stuartw
Member
Collapsing helmsman\'s seat-MBM(Nov)- letter
I found the letter from a reader about a cabin sole socket giving way, quite disturbing.
Naturally I felt sympathy for the man's wife, and the possibility of a more catastrophic end. However, I was disturbed to think that somebody thought that those sockets were capable of supporting a person on the end of a 2 ft support tube.
To last a year was a miracle.
Those sockets are for tables, weighing only a few kg. Sometimes they are used to convert tables to beds, but most of the forces are vertical, with short support tubes.
In the helmsman's seat scenario, a 140 lb person would exert a bare minimum of 280 ft. lbs of torque on the socket. with the boat gently pitching or rolling. That figure could be 2 - 3 times that in a lively sea.
The aluminium cup would have massive shear stresses on the bottom, which will eventually give way.
A bucket seat, on that length of tube, should be supported by a large casting with clamp and a four-point fixing to the cabin sole. The tube will sit in a deep socket, clamped, and the four feet will take the load.
Quite frankly I am amazed that MBM agreed to print that letter. Surely somebody technical, vets letters? To me, publishing a letter that warns people not to do something, when that something is fundamentally flawed, is wrong.
Maybe the lesson to be learnt here, is that manufacturers of such equipment, should stipulate the uses to which they can be used.
Having said that, maybe they do, in this case.
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I found the letter from a reader about a cabin sole socket giving way, quite disturbing.
Naturally I felt sympathy for the man's wife, and the possibility of a more catastrophic end. However, I was disturbed to think that somebody thought that those sockets were capable of supporting a person on the end of a 2 ft support tube.
To last a year was a miracle.
Those sockets are for tables, weighing only a few kg. Sometimes they are used to convert tables to beds, but most of the forces are vertical, with short support tubes.
In the helmsman's seat scenario, a 140 lb person would exert a bare minimum of 280 ft. lbs of torque on the socket. with the boat gently pitching or rolling. That figure could be 2 - 3 times that in a lively sea.
The aluminium cup would have massive shear stresses on the bottom, which will eventually give way.
A bucket seat, on that length of tube, should be supported by a large casting with clamp and a four-point fixing to the cabin sole. The tube will sit in a deep socket, clamped, and the four feet will take the load.
Quite frankly I am amazed that MBM agreed to print that letter. Surely somebody technical, vets letters? To me, publishing a letter that warns people not to do something, when that something is fundamentally flawed, is wrong.
Maybe the lesson to be learnt here, is that manufacturers of such equipment, should stipulate the uses to which they can be used.
Having said that, maybe they do, in this case.
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