bridge clearance??

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pdecat

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Is bridge clearance on a non arched bridge measured at the bottom of the bridge or to the bottom of the bridge center lights??
 
if you are worried about those few inches of difference, its probably too tight!
 
true but I need to know anyway.Lightws look to be more like a foot or more.
 
I would t be surprised if lights have been changed over the years so while the charted clearance should be under the lights, it may not be. Usually you re just off center the clearance will be almost the same, especially since you re asking about a non arched bridge
 
Charted clearance is measured on centerline at mean high water. Call your local DOT and ask. You can always take a 100' tape measure (or whatever length is appropriate) with a few diving weights clipped to the dumb end of the tape. Drive over the bridge you have questions about, pull over to the side, get out, drop your tape, note the time (to compensate for tide) and measure for yourself. I've done that on several bridges where the DOT wasn't sure or gave conflicting numbers. Just don't do it during rush hour and nobody cares.
 
As L. Kith sez, the charted height will typically be the center at MHW, but you will see signs on a lot of narrower bascule bridges indicating that height is for low steel and to to add a specified amount at center. Think about it, a large vessel or barge needs to know what minimum height is, height at center doesn't do them much good if they are pushing a big cube. So best practice is to call the bridge on the phone and get the "skinny" if it is tight for you.
 
In some counties, like Palm beach, the gauges only show clearance at low steel but when you ask the tender how many extra feet at center their reply is "we are not allowed to tell you"!!

In these days of heightened security I don't know if I d try to measure the bridge..
 
"In these days of heightened security I don't know if I d try to measure the bridge."

Good Point!
 
About a year ago I was involved in the movement by barge of a large piece of a nuclear reactor component (no fuel) and I measured bridges in four (4) states inclusive of Florida, never once had a issue anywhere. You guys must look suspicious!
 
That's odd ! The drawbridges here on long island give the height at the lowest point of the bridge ! Not centerline but lowest point off to the sides .

Rob
 
All the same here,that's why I said non arched. It is a freeway bridge overa small canal with the usual center lights hangomg down so I don't think I'll be able to walk along the freeway to measure That's why I want to. Know if the clearance is to the bottom of the bridge or lights. Keith you must have experienced those lights on swivels, what did you find?
 
Bridges are built and managed by various government entities, it really varies. If the span is not arched, how dies it matter? Just cross off center
 
Pascal :
It is the fixed bridge to Loggerhead marina st. petersburg fl. Active captain shows old MHW 21' sign board says 20' probably MHHW. Chanel is narrow 25'??? width not even charted. I'm going to try it but if the hanging light is actually at 20' as show on the sign board then my nave light will pass the light if I go off center as you suggest. Standing on the bow it will be very hard to be certain of the clearance.
 
Go at low tide for extra clearance and there is current pick either slack or running into the current

Depending on layout, you re better off having a spotter looking thru a FB hardtop hatch or siting on an aft deck hardtop. It s hard to spot from the bow
 
Maybe a TV camera on the mast? I've a couple low (and unmarked) bridges to deal with and I've thought of it more than once when I don't have someone to check for me.
 
The swivel lights I have encountered don't hang below the lowest edge of the bridge. If they are swivel lights just slide them to one side as you pass under. Rig a "scrape stick" at your bow that will contact the obstruction first if the clearance is that close. You can always haul a few fat girls as temporary ballast, just go to a Wally World and ask "anybody want to go on a boat ride"?
 
Willie: i like the camera Idea if I am going to stay there. Do you have any suggested brands/models??
 
Sorry I don't, I think any decent outdoor "security" camera would work, most are 12v and will work with any TV, which you probably already have on board.
 
If you re going to play chicken with a bridge using a camera :) the most important thing will be a large readable monitor...may still be hard to read
 
Well I hook my back-up camera to my 21" TV, which works pretty well. As far as the "bridge TV" goes, mount the camera at the absolute highest point of the vessel... If you can SEE under the bridge, you can GO under it. If you can't, you can't. That's the same policy we use when I ask the Mate go up on the roof and watch. A person standing on the roof of my boat is higher than any part of the vessel including the mast.

If there's still any doubt, it's time to stop and wait for the bridge or the tide.
 
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