Bridge Clearance

[hallejj]

So here is how this question came up for us: the standard route into Poulsbo WA from the north passes through Agate Pass. a narrow channel spanned by a bridge. The NOAA chart and its electronic offspring show the clearance as 75 feet. Our boat has an air draught of 66'7" minus a couple inches for load and plus a foot or two for electronics. So we have more or less 6' of clearance, right? Well, maybe not. The tide table for Port Madison (less than a mile away) showed plus 10.8' at the time of our arrival, which led to the obvious question: 75' above what? On the assumption that no sensible chartmaker would use anything other than a fairly high tide as a datum, we slowed down to 1kt., held our breath and passed under without any ugly grinding noises.

I can't find any official confirmation of this but a related thread contains numerous assertions that the datum for bridge clearance in the US is Mean Higher High Water or Mean High Water depending on local tides and that would not be surprising but it still does not answer the practical question as it presents itself to anyone trying to sail under a bridge. This is because I have no idea what MHHW is at any point on the water nor any idea how to find out. If I am trying to determine if I have enough depth to proceed, I have all the information I need because the applicable datum is MLLW, which is the reference number for all published chart depths. I also have a depth gauge which tells me what the actual depth is under my keel. If I am going under a bridge, I think I can assume that the published clearance is above MHHW but I have no way of knowing what that is and no sensor that gives me the exact height of my mast or any overhanging obstacle. And, as everyone who has done it knows, if you are going under a bridge in a sailboat, looking at the mast will convince you that you are about to crash whatever the clearance is.

Other than just leaving a generous margin for error, is there any way to actually figure out what the clearance is using current tide charts and published clearance data?

[requiem]

Hah! Yes, I feel your pain; unfortunately the needed information is only on the paper charts, not the ENCs. (Somewhere in that other thread I think I posted the reply from NOAA.)

On the paper chart (or the PDF version) you will find somewhere a tidal information table that will give the values at a few different named locations for MHW, MHHW, and MLW as offsets from MLLW. There will also be a paragraph labelled "heights" that will say something like "elevations of bridges in feet above MHW".

[slugsgomoo]

I'm roughly ~65-68' and I don't really enjoy going under the Agate bridge myself- My charts I'm pretty sure show MHW, so any + tide you'd have to subtract from the bridge height (if I'm remembering right). I hate to fight the current, so when we were moored up there, I always tried to hit the exit on max ebb, and the entrance just after the shift from slack to flood.

[Nicholson58]

Mean high water is typical chart notation.

You should be able to find this on either raster or vector electronic charts. It’s clear on paper charts.

We never go without paper. You should be able to find paper chart books on line. Does Richardson cover your area? Even last year’s book is good backup.

[requiem]

Originally Posted by slugsgomoo

My charts I'm pretty sure show MHW, so any + tide you'd have to subtract from the bridge height (if I'm remembering right)

Add the MHW offset, subtract the tide. So, if MHW is 6', there's a 4' tide, and a 75' bridge clearance, you have 77' of space.

[hallejj]

Originally Posted by requiem

Add the MHW offset, subtract the tide. So, if MHW is 6', there's a 4' tide, and a 75' bridge clearance, you have 77' of space.

I agree with this assuming, as everyone seems to agree, that MHW is the datum for bridge heights reported on NOAA charts. For anyone who cares, I have empirical evidence that, when the tide at Port Madison is 10.8', there is at least 68' of clearance under the very center of the center span.

[carstenb]

Once again a good argument for using LAT and HAT not screwing around with MHHW, MHW MLLW or MLW.

[hallejj]

Just checked the NOAA chart for Agate Pass and sho' 'nuff there is data for selected locations for both MHW and MHHW. Looks like my original passage that created one of the higher pucker factors of my sailing career was actually a breeze. Thanks everyone for the enlightenment.

[Stu Jackson]

Originally Posted by hallejj

Just checked the NOAA chart for Agate Pass and sho' 'nuff there is data for selected locations for both MHW and MHHW. Looks like my original passage that created one of the higher pucker factors of my sailing career was actually a breeze. Thanks everyone for the enlightenment.

Originally Posted by hallejj

I can't find any official confirmation of this but a related thread contains numerous assertions that the datum for bridge clearance in the US is Mean Higher High Water or Mean High Water depending on local tides and that would not be surprising but it still does not answer the practical question as it presents itself to anyone trying to sail under a bridge. This is because I have no idea what MHHW is at any point on the water nor any idea how to find out.

So glad to hear you've evolved.

[hallejj]

Originally Posted by Stu Jackson

So glad to hear you've evolved.

Actually, not yet fully evolved because I still don't know for sure whether the official datum is MHW or MHHW but I'm ok with that because, if it matters, I don't want to be anywhere near there anyway.

[requiem]

[wingssail]

Originally Posted by Stu Jackson

So glad to hear you've evolved.

Now let's add a further complication: If you are trying to get under the Hood Canal floating bridge where the clearance (on the east end) is 49.4 feet and there is a 10.7 foot tidal range (MLLW to MHHW) then you need a zero tide to obtain 60ft of clearance. A minus 3 foot tide will give you 63 feet of clearance.

But presumably that 49.4 feet is is the minimum under that span (the part closest to the floating section of the bridge) and there is more air clearance closer to the land. BUT how much? And...it is shallower there too, so where do you go, close to land for the air clearance but risking running aground, or farther out where you have more depth, but less air clearance?

I've been through, under sail no less, at very low tide.

Oh screw it, just ask for them to open the span.

[hallejj]

Originally Posted by wingssail

Now let's add a further complication: If you are trying to get under the Hood Canal floating bridge where the clearance (on the east end) is 49.4 feet and there is a 10.7 foot tidal range (MLLW to MHHW) then you need a zero tide to obtain 60ft of clearance. A minus 3 foot tide will give you 63 feet of clearance.

But presumably that 49.4 feet is is the minimum under that span (the part closest to the floating section of the bridge) and there is more air clearance closer to the land. BUT how much? And...it is shallower there too, so where do you go, close to land for the air clearance but risking running aground, or farther out where you have more depth, but less air clearance?

I've been through, under sail no less, at very low tide.

Oh screw it, just ask for them to open the span.

Legend has is that a member of the Portland Yacht Club whose air draught was a couple feet too high to go under the Interstate Bridge without halting traffic would nevertheless get through by building up as much speed as possible and, at the last minute, cranking in the main and getting as much temporary heel as possible. There are a number of things I am way too old to do and that's one of them. Going under the Hood Canal Bridge is probably another for the next life.

[Jammer]

Originally Posted by hallejj

Other than just leaving a generous margin for error, is there any way to actually figure out what the clearance is using current tide charts and published clearance data?

Well, there are bridge boards. While not perfect, they are not subject to errors of arithmetic or datum.


For a bridge you transit often, you can make your own determination of clearance based on when your antenna does and does not hit low steel.

[Chotu]

Originally Posted by Jammer

Well, there are bridge boards. While not perfect, they are not subject to errors of arithmetic or datum.


For a bridge you transit often, you can make your own determination of clearance based on when your antenna does and does not hit low steel.

Right??

I just use the chart as a guide. I use the bridge board as the truth and put in a nice fudge factor.