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Understanding the Rule of Twelfths Formula for Tide Predictions in the San Juan Islands

Disclaimer #1 
I have read this little tool rule many times by different authors and each time have come away confused and surprised at how such a simple idea can come out so convoluted and nonsensical. I wrote my clearly understandable version below and tried it on Linda who knows and understands the rule only to hear  "Your writing is  confusing, why did you say tidal range?"
Using the Rule of twelfths when anchoring
"The Rule of Twelfths"
 First, lets think in terms of where we will apply this tool.  For me it is when I need to anchor and I don’t know how far the tide is going to drop or rise.  Will my keel touch bottom in two hours, will the anchor drag while we sleep?  These important questions need answering before setting the hook.

       To employ the rule you must know some approximate facts first. #1, what is the tidal range in the area, ie. if the tide book states high tide is 8.5 feet and low tide is 1.5 feet, the total range the water rises or falls is 7 feet. As you know every day is different and varies by area so our first fact is just an approximation.  #2, we need to know what time high or low tide is forecast.  These two facts are all we need to know and we can arrive at the anchorage and make a good estimate of what to expect.

       Most areas have a six hour duration from low tide to high tide so we need to think in terms of six one hour segments. During the first hour the water rises or falls slowly. During the second hour the rise or fall picks up speed.  During the third and fourth hours, the water is moving at its fastest rate, and the fifth and sixth hours are slower and mirror hours one and two.  That’s the cycle that is repeated regardless of area or total range.  Slow at first, picking up speed, very fast, then slow down, and come to a stop, in six hours and then repeat.

       So now we have a six hour period broken into one hour segments, it’s time to assign a value to each hour and the following is the backbone of the Rule of Twelfths  123321, (each number represents one hour) yes they do add up to twelve. In the first hour after slack water, the water will move 1/12 of the range, in the second hour the water will move 2/12 of the range, or double the first. In the third hour the water moves 3/12 of the entire range. So this means that three hours after slack, or three hours  before the next slack, the water has moved a total of 6/12 or halfway through its range.  We already know that at the halfway point the tide is moving at its fastest and is halfway in or out, so no surprise there.  When we arrive at an anchorage exactly halfway between high and low tide, all we need to know is the total range and we can easily determine how much more or less to expect.  Where this tool helps most is during the second hour or fifth hour, referring back to our rule of 123321 we see the fifth hour totals 10/12, and the second hour total 3/12 of the range. By reducing fractions we easily understand that in the fifth hour the tide has moved 5/6 of the range and only has about 1-2 feet still to go. In the second hour the tide has only moved  1/4 the range and so still has about 4-5 feet to move.

       Armed with this knowledge you can easily determine how much the water will rise or fall and anchor where the depth is best for your boat.  So remember 123321,  know the tidal range in your area, and what time high or low tide is forecast. The rest is just fractions.

Disclaimer #2
       As a practical matter, I don't park the boat with just a foot of clearance under my goodies unless I have a good reason, such as I'm at a dock at low tide and I know whats coming.  Swinging at anchor with only a foot to spare is asking for it in some locations.  For you naysayers consider what happens to your boat when a big swell comes your way, that's right it is followed by a trough.  Well not only does a one foot trough take away your clearance, but it slams you violently on the bottom to tell you so.

       So, even though the Rule of Twelfths is a fun exercise, and I recommend everyone understand the ups and downs of tides, (ha) I still look for lots of depth at low tide, set my anchors very well, and sleep even better.

Here is a link to the NOAA tide tables  > NOAA tide tables


My shore power cord got so hot the plastic plug melted and had to be pried off the connection

Where have we been for the last sixty days?      

       I have felt bad for not posting recently, but not anymore.  Just as I sat down it occurred to me that many of us are in the same situation. Winter projects, holidays etc. Now that I have time to write, some of you may have time to stop by, and the fact that two months has gone by is not an issue.

         One of my projects was to keep my boat from freezing solid and being damaged. So far so good.  We went for a Thanksgiving weekend cruise in freezing weather, which was my last post about the stove huffing and puffing soot all over the place. You know soot becomes snoot when it gets wet or rubbed or wiped or touched. It also seems to permanently sink into oxidized fiberglass.  Bad news all around, snoot is. The good news is that it cleans off  painted surfaces really nice and almost cleans off waxed surfaces.

        On New Years we went for another cold weather outing and the oil stove worked flawlessly now that I know to pay better attention to the burner.

      Now about the hot wire,  and warning  for all of us that know better.  My 30 amp power plug got so hot that it partially melted the plastic and I had a really hard time removing the plug from the receptacle on the boat.  

        The heat was caused by two things #1 the plug terminals must not have  made a good connection so I'm not getting 30 amps, instead I'm getting heat.   #2 the electric heater and battery charger in the boat probably combine for too much power draw, and with the cold weather I'm lucky I caught it before - poof - a different kind of snoot appeared where the boat is/was. My advice to myself is to check both ends of every cable connection for heat. What worry's me are the buried connections I can't get to.

Here's proof it can happen  - Clickety click   >>  >>  fire damage

Fire and smoke damage in our San Juan Islands boat
This picture shows smoke damage.  My first thought was that I could simply clean  the surfaces.
Think again skipper -- some surfaces clean really well and some become etched.  Smoke is extremely corrosive, and in a closed boat fire, flames may not be present but the smoldering fire pressurizes and forces smoke into and behind every nook and cranny.  Nothing escapes, not the field glasses, not the radio,  not the switch  panel or any electrical terminals .

      Within six months of this fire - all electrical switches, contacts failed.  Wires survived, but all had to be re-cut and new terminals crimped.  (The adjuster totaled the boat)

The cause of the fire was an overheated extension cord.  You can see the orange cord in the picture  So regrettable and preventable.