page forty-two

The Hold

Forty-one rooms and nothing has ever had to float. Today's number fits out a small merchant sailing ship, piles a dozen lots of cargo on the quay beside her and gives you her stowage plan. Put the heavy things low and she is safe and unbearable. Put them high and she is comfortable until the first squall. Somewhere between the two is a ship that sails.

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Her profile, bow to the right. G is where her weight acts, B where the sea holds her up.

The stowage plan. Pick a lot on the quay, then tap a space. Tap a stowed lot to move it.
On the quay, by port of discharge in the order she calls
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Her figures
    The owners' conditions
      End on, in a swell. M is the metacentre: while G is below it, she comes back.
      Ship
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      The arrow keys walk to other days. Every day is a different ship, a different cargo and a different set of ports.

      What is real here and what is not

      The physics, the rules of thumb and the trouble they were invented to prevent are real. The ship, her owners and her cargo are the day's number.

      1. 01

        Bouguer's metacentre

        Pierre Bouguer named the metacentre in his Traité du navire of 1746. Heel a ship a little and the centre of her underwater volume, B, swings out to the low side; the line straight up through the new B crosses her centreline at M. If her centre of gravity G is below M, her weight and the sea's push make a couple that rights her. The distance GM is the whole of her small-angle stability, and the only part of it the person loading her controls is how high G sits.

      2. 02

        Stiff and tender

        A ship rolls like a pendulum, and the period goes as her beam over the square root of GM. In feet the usual rule of thumb is T ≈ 0.44 B / √GM seconds, which this page uses. Too little GM and she is tender: she lies over and comes back slowly. Too much and she is stiff, snapping upright so quickly that she strains her rigging, shifts her cargo and wears out her crew. Stowing weight higher on purpose to ease a ship that is too stiff is a real and old practice.

      3. 03

        Stowage

        A mate's stowage plan has to satisfy the cargo as well as the physics: the first port's goods on top where the stevedores can reach them, casks of oil, ale and brine kept off anything that spoils wet, and nothing stowed over a void that can collapse in a seaway. Trim matters too; sailing ships were usually loaded a little by the stern so the rudder bit and the bow rose to the sea. Samuel Plimsoll's campaign and the Merchant Shipping Act of 1876 put the load line on British hulls because so many were not.

      4. 04

        How the day is drawn

        The ship's length, beam, light weight and the height of her metacentre at her loaded draught are drawn from the day's number, and the metacentre is set so that the obvious plan (heaviest lots low) leaves her too stiff. The heavy lots are dealt round the ports so the first port always has something heavy to put somewhere. Before the page shows you a cargo, a simulated annealer searches the hold until it finds a plan that meets every condition; that is the mate's plan you are shown after you sail. Trim is computed from moments about midships and drawn five times over so you can see it.

      Forty-two rooms, and one number doing all the work. Today it builds a ship, loads the quay beside her and lets you find out why nobody puts all the iron at the bottom.