Double Checking My Math - Bridge Height

DrB · #1 (**permalink**) 10-11-2011

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?

Thanks.

DrB

jackdale · #2 (**permalink**) 10-11-2011

That should be correct. Double check the Title Block on your chart. You might also check with folks in the area. Also check the depth to ensure you have enough water under you.

celenoglu · #3 (**permalink**) 10-11-2011

How do you get 80 ft of mast under a 65 ft bridge? [VIDEO]

This might work

denverd0n · #4 (**permalink**) 10-12-2011

Your math, and your assumptions, are generally correct. But there are exceptions!

The advice to check with locals is key. Only those who go under the bridge regularly will know if the chart information is accurate and up to date.

saillife · #5 (**permalink**) 10-12-2011

Does the bridge have a pedestrian walkway? Might just want to take 60' of line (allowance for standing on the bridge) and see if it reaches the water.

ronbo1 · #6 (**permalink**) 10-13-2011

Call a local tow boat service for their view.
While they're careful about liability issues their advice may be useful.

Ronbo

monark192 · #7 (**permalink**) 10-13-2011

From your perspective at the helm, it will look like there is no way you will get under as you get close. Got to trust the math / research / local advice.

jackdale · #8 (**permalink**) 10-13-2011

Quote:

Originally Posted by monark192

From your perspective at the helm, it will look like there is no way you will get under as you get close. Got to trust the math / research / local advice.

Absolutely

I often go under a set of powerlines that are 38 meters above HHWLT. It is not usual for crew to think we are going to hit, especially in the dark when tey see the lines with a light.

DwayneSpeer · #9 (**permalink**) 10-13-2011

I stopped looking up when going under bridges a long time ago. My heart just wouldn't take it! However, I did in fact, hit the clearance (navigation?) light hanging under a bridge once. That's how close it was.