Four feet under the pull

Dear Mr. Field,

You asked after Millwall, and I will tell you plainly what the launch taught me. On the third of November the Great Eastern moved four feet under the full pull of the chains and then stopped. We had calculated the friction of the ways at a figure we could defend on paper. The timber cradles were greased to specification. The ship did not care for the specification. She sat on the slipway through the winter while I tried every explanation I could construct - the tide, the grease, the bearing surface of the brattices, the alignment of the ways themselves.

Here is the thing I got wrong, and I want it on record before the formal report closes the matter. I kept looking for the single fault. A tunnel man’s habit: when the rock resists, you assume one wrong assumption and you dig for it. But the launch was not a tunnel. The resistance was not one load path failing. It was the accumulated friction of the whole system - every cradle, every way, every ton - and no single measurement would isolate it because there was no single cause to isolate. The hydraulic rams solved it in the end, not by finding the fault but by supplying force enough to make the fault irrelevant. I spent from November to January searching for a diagnosis that did not exist, when the correct question was how much force the condition required.

The worst-case load is stated before the design; that rule I have always held. The rule I failed to hold is the one on the far side of it: once the worst case is supplied and the structure still will not answer, stop diagnosing and change the operating condition. There is a boundary past which further inspection returns nothing. The lamplight walk before the first train is an obligation. The lamplight walk at two in the morning, repeated for the ninth time, is a man avoiding a decision.

So I amend my own thinking. I believed the engineer’s duty was to find the cause before acting. I now believe the duty is to distinguish causes that can be found from conditions that must simply be carried, and to know which one you are holding before you spend another month on the ways.

Yours, I.K.B.