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Over the course of the past year or so we have seen a large number of well-documented collisions between vessels, ranging from kayaks to tankers to warships. The common thread between each of these seems to be a simple lack of a vigilant and competent lookout, on at least one—and usually all—of the vessels involved.

So let’s see what our actual requirements are for keeping a lookout and what is considered generally prudent seamanship. In this case, the Rules of the Road are simple and explicit—and the international and inland rules are identical.

Rule 4, in this context, simply tells us that everything in Rule 5 applies at all times—day or night, fog or sunshine, no exceptions.

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And Rule 5 itself begins with the clarification, “Every vessel shall, at all times....” This is pretty unambiguous. Whether we are underway, at anchor, or tied to a mooring buoy, and whether the weather is clear or foul, we must always, always maintain a proper lookout. The only exception is when we are actually moored fast to a dock, pier, or wharf.

“Maintain a proper look-out by sight and hearing.” The lookout must be able to see and hear, unobstructed, 360 degrees around the vessel. Depending on the configuration of the vessel, this may well require more than one lookout on watch. In some cases, if they are not distracted by the responsibilities of driving the vessel, the helmsperson may serve as one of the lookouts.

“As well as by all available means appropriate.” This includes radar, ARPA, and AIS. These supplement but can never replace our visual lookout. Nonetheless, if we have an operational radar, we must be using it “to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects,” per Rule 7.

“In the prevailing circumstances and conditions.” Weather, visibility, and traffic density may dictate that we need even more lookouts than the minimum already described.

“So as to make a full appraisal of the situation and of the risk of collision.” This is crucial: Our lookouts must be as knowledgeable about the rules of the road, navigation, and collision avoidance as the captain is. Otherwise they aren’t lookouts at all, just passengers with a better view.

Rule 5 requires that we maintain a lookout “by all available means appropriate.” Rule 7 specifically calls out the use of radar if we have operational radar onboard.   

Rule 5 requires that we maintain a lookout “by all available means appropriate.” Rule 7 specifically calls out the use of radar if we have operational radar onboard.   

Basics of Contact Observation

Even if you are a solitary helm/lookout watchstander, it is sometimes helpful to think about a newly acquired contact as if you were the lookout reporting it to the helm station.

A simple mnemonic for the information we want for evaluating risk of collision is “TARSEC,” which stands for type, azimuth, range, speed, estimate CPA, correlate. A little about each of these meanings:


This is your initial impression of what type of vessel you are observing. “A ship” is less useful than “a container ship,” and “a container ship with a blue hull” is more useful still because you may be dealing with more than one. Why does the type of ship matter to a recreational powerboat? Because different types of ships behave differently, travel at different speeds, and will go different places. For example, a container ship will almost certainly be traveling at about twice the speed of a bulk ore carrier.

Do we know more from her lights or day shapes? Is the vessel restricted in her ability to maneuver? Some details may not be apparent initially, so update your contact appraisal as more information emerges.


“Azimuth” just means a vessel’s bearing along the horizon. We could, of course, just say “bearing,” but that makes our mnemonic clunkier. In any event, what is the relative bearing of the contact? We can think of this in terms of degrees or a clockface, or even compass points if we’re so inclined, so long as everyone on the boat is speaking the same language (in units). Bearings should always be given relative to your own bow, as your lookouts might not always have access to a compass.


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Estimating distances on the water can be challenging. Practice, especially while comparing visual and radar ranges in clear weather, definitely helps. As with azimuth, you have a choice of the units you can use to report distances—some use nautical miles, some use kilometers, others use thousands of yards or meters. But because most modern radars are calibrated in nautical miles and we’ll be correlating our visual contacts to that, we’ll use nautical miles for this article.

An important tool for estimating distance on the water is knowing the distance to the horizon based on your height of eye.

Due to the curvature of the earth (yes, it’s curved), the greater our height of eye above the water, the greater the distance to the horizon. If we are in a kayak, with a height of eye of 3 feet, the horizon would only be about 2 nautical miles away. From the wheelhouse of a trawler, with a height of eye of 12 feet, the horizon would be about 4 nautical miles away. Knowing the actual height above the waterline of your wheelhouse windows, and also any other likely lookout stations, is useful in many other ways as well.

lookout 4 copy

So, if the horizon is 4 nautical miles away, a vessel exactly on the horizon is 4 nautical miles away as well. If a vessel is significantly closer than the horizon, we will see all of the hull in front of the horizon; we call this contact “hull-up.” If the contact is half-way to the horizon, its range is 2 nautical miles. Conversely, if a contact is beyond the “hill” of the horizon, part or even all of the hull will be obscured by it. We call this contact “hull down,” which in this case means that it is greater than 4 nautical miles away.

How far we can see a contact beyond the horizon is a function of its height. The masts of a ship with a masthead height of 100 feet can be seen 12 nautical miles beyond the horizon. This figure, added to your 4 nautical miles to the horizon, would place the ship 16 nautical miles away, which seems like quite a long distance away and hardly a concern. But if that ship is approaching us at 25 knots, and we are approaching it at 15 knots, our relative closing speed is 40 knots. On a collision course, we would hit in 24 minutes.


So how do we know the ship is going 25 knots? Certainly at that range it’s a bit of sloppy guesswork, especially because at that point we only have its masts for identification. As the ship gets closer, our speed estimate can improve.

This is why it’s important to identify the type of vessel as early and as accurately as we can. Weather and conditions being optimal, we can expect a container ship or a cruise ship to be running about 25 knots. A car ferry might run about 20 knots. A tanker or bulker might run about 15 knots. A tug and barge might run about 8 knots.

Even a slow-churning 8-knot tug, oncoming at 5 nautical miles away, will have a combined closing speed of 23 knots if you are going 15. You could still collide in only 13 minutes.

And here you thought those silly word problems in seventh-grade math were pointless.

Closest Point of Approach (CPA)

Advancements in radar technology have made target acquisition and tracking much easier with integrated AIS and Doppler radar.   

Advancements in radar technology have made target acquisition and tracking much easier with integrated AIS and Doppler radar.   

Our visual lookout usually doesn’t need to estimate closest point of approach with great precision—that’s what radar is for. But we do need to have a sense of whether the contact will cross ahead or astern of us, whether it will pass down our port or starboard side, and if we need to maneuver to give them more room to do so. Generally speaking, if the contact is well to the right of your bow and drawing further right along the horizon (not objects on shore), or well to the left of your bow and drawing further left along the horizon, you are probably not developing a risk of collision.

On the other hand, if the vessel is on any azimuth and is not appreciably changing azimuth over time, this is called a “zero-bearing rate” contact and must be considered a possible collision threat.

With a decent estimate of the other vessel’s range and speed, we can also calculate roughly when we will reach CPA with the contact. This is especially critical when we are interacting with multiple contacts, which in a busy harbor is nearly always. And in a busy harbor, it is easy to become overwhelmed with the number of contacts. The ability to quickly estimate CPAs will help us focus on the contacts that actually pose a threat of collision.


Correlating our visual contacts with radar and AIS, when available, is fundamental to good seamanship and collision avoidance. Often, even in clear weather, radar will detect small contacts earlier than you will be able to see them. Clear communication between the visual lookout and the radar lookout is critical for collision avoidance.

Radar observation will provide a much more precise estimate of target range, speed, and CPA. Communicating this more precise data to your visual lookouts allows them to better assess information that the radar observer may not have, such as light configurations. Target maneuvers are almost always more readily observed visually than by radar, as are relationships between vessels such as a pilot boat intercepting a tanker.

No single source of contact information is best for all purposes. This is why we have multiple sources. If our different sources are not in agreement, slow down and figure out why. In nearly every collision, the correct information to avoid it was available and correlated, but it was unheeded or misinterpreted.

Good watch!