148 seamanship secrets
THE DANGER OF USING “SCANTY INFORMATION”
Rule 7, Risk of Collision, warns the mariner not to be lazy. The rule explicitly
states that you should make no assumptions based on “scanty information.”
If you sight a contact at 6 miles, don’t blow it off. Start tracking the bugger
as if it’s an enemy U-boat! Here’s why.
In the Straits of Florida the big boys are doing 25 knots, riding the Gulf
Stream. Let’s put one of these vessels into play with a 5-knot sailboat traveling
eastbound for Key West.
A 25-knot freighter overtakes the 5-knot sailboat at 20 knots (25 knots –
5 knots = 20 knots).
Sighting the freighter at 6 miles, you have 18 minutes to maneuver (60 minutes ÷
20 knots = 3 minutes per mile; 3 minutes × 6 miles = 18 minutes).
Since you only have 18 minutes (0.3 hour) to maneuver, you must act
immediately to increase the passing range to 1.5 miles (5 knots × 0.3 hours =
1.5 miles).A distance of 1.5 miles might sound large, but that is not the case. These ships
eat up that distance in the blink of an eye. You must open your range to more
than you think you need. If the ship changes course for some reason, you’ll
need even more time to maneuver to avoid collision.
SEA-CRET TIP
X Train your crew on basic radar plotting and drift bearings. If you’re
below deck and off watch, make sure they know when to wake you.
On many large vessels, the watch must notify the skipper if a radar
plot shows any contact that will pass within 2 miles.Radar Scope Plotting
Plot directly onto the glass or plastic dome cover—called a scope—on your radar.
Th is direct surface method, called radar scope plotting or radar contact plotting,
gives fast, accurate results for collision avoidance. All radar plotting begins with
a determination of another vessel’s closest point of approach, or CPA. Once you
acquire a target, begin a scope plot to answer theses four vital questions.
- How close will the contact come to your vessel?
- Is this a meeting, crossing, or overtaking situation?
- How much time do you have to maneuver?
- What is the best action to avoid collision?