So what is your understanding?

[RWSlater]

I was involved in a discussion the other day about bridge clearance, and I'm interested to hear what your thoughts are.
The discussion was, when you come up to a fixed bridge and the clearance is indicating say 63 feet, what is it 63 feet from. I heard that the clearance is from the current water height to the bottom of the bridge structure at its highest point (arched). I've also heard that it isn't that but the distance from the current water height to the light that hangs under the bridge.

What say you?

 

[Gunni]

The light marks the location where the stated clearance exists. There is a defined channel and if the NTM states 65 feet you will have that under normal circumstances (short of a full flooding apocalypse).

 

[SFS]

I think the presumed discrepancy is embedded in your question: "the light that hangs under the bridge" (emphasis mine). Many people feel that "the light" extends below (closer to the water) the actual bottom of the bridge. Others argue that it does not. In my experience, I have seen at least one bridge where a mast that would clear under the span would not clear under the light, but I suspect that is rare. The light is there to mark the best clearance, why would the engineers reduce that clearance by hanging the light "incorrectly"?

 

[Head Sail]

Up here in the PNW, where the tides run 12 ft, it has been my assumption that the stated clearance is from the high water mark (minimum clearance) to the top of the bridge. It would make no sense if from the current water level since that is constantly changing.

 

[Benny17441]

It is to the highest arched point on the underside of the bridge. Most bridges will have markers indicating the height from the water to the high point as that distance varies with the tides. The light is just a light to aid in nighttime navigation but usually is also hung at the high point (I know as I once hit one)

 

[Terry Cox]

Most all navigation aids that I've seen where tide state becomes a part of the measure, such as with bridge clearance, mean high water would be the base line clearance referenced in charting information.

https://www.nauticalcharts.noaa.gov/mcd/learnnc_bridge.html

 

[SFS]

It is to the highest arched point on the underside of the bridge. Most bridges will have markers indicating the height from the water to the high point as that distance varies with the tides. The light is just a light to aid in nighttime navigation but usually is also hung at the high point (I know as I once hit one)

@Benny17441 - I just want to confirm that I've understood correctly. You are saying that the light, even though it was intended to mark the point of "most clearance", was hung in such a way that it robbed you of some of that clearance. In other words, the light hung down (closer to the water) from the bridge, rather than being even with, or higher than, the lowest point on the bridge.

 

[Gunni]

On lift and draw bridges you have to wait for the light(s) to go green before you get clearance under the bridge.
https://www.uscg.mil/hq/cg5/cg551/Lighting.pdf

 

[RWSlater]

@Benny17441 - I just want to confirm that I've understood correctly. You are saying that the light, even though it was intended to mark the point of "most clearance", was hung in such a way that it robbed you of some of that clearance. In other words, the light hung down (closer to the water) from the bridge, rather than being even with, or higher than, the lowest point on the bridge.

I know the bridge in the ICW near St Lucie, depending on the tide will indicate I think 62 or 63 feet bridge clearance and from what I can remember the red light in the center hangs lover than the bottom of the bridge by some amount. So that's how the discussion got started. One person said it was to that light the other said it was to the bottom of the bridge. Some red center lights don't hang down from inside the bridge, they are on the outside of the bridge.

 

[uncledom]

The clearance is Mean High Water to the bridge. The light is an Aid to Navigation and I always sail just off to one side of it as some do hang lower than the bridge structure. IF there is a lunar high tide or the wind is strong from some particular direction the height can be substantially reduced. I have had problems on the ICW due to both wind and lunar tides.

From Coastal Pilot 1:
Vertical clearances of bridges and overhead cables
are in feet above mean high water unless otherwise stated;
clearances in Coast Pilot 6 are in feet above Low Water
Datum unless otherwise stated

 

[RWSlater]

So have we decided that if the light hangs down one foot below the bridge's highest point, and the bridge clearance indicator (My grand daughter calls it a ruler) says clearnece is 62 feet at that moment, and my mast is 61 ½ feet above the waterline, I could possibly hit the light?

 

[uncledom]

I would have to vote yes, you might hit it.

 

[njlarry]

Yes you will hit it. Bridge clearance is to the bottom of the girder. Think of the light as just an accessory, like the antenna on your mast.

 

[Gunni]

With a hypothetical 6 inch clearance I wouldn't be looking at the light, but rather at the clearance board on the bridge side piling. That is where you see the measured clearance from current water level to the bridge structure. You may in fact be passing during a tide that is anything but "mean".

 

[RWSlater]

With a hypothetical 6 inch clearance I wouldn't be looking at the light, but rather at the clearance board on the bridge side piling. That is where you see the measured clearance from current water level to the bridge structure. You may in fact be passing during a tide that is anything but "mean".

Correct. I don't look at the light, I do look at the ruler on the piling. The only reason I would look at the light is to avoid it if it does hang down below the bridge.

 

[LeslieTroyer]

Just to confuse things I believe Canada uses a different reference (mean flat tide) and not high tide.