Download Lifeboat Navigation - Marion to Bermuda Race

Lifeboat Navigation: Back to Basics
by Bill Biewenga
P.O. Box 686
Wellfleet, MA 02667
Cell: 401-578 4901
E-Mail: [email protected]
Whether you’re in a lifeboat or the electronics on your 120 foot motorsailer have just been
smoke-tested in a lightening strike, knowing a few navigational tricks can come in very handy. As
navigator, you’re still responsible for getting your vessel from Point A to Point B as safely as
possible. Having a navigator’s bag of tricks may not only help in that responsibility, it’ll also help
you to feel more at home as you venture across oceans.
First and foremost, you need to know where you are. Keeping an accurate, up to date log
will go a long way in getting you started correctly if your instruments begin to blink, smoke, or
melt down. While offshore, you should make log entries at least every 3 or 4 hours, and as you
close with a coast line, you should make entries at least every hour. In coastal navigation
situations, you should plot your position on a paper chart more often as you go on soundings. Then
if you lose your electronics, you have a good place from which to start your DR (dead reckoning).
As a minimum, the log entries should include date and time, position, speed, heading, wind speed,
wind direction and any other relevant information.
In an age of electronic navigation, we all become increasingly dependent on GPSs. Having
a battery operated, handheld unit or two with spare batteries could be a godsend in a time of need.
If you have a remote antenna, charging unit, and an interface capability with your on-board
computer, you may go a long way to making an emergency transition relatively painless. Keep in
mind that some instruments like Standard C communications devices also have a built in GPS that
can be utilized if your main unit goes on holiday.
Sextants still work, even in the Information Age. Without any form of electricity, it may be
the only source of positioning information. Having the necessary tables and a working knowledge
of how to use a sextant could be a very worthwhile resource.
If we know where we are, the next bit of information that will be of interest to us will be
which way we’re going. There are a number of tricks to figure out our heading. Some are more
obvious than others are. The first choice would be to find the North Star at night and see how our
course compares to its location if we’re in the Northern Hemisphere. If our compasses have been
remagnetized in a lightening strike, we can head our vessel towards Polaris, noting the compass
bearing to the star. Allowing correctly for magnetic variation, are you pointed toward true north?
If not, how far out are the compasses. If locating the North Star is a problem or as an additional
check use Orion’s Belt if it’s low enough on the horizon. In the constellation Orion, the uppermost
star in the belt rises almost exactly in the east and sets almost exactly due west.
In areas south of the Caribbean, where Polaris may not be visible, the Southern Cross can
help to guide the way and will indicate south. The south celestial pole is located approximately at
the intersection of a line through the longer axis of the Southern Cross with a line from the
northernmost star of Triangulum Australe perpendicular to the line joining the other two stars of the
triangle. No other conspicuous stars mark this spot. Star charts can provide a very exact means by
measuring the accuracy of your compass. Even a copy of Reed’s will provide a table of the sun’s
true bearing at sunrise and sunset for a given latitude and declination.
Of course during the day, we know that the sun rises somewhere in the east and sets
somewhere in the west. But a few hours after sunrise, especially in the tropics, determining
direction using the sun can be refined for our bag of tricks. To help determine direction during the
day you can use a watch with an hour hand if you don’t have a functioning compass. Point the hour
hand toward the sun. An imaginary line halfway between the hour hand and an imaginary line to
the 12 will point approximately toward south. Naturally, a compass is more accurate, but at least
you will have an idea of how to check your deck compass to make sure it hasn’t been totally
rearranged by the lightening strike. And if you don’t have a compass, your watch could be the next
best thing.
If we know where we are and where we’re heading, the next thing we’ll need to know for
our DR is how fast we’re going there. How can we determine speed without benefit of electronics?
On some of my early deliveries, a mere 25 years ago, boats often carried taffrail logs to determine
speed and mileage. Spinners were towed behind a boat, and as the line twisted, an odometer
recorded the information. A mechanical device, the taffrail log wasn’t dependent on electricity, but
the spinners were susceptible to being eaten by sharks on occasion. Those unaccustomed to sextant
and taffrail log would be amazed at how accurately landfalls could be predicted. And that form of
navigation could be performed without benefit of electricity.
In most cases, however, the taffrail log has become extinct as an available navigational tool.
Returning again to basics, speed can be quickly calculated by tossing a floating chip
(biodegradable, please) off the bow and timing how long it takes to float a given distance beside the
vessel. Speed is a measurement of distance divided by time in which 1 foot / second = .5925 knots.
If you multiply both sides of the equation by the same number, the new values will also be equal.
If, for example it takes a wood chip floating down the length of a 60 foot vessel 5 seconds to
complete the distance, you merely multiply 60/5 x .5925 to find the speed in knots. The boat would
be traveling at 7.11 kts approximately. If you etch the equation “1 foot/second = .5925 kts” in the
bottom of the nav table top, you’ll always be able to figure out your boat speed. Doing the timed
procedure several times and averaging the results may help to provide a bit more accuracy.
Now that you know where you are, and how fast you’re going someplace, you’ll probably
still be interested in the situation surrounding you such as wind speed and the distance off objects
on the horizon. Observation will help answer a number of questions for the navigator. Wind speed
can be approximated using the Beaufort Force Scale. Originally developed to provide a scale based
on visual observations of the sea surface, you can get a very rough equation of wind speed by
subtracting 1 from the number of the Beaufort Force and multiplying that by 5. The resultant
number will be a rough approximation of wind speed in knots. (i.e.: Force 7 is about 30 knots +/((7-1) x 5 = 30). Also it can be helpful to understand that whitecaps begin to form in about 11 or 12
knots of true wind speed. When the water temperature is very cold and the air temperature is rather
warm, such as in the morning during the spring, wind at the masthead may not match the wind
speed on the water’s surface, however. Those conditions are conducive to wind shear.
Radar certainly has its advantages in determining how far an object is away, but you can
still get a good idea of how far away a ship or landmass may be by using the following formula.
Distance to the horizon in nautical miles is approximately 1.15 x the square root of h, where h is the
height of the eye in feet. In other words, if the height of your eye is 9 feet above the surface of the
water, the horizon is about 1 1/7 x 3 = about 3 ½ nautical miles. If you were trying to see the top of
a mast that was 81 ft high, you could calculate its distance by the same formula. 1.15 x square root
of 81 = 10.35 nm to an observer with a height of eye of 0. At a height of eye of 9 feet, the top of
the mast would be breaking the horizon at about 10.35 nm + 3 ½ nm = about 13.8 nm.
There are of course a number of 20th Century navigational tricks to draw upon. I’m sure
many of you have heard about navigators following jet contrails to Hawaii, but I’m not sure how
many navigators found Tahiti that way by mistake. From offshore, Atlantic City’s glowing
electrical lume can be seen about 20 miles or more. Traveling through the Bahamas and other
places, you can even smell charcoal being made several miles offshore. For depth, breaking waves
can warn you about shallow water when compared to a calmer sea state near by. Your senses and
keen observation can tell you quite a bit.
One of the more amusing anecdotes tells about “potato navigation” in Maine’s coastal fog.
“Send the person with the strongest throwing arm up to the bow with a 10 lb. bag of potatoes,” the
story goes. “As you approach the anchorage, he should throw a potato every 30 seconds. As long
as he hears a splash, you’re not too close to the shore. When he hears a thud, you’re too close.”
While the story may be a bit of a joke and distort prudent navigation, a point can be well taken.
Listen to what is happening around you as well as watch. And in a time of need, there can be more
to navigation than mere electronics.
END
Bill Biewenga
Tel: 401-847 0867
Fax: 401-847 2698
Website: www.WxAdvantage.com