Join Date: Mar 2007
Location: New England
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Double Checking My Math - Bridge Height
My boat has an estimated 50' air draft (mast height from the water). I need to go under a bridge that has Vertical Clearance of 49' based on the latest chart that I have.
It is my understanding that the Bridge Height is based is the minimum height at MHW (Mean High Water). So at an average high tide, the distance between the water and the lowest part of the bridge is 49'.
The tides here are on average about 8.5 feet, so at low tide (MLW), the minimum distance under the bridge is 49' + 8.5' or 57.5 '. Therefore, if I go under the bridge at MLW , I should have approximately 7 ft of clearance.
Please confirm if this is correct. I'd hate to get to the bridge and then find out that I totally screwed it up.
Also, how accurate are these bridge heights? Do the underestimate the height somewhat so that say a 49" bridge height could be almost 50', but since it isn't 50' yet, they can't say it. It would never be less than 49 and reported 49, correct?
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Last edited by DrB; 10-11-2011 at 01:32 PM.