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Trireme Rowing Geometry, 6th Century B.C. (Time-Life Books: The Ancient Mariners, Page 71)

Sunday, December 18, 2016

Piloting Part 5: Dealing with Currents


Introduction


Ignore this post if you ONLY row/paddle in landlocked lakes or reservoirs... there are no currents. But if you ever even think about rowing in rivers, or bays/estuaries that are affected by tides, then continue on.

Tide and Currents


The tides are the result of the gravitational pull of the moon and sun…  the moon’s gravitational pull is about twice that of the sun. The gravitational pull causes a ‘bulge’ in the earth’s waters, about 18 inches (46cm) in mid-ocean. There is a corresponding bulge on the opposite side of the earth, and two ‘hollows’ 90° from the bulges. These bulges and hollows circle the earth about every 24 hours and 50 minutes.

But we don’t typically row in the middle of the ocean… we travel in bays and estuaries connected to the oceans. The bays and estuaries have water levels that attempt to keep up with the bulges and hollows in the oceans. They do keep up, but there are delays while the bays and estuaries fill up with water and then empty to match the bulge/hollow as is passes. This emptying and filling results in tidal currents.

The tidal currents, though caused by the tide, are not necessarily in sync with the tide. This means that the strongest tidal current may not be (and in most cases seldom is) at the half way point of the flooding or ebbing tide.

How do we know how strong the current will be, and when will it occur?

Current Tables


In the United States use NOAA Current Predictions: select the state, then the ‘station’. The image below is what will be displayed. At the bottom of that screen, you can change dates to be displayed as well as other parameters.

In the chart heading is the ‘true’ direction of the current, e.g. “Mean Flood Dir: 30° (T)”. (Subtract your local declination to convert 'true' to 'magnetic' direction of the current.) The peaks and valleys of the chart are also given in a text file on the right side of the screen. The speeds, in knots, are the maximum estimated speeds.

Example NOAA Current Table

Outside of the United States, NOAA provides a listing of resources for tide and current information by country.

Using the Currents


The current predictions (how fast and when) are for a single designated point. The current speed can be very different just a short distance away.  What are the factors that affect these differences in predicted speeds/times and how do you use these differences while you are rowing?


  • If a confined waterway (e.g., between two islands) becomes narrow (in width and/or depth of water), the speed of the current increases in proportion to the amount of narrowing. For example, if the waterway narrows 50%, the speed of the current doubles. Another application: Rowing under a bridge… stay equal distance from the two abutments to avoid eddies that occur as the faster current flows around the abutments.
  • The current is slower closer to the shore in a wide bay or estuary because the water is shallower. If you are rowing with the current, stay in the middle or deepest water where the current is stronger. If rowing against the current, stay closer to the shore.
  • The current is stronger on the outside of a bend in a narrow waterway, such as a river… and slower on the inside of the curve. If you are rowing against the current, stay on the inside of a turn… and if rowing with the current, stay on the outside of the turn.
  • A point of land protruding into a waterway often creates a ‘back eddy’ (reverse current) downstream of the point. Heading downstream, row well outside the point to avoid the back eddy. If going against the current, stay inside the point as long as possible to take advantage of the reverse current.
  • A steady wind blowing over a body of water for at least half a day will start a surface current flowing at a speed of about 3% of the wind speed. So an extended 20 knot wind will create a 0.6 knot surface current. Note that the waves created by this wind will increase in size if the tidal current is in the opposite direction.

How Fast is the Current?


When we do a look-up in the ‘current table’ (example above), the speed value is the estimated fastest speed of the current. As you can see from the discussion above, current speed varies a great deal for a number of reasons.

One way of determining  current speed is to use ‘speed made good’, which is your actual speed (measured along the shore… i.e., between fixed points) in comparison to your speed in the water.
Example: You are rowing at 3.5 knots and you row past two buoys that are 0.9 miles apart (according to your chart) in 27 minutes. You remember the ‘sixty D street’ formula (60D=ST), solve for S, giving you Speed = 60 x 0.9 / 27, which is 54/27, or 2 knots speed made good. This means the current is 1.5 knots against you (3.5 – 2 = 1.5).  
Example: But if it only took you a little less than 11 minutes to go 0.9 miles (still rowing at 3.5 knots), speed made good would be 60 x 0.9 / 10.8 which is 5 knots speed made good… you’re getting a 1.5 knot boast in speed from the current.
Two issues with this technique: You have to do some math in your head (the real world usually doesn’t come out in nice round numbers like these examples). And second, we have not accounted for wind. We’ll talk more about the effect of wind on rowing in Part 6 of this series.

Another way to determine current speed is to measure the current directly:
Current speed = 0.6 x Feet per Second.

Example: If I’m anchored in my 15 foot boat, and a twig floating in the water takes 5 seconds to go by my boat, the speed of the twig (current in this case) equals 0.6 x 15/5, or a 1.8 knot current. Note that this formula can also be used to measure boat speed…
Example: I’m rowing in Newport Harbor (Rhode Island) and spot a 12 Meter anchored… typically they are 70 feet (21.3m) long. It takes me 10 seconds for me to row by it… my speed is 0.6 x 70 /10 or 4.2 knots.
Example: I’m rowing Barnegat Bay early morning, no wind, at slack tide. I see a channel marker in my mirror… it takes 3 seconds for me to pass the marker from stem to transom (15 feet)… I’m rowing at 3 knots. (0.6 x 15 / 3 = 3 knots)

Crossing a Current 


The current table gives the estimated highest speed of the current, typically in the middle or deepest part of the waterway. The current becomes progressively less as you approach the edges of waterway. Because of this variability, it is difficult to predict where you’ll end up when you cross a current.

Let’s say you want to cross a river and end up at point X on the other side. In general terms, there are four ‘strategies’ to get to point X:


  1. Aim for point X (let’s assume 85°m) and start rowing. You’ll end up an unknown distance downstream (in the current direction) from X. Not a good strategy.
  2. Row upstream close to the edge of the waterway where the current is weakest. When you are ready, head across the waterway at 85°m. The current will take you downstream toward X. Where you’ll end up depends upon how far upstream you went before crossing. But you’ll be better off than using strategy 1.
  3. Use a range (two landmarks in line with X, an LOP). As the current keeps pushing you downstream of the range, change your course upstream. Continue to adjust your course (upstream or downstream) to stay on the LOP. This strategy, combined with strategy 2., has the advantage of landing at X and of minimizing the amount of upstream rowing/paddling that must be done in the strongest current.
  4. Do the following calculation to get the upstream angle (called the Ferry angle) you must apply to your course in order to get to X. Ferry angle = 60 times current speed/rowing speed.
Example: If the azimuth to X is 85°m, current speed is 1.5 knots (at about 180°m) and your rowing speed is 3 knots, then the ferry angle is 60 times 1.5/3, or 30°. This means you should row/paddle on a course of 115°m (85° +30°). And you’ll end up at X… well, not really. Because the current speed is not constant all the way across the waterway. You’ll probably end upstream of X, which is usually not a bad thing. 


In this post, we’ve discussed currents and how to deal with them. In the next post in this series on piloting, we’ll focus on how to deal with wind... but first we'll show you another John Welsford design and how it was used in the Texas 200.

The fine print...


I’m not a professional pilot. I try to be accurate and I check my information, but I’m not perfect. This post is for information purposes and is intended to be only a starting point for learning the skills of piloting. As with any activity with a small boat, there is always the opportunity for ugly surprises. Practice the skills under ideal circumstances and you’ll increase the probability of being able to use the skills during an ugly surprise to keep you and your boat safe.

2 comments:

  1. Figuring out and timing the currents is an interesting part of row cruising in tidal estuaries. Timed right, a 1.5 kt flow is a big boost for a 3 kt rowboat, and it is 3 times the speed of making a mistake and trying to row against it (4.5 kt instead of 1.5 kt).
    Current flows are not in phase with tide heights, so you have to find current tables, try to understand the streams and basins, and/or just go out and see what happens.
    Current may be maximum when the tide height is changing fastest (mid tide) on a deep channel without large basins. If a narrow or shallow channel is feeding into a large basin, the current can be greatest near the high and low tides. Large bodies of water can have strong inertial effects, such as vortices, that last for hours. The Canadian Current Atlas of the Salish Sea area shows many of these.

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  2. Hi Rick... Thanks for the insight from your own experience...
    Have a wonderful, and safe, Christmas and 2017... Make that also 'healthy' for 2017!
    Tom

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