Bridge clearance: MHHW?

[foojin]

Trying to schedule a meetup near Port Townsend, WA for folks from both sides of the border.

On the NOAA charts I have, depths indicated are MLLW.

Am I correct in assuming the figure indicated for vertical bridge clearance is MHHW?

[Shrew]

Correct. It would indicate air draft at either MHW or MHHW.

[Kelkara]

I only have Canadian charts, but they specify:

Depths:
Canada datum = Lowest Normal Tide
USA datum = Mean Lower Low Water

Spot Elevations and Clearances:
Canada datum = Higher High Water Large Tide
USA datum = Mean High Water

So the datum changes across the border on the charts ... the same probably happens on the USA charts.

[Hudson Force]

I thought bridge clearances were listed at mean high water (MHW); however, it's been my experience not to trust them. There are variations when the low steel is arched and even cases where the adjacent paths are higher than the center span. Local knowledge is often best. Sometimes the calibrated clearance boards at the fenders don't agree with the charted clearance when you read at the high tide scum line on the calibrated scale.

I've had the occasion to climb up to the catwalk and signal my wife as she has backed through in reverse, though there are trespassing/security concerns. I've also seen bridges where the parallel I-beams under the road bed are not all at the same height.

Once I clear a close fit I diagram or photo the fenders with notes in my log to help with my next passage. I notice that the pair of bridges in the Cape May Canal are listed at 55' MHW, but they are very frequently down to near 50'. Some places are subject to far more variation than others, especially up rivers or estuaries subject to run-off.

[foojin]

Originally Posted by Shrew

Correct. It would indicate air draft at either MHW or MHHW.

Cheers and thanks to all.

[charliehows]

I've been wondering about this for some time myself – let me give an example; on the tide chart the following tide heights are given; High 1.65, Low 0.46, High 1.44, Low 0.49; The map indicates a bridge height of 12.3 metres; my mast height is 12.67 metres. I've never been able to figure out from this how to calculate when or even if my mast will fit under this bridge – my guess has always been go under with great caution at lowest possible tide.
Anyone ?

[denverd0n]

It will say on the chart whether the heights are MHW or MHHW. Unfortunately, if you are using any of the electronic chart tools, they typically cut off the "legend" portion of the chart and quilt the next chart into its place. You either have to use other chart viewing software to view the individual chart, or you have to eyeball a real, live, paper chart.

[senormechanico]

I previously posted on CF about my experience with the Hood Canal Floating Bridge.

I found AFTER THE FACT from the Washington State Bridge guys in Olympia that it's the only bridge in the state where the numbers posted are for LOW TIDE !!!!!!!!!!!!!!!!!!!