Measured miles were critical in the days when the Firth of Clyde was a major shipbuilding area, all newly built or re-engined ships had to undergo sea trials before being handed over to the owners. It was on the basis of these trials that the owners decided whether or not the ships were up to specification and agreed to accept them. One of the most important tests was the maximum speed test, carried out over a set distance.
This is now relatively simple, as the course can be set on the autopilot, and the maximum speed determined using GPS. Before the invention of this technology, things were more difficult and involved an officially recognised measured mile, marked out near Skelmorlie. Ships were required to cover a set course between the marker at full power, a lookout with binoculars would start a stopwatch when the first two markers lined up as the ship passed, and stopped it when the second pair of markers was passed, allowing the speed to be calculated. For greater accuracy these tests were only carried out in light wind conditions, and repeated in the opposite direction to minimise any directional effects. Testing was carried out during the hour of slack water at high and low tides.
Before a new owner would agree to take possession of a vessel constructed in one of the Clyde shipyards, it was necessary for the builder to conduct sea trials, and prove the vessel performed to the specifications laid down in the purchase order. These trials were conducted with the vessel steaming at full power over a 13.666 nautical mile (15.736 statute mile) course between the Cloch and Cumbrae Head lighthouses, in a procedure known as Running the Lights. Tables from the period show that at 14 knots the run takes 58 minutes 34 seconds; at 20 knots, 41 minutes. Having completed one run, the vessel would then turn around as quickly as possible and repeat the run. This reversal was intended to negate the effects of any wind and tidal input to the vessel's performance. In reality, the time taken to complete each run, stop, and turn the vessel for the second run meant that wind, tide, and weather could all have changed from those current on the first run, thereby negating the averaging effect of the two runs.
To solve this problem, the test distance was reduced, with the steamer pier at Skelmorlie marking one extent, and the Skelmorlie Hydropathic Hotel, south of Skelmorlie Castle, the other. Measuring 6,080 imperial feet, this nautical mile came to be regarded as the most important Measured Miles in Britain. The nautical mile was later redefined within the international system of units, losing approximately 10 feet in its length, defined as 1,852 metres.
The measured mile at Skelmorlie was notified on July 4, 1866, in a Notice to Mariners, No 36. The unlit beacons marking the mile were described as a single pole, 45-feet high, with arms 10-feet long forming a broad ( V and ‘inverted’ V ) angle 15-feet from the base, the whole being painted white. Once the 'V' and the inverted 'V' were aligned they became an 'X', at which point the tester's stopwatches would be started or stopped dependent on whether the start or finish of the run was being recorded. From the precise time recorded for the run, and the known distance between the beacons, the results were read off from standardised time and distance tables published in almanacs.
Prior to running the measured mile, vessels would be brought into a steady state of motion, maintaining steering and power constant to avoid any influence and distortion of the final speed calculations. Prior to passing the markers, vessels would always run a straight and steady course for up to four miles, allowing course and speed to be stabilised.
At the end of each run the vessel would turn and repeat the run in the opposite direction, using the same engine power and revolutions. This procedure was intended to cancel out any affects arising from wind, weather, and tide, once an average speed was calculated over the two runs. It was customary for at least two return trips to be made over the course, with results being deemed acceptable if they were in agreement. Variations are reported to have been in the order of 0.5%.
The Skelmorlie markers are no longer depicted on OS maps, suggesting that they have fallen into disuse.
The Arran measured mile is actually two consecutive miles, marked by three sets of pylons or beacons on the north east of the island, between South Sannox and Corloch. The beacons are illuminated, and maintained by a marine service company operating out of Rhu Marina.
The northern mile measures 1.8544 kilometres, while the the southern is 1.8514 kilometres. The Arran mile has more manoeuvring space at each end of the run, compared to Skelmorlie, making it easier for vessels to carry out their test runs quickly, before the weather and other conditions have a chance to vary. Procedures at Arran remove the stop and turn manoeuvre, allowing the vessel maintain speed and perform a turn in order to reverse its direction for the second run.
In 1937, a question was recorded in Hansard, regarding the measured mile off Arran and its operation:
Royal Navy (Measured mile, Isle of Arran).
Sir C. MacAndrew
asked the First Lord of the Admiralty whether he has yet received a report from his expert who visited the Sannox measured mile on the island of Arran at the beginning of June; and, if so, what steps are going to be taken to alter the positions of the beacons?
A report has been received on the observations made at Sannox Pier during recent runs by His Majesty's ships on an experimental track placed as far North of the existing track as was considered practicable. This report show that the use of the experimental course would only very slightly improve the conditions at Sannox Pier; and after the most careful consideration, it has been decided that the considerable expenditure which would be involved in moving the trial course to the experimental position cannot therefore be justified.
Arrangements will, however, be made to avoid, as far as possible, runs on the measured mile between 2–3 hours before high water and high water. With this precaution, in addition to the existing instructions that wherever possible the Northern half only of the two miles course is to be used, and that notice of trials is to be given to the Arran Barytes Company, I am satisfied that all practicable steps will have been taken to reduce local inconvenience to a minimum.
QE2 visit 2007
When the QE2 returned to the Clyde in September 2007, marking the 40th anniversary of her launch from the Clyde, one of the procedures she observed was a full-power run over the Arran mile, recreating part of her original sea trials in 1968.
St Abb's measured mile
A set of four Admiralty Distance Poles is is identified on OS mapping, and also marked as a measured mile. The poles lie on the southern coast of the Firth of Forth, on rough ground and cliffs to the west of St Abb's.
The sea area off St Abb's Head is understood to have been used for testing of vessels from the shipyard to the west.
Skelmorlie north markers
Skelmorlie south markers
- Navigation Light Contract Old link - dead 2008.
- Navigation Light Contract
- QE2 40th anniversary, The Herald, September 20, 2007
North beacons Corloch:
- Arran North beacon upper NR 99506 48658
- Arran North beacon lower NR 99562 48700
- Arran North shoreside beacon NR 99652 48763
Mid beacons Laggantuin:
- Arran mid beacon upper NS 01126 47535
- Arran mid beacon lower NS 01154 47554
- Arran mid shoreside beacon NS 00893 47368
South beacons Sannox Bay:
- Arran south beacon NS 01184 45303
- Arran south beacon NS 01286 45376
- Arran south shoreside beacon NS 01846 45774
St Abb's measured mile Admiralty poles:
- St Abbs Admiralty pole west-north
- St Abbs Admiralty pole west-south
- St Abbs Admiralty pole east-north
- St Abbs Admiralty pole east-south
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