Appendix 4
In principle, the inclining experiment is very simple, but in the real world few things are simple and almost everything can go wrong to give seriously misleading results. A weight (w) is moved across the deck through a distance (1) and the heel (θ) produced is measured. Then -
W.gm.θ = w.1
Where W is the displacement and gm the metacentric height, both at the time of the experiment. Since the position of the metacentre can be calculated from the geometry of the ship, the position of the centre of gravity can be deduced.
The RN College notes had a series of ‘Do’s and Don’ts’, all based on painful experience:
•Make sure the ship is afloat. Many shipyard basins were shallow and if the bottom was soft mud the ship could move but not to the correct extent.
•Ensure the ship is upright and close to the design trim.10
•Check the water density fore, aft and amidships. It was not uncommon on riverside berths to have salt water fore and aft and ‘fresh’ water from a sewer amidships.
•Measuring the draughts is difficult unless the water was dead calm. If the surface is moving more than about 6in, give up. Check amidships – if there are no draft marks, measure down from deck edge.
There would be four equal lots of ballast, two on either side. One lot would be moved across and the angle measured using a very long pendulum (actually, two as a check). Then the second lot would be moved and the new angle measured. If this was not twice the first, you had a problem. One and then the other would be moved back and then the other two lots were moved across. While this was going on, all people on board had to remain still in marked positions. In the bigger shipyards the ballast would be cast-iron blocks with their weight cut in to them, but in small yards there might just be four skips of scrap with their alleged weight chalked on the side.11
At this point, the displacement and metacentric height as inclined was known and the real problems began. One had to estimate the weights (and their position) on board which would come off before completion – workers, their toolboxes, temporary wiring, etc and the ballast. Then one estimated the weights to go on to complete – stores, fuel, crew and effects. Most errors probably lay in this part.
An inclining experiment was a very good experience for a young assistant, with a management task followed by scrambling round the ship learning where every thing went.12 Then there was a lengthy calculation, which was prone to error. In the author’s first job he had an inclining about every 2 weeks and it had to be complete and written up before the next one. Years later, when checking such experiments the author was assured that with the aid of a computer programme the work could not be carried out in less than 3 months!13
10 My first inclining was of an ‘Eddy’ class tanker with considerable heel and heavy trim trim. There was no way to bring the ship to a design condition, but it was possible to calculate the actual position of the metacentre – a lengthy and tedious calculation but it made a foot difference in GM.
11 It was usual to throw a bucket of whitewash over the scrap after weighing to prevent additions or removals.
12 I still remember with pleasure visiting the bullion room of the Royal Yacht to estimate the weight of gold and silver on board!
13 In the early years there was an almost religious feel to an inclining, and the answer (GM) was expected to be accurate to about 1/100ft, a ridiculous pseudo accuracy. Inclining the trials vessel Decibel in poor weather I reported the GM as ‘about 32ft’. Luckily I had an understanding ADNC and after a tough examination he accepted my view that it did not matter if it was 33 ft or even 31ft.