This page assumes some general understanding of sailing, so if you’re not a sailor, or you haven’t read Arthur Ransome’s Swallows and Amazons books, you should probably read Sailing first!
In Birgom’s time ports are neither as large nor as deep as today. This is partly because of a generally lower stage of development, but mostly because it’s not worth making huge efforts to build ports that will quickly become obsolete as sea level goes down. Consequently large ships simply don’t come into port; they anchor offshore and wait for smaller vessels to transfer passengers and cargo. Large ships are nonetheless the most economic long distance transport, in both capital and labour costs.
Manafariiba and the other Manafa line ships are designed to sail in the open sea, with plenty of depth and plenty of sea room. They are much the largest ships anywhere in the world in Birgom’s day, but many other, smaller, more conventional ships are also too big for most ports.
Most of the other ships in the book wouldn’t have looked out of place in the sixteenth or seventeenth century AD in the real world – the other exception is Vinhaassa, but wait until you meet her in the book before looking at her page!
The first diagram shows why conventional boats heel over as the wind pushes on the sail(s).
The second diagram is a rear view cross-section of Manafariiba sailing close hauled, showing one sail and two keels (in colour – the grey ghosts show the rigging when the wind is on the other side). Two more keels belonging to this sail are directly behind the two you can see. The sail flies like a kite, instead of being attached to a mast. The mainsheet takes most of the tension, attached just forward of the centre of the sail, to the point where the cross spars cross; the control lines take a much smaller part of the tension, and give the sailor (the flyer) control over the sail.
The mainsheet and control lines feed between rollers in eyes on each end of a swing arm and thence to winches. The winches are connected to all the other winches for all the other sails, and for the ship’s anchors, with a system of variable ratio belts and pulleys, under control by the captain so that he can allow some sails to fly closer to the ship, or further away, by adjusting the tension in the main sheet.
An expert flyer can get a pilot sail up entirely by hand. Once it’s up, it generates the initial tension that helps get the second sails up. The greater tension produced by the second sails then helps to get the full sails up.
Whichever way the sail is pointing, the swing arm is pulled to the same line as the force on the sail. The keels are attached to the end of their own swing arms, which are mechanically linked to the sail’s swing arm, so the keels are always pointing in the opposite direction to the sail (laterally and vertically, but not fore and aft).
This means that the line of the force on the sail is exactly opposite to the combined forces on the four keels, so there is exactly zero heeling moment*. These ships do not heel at all in a wind!
(Of course the forces don’t cancel out in the fore and aft direction. The sail pulls the ship forwards more strongly than the keels pull it back, whether sailing close-hauled, on a beam or broad reach, or running.)
The flyer has to keep the sail aloft by pulling on the control lines, either helping the winches pull them in, or pulling against the winch. To tack, he (almost all flyers are men – but Mkembi, the “grandmother” of the ship, is a real expert flyer, and the main teacher of the skill) has to fly the sail right overhead and down to the other side of the ship.
To run before the wind, he has to fly the sail forwards, taking care not to tangle with the other sails! This is shown in the third and fourth diagrams. The third diagram shows one sail and the matching keels viewed from behind. The fourth diagram is a side view of the whole vessel, with all six sails (I’ve not bothered to show the billowing of the smaller sails). You can only see two of each set of keels, because the others are directly behind them.
The fifth diagram shows a plan view of Manafariba sailing close hauled. Magically, you can see the keels and their swing arms through the ship. Really of course they’d be hidden.
All these diagrams show the sails relatively close to the vessel, to keep the diagrams manageable. Really the sails would usually be flown considerably further out, catching more wind higher above the sea, and giving the flyers more time to react to any erratic movement in the sails. And of course there’s the business of sending a flyman way up, harnessed to a full sail, to get a good view. The flyer of the rear second sail has to fly his kite off line if you’re running straight downwind when you do this. An accident waiting to happen...
Later, larger Manafa line ships have even more sails and keels, but work to the same basic design.
Rudders, sea anchors and anchors are not shown. They’re pretty conventional! The anchor chains are connected to the same tensioning system as the sails.
* A moment is a twisting force, equal to the two opposing forces multiplied by the distance between their line of action. In this case, that distance is zero, so although the forces are not zero, the moment is zero and there’s no twisting.