Monday, March 24, 2008

Transverse stability can't get enough of it! On Friday, the owner said the filmmakers had promised to put up a dedicated Cangarda site, but I don't know if that will ever happen. In the meantime, I will attempt to bring you up to date. Thomas asked about the near capsize at launching. It is true. It happened. Here is a shocking photo:John Horton (on the rail) probably hadn't anticipated this event, and is trying to decide whether to hold on or leap free. Andrew and Jody were in the Zodiac and are backing like mad. Yeah, it looks funny now, but it was serious and could have been far worse. I just read the article you cited: Evidently, some people, including the owner, didn't fully understand what happened. As you point out, he seemed to think that this was proof of the vessel's inherent stability. It's not.

This is classic launching instability and, until recently, has been a standard part of elementary naval architecture instruction. Today, most vessels are launched parallel to their design waterline either with a travel lift, a syncholift, or by allowing water to enter the building dock --never losing transverse stability. Years ago, most vessels were end launched (stern first) off an inclined railway or set of building ways. Launched this way, as soon as the stern floats, the ship is only supported at the bow, and would lose all transverse stability if inadequately suported. The forward poppets have to be strong enough to resist that lateral load.

If a vessel were launched perfectly straight and there were no asymmetry, then it could slide all the way to the end of the ways with no incident just as a riderless bicycle could be sent down a hill with a perfectly straight push. The farther the ship makes it down the ways, the more form stability it gains...its a race between the stabilizing and destabilizing forces. Here's a link to a video of a launching in 1907 that went wrong:
The instability and the overturning moment can be calculated. Before computers that calculation was difficult and rarely done, now (with the right software) it is easy and accurate. Long, slender vessels are more prone to overturning than short, wide ones (with LCB well aft of amidships). Cangarda was launched by a very interesting and energetic housemover who had experience launching large fishing boats with the same gear and method. The gear consisted of a massive, multiwheel dolly under the stern, and a steerable dolly that supported the bow at a single point. When I told Jeff and him that Cangarda would capsize the mover said, "I was thinking about some kind of tie down, but I didn't want to eff up the guy's paint."

"Well. His paint is going to get effed up a whole lot more when he capsizes on the ways".

The mover's gear was old and primitive, and watching his crew and he move Cangarda out of the yard brought to mind a horde of ancient Egyptians hauling a giant obelisk with ropes and rollers.

To make a long story a little shorter, prior to the launch the housemover added sufficient lateral support to the bow dolly, but when the stern floated free, the rusted old dolly itself split apart. Luckily, the mover was driving the truck, and when he saw his dolly start to rip apart, had the presence of mind to push Cangarda down the ways as fast as possible, thereby preventing a disaster too heinous to contemplate.

What does this say about Cangarda's transverse stability or ability to weather a severe storm...nothing.

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