The Bilge Pump
All wooden ships leak to some degree, generally associated with the working of the ship in a seaway and the resultant loosening of the seams between the hull planking. This water accumulates in the bilges and must be pumped out.
Figure 1. Chain Pump Schematic
The basic form of bilge pump that was used form the 15th century is the chain pump shown in the diagram in Figure 1. This uses a continuous chain of discs, with leather washer around the rim to act as a seal, and running through the tube; the open bottom of which is positioned below the level of the water to be pumped. As upper gear is rotated, typically using crank handles, the chain is rotated, water entrapped in the tube by the discs and brought to the upper decks where it can be discharged.
Figure 2. Chain Pump Operation
In early versions, the tubes were made by boring out tree trunks, typically from elms, leading to them being referred to as Elm tree pumps, though these were replaced over time with cast iron cylinders.
By the early 17th century, ships would typically have two pumps installed alongside each other on the lower deck aft of the mainmast with 3-deckers having an additional part installed by the later part of the 17th century. By the mid 18th century, all ships with more than 70 guns were fitted with 4 pumps.
By the late 18th Century, the basic chain pump was increasingly considered as inadequate and various new designs were put forward to improve performance. In 1769 the Coles-Bentick pump, which used a new chain design using cast iron links was developed, providing a significantly greater discharge rate.
The pump seen on Unicorn and shown in Fig. 3 is a Downton Pump, patented in 1828. This is a variant on the lift or suction pump shown in Fig. 4 and uses a series of buckets connected to the wheel driven crank to provide a sequential lift and improve flow.
(a) (b) (c)
(a) The Downton Pump (L Hebbert, The Engineer’s and Mechanic’s Encyclopedia Vol II, Thomas Kelly, 1836)
(b) Pump on Unicorn
(c) Pump on Unicorn
Figure 3: The Downton Pump on Unicorn
Figure 4: Operation of a Lift Pump