After a conversation with a friend about being aware of your surroundings, especially at night, and recounting some very close calls, I decided to share some thoughts on judging distance.
We know that a vessel with a constant relative bearing and a closing distance is going to end up on our fo'c'sle, which is never a good thing.
I understand this might sound a bit flippant, but I've witnessed a few occasions where the lateral mark navigation light being observed was much closer than the skipper of the vessel I was following thought, as they gently grazed it while passing. Top tip, if you can see the light reflection on the water you are inside 200m.
While the universal use of GPS and AIS has undeniably improved safety at sea, the issue I've observed (as also noted in MAIB accident investigation reports) is that many collisions and incidents occur because there is too much focus on the chartplotter and not enough on observing the surroundings. After several discussions on the pontoons in the marina, it became clear that some are blissfully unaware that vessels under 300 tonnes do not need to carry or operate AIS. A visual watch is crucial to avoiding hazards not displayed on the screen, such as other small craft, lobster pot floats, and debris carried by the tide that could damage the boat. In my operating area, on a spring tide, it is common to encounter large trees heading your way on the ebb tide.
Being able to judge distance other than relying on the chartplotter is, in my opinion, a vital skill for any skipper.
Here are a few thoughts on judging distance, most of which will require practice to hone those skills. Let's start with some relatively straightforward ones.
- When looking at the shoreline with normal eyesight, if you can see people with the naked eye, you are likely inside a mile. Beyond that, they would be indiscernible as people.
- When looking at the base of cliffs, if you can see the surf (on a moderate day), you are approximately 3 miles away (refer to the almanac distance of the horizon table).
- Looking at a ship in the distance, the relative bearing will likely change slowly. At around 3 miles, you need to decide if any avoiding action is necessary. A ship traveling at 18 knots will cover that distance in 10 minutes, while at a more sedate 10 knots, it will take around 18 minutes. Large vessels, ranging in height from 40 to 70 meters, will appear over the horizon at approximately 15 to 20 miles away.
In my operating area, the Bristol Channel, it is a busy commercial area with vessels ranging from pleasure craft, tugs and dredgers, all the way up to very large car carriers, I often challenge my students to calculate when approaching vessels will be on our beam, using Marine radar gives them all the AIS information they need, I provide them with our speed, and the rest is simple maths. Closing speed and time sometimes surprise them at times but are worth exploring if you haven't done it before. Returning to our ship, if you can see the waterline, it is around 3 miles away (10 minutes at 18 knot and 15 minutes at 12 knot transit speeds).
With a height of eye of 3 meters, the horizon is 3.6 miles away (at 2.9 meters, this drops to 2.9 miles). If you can see the bow wave and the waterline at that height, the object is within that distance. Although judging distance is subjective, it's worthwhile spending some time determining an accurate height of eye for the vessel you are operating. With time, perseverance, and the ability to cross-check using your chartplotter, radar, or AIS, your calculations will become more accurate. Reed's Almanac has a table providing the distance of the horizon from different heights of the eye (in my version, it is on page 44).
Doubling the angle off the bow is a useful skill to have. For example, if there's a lighthouse on your passage, and 1 mile to the southeast of it, there are overfalls you want to avoid. If you're on a heading of 240º and take a compass bearing of the light, noting the distance log, and continue until the light bears 300º, twice the original relative bearing from your heading, the distance traveled between the two bearings (corrected for tidal stream) is your distance from the lighthouse at the second bearing. In this instance, the distance off is a safe 1.4 miles, requiring no adjustment of course.
The rule of 1 in 60 is useful when looking at bearings. If you know the range of an object or transit marks, you can use this to estimate how far off track you may be. The rule states that over 60 miles, one degree of error equates to being one mile off course. Scaling it down, over 1 mile, one degree is equal to a distance of 30 meters.
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In this picture the 2 buoys have 20º between them, the range to the buoys is 1 mile (20º x 30m = 600m) |
To become competent at estimating distance with any degree of accuracy, these few skills whilst needing a little work will help you become safer at sea. With many of the fair weather mariners cleaning and changing elements of their vessels on their hard standings, it is a bit quieter out there. Use this time to get out and have some fun.
Have a great day.