This bridge, opened around 1908, has an opening span with two leaves that
roll up. It can still be opened on request - but with constraints and advanced warning.
Here I attempt to estimate the air draught when closed.
Construction details were a 120ft clear opening span between timber dolphins. The dolphin deck to be 7 ft 3" above HWS.
An image taken on 1 sept 2017 at 10.22 GMT when Heysham tide-gauge read 5.3m:
From the image the ratio of height (sea-level to highest clearance level) to
width is about 0.26. Using width 37m then gives height 9.6m. At HWS the sea-level would be
approximately 4.0m higher: so clearance at HWS would be 5.6m.
As a cross-check one can work out the height of the dolphin deck over HWS similarly - getting about 2m. This checks the design specification of 7.25ft quite well.
In conclusion at HW springs, the clearance (air draught) should be around 5.6m. This checks with my observation that several of the motor vessels moored the N side of the bridge have larger air draught than MARLIN (4m).
Please use these results only as an indication - and check carefully before passing under the closed bridge.