Understanding Cape Cod Canal Tidal Currents (with Daily Charts)

Dave Kaiser (owner – Mattapoisett Boat Yard) texted me Tuesday and asked me how to estimate the tidal currents in the Cape Cod Canal.

I haven’t a clue, so I decided to research it and put it on My Buzzards Bay for anyone else looking for answers.

An unfavorable tidal current isn’t a big deal for Vigilant. With our 715 HP Cummins, the only impact is burning a couple of extra gallons of diesel when the tide’s not your friend.

In Dave’s case – on his 46′ Grand Banks Trawler, it can be a big deal. In a really bad head current, you can get pushed backwards over 6 knots. and The Sarah Francis is a 9 knot trawler and that means a lot of tough sledding!

The 2 1/2 Hour Maximum

In case you didn’t know, the Coast Guard has set a maximum time for traveling the canal:

All vessels, including pleasure vessels, must be able to transit the land cut portion of the canal, between the Cape Cod Canal Control Station in Buzzards Bay and the East Mooring Basin in Sandwich (approximately 5.9 NM) within 2 hours and 30 minutes against a 6 knot foul current. 

While I doubt anyone times your passage, the fact is fighting a head current for hours is no fun. Our 26′ Boston Whaler, Tenacity (37′ Back Cove), and Vigilant (41′ Back Cove), all handled the head currents pretty well. The Mean Kitty (Hydrasport 33) did not.

Draining The Buzzards Bay Bath Tub

I think it’s important to understand where these tidal currents come from in order to deal with them. For years I’ve been told that the two major Atlantic Ocean currents – The Gulf Stream and The North Atlantic Drift converge in Buzzards Bay. This is why the water temperature is 10-15 degrees warmer here than the north side of the Cape.

It’s also why the Cape Cod Canal (and Woods Hole) currents are so strong.

Here’s a condensed version of a rather extensive explanation that comes from someone named Steve from Striper’s Online:

  1. When the current is moving from east (Sandwich End) to west (Bourne End), the water level is dropping for about 4 hours and anyone heading out of Buzzards Bay up the canal will face an unfavorable tidal current.
  2. After 4 hours, the current goes slack for a bit, and starts moving in the opposite direction – from the west (Bourne) to the east (Sandwich) as the water level in Buzzards Bay drops for about another 2 hours (give or take) – Draining The Bath Tub.
  3. While the current is still moving east (from Buzzards Bay toward Sandwich), the water level in the Canal starts to rise. About 4 hours after the low tide, you get another current slack, and then the pattern repeats itself.

The reason this happens is because the water levels (high and low tide) in Buzzards Bay occur about 2 1/2 – 3 hours before the same high or low water occurs in Cape Cod Bay.

In addition, the tidal difference from high to low in Buzzards Bay is only about 4-6 feet, whereas the tidal difference between high and low in Cape Cod Bay can be anywhere from about 8 feet to as much as 12 feet plus (depending on the moon cycle) which further exasperates the tidal current.

When you get to somewhere around the RR bridge, the lag between the current change and the water level change gets less, and if you go west from there to Wing’s Neck, the lag is even less…

As you can see, there’s a lot of hydro dynamics going on in The Cape Cod Canal!

The Charts

The big time and tide differences between Buzzards Bay and Cape Cod Bay make the standard tide charts pretty useless – unless you line up the high/low for Bourne and compare it to the high/low for Sandwich.

The Army Corps of Engineers publishes a chart that actually does this and it makes it all somewhat easier to understand.

Here is the chart for August, 2018 (you can follow the link above to seen the entire year).

Cape Cod Canal August 2018 Tide Chart

Last Saturday (8/11/18) we went from Bourne to Sandwich around 3:30 PM. If you look closely at the above chart you’ll see that the tide at The Railroad Bridge (Bourne) was turned from West to East (favorable) at 10:24 AM and then reversed at 4:18 PM.

tide-8-11-18.png

This meant that we were at the end of the tide change and we had little current either way.

On Sunday we headed for Padanaram around 10:00 and here’s that tide change.

Tide 8-11-18

As it turned out, we faced a minor unfavorable tide for the first half of the trip up the canal and then nothing at all. As the chart shows, the tide turned west from the Railroad Bridge (Borne) at 10:24 which is about right.

When is The Worst? – Look To Ebb and Flow…

Now we’re getting into the more interesting stuff. There are 4 times a day when the tides are slack – meaning they’re not flowing particularly strong in either direction. Based on my experience last weekend, this doesn’t last too long.

What everyone is looking for is that extended period of time when the current is your friend and it flowing 5 knots in your favor.

I found a nice chart that includes the Ebb and Flow that includes the knots of current at The Tides Near Me website.

Tides Near Me CCC

This is really the info my friend Dave needs to know. At 3:36 AM and 4:06 PM today, they EBB will max out and at 10:00 AM and 10:18 PM, FLOW will max.

So the only question left is – which direction is EBB and which is FLOW?

The answer is that FLOW means that the current of running from Buzzards Bay to Cape Cod Bay and EBB means the current is running from Cape Cod Bay to Buzzards Bay.

This means that yesterday (August 15, 2018), the best times to head down the Canal from Buzzards Bay would have been 10:00 AM or 10:18 PM; the worst would be 3:36 AM and 4:06 PM.

Now you know it all!
Dave

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