Here is a table showing (in metres) the dimensions of locks on the Shannon Navigation.
|Lock||Width||Mitre to mitre||Mitre to sill||RL upr sill OD||RL lwr sill OD|
|Clarendon, Boyle Water||9.14||33.53||31.09||42.36||41.18|
|Drumshanbo, Lough Allen Canal||4.50||26.70||21.00||48.16||46.48|
|Drumleague, Lough Allen Canal||4.22||20.52||19.68||48.16||45.11|
|Battlebridge, Lough Allen Canal||4.11||23.16||22.45||45.04||41.07|
|Albert, Jamestown Canal||9.14||33.53||31.09||41.07||39.31|
|Richmond Harbour, Royal Canal||—||—||—||—||38.28|
I don’t know the provenance of these figures: I suspect that they were measured, or derived from plans, during the late twentieth century, but before the River Suck was made navigable to Ballinasloe: the table does not include figures for Pollboy Lock.
A handwritten note says that 2.71m should be subtracted from the sill heights to give Malin OD. These figures should not be used for navigation.
Note the interesting figures for Drumshanbo and Drumleague: Drumshanbo is a double-acting lock, capable of being used whether the level of Lough Allen is above or below that of the canal. The extra gates may explain the low proportion of usable (mitre to sill) to full (mitre to mitre) length.
Could those measurements be in feet? and what does RL lwr sill OD etc mean?
Those figures are in metres and were probably translated from feet for a working document. It may be possible to find figures in feet, possibly from the Shannon Commissioners’ files, but I don’t have them.
A lock has two sets of gates, an upper [upr] and a lower [lwr] and each has a sill. OD is the Ordnance Datum, a set point from which heights can be calculated. Nowaways, the official Irish ordnance datum is the mean sea level at Malin Head https://en.wikipedia.org/wiki/Ordnance_datum but on inland waterways many of the heights of water levels etc were calculated in the nineteenth century by reference to the older Poolbeg OD and these figures are still found in use today.
“RL” was on the source document but was not explained; perhaps it means “reference level” or something.
Many moons ago I spoke to a gentleman who walked with his from Castletroy, I presume Plassey, up some canal. He had to fight his way through bushes and trees but he did come across what he described as Ireland’s only triple lock. Judging by the table above, it doesn’t look that way, maybe he meant with three gates. Is there even such a thing as a triple lock?