◊ Rules of Thumb

Rules of Thumb and Other Useful Tidbits

This section assumes a basic familiarity with sea kayaking navigational parameters and terminology. These 'Rules of Thumb' have been chosen for their utility. In all cases, there will be situations where they do not apply. For example, a complicated confluence of tidal streams can make certain rules not even approximately correct. Local knowledge will typically trump these rules. Similarly, unusual weather can sabotage these rules.

As I come across unfamiliar or new rules I will check them out and then add them if I deem it appropriate.

Rules Related To Currents


50/90 Rule
:

From slack to slack is roughly 6 hours with the max current speed occurring halfway through i.e. from slack to max current is roughly 3 hours.
  • Starting at slack, the current reaches 50% of max by the end of the first hour
  • then 90% of max by the end of the second hour
  • and 100% of max by the end of the third hour.
The current speed then begins to decrease
  • reaching 90% of max by the end of the fourth hour
  • then 50% of max by the end of the fifth hour
  • and slack at the end of the sixth hour.
Of course, we said slack to slack takes roughly 6 hours and you must adjust appropriately if it is different. For example, if it were 7 hours, each hour period in the description above would be replaced by 70 minutes. A mnemonic way to write the rule for those who think easily with numbers:
0:50:90:100:90:50:0

Also note that these speeds are estimates for the instantaneous current at the end of the associated hour.

Rule of Thirds

This rule also has to do with the speed of current and deals with the average speed over an hour and not, as in the prior rule, the instantaneous speed at the end of the hour. Again, for ease of understanding, we assume from slack to slack is roughly 6 hours with the max current speed occurring halfway through i.e. from slack to max current is roughly 3 hours. That MAX is the basis for calculations in the rule. The rule is:
  • during the first hour after slack, the average speed of the current is 1/3 of MAX
  • during the second hour, the average speed of the current is 2/3 of MAX
  • during the third hour, the average speed of the current is 3/3 of MAX
  • during the fourth hour, the average speed of the current is 3/3 of MAX
  • during the fifth hour, the average speed of the current is 2/3 of MAX
  • during the sixth hour, the average speed of the current is 1/3 of MAX

A mnemonic way to write this rule for those who think easily with numbers:
1:2:3:3:2:1

Comparing the 50/90 Rule and the Rule of Thirds

The 50/90 rule deals with instantaneous speeds and the end of the hour, while the Rule of Thirds deals with average speeds during the hour. I tend to use the 50/90 rule for traversing narrows where my concern, at my skill level, is avoiding high current speeds. The Rule of Thirds is more useful for estimating the progress you make over an hour, essentially giving you an idea of drift. It would be useful on longer, wider channels when you want tidal drift information to help estimate your trip progress.

Keep in mind that both are estimates, but good enough for kayakers. If one does a linear interpolation based on the 50/90 Rule values to estimate the average values for each hour one gets the following table:

hour

end of hour speed

avg. current speed

1

50% of MAX

25% of MAX

2

90% of MAX

70% of MAX

3

100% of MAX

95% of MAX

4

90% of MAX

95% of MAX

5

50% of MAX

70% of MAX

6

0% of MAX

25% of MAX

The Rule of Thirds should be compared to the 3rd column and does not match very well. Interestingly, if instead of linear interpolation, one uses a sinusoidal model the 3rd column results are within a percent of those tabulated above and the 2nd column is reasonably close as well. In any case, these rules are useful for kayakers and often good enough.

Note: The sinusoidal data can be seen in a discussion by Haris (October 27, 2009) on the web site of the Chicago Area Sea Kayakers Association.

Duration of weak currents near slack:


Near slack, what is the duration of relatively weak currents? Of course, about half that duration is just before slack, and the other half just after. Touring kayakers of modest skill typically want to traverse a tidal rapid during this time of weak current. Unfortunately, the higher the max current, the shorter this time. Here are some rough estimates, choosing to define weak current as a half knot or less.
  • max current = 2 knots, weak current duration = ~ one hour
  • max current = 4 knots, weak current duration = ~ one half hour
  • max current = 8 knots, weak current duration = ~ one quarter hour
We can get more mathematical. We'll start by defining slack as a current speed of less than 0.5 knots. With a bit of thought you should conclude that the greater the speeds of max flood and max ebb, the less is the duration of slack. The tables give the 'exact instant' of slack, call it slack0. Then we calculate the duration of slack around that time by figuring the amount of time when the current is less than 0.5 kt before slack0 and adding the amount of time when the current is less than 0.5 kt before slack0. The formula is, in each case,
  • slack interval = (60/max_speed) minutes where max_speed is in knots and is either at max flood or max ebb, whichever is appropriate

The formula above assumes 3 hours between slack and the adjacent max. If that is not close, the formula should be replaced by
  • slack interval = (T/max_speed) minutes

where T is a third of the time in minutes between slack and the adjacent max.


Rules Related To Tides


From low tide to high tide takes roughly 6 hours ... and from high back to low, another 6 hours. If the 6 hours were exactly that (which isn't quite the case), there would be 2 high tides and 2 low tides per day. In the tidal locations I have paddled, the high tides are at different levels from each other - as are the low tide levels. This leads to the descriptive phrase, 'mixed semidiurnal tides'.

The sea level change from low to high tide, or high to low tide, is called the 'tidal range'. The level change is not linear, looking more like a sinusoidal shape. The 'Rule of Twelfths' gives a quick estimate of the expected tidal level at various times during the level change.

Rule of Twelfths

  • In the first hour the tide level changes by 1/12 of the range.
  • In the second hour the tide level changes by 2/12 of the range.
  • In the third hour the tide level changes by 3/12 of the range.
  • In the fourth hour the tide level changes by 3/12 of the range.
  • In the fifth hour the tide level changes by 2/12 of the range.
  • In the sixth hour the tide level changes by 1/12 of the range.

Note that the above assumes 6 hours; adjust for the actual time interval. On the average, it is closer to 6 hours 12 minutes - but you should still adjust for the actual time interval.

Time of Tidal Highs (or Lows) from One Day to the Next - The 50 Minute Rule

From what was said earlier, we know that from one high tide to the next takes about 12 hours. A more precise number is 12 hours and 24 minutes. So if the first high tide today occurs at 2 AM, then tomorrow's first high tide will occur at 2:48 AM. This is, of course, approximate - but the rule we can state is that the tidal highs and lows each occur about 50 minutes later than the day before.

Rules Related to Navigation

Using a GPS unit obviates the need for many rules of thumb related to navigation, but GPS units can fail for various reasons. Further, learning and understanding such rules help develop an intuition that can be useful in unexpected situations e.g. recognizing that your GPS or compass is starting to malfunction. The rules advanced in this section would profit from simple illustrations, but such are not in my plans. So I suggest you read through a simple navigation boof with illustrations e.g. Ray Killen's Simple Kayak Navigation. The rules below would then become reminders. In any case, I find the selection below useful.

Using Ranges (aka Transits)

If i'm not just meandering, but am paddling toward a particular next destination, then I like to paddle there in a straight line. A novice might point the kayak bow at the goal and just keep it pointed like that until the goal is reached. But if there is a wind or current from the side, the kayak will travel in an arc that is definitely not the shortest path. If you have a deck compass, you could initially take a bearing on the goal and then make that your heading. If you don't have a compass, you can take a range and keep heading so the range is followed.

But what's a range? It's two aligned stationary visible objects, one in the background of the nearer object. For example, let's say our target is a landing quite close to that lighthouse in the distance. Choose the lighthouse as our near object. Then find something further but almost exactly lined up behind the lighthouse e.g. an ugly clearcut on the side of a more distant hill. That clearcut is your second object and now you have a range. As you continue to paddle toward the goal (the landing close to the lighthouse), keep your two objects in alignment and you'll be paddling in a straight line. If you can take a range, it is perhaps somewhat easier than using a compass. Of course, you cannot always establish a range, for various reasons.

Corrections are easy. For example, if the more distant of the two objects seems to be drifting left, then you are drifting left and you correct by moving to the right to reestablish the correct line of travel. Note that as you near your goal, the further object may become obscured and you may need to pick a new range.

Aiming Off

Let's explain this concept by example. We're to meet Jane and Nazrat (her rather backward boyfriend) at the campground on Blunder Island. We know the campground is somewhere on the nearest side of Blunder which presents us with a 4 mile stretch of beach. The campsite is actually hidden in the trees and can only be seen when you're just about on the beach. Although we can't see the campground, we know its coordinates from our chart and can pick the 'correct' compass direction and make that our chosen course. Our navigation likely won't be precise enough to hit the correct spot exactly. So when we hit the beach and don't see the campsite, which way do we go ... right or left? And how far should we go, before we decide we picked the wrong direction?

Aiming off is a way to avoid this problem. We proceed as before, but pick a compass direction that deliberately hits the beach off to one side or the other. Let's say we aim off to the left of the campsite. It must be far enough off so that we cannot accidentally end up off to the right of the campsite. Now when we hit the beach, we know we are to the left of the campsite and just paddle along the shore to the right until we see the campsite.

Doubling the Bow Angle

This rule follows from simple high school trigonometry (proof left as your homework assignment). We'll proceed again by example. We assume throughout the example that the kayak keeps the same heading. Let's say there is a lighthouse off to the right and we want an estimate of our distance from that object.
  • Step 1 - Take a bearing of the lighthouse and calculate the angle between your heading (the bow). Memorize that angle or use your grease pencil to jot it down on your foredeck.
  • Step 2 - Keep a record of the time and paddle (at the same heading, of course) until the bearing of the lighthouse indicates that your bow angle is double the value of the angle from Step 1. Record the time it took to double the bow angle.
  • Step 3 - As an experienced paddler, you should be able to estimate your average speed during Step 2 and be able to calculate the distance traveled. For example, let's say your average speed is about 3 knots and the time elapsed during Step 2 was 20 minutes. Then you know the distance traveled in Step 2 was about 1 nautical mile.
  • Step 4 - Here's the trigonometry. The distance traveled during Step 2 and the distance from you to the lighthouse at the end of paddling in Step 2 are the same. In fact they form two sides of an isosceles triangle. So, from the number example in Step 3, you are now about 1 nautical mile from the lighthouse.
Note: Draw a picture here; it will really help.


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