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Taming the Exuma cuts

Nov 9, 2007

To the editor: Anyone who has sailed the Exumas chain in the Bahamas, knows about cuts. Anyone who is new to sailing in the Exumas will soon need to learn about the cuts.

The Exumas chain in the Bahamas is a land mass that roughly runs northwest to southeast for just under 100 miles. This narrow strip of land is broken up by a series of cuts, or passes. These cuts between two land masses break the chain into more than 300 various cays, islands and rocks to make this a voyagers’ paradise. Grand Bahama Bank is shallow and exposed to the west, while Exuma Sound drops off very quickly to thousands of feet in depth, and is exposed to the east. The cuts provide voyagers with a way to travel from the banks side to the sound side. They are also where these two large bodies of water exchange water when the tide changes, and that is where it gets interesting.

In the Exumas, the cuts tend to be rather narrow, so the amount of water flowing through a cut can be quite impressive. We have surfed through cuts at more than nine knots measured by speed over ground (SOG), with the tachometer showing a rather conservative 2,300 rpm. We have also powered out of a cut at 3,200 rpm only to see a meager 2.9 SOG on the knot meter.

Add a little wind opposing the current flow and you can very quickly develop 6- to 7-foot seas in a very confused state combined with a vicious current, making for a dangerous situation with land on both sides of you.

Sit in any voyager’s bar in the Bahamas and you will soon hear tales about this or that cut. Usually these stories start with the strength of the current, move quickly to wave height and frequency spiced with a couple of white knuckle experiences, and end with a recommendation to use a different cut. More times than not, I believe the problem is not the cut, but the skipper’s timing of when to run it.

The Explorer Chartbook for the Exumas lists more than 30 different cuts, and in addition to very good, detailed charts, it provides waypoints and a short description for each. Although very informative, this is not enough information to make a prudent decision about when, or most importantly, when not to run a cut.
When it comes to safely running the cuts, timing is everything, so I developed a simplified way to decide when to take Jade Moon, our 1991 Freedom 45 CC sloop, through one. My goal quite simply is to avoid the very worst of times to run a cut.

My method is based upon the “rule of 12ths.” According to my Explorer Chartbook for the Exumas (where I first came across the rule of 12ths), at its simplest the rule says that in the Bahamas the tide will change every six hours, and by breaking the volume of water into 12 equal parts, you can quickly gauge what the height will be at any time.

The rule of 12ths says that in the first hour of a new tide the water will rise one-twelfth of the tidal range. In the second hour the water will rise two-twelfths of the tidal range. In the third and fourth hours the water will rise three-twelfths or 25 percent of the tidal range each hour. In the fifth hour it is back down to two-twelfths and in the last hour only one-twelfth. Obviously, the action occurs during the third and fourth hours of a tide, when the water rises 50 percent of the total tidal range.

Although the rule of 12ths is intended to gauge the height of a tide, I use the timing of the tidal change and the rule of 12ths to gauge the volume of water that will be flowing through a cut. So, we try to time our running of a cut to the first or last hour of a tide, but, most importantly, we avoid the third and fourth hours of a tidal change when the water is really pouring through.

To help me do this quickly, I developed a simple fill-in-the-blanks worksheet that I keep at the nav station to use when planning any trip that involves a cut. I created the worksheet in Excel, and print out several blanks at a time.

A good example is a worksheet that I worked up for May 9, 2007, when we were leaving the anchorage at Black Point heading to Cambridge Cay. Although we had anchored on the banks side and could have stayed in those protected waters, we wanted to go fishing, so we needed to go to the sound side and then back to the banks side for our evening anchorage. This meant leaving through Dotham cut, and arriving in Cambridge through Bell cut.

The Exumas are not very populated, and you cannot get specific tide times for the individual Cays, not to mention the cuts. I start by filling in the times for all four tides for the day, based on tide tables for Nassau, which are readily available, in column one.

Next, I modify those times to a localized tide time by adding 30 minutes for the difference between Nassau and the central Exumas. The 30 minutes is an estimate garnered from talking to numerous cruisers and a number of local fishermen. A word of caution, although tide tables show exact times and heights, it ain’t so. There are a great number of variables, and the best you can hope for is to get in the right ballpark.

Once I have localized the tide times, I fill out the localized time in column three, starting new for each tide, and I round times off to even numbers. This way any errors do not get compounded as the day increases. Lastly, in column four I note whether the tide will be ebbing or flooding.

Note that I only do this work for those hours of the day when there is enough light to safely be moving the boat. No need to do it for early in the morning, or late afternoon or evening, times when you should be securely anchored. To be moving safely in the Exumas you need overhead light so you can “read” the water.
The 12ths calculations are preprinted on my worksheet, as are my go or no-go times. With the worksheet filled out I look at the wind and wave direction forecast to make sure I am not going out on an ebbing tide, faced with wind and waves blowing into the cut, opposing the tide. This combination can make for serious waves in the cut.

Looking at my filled out form, I can quickly see that I should go out the cut between 0930 and 1130. With the wind blowing from the east, the tide will be coming in and the wind will blow with it keeping wave height down. The current will be against us, but conditions will be benign. Two hours later there could well be too much current against us.

As far as our arrival at Bell Cut, the overall trip from cut to cut is just under 15 miles. Assuming we average a little more than five knots (our average SOG while trolling), the trip should take us just over three hours. If we leave Dotham Cut at 10:30, we will arrive at Bell Cut around 13:30, slightly later if we have any success fishing. Perfect timing, as we will ride a modest flood tide through the cut with the wind at our back. 

For me the key is to avoid being in a cut at the worst time. Every six-hour tidal cycle has two hours that you want to avoid. There is no excuse for being in a cut during the worst conditions; just use the rule of 12ths to avoid the rough water. n

–  Wolf sails Jade Moon, a 1991 Freedom
45 CC, with his wife Linda Gilbert, out of
St. Petersburg, Florida.