The Workings
The information below is in this order :
Windshaft / Sailstocks / Sails / Main Wheel & wallower gear / Millstones / Cap Structure / Materials
Windshaft :
The windshaft is the central shaft which projects out externally and through which the sail stocks are fixed. The shaft thus rotates when the sails are driven by the wind. The shaft has a neck nearing just behind the sail stocks and tail bearing at the other end. It is slightly inclined so that the thrust force from the sails is pushed back into the structure.
The photograph above (c1907) of the north windmill shows the start of the decline with the cap roof gone exposing the windshaft to the elements. It can be seen that the shaft is inclined in its bearings so that the sails are not vertical and can catch the wind more efficiently.
The sketches above show the gradual decline of the north mill. The internal wooden components gradually decayed until the tail of the windshaft fell into the tower itself. Initially supported by both sailstocks, only one was left by the 1960's.
The Windshaft from the North Windmill was finally removed from the windmill tower in 1983 when it had became potentially unsafe.
The photograph (from Mill Archive) shows the shaft "suspended" by the remaining one sailstock. The latter was removed as well and both have been removed from site and stored externally in the rear garden of the nearby Portland museum ever since.
The removal of the windshaft and sailstock was recorded by the local Dorset Evening Echo in November 1983.
The crane was provided by a local company (Kingston Minerals). The shaft was reported to be of oak with a weight of approximately 14 cwt (711kgs).
Weymouth & Portland Borough Council who operated nearby Portland Museum then placed both items in the rear garden of the museum. They were inspected by mill expert Martin Watts who strongly recommended that they were protected from the elements. This unfortunately went unheeded leading to much decay since 1983. (nearly 40 years ago !)
RC recorded and measured them in 1991.
It is understood that the only other surviving windshaft of this type is in the Bridewell museum in Norwich (not seen by RC)
Photographs for the Echo were taken by local photographer Reg Vincent (original prints in Portland Museum)
The windshaft from the north mill stored externally in the rear garden of Portland museum (RC photo 1991)
Windshaft - note iron tail bearing shaft, mortices for main wheel and sailstocks and neck bearing . On right is the remaining sailstock.
(Photo RC 1991)
Plan view showing windshaft in situ
Measured drawing / sketch of windshaft (RC)
Right : Diagram showing the overdriven type of simple gear arrangement in the windmills :
The Windshaft is 4368mm long with a cast iron tail bearing projecting at the tail end. There is a cast iron band around the head end.
The main neck bearing is recessed into the square sectioned shaft (main body is 425mm square). The bearing has metal "strake" plates (each 75x175mm) around its circumference to act as wearing surface. The bearing was probably crude with a stone bearing block lubricated with say animal fat.
The shaft is in oak and calculated to weigh about 640kg (12 cwt or just over half a ton).
The sail stocks are fixed through the two forward mortices whilst the main wheel (compass type) has its arms fixed through the rear mortices.
Such shafts would have lasted about 100 / 150 years depending on wear and exposure. It would have been a major operation to replace the shaft requiring the cap roof to be removed and then reinstated after the renewal.
The Sailstocks :
Only one sailstock still survives. It is in oak and in one long piece (10.65m - 35.5 feet). (see above)
At the centre (where it is housed in the windshaft mortice), it is 187 x 225mm in section. It is tapered to each end where it is 100mm square in section.
Left : The sketch shows half of the sailstock and how both sailstocks would have been housed within the front (poll) end of the windshaft.
Millstones
The only millstone to date that has been found that could have perhaps come from the windmills is lying in a field near the Old Lighthouse, Southwell.
This has not been studied in detail. A stone cistern is lying beside it.
Above : A typical millstone showing grinding face
Materials used in the windmills :
Stone : Obviously stone was plentiful on the island together with the expertise to build the stone towers. The stonework was local coursed Portland rubble, with some ashlar quoins to openings and the top circular kerbs.
Timber would have been an essential item for the construction of the windmills. All timber would have had to be imported onto the island as there are no natural sources whatsoever. Timber could account for about 20 % of the total cost of the mill. Oak would have been used in many components including the windshaft, sailstocks, main beams, main wheel etc. The laths and lighter framework for the sails would have probably been made using ash. Crabapple wood was often used for the wooden teeth or pegs of the main wheel and sometimes for the staves of the wallower lantern gear wheel.
Metals : Iron would have been worked by the local blacksmith to make brackets, strengthening pieces, bearing strakes etc. Brass was used for bearing faces in later mills but it is doubtful that this was used on Portland.
Bearings : Marble was sometimes for housing windshaft bearings. This was certainly the case in the Rhodes windmill studied.
Sailcloth : Canvas would have been readily available on the island due to the obvious use of boat's sails etc.
Lubricants : Tallow from animals (many sheep on the island) was often used to lubricate moving parts and bearings. Soap substances were also used.
Millstones : Millstones were sourced from Bodmin Moor & Dartmoor in the southwest and perhaps the Forest of Dean, parts of Wales and, of course, the Peak District further afield. There are records of French Burr millstones being imported into Weymouth and it may be that given the proximity of France to the Island that these were used.