong.
Any accountant will tell you that, when adding up numbers ,get one wrong, and everything you do from that point on is wrong. Miss a huge factor like buoyancy in deck shape and structures, and everything you calculate from that point on is wrong.
Skinny keels fold over, regardless of what you put inside. Perhaps these boats are designed to the standard plastic boat standards, where they are simply expected to break up if they hit anything.

Last edited by Brent Swain; 04-24-2013 at 08:15 PM.
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