Design Blog: Wheel, Tiller & Rudder
The combination of the wheel, tiller and rudder provides the means by which HMS Unicorn would be steered.
Despite their crucial importance to the sailing of the ship, they would, at first glance at least, seem to offer little in the way of a challenge to the designer. In fact, this is not the case, and the arrangement of the wheel, tiller, and rudder, and their respective geometries presents significant problems that the designer must accommodate.
To understand the operation of the wheel, tiller and rudder and the issues involved, consider the arrangements shown in the figure. The wheel is connected to the tiller by rawhide ropes, which are less liable to stretch, and turning the wheel then acts to move the tiller, and hence the rudder. This movement of the rudder would produce a turning force which itself results from the momentum change of the water flowing past, and being deflected by, the rudder.
Steering the Ship
The geometry also means that moving the tiller to the port side would cause the ship to turn to starboard. This gave the apparently contradictory result that an order to ‘port the helm’ resulted in a turn to starboard!
This apparently simple situation is however complicated by the fact that, as can be seen from the figures, the total length of rope connecting the wheel with the tiller will change with the position of the tiller, in the case of the geometry of (a), this total length will decrease, while in the case of (b) it will increase.
This change in rope length requires that the designer developed a means of either providing or taking up this additional rope to maintain an appropriate tension in the tiller ropes, and in the case of HMS Unicorn this was achieved by using a tapered barrel as shown in the figure, and which is clearly visible in the photograph of the wheel.
The rudder can be seen mounted at the stern of the ship where it is hung onto the gudgeons by the pintles to enable it to pivot under the action of the tiller.
The form of rudder used on Unicorn, in common with all ships of the period, is referred to as an unbalanced rudder with all of the rudder surface area aft of the pivot point. This means that as the rudder is turned from the amidships position, the force on the rudder increases, as does the effort required to turn it.
This presented the designer of the wheel, tiller and rudder system with the additional requirement to choose a geometry which reduced the force required at the wheel to the minimum practical. It also restricted the size of the rudder that could be used, which reduced its ability to turn the ship.
Nevertheless, the wheel would often require several men to turn it, and in severe conditions, a series of tackles, known as relieving tackles, would be installed at the tiller itself to enable additional effort to be applied.
In contrast, a modern balanced rudder would have part of its surface ahead of and part behind the pivot point, reducing the force required to turn the rudder.
Blog by Professor Emeritus David Bradley