Robert Sherer, author of 2015 ICW Cruising Guide: A guide to navigating the Atlantic Intracoastal Waterway and frequent contributor to SSECN, has granted us permission to share a link to his Intracoastal Blog: http://fleetwing.blogspot.com/

Especially for those of you heading north for the summer, you will find many informative and entertaining reports on the waterways ahead of you. Such as this discussion of bridge heights near Cape May, NJ.

Bridge Height on the Cape May Canal – 58 ft at MLLW

Is your height less than 58 ft? If so you may be able to take the Cape May canal between Cape May and Delaware Bay. There are three factors to consider:

1 – The tide. It’s simply the predicted tide out of a tide table, easy to find for any time with a charting programs that allows the time to be varied with a display of the tide height. For the bridges, I use the Cape May Harbor tide station since it’s much closer to the two bridges than the ferry terminal station.

2 – The actual vs predicted water level. NOAA maintains a family of tide stations that show the actual water level vs the predicted water level. They are not generally known but there is such a station at the ferry terminal at Cape May. It will display either a graph or a spreadsheet showing the actual vs predicted tide height. Most helpfully, the data can be accessed in real time over the internet at NOAA Cape May water level site. Here’s a shorter link: tinyurl.com/zpzojfc. The water level varies due to weather conditions such as a strong on-shore wind blowing water into the bay or a heavy rain upriver when the flow reaches Cape May. An easterly 15 kt wind will easily raise the water level a foot as will heavy rains up the river. During tropical storms and winds greater than 20 kts the water level can be dramatically higher, 2 to 3 ft or more.

3 – The real bridge height. Unfortunately, data does not support a 55 ft bridge height at high tide provided the height boards are accurate, see photos.

The water level as reported by NOAA at the ferry terminal station was running 0.7 ft above the predicted tide level. There had been heavy rains up the river the day before. So what is the real bridge height? Over the years I’ve used 58 ft at low tide as the starting point for figuring passages. Take a look at the low tide picture. The total water level above MLLW was 0.4 + 0.7 = 1.1 ft. If you add that 1.1 ft to the displayed number on the height board (57.2 ft) then you get pretty close to 58 ft at 0.0 MLLW with no water level delta (actually in this example 58.3 ft). With the three numbers I’ve developed an equation for computing bridge clearance:

Clearance under bridge = 58 ft – Tide Height – Water Level Delta of predicted vs actual

As long as your mast height is less than the clearance from the formula above, you’ll clear the bridge.

Note that the tide height and water level delta can be positive or negative numbers. The equation works either way. The crucial bit of information is the real bridge height of 58 ft at MLLW. The number started out from my experience with a dozen transits and it looks to be a little conservative by 0.3 ft. I haven’t yet confirmed this but you probably have a few more inches if you avoid the red light hanging down from the middle of the bridge. As I said earlier, I’ve used this formula a dozen times with success. In fact one year I was going through and was called over the VHF to STOP! (their emphasis), I was going to hit! This was before height boards were added. I double checked my figures and proceeded onward with no problem. The experience does highlight the difficulty in judging bridge heights as you approach a bridge. You would swear you are going to hit, your angle of view will lead you to believe that – but you won’t if you follow the formula.

Standard Disclaimer: the captain always take full responsibility for his actions, I believe the above advice to be accurate and I’ve followed it myself many times in my sailboat with a 55 ft 3 inch mast, once with the height board reading 56 ft.

Robert Sherer