Iron Cove Bridge Clearance

A few years ago there was a thread on here about clearance under the Iron Cove Bridge. As we live near it now I bought a laser distance measurer and walked up today to measure it from the southern end. I calibrated the measurer at home on known distances and it was spot on.

The exercise was to see if MB can fit under (it can't) but it became a bit of an exercise in tides and navigation.

The tide was 0.8 metres at Fort Denison and falling. I didn't do a calculation for tide differences up harbour but it was probably nearer 1.0 metre at the Iron Cove bridge.

The Port Guide (dated 2015) doesn't mention the new bridge which opened in 2011and gives the following guidance:
"1.13.1 Clearances are given in metres above chart datum which is Indian Spring Low Water (ISLW) (being approximately zero at Fort Denison tide gauge)."I note ILSW is described as rarely occuring.
and
"1.13.5 Iron Cove Bridge - Seven fixed spans, minimum clear horizontal opening is 47.8 metres, maximum vertical clearance is 14 metres but travelling gantries, when in use, decrease the vertical clearance to 12 metres."

The old bridge is higher so I only used the new bridge measurements.

I measured the new bridge at 12.5 metres. With the ILSW zero tide you get 13.5 metres MAXIMUM clearance (12.5 plus the 1.0 tide to ILSW). No good for MB.

The chart gives a MINIMUM clearance on Iron Cove bridge of 11.8 metres at HAT. The only HAT I found for Sydney Harbour was 2.1 metres at Middle Head. Assuming HAT is the same at Iron Cove MAXIMUM clearance would be 13.9 metres at ILSW. This agrees with the Port Guide.

However HAT would be less way up harbour at Iron Cove. Therefore my measurement of maximum clearance at ILSW of 13.5 metres may be more accurate and indicates that HAT is probably closer to 1.7 metres at Iron Cove (the 13.5 clearance at ILSW I measured less the 11.8 minimum clearance at HAT on the chart).

So is the Port Guide using the 2.1 metre HAT at Middle Head and therefore wrong in saying there is 14 metres clearance. Is the chart deficient by not describing where the HAT measurement is taken from?

Or, more likely, did I cock something up after having a few beers?

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sirgallivant said..
it could cost a lot of boat dollars for one who mis-calculates the clearances.

I've seen it happen. We used to get the 28'er in under the Iron Cove Bridge, but only at about half tide or less (IIRC). One day we were sitting at the balcony of Dobroyd Aquatic Club when something that looked like a Catalina 42 seemed to just plough straight into the bridge. It would have collected the bridge at about top spreader height or lower, so it wasn't even a near miss. I didn't hear anything about it so I assume no one was hurt when the rig came down, but it could have been a tragedy for a motorist.