How to Judge the Tide and Not Run Aground

Would you buy a boat known to have grounding damage?

One thing that would make me walk away fast from a boat for sale are the words “grounding damage.” Running your boat aground is a most unpleasant experience. Take my buddy Lee as an example: An avid fisherman, Lee took his 25-footer into the ocean looking for bluefish.  He was approaching the channel to take him home, he saw the buoys close at hand, his boat was equipped with a GPS — yet run aground he did.  The ruined lower end of his motor cost $3,400 to replace—no insurance. He was sufficiently embarrassed to vow it wouldn’t happen to him again—at least in this spot he thought he knew so well.

I can commiserate; I’m ashamed to say I’ve run aground numerous times. Oh sure, a few times it was an uncharted hazard, but mostly it was caused by not paying attention. There are always excuses: We had a chart, but not on deck. The GPS was set to the wrong range. The buoys were moved by the current. We weren’t quite sure of the tide conditions. But no excuse matters much once your vessel is stranded.

So how to avoid those embarrassing, costly, sick-to-the-stomach OH… @#$%$ moments?  There’s an old adage that the best pilots know where all the rocks are because they’ve touched them all. To boat is to explore, and to explore means taking chances.  But you can reduce those risks to boat, bank account, and pride.

There are three basic things you will need to learn in order to stay off the rocks:

  • How to gauge the height of  the tide and state of the current in your location
  • How to identify hazards on a chart
  • How to plot your position on a chart so you know where you are in relation to the hazards

Today I’ll focus on the first of those basics: knowing how much water  is under your boat. I’ll address chart symbols and plotting positions in future blogs.

(By the way, although I’m obviously referring to tidal locations here, freshwater boaters also need to be aware of water level and any possible currents in their areas. An old tree stump in a lake where the water level is abnormally low is just as dangerous as a rock on a tidal coastline.)

Most coastal locations on earth have two high-water and two low-water tides each day; these are referred to as diurnal tides. The chief cause of tides is the gravitational effect of the moon and to a lesser extent that of the sun. Depths marked on charts are at  “mean lower low water,” which involves an average of the lower of the two daily lows. The point is, the actual depth of water is the amount marked on the chart plus the height of tide at a given time. In extreme astronomical or weather conditions you should be aware there could be even less water than marked on the chart. Today I’ll show you how to judge the height of the tide at a given time, assuming that you have consulted the tide tables to learn the times of high and low water.  Knowing how much water is under your keel obviously is a key to staying afloat.

To estimate the height of tide at a given time, we use “The Rule of Twelfths.”  This example is taken directly from the Eldridge Tide and Pilot Book, which gives tide tables for the East Coast of the U.S. Since the average interval between high and low (or low and high) is about six hours, we can divide the cycle into six segments of one hour each. On average the tide drops according to the following fractions.

Hours after High Drop fraction Drop % % of High
High —– —– 100%
1 hour 1/12 8% 92%
2 hours 2/12 17% 75%
3 hours 3/12 25% 50%
4 hours 3/12 25% 25%
5 hours 2/12 17% 8%
6 hours 1/12 8% Low

Apply the formula to get heights by the hour at a specific place, say Southwest Harbor, ME, as follows:

Tide Fraction SW Harbor, ME
MHW 10.2 ft
1 hour 1/12 9.4
2 hours 2/12 7.7
3 hours 3/12 5.1
4 hours 3/12 2.6
5 hours 2/12 0.8
MLW 1/12 0.0

Fortunately, the tide tables are calculated by considering all the astronomical reasons for tide differences throughout the year, but again, remember that extreme weather conditions can also have an effect on the height of the tides.

I don’t know about you, but given the amount of time, money, and effort I lavish on my boat, the idea of running it aground makes me positively sick.  So now that you know how to determine depth of water by adding the height of the tide at a given time to the charted depth of water, I have a few more suggestions. Depthsounders are helpful in telling you what is directly under you, but you should have an understanding of how your depthsounder is calibrated.  Is depth being measured from the sounder’s transducer or has it been adjusted to measure depth from the lowest point of the keel or prop? Know how your sounder is calibrated. Also, most sounders typically have an alarm feature that can be turned on to alert you when things get too close. You can set the clearance at which you want the alarm to come on, and I recommend you leave the alarm on. Remember, depthsounders look down but they can’t look ahead—so slow down in tight quarters.

Skimming along the surface of the water in our beautiful craft on a sunny day seems so carefree. But staying safe from those unseen hazards below the surface takes a little doing. Specific knowledge about piloting your vessel, estimating depths, and a little vigilance will maintain your boat’s resale value and reduce those risks to boat, bank account, and pride. Safe boating.

Written by: Peter d'Anjou

A USCG licensed captain and former merchant mariner, Peter d'Anjou is now a freelance writer and editor. A one-time executive editor at Sailing World magazine, he writes about his passion for racing and boating. Having managed a large yacht repair facility in the NE U.S. his background in boat construction and repair translate to the practical side of boat ownership.


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