Ralston ship's stability and trim Indicator
Datec 1935
Object number00055663
NameIndicator
Mediummetal, wood
DimensionsOverall (open): 530 × 571 × 431 mm, 15500 g
ClassificationsTools and equipment
Credit LineGift from University of New South Wales Naval Architecture Stream
DescriptionCustom made Ralston ship's stability and trim indicator for the MV POOLTA. The instrument is inside a wooden box that has a duralumin etched tray with the profile plan of the MV POOLTA. The plate is attached to a frame that can be raised or lowered through levers on each side. There is a trim balance weight, slacker flaps and a GM balance weight. On the upper side of the box, there are two slides that show the ship’s trim and stability. A slide on the left has a draft scale as well as instructions and notes to use the indicator accurately. There are two keys attached inside the box’s lid.HistoryThe Ralston ship’s stability and trim indicator was used as an analogue method of distributing a ship’s cargo mass to ensure correct fore and aft trim and transverse stability of the vessel before it left for a passage at sea.
The indicator was a clever, practical system that involved simply balancing weights on a plate over a set of twin scales built into a handy case, and specifically designed or formatted for each individual vessel.
A plate depicting a schematic profile view of the vessel with its tanks and cargo spaces defined in outline is located on the surface, and the plate can be balanced fore and aft or vertically as separate actions.
Weights for the cargo are placed in their location, and any water/fuel/ballast or waste tanks are able to be included or excluded depending on their fill or empty state. Using two sets of levers to raise the plate, it balances either fore and aft or vertically, but not at the same time. Slides parallel to either the vertical or the horizontal axis then locate where the centre of gravity is in either direction.
If the centre of gravity is too far aft or forward in the horizontal direction, adjustments to the loading or ballasting can be made until it is an acceptable position. The vertical scale shows the GM or metacentric height for the loaded condition. This is a measure of the ship’s stability that relates to the method used to calculate it from a set of plans, where the vertical distance between the actual centre of gravity and the vertical point above the heeled centre of buoyancy that is on the ship’s heeled centreline, which is known as the metacentre. GM or metacentric height is a key indicator of a ship’s stability and roll characteristics, and understanding this is vital to a ship’s safety at sea. If the GM is insufficient for a safe operation, then the load or ballasting can be adjusted to correct this.
This example of a Ralston stability and trim Indicator has been customized to suit the MV POOLTA, a diesel cargo vessel built in 1959 for the Union Steamship Company of New Zealand. It was 76 m long, and was the last of the Leith (Scotland) built ships for the company.
SignificanceThis Ralston ship's stability and trim indicator assists in interpreting the story of classical and practical naval architecture. It was originally part of the University of NSW Naval Architecture course and demonstrates classical methods of engineering and naval architecture using hand calculations and practical models testing to develop and assess a ship’s design and subsequent operation.
post 1945
