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post #3169 of Old 01-13-2014
Brent Swain
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Re: Pros and cons of steel sailboats

Originally Posted by MikeJohns View Post
They have absolutely nothing similar to a boat hull. Not the shape nor the loads. Forget drink cans and gas bottles we are talking boats here.

Brent you have been told before why pressure vessels are invalid comparisons to boat hulls. Engineering isn't a belief system if you come up with an idea you have to show it's valid. Just saying something repeatedly doesn't make it a fact.

Pressure vessels are a completely different scenario in scale, shape, relative plate thickness and in the level of restraint of the ends. They are also carefully designed for their purpose that includes the end cap shapes. End cap shapes resist buckling but they bear no resemblance to the shallow curves of a boats lines.

In a metal boat hull a huge amount of reserve strength is thrown away if the plate cannot ultimately form a membrane under large strain. If you have tension of one side and compression on the other there is a large amount of material contributing very little to resist collapse. But pre-compressed tension members make this even worse and that's why your designs are so weak for the material they use and why a small amount of additional material added sensibly to your design adds a large amount of strength.

This is a long way from the marketing hype claiming extra strength with less material. Just a classic case of the emperor’s clothes.

Here's Bernard Moitessier's steel sailboat "Joshua". But Joshua was extensively transverse framed so Brent was misinformed again.

The damage at the turn of the bilge is from the plate stretching between the close transverse frames as the hull impacted on a rocky shore. You can see that where blows hit the transverses no damage occurred, where the blows struck the plate ( repeatedly) the plate has stretched between the transverses absorbing the impact energy, the transverses also limit the extent of the damage.

Stretching is significant because to stretch the plate past yield it must be in tension. You cannot develop tension without being able to develop a membrane stress by restraining the plate with supporting structure. Buckling collapse uses significantly less impact energy.

You can see how close the transverse frames are. Gringo mentioned before was also extensively transverse framed with a similar transverse frame spacing as Joshua.

Steel is immensely tough but you have to design with it properly to get it's full strength. If you want to get full tensile strength of the plate in can only develop that in TENSION !
Have you ever been to Cabo? I have, and have walked on the beach where Joshua went ashore. Not a rock in sight, only sand . Again you distort the facts to suit your argument, just as your claim that Winston's boat had extra framing and plate thickness, compared to what I had designed. That was another fabrication to support your argument. She had exactly the same structural and plate thickness I designed in her . She came home with zero dents . Had she transverse framing ,she would have been a mess of dents. Without framing to stretch against ,she simply springs back.
I met a guy named Welch, in Tonga who built a Spray in Victoria. In Hawaii she had blown aground and became severely dented between frames. The combination of framing and lighter plate was the main cause of the sever denting between frames . Your drum skin theory would require the plate to be fully welded to the frames and stringers. This I show military ships are done and you can se every frame and stringer in them from a quarter mile away, the distortion is that bad,.
So just tell the sea it is a pressure vessel, and the steel will decide to behave differently, and the sea will treat it more kindly?
Ya ,sure Mike!
Your problem in comprehending the principle, is you have a blind spot when it comes to the fact that the longitudinals are welded to the plate , fixed solid points. They are not floating free! Until you get that, you are mentally incapable of understanding the structural principles of the stiffness added by longitudinals.
Any curve stiffens and strengthens a hull. The longer the span the greater the strength it gets from the ever increasing amount of curve.
How many fibreglass boats have transverse frames in their topsides. If they did where would they crack? At the frames! How do they get their strength. Shape!
A friend bough a 35 ft Endurance fibreglass bare hull from Fraser Yachts . When he asked the owner and builder of dozens, over many decades, if he could leave out some bulkheads, the owner told him "Good idea. The only place they ever crack is at the bulkheads."
As I have pointed out ( completely over your head, it seems) you get the greatest benefit from structural running along the curve, and the least running it across the curve, in open ocean. In a hard chine boat, the curve runs longitudinally, in a round bilge boat the curve runs transversely . So for a round bilged boat in open ocean , transverse framing is best, until you hit a hard, sharp object while underway, with a forward motion, at right angles to the frame. Then when the sharp object hits the frame the frame gives the plate something to stretch and tear against, drastically increasing the odds of holing.
So tell us Mike. How many steel boats to your own design have you built ?How many have been built ? What is the greatest number of decades you have been cruising in and maintaining any one of them? Why have I never encountered one of them ? How many of my boats have you seen, either cruising or being built /
No answer means NONE!

Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"

Last edited by Brent Swain; 01-14-2014 at 08:01 PM.
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