Image: copyright JS Mitchell, 2020
The new slipway carriage at the The Maid of the Loch’s Balloch Slipway will form part of the steam-powered winch house and underwater railway infrastructure. The patent slipway is on three rails with a double centre rail enclosing a rack for locking the carriage on a 1:18 gradient. The outer rail centres are 5.79m and the new carriage replaces two previous timber iterations, the first being in 1901. It is believed that this is the only remaining working steam slipway in the world.
The new 50 tonne carriage is in steel but with structural timber elements on the outrigger chassis amounting to 5 tonnes of (sustainably certified) opepe, a West African hardwood. The original 80 cast iron bogies, running on 240 cast iron wheels, have been reincorporated, along with the riveted outrigger arms, silt ploughs and tow bracket. The all-up weight of the carriage and ship, the “Maid of the Loch” will be c.600 tonnes and the pull required from the steam winch will be 100 tonnes at 1 metre per minute; the speed being limited by the boiler capacity.
The carriage design had to stay within the 50 tonne weight limit requirements, be able to bear the weight of the 550-tonne ship and the tension load of 100 tonnes. It also had to comply with requirements of appearance and working principles, which have been achieved by using solid timber outboard lower chassis and the original running gear. Lightness is achieved by using longitudinal universal columns (H beams) to form the correct visual ‘mass’ but being internally hollow. The 51m carriage is made up of five coupled articulating units which can follow height variations on the track while keeping all the bogies in contact.
Inter-chassis stiffening is vital, so light weight diagonal stiffeners, held by single steel pins to allow flexing are repeated the full length of the structure and 90 degree cross brace are included to prevent the structure ‘lozenging.’ All of this has followed the principle of strength with controlled flexibility, weight suppression and of course, appearance.
JS Mitchell ACR FIESiS, Conservation Engineer on PS "Maid of the Loch" and Balloch Steam Slipway