Bridge Clearance

[pnw2022]

I know this is an old post, but I just came in through Agate Pass here in the PNW earlier this evening on the way to Poulsbo. Had to catch the flood tide from Port Townsend.

I am a big believer in mathematics, and generally trust the data provided on charts.

Meanwhile, I always have a terrible time judging bridge heights and often just blindly trust the charts, math, and have to look away when going through (obviously avoiding low hanging items off the bridge like lights and such).

Meanwhile, I was pretty stressed out this time. I swear, there was only a couple feet looking up directly vertical. Where:

Air height on the boat: 63', call it 65' with the VFH and 68' for safety.
Bridge height: 75', off MHW (from the chart)
MHW to some local areas ranges from 7-11', so presume 5' for extra safety.
Max tide 12' at nearish stations, used the engine to arrive a little early so only 10' for extra safety.

75' bridge + 5 (from chart, conservative choices for safety) MHW => 80' at a local zero tide.
80' bridge off local zero tide + 12' for worst case nearish tide station => 68' clearance.

68' mast height (with additional safety) => all should be fine?

Looking up while going through (trusting the math), OMG!

Anyway - yeah, all was good, but way too close for me. But I always seem incapable of dealing with watching my mast while we go underneath bridges unless they bridges are at least 200' or more

I have toyed off and on about buying one of the golfer range finder units. Mostly for shenanigans at tight anchorages, but for sure now so I can find a nice time around low tide when I am going against the current and can measure the bridge height from the boat deck and double check the math and datum.

I can't help but wonder also if the MHW stats from the other stations, which seem they would have very high tides - if when all this water is being squeezed through a tight pass like that, if maybe it builds up additional height right there? I don't know enough about fluid dynamics.

[carstenb]

Originally Posted by pnw2022

I know this is an old post, but I just came in through Agate Pass here in the PNW earlier this evening on the way to Poulsbo. Had to catch the flood tide from Port Townsend.

I am a big believer in mathematics, and generally trust the data provided on charts.

Meanwhile, I always have a terrible time judging bridge heights and often just blindly trust the charts, math, and have to look away when going through (obviously avoiding low hanging items off the bridge like lights and such).

Meanwhile, I was pretty stressed out this time. I swear, there was only a couple feet looking up directly vertical. Where:

Air height on the boat: 63', call it 65' with the VFH and 68' for safety.
Bridge height: 75', off MHW (from the chart)
MHW to some local areas ranges from 7-11', so presume 5' for extra safety.
Max tide 12' at nearish stations, used the engine to arrive a little early so only 10' for extra safety.

75' bridge + 5 (from chart, conservative choices for safety) MHW => 80' at a local zero tide.
80' bridge off local zero tide + 12' for worst case nearish tide station => 68' clearance.

68' mast height (with additional safety) => all should be fine?

Looking up while going through (trusting the math), OMG!

Anyway - yeah, all was good, but way too close for me. But I always seem incapable of dealing with watching my mast while we go underneath bridges unless they bridges are at least 200' or more

I have toyed off and on about buying one of the golfer range finder units. Mostly for shenanigans at tight anchorages, but for sure now so I can find a nice time around low tide when I am going against the current and can measure the bridge height from the boat deck and double check the math and datum.

I can't help but wonder also if the MHW stats from the other stations, which seem they would have very high tides - if when all this water is being squeezed through a tight pass like that, if maybe it builds up additional height right there? I don't know enough about fluid dynamics.

when going under bridges it ALWAYS looks like your mast is going to whack it. I recall going under a bridge with 90 ft clearance (our mast + antenna is 67) and I was damned sure we were going to hit - but didn't. it is all an optical illusion

[SailingHarmonie]

Us old fogey sailors know about a tool called “The Coast Pilots”. Which have a huge amount of information in them that could never be all included on a chart.

Including bridge heights and datums, right on page 1.

Download for free for the US Office of Coast Survey. Read it, and learn a huge amount about your local cruising grounds you never knew.

[Leadfree]

I was beaten to it, but yes, the Coast Pilot is the answer for your question.


https://www.nauticalcharts.noaa.gov/...lot/index.html

[requiem]

Regarding Agate Pass...

Looks like you'd have plenty of space there... MHW at Brownsville is reported as 10.96', and 10.07' at Edmonds, so that's 85-86' of space at 0 tide, or 73-74' with a 12' tide. If you're really curious you might sit through a tide cycle or two to compare what you see against what's predicted.

Regarding the Coast Pilot...

I increasingly get the sense that more people are considering charts as sufficient for navigation in themselves and being less aware of supplementary data such as the Coast Pilots. This is particularly important since survey data is often quite ancient and errors still exist here and there.

However, I don't see the MHW to MLLW offset listed in the Coast Pilot. Both the charts and the Coast Pilots do list both bridge heights and what to use as the datum. For the last bit I needed to look up nearby tide stations and click the Datums link at the bottom of each page.

[Jammer]

I pulled some photos and it is clear that Agate Pass Bridge lacks bridge boards. If I were a boater in the area I would be tempted to write to WSDOT and suggest that safety would be improved if they were to install them.

Since there is lawful pedestrian access to the bridge, you can measure the clearance by dropping a weighted tape measure from the rail to the surface of the water, then subtracting the distance from the rail to low steel. The rail to low steel distance is small and could be measured with the aid of a boathook or similar object. I have done this myself with local bridges where the coast pilot listing is doubtful (as when a bridge has been replaced but the listing has not been updated), unclear (such as clearance listings only to the 2% flowline), or missing entirely.

[pnw2022]

As somebody noted earlier - the Coast Pilot. Yes, I actually have a copy onboard, 51st edition - 2019. No clue why it never occurs to me to actually look stuff up sailing around within the Salish Sea. I guess I usually just go to my regular places and/or ask around ahead of time, etc. It was definitely an awesome tool coming back up the coast from San Francisco some years ago. It kinda went back on the shelf when we got back and sat there since then. Thanks for the reminder - it is an awesome additional tool!

I took a peek, nothing interesting in the Coast Pilot that I didn't already know from local knowledge already - and the bridge is noted at 75 feet.

Separately - no clue why they don't have those bridge signs here. I don't think I have ever seen one in Washington State, at least on the salt water side of things. Possibly not enough overall economic traffic to justify the cost? As you might surmise, I generally only go under bridges that are very tall anyway and never really look for the signs since it's already a no-brainer with the math.

Anyway, my post earlier, it was more about how weird it is (at least for me) dealing with the bridges in general. My logical mind understands math, geometry, reliable/trustable data points - and yes, even trigonometry and calculus while looking up going underneath Meanwhile, when I look up, everything in my body says "you are going hit it!". Haha. And I am fine with airplanes, motorcycles and a wide variety of other things. Just something weird for me about sailboats, masts and bridges I guess. Maybe I watched too many YouTube videos of people not getting it right?

I am thinking about going through the Hood Canal bridge next month. Luckily for me anyway, that is a floating bridge that opens sideways so no airheight concerns. Brand new local territory for me - so thanks again for the reminder that the coast pilot is sitting on the shelf.

Cheers.