My wife and I brought our boats up to St. Margaret’s Bay, Nova Scotia for a couple of weeks and we’re really enjoying it. But I have a question about the tides that I hope one of you experienced sea kayakers can answer.

Of course up here the tides are semi-diurnal and I would expect the daily progression of them to be related to the lunar day of 24 hours and 50 minutes. But successive high and low tides are not exactly 12 hours and 25 minutes apart. In fact they vary quite a bit, sometimes as much as 12 hours and 36 minutes apart. I understand that undersea topology and the shape of the bay would delay tides but I would expect this to be consistent. Can anyone give me some insight to what I am missing?

other factors

The 12 hours and 25.2 minutes is an AVERAGE of the tidal sequences in a daily cycle, not an exact period for each. There are other factors that affect the individual tidal events including lunar position and the affect of regional weather patterns in estuaries and on whirlpools and other deep water movement. Here’s a copy/paste of an explanation of cotidal lines and their affect on short cycle timing:

Because the M2 tidal constituent dominates in most locations, the stage or phase of a tide, denoted by the time in hours after high water, is a useful concept. Tidal stage is also measured in degrees, with 360° per tidal cycle. Lines of constant tidal phase are called cotidal lines, which are analogous to contour lines of constant altitude on topographical maps. High water is reached simultaneously along the cotidal lines extending from the coast out into the ocean, and cotidal lines (and hence tidal phases) advance along the coast. Semi-diurnal and long phase constituents are measured from high water, diurnal from maximum flood tide. This and the discussion that follows is precisely true only for a single tidal constituent.

For an ocean in the shape of a circular basin enclosed by a coastline, the cotidal lines point radially inward and must eventually meet at a common point, the amphidromic point. The amphidromic point is at once cotidal with high and low waters, which is satisfied by zero tidal motion. (The rare exception occurs when the tide encircles an island, as it does around New Zealand, Iceland and Madagascar.) Tidal motion generally lessens moving away from continental coasts, so that crossing the cotidal lines are contours of constant amplitude (half the distance between high and low water) which decrease to zero at the amphidromic point. For a semi-diurnal tide the amphidromic point can be thought of roughly like the center of a clock face, with the hour hand pointing in the direction of the high water cotidal line, which is directly opposite the low water cotidal line. High water rotates about the amphidromic point once every 12 hours in the direction of rising cotidal lines, and away from ebbing cotidal lines. This rotation is generally clockwise in the southern hemisphere and counterclockwise in the northern hemisphere, and is caused by the Coriolis effect. The difference of cotidal phase from the phase of a reference tide is the epoch. The reference tide is the hypothetical constituent “equilibrium tide” on a landless Earth measured at 0° longitude, the Greenwich meridian.[18]

In the North Atlantic, because the cotidal lines circulate counterclockwise around the amphidromic point, the high tide passes New York Harbor approximately an hour ahead of Norfolk Harbor. South of Cape Hatteras the tidal forces are more complex, and cannot be predicted reliably based on the North Atlantic cotidal lines.