boats at low tide... Damaged?


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Posted by Ed Kiser on December 27, 2003 at 22:08:26 from 64.12.96.8 user Kisered.

I have just been looking at the CORNWALLCAM which today had a great set of photos showing the tidal areas of the coast. The tide was out leaving boats stranded on the mud. They were tilted way over on one side. Assuming that these boats are in their normal mooring spot, and then each day as the tide comes in an out, these boats are alternating in their mode of repose, sometimes afloat on the water, other times tilted on one side resting on the mud.

A boat is a structure, built to float. Its weight is distributed within its structural members of ribs and planking, with the water being the support, providing its lift across the entire bottom, the load being evenly supported. Then comes the time this boat is canted over sideways on the mud, with no water to support it. Now the weight of this structure is shifted from all along the bottom with the boat evenly supported by the water to just a few contact points, possibly as few as three points, depending on the even spreading of the mud. Essentially the keel has most of the load, and some small point along the side that keeps the tilt from going over even further. We are in a different support state.

Now the boat is somewhat like a SEESAW, sometimes called a TEETERTOTTER, where the loads are essentially the weight of the two children sitting on the extreme ends of a board, and the whole thing is supported at one point there in the middle. The board is in a state of a twisting tension, which in an extreme state, as when the kids involved are too heavy, can result in the breaking of the board.

In the boat situation, the weight of the entire structure is likewise bending that structure about those few points of contact with the ground, causing a twisting and bending of the structure. It would seem that such flexing on a daily basis would be apt to put strain on the seams, on the fastenings, thus weakening the watertight integrity of the boat, thus producing a leaking vessel.

In other words, isn't it bad for a boat to be put back and forth between floating, and then sitting on the bottom, that state changing several times a day? Wouldn't it be better with respect to its structural ability to remain intact to either moor the thing in water deep enough to still be in water even at load tide, or to take the boat out completely and leave it on some sort of trestle, where it stays at rest, without the daily flexing back and forth. It just can't be good on the boat to keep bending it with each tide.

In Picts and Martyrs, Chapter 10, the D's are putting the pigeon cage into Timothy's boat that he has pulled up onto the shore on the lake side of the promontory. I quote:

The long narrow boat listed sideways with a loud creak as
Dick climbed in.

"It's all wrong pulling her so far up on the stones," he said.
"But I suppose he was thinking of the wash from the steamers...

We see here the flexing, caused by the shifting load (Dick walking through the boat) while the boat itself has part of its weight concentrated upon the water in the stern, but with the front's weight being on a single point on the stem as its nose is pulled up on the rocks. Dick says, "It's all wrong" to have a boat thus suspended between both ends like this. That "loud creak" to me represents a shifting of timbers and of fastenings, indicating that some minute degree of damage has been inflicted on the structure. Something has moved, shifted, because of this mode of mooring.

If it is "all wrong" for Timothy's boat to be thus placed, is it not also wrong for a boat to be left to founder on the mud during low tide?

Or is my physics all wet here?

Ed Kiser, South Florida



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