Mark Wallace's Black Skiff

Sunday, October 30, 2016

Piloting, Part 2... Preparation


Google Maps

Google maps are available for virtually every location on earth. The resulting maps show roads and cities, in very fine detail as you zoom in.

The “Earth” option (click on lower left corner button, “Earth”) displays a satellite view of the map (or a 3-dimensional globe with the location highlighted.)

You can also enter a latitude and longitude. For example, enter “41 26.00 - 44 13.10” (without quotes) in the search field and it will show you a location in the middle of the North Atlantic Ocean. This would be useful if you are planning an offshore cruise and intend to mark your proposed course on the map.

Waterway Guide

Waterway Guide can also be printed (and waterproofed). They show anchorages as well as other features such as navigation buoys (click on the "I'm looking for..." drop down button).  Areas covered are Great Lakes, Lake Champlain, Saint Lawrence River, Georgian Bay, North Channel, US East Coast, Bahamas, Cuba, Inland rivers Chicago to Mobile and Gulf coast Florida Keys to Texas.

Tide and Current 

Tide and current tables from NOAA are only for the United States


Windfinder provides wind, temperature,  precipitation, cloud cover for 40,000 ‘stations’ around the world.

The iPad app (Windfinder Pro) displays, on a 3-hour basis, wind direction/speed, cloud cover, temperature, wave direction/height, and tide condition. All this information for ‘today’ and forecast for the next 9 days. Access this the day before you start and print.

Magnetic North (compass), True North (charts) & Declination

Charts and maps, including Google maps, use ‘true’ north (top of the map/chart is toward the North Pole). Actually, they use ‘grid north’, but for our piloting purposes, assume they are true north. However, compasses use ‘magnetic’ north. Azimuth readings taken off a chart (a true azimuth) must be converted to a magnetic azimuth… declination for the location is needed to do the conversion.

What is Declination? 

Declination defined by NOAA.
“Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. Declination is positive [if magnetic north is] east of true north and negative when west. Magnetic declination changes over time and with location.“

Finding Declination

Go to this site to find the declination anyplace in the world and enter the location of your ‘cruise’.

Calculation example:

If the declination is 12°, 34'' W, I’d use -13° for our piloting purposes.  Here are the formulas to use:

Magnetic North = True North – Declination
If the azimuth on the chart is 327°t, then
Degrees magnetic = 327 minus (-13)
X°m = 327 - (-13)
= 340°m, the heading I would use on the compass.

True North = Magnetic North + Declination
If my compass reads 340°m, then
Degrees true = 340 plus (-13)
X° t = 340 + (-13)
= 327°t, the azimuth I would plot on the chart.

A Row Cruise Example

Let’s say I’m planning a row cruise in Barnegat Bay (New Jersey), exploring from Manahawkin Bridge north to Barnegat Light. I’d go to Google Maps. Using the built-in ‘distance tool’ (see Piloting: Part 1… Equipment for how to use this distance tool). I’d mark a proposed route . However, I’d want to explore the wildlife refuge on the Western shore north of the bridge, so the actual miles will probably be greater than the 30.4 miles I plotted.

Google Map of Proposed Route 

The launch site I’ll use is on the east end of Cedar Run Dock Road, so I print a chart, using the Waterway Guide that includes the launch site and Manahawkin Bridge.

Waterway Guide Chart: Launch Site to Manahawkin Bridge

 Rowing north in the Bay, I’ll go from Manahawkin Bridge to Sedge Island and I print that portion of the route.

...Manahawkin Bridge to Sedge Island

The next chart I print goes from Sedge Island up to Barnegat Inlet, where the lighthouse is located. The Waterway Guide shows two anchorages, one in an enclosed bay west of the light, but only accessible from the north and a second at Conklin Island. I’m not sure which anchorage I’ll use, depending upon how tired I am and which way the wind will be blowing.

...Sedge Island to Barnegat Light
Since I may be exploring around Barnegat Inlet, I also do a more detailed chart of the area.

Detail of Barnegat Inlet Area Along with a Magnetic Azimuth ("81 m") from Conklin Anchorage to the Light

I do want to explore the Wildlife Refuge, so I print a chart that covers most of the Refuge.

Edwin B. Forsythe National Wildlife Refuge

Adding 'Compass Azimuths' to a Chart

When planning a cruise, I may want to not only mark the chart with my proposed route, along with detail charts throughout the route, but also to add compass headings for my proposed route and bearings to land based objects that are on the chart and will be visible.

To do that, I must convert the azimuth reading from the chart to a magnetic reading I'll use on my compass. To get the magnetic azimuth, use the formula:
Magnetic = True minus Declination.
I remember this with "magnetic-minus" (M and M). So I take the chart (true) reading and subtract the declination... for this area, the declination is 13° West, or -13. An example is in the chart below.

From the chart, the true azimuth from just northwest of Barnegat Light to the mouth of Forked River is "328°t". Applying these numbers to the formula, m = 328 - (-13) and therefore the magnetic course is 341°m. Disregarding wind and current, if I maintain a heading of 341°m from the tip of the peninsula northwest of the Lighthouse, I'll be at the mouth of Forked River.

But please note, if I use the true azimuth (from reading the chart) on my compass, I'd follow the dotted line (328°m) and end up south of Forked River.

Example of not applying a declination correction

Tide and Wind Charts

The tidal range at Barnegat Inlet is 3 to 4 feet (.9m to 1.2m). I’ll print the tide chart for each day I’ll be on the water just before I leave.

Tide Chart for Barnegat Inlet

Also, the day before I leave, I’ll print the Windfinder Table that gives me a good indication of the weather to expect.
Example of a Windfinder Table

Other Preparation

One of the preparation steps you can take that will pay ‘piloting dividends’ for ALL your row cruises is to calculate rowing speed under various conditions.

Example, using my iPhone Cyclemeter app, I rowed a half mile in 8”35’’’ for a speed of 3.74 mph (a little above average ‘cruising speed’ for me) using 166 strokes, which works out to 15.9 feet per stroke (let’s call it 16 feet per stroke) and a stroke rate of 19 strokes a minute. Of course, how tired I am, wind and current conditions all will affect the outcome, but setting base line numbers (more than just this one base line)  for different conditions gives me the ability to estimate distance using just time or number of strokes. We’ll talk more about estimating distance in a later post in this series on piloting.

The ‘speed’ formulas to remember are:

  • Distance (in miles/kilometers) equals Speed (in miles/kilometers per hour) multiplied by time (in HOURS)
  • D = ST (Think “D STREET”)
    • (Divide both sides of the equation by T results in Speed = D/T)
    • (Or divide both sides by S results in Time = D/S)
  • 60 times Distance (in miles/kilometers) equals Speed (in mileskilometers per hour) multiplied by time (in MINUTES)
  • 60D = ST (think “60 D STREET”)
    • (Divide both sides of the equation by T results in Speed = 60D/T)
    • (Or divide both sides by S results in Time = 60D/S)

Water ‘proof’ the charts you’ve printed out with a clear finish  such as Rust-Oleum Clear.

To keep the charts dry, yet available, make a water resistant chart holder.

For example, varnish or paint a plywood base 8½” by 11” (assuming the charts/tables you print are on 8½” by 11” paper). Cut a piece of clear acrylic (Lucite, Plexiglas, etc.) 9” by 11½“. Glue a ¼“ wide piece of self-stick weather stripping around all four sides of the bottom of the acrylic so that the plywood base, and charts, fit inside the weather stripping and up against the acrylic. Hold it all together with spring clips. The weather strip will prevent rain/spray from seeping under the edge of the acrylic (though it will not protect the charts under the acrylic from total immersion).

Chart Case

The fine print...

What we’ve discussed here is the basic cruise preparation from a piloting point of view… Preparation of your boat, your fitness, your food and your equipment all must be done in addition to the piloting preparation introduced here.

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.

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