Mean High Water

Life Aboard

Mean High Water and Vertical Clearance

Can I Slip Under that Bridge Right Now?

by Larry Shick s/v Moira

Mean High Water or MHW and Vertical Clearance

Bridge clearance changes with the tides.

How to know when it's safe to proceed.

We know that bridge heights are measured in feet above Mean High Water. On any given day, if you are going to transit a bridge near high tide, you'll need to know if the tide level is above MHW. Will you have the charted 65 feet (or 64 feet, or whatever)?

You can find the height of MHW above MLW by going to, click on "Products," click on "Datums," and find your station of interest. If you are looking at a subordinate station, you may need to refer to the corresponding reference (base) station. That page will give you many datums for the station, most of which are of little interest to the sailor.

The heights of the various datums for any given station are all shown relative to an arbitrary zero height for the station. So MLW may show on the page at (say) 3.75 feet (relative to the sensor), and MHW at 8.23 feet (relative to the sensor). Subtracting MLW from MHW (relative to the sensor) gives you a value of MHW of 4.48 feet (relative to MLW), so any tide height above 4.48 feet is higher than MHW and you should watch out for reduced clearance.

Now that you've learned the simple method of knowing height clearance at MHW, you shouldn't ever get stuck under a bridge.