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scraph 03-30-2011 07:02 PM

Posting a proper visual lookout: A discussion
 
Rules of the Road, rule 5, states "Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision."

Believe it or not, this rule does not require a look-out to be topside 24/7 making continuous visual sweeps. However, it also does not abdicate a sailor from keeping a proper lookout simply because he has radar and AIS installed. I would like to share a technique of maintaining a proper lookout that I have not seen discussed in any bookstore available books on seamanship.

My discussion will be addressed towards open ocean sailing with excellent visibility (but the technique can be adapted as necessary for other conditions). On the open ocean, we can expect displacement-hull vessels to be operating and as such, the longer of them are the faster. Therefore, one collision risk at sea is with a large ocean going tanker. Of this class of ships, we can estimate a masthead height of 120ft and a cruising speed of 12kts (distance to horizon for 120ft is 13.4 nautical miles).

Let's suppose that we are cruising along at 6 knots and from the cockpit our height of eye to scan the horizon is at 8ft above sealevel (distance to horizon for 8ft is 3.5 nautical miles).

Visibility supporting, we should be able to gain first sight of this large tanker at a distance of 16.9 nautical miles, 10ft of its mast at a range of 16.4nm, and 50% of its height at 13nm.

Now, lets talking about collision. The worst case risk of collision is if this tanker is headed directly towards us and we are heading directly towards them. With our combined speeds, that would put our worst case closing rate at 18kts (12kts(theirs)+6kts(ours)). If our look-out goes below and takes his eyes off the horizon the moment that a tanker appears on the horizon and we are both closing one another at 18kts then it would take just over 56 minutes for us to collide (16.9nm / 18kts = 56.3 min). Obviously, collision is not acceptable so 56 minutes is too long for a look-out to take his eyes off the horizon. Lets pick a minimum safe range of, say, 1nm. To ensure that is maintained, our lookout cannot go below for longer than 53 minutes (15.9nm / 18kts = 53 minutes). 53 minutes is an odd interval to maintain a lookout on so ... lets make it 45 minutes. At a 45 minute 'look interval', we ensure a worst case warning of 3.4nm. The look interval is the time required between a look-out performing subsequent full 360-degree scans of the horizon.

Now that we have come topside with a large tanker heading directly towards us with 3.4nm of warning, we turn off our current course by 90 degrees. The time required for the tanker to close that last 3.4nm is roughly 11 minutes. In that eleven minutes, our new course will carry us 1.1nm away from the tankers path when he crosses behind us.

In this case, worst case collision course, maintaining a 45 minute 'look interval' will provide a CPA of roughly 1nm from this hypothetical tanker.

Another collision possibility, lets suppose, is another sailboat just like ours who is not maintaining a proper look-out. Let's take a Beneteau First 36.7 (masthead height 55.75', hull speed 7.7kts). To maintain a CPA of 1nm after avoidance from this contact would require a 'look interval' of 48 minutes (the math on this case is up to you). Since 45 minutes is the shorter of the two calculated look intervals then it will keep us safe from collision for both.

This sort of technique to maintain a proper lookout is used widely and is sufficient to 'maintain a proper lookout' as is required and prudent. The method to calculate a look interval can be modified to account for lowered visibility (if visibility range is less than the combined distance to horizon) by replacing the 16.9nm range from above with the visibility range in the calculations. Coastal traffic can be compensated for by assuming collision with a different class of vessel (for example, a coastal tanker or small freighter when nearshore) and making appropriate assumptions of likely cruising speed and masthead height. Our actual vessel speed, being known, should be substituted for the 6kts used in the above examples.

This method can even be used for extreme navigation conditions such as heavy fog. You might find, given heavily reduced visibility, that your safe look interval comes out to be a matter of seconds and that safe CPAs can't be maintained any less than visibility range itself. Of course, if CPA can't be maintained less than 20ft (because visibility is 20ft) and your look interval comes out as 2 seconds ... you should probably be anchored out of the channel anyway.

Grendler 03-30-2011 07:28 PM

Quote:

Originally Posted by scraph (Post 714813)
Rules of the Road, rule 5, states "Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision."

...

This method can even be used for extreme navigation conditions such as heavy fog. You might find, given heavily reduced visibility, that your safe look interval comes out to be a matter of seconds and that safe CPAs can't be maintained any less than visibility range itself.

Correct and interesting theoretical calculations.

Real life data shows that boats move at speeds of 30+ knots being 10feet high...

My rule of thumb is to keep a constant watch on board while close to the land (min. 2 people at night). When cruising >100NM from the land and knowing no fast ferry routes are going nearby we can all go down for lunch but with the radar alarm on and not more than 15 minutes without anyone on board.

JohnRPollard 03-30-2011 07:52 PM

Quote:

Originally Posted by scraph (Post 714813)
Rules of the Road, rule 5, states "Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision."

Believe it or not, this rule does not require a look-out to be topside 24/7 making continuous visual sweeps....

Can you cite a source for that interpretation? The language you cited clearly states otherwise.

Quote:

The worst case risk of collision is if this tanker is headed directly towards us and we are heading directly towards them.
In my mind, the worst case is that you missed the freighter in your horizon scan, when it was still hull down, especially during the daytime. Maybe you were out of sync on the wave crests, maybe there is distortion at the horizon from humidity, etc.

As for the closing time, you seem to be grossly underestimating the speed of modern tankers and freighters. Speeds in excess of 20 knots are common, and some are even moving over 30 knots. As are many naval vessels. For purposes of your calculations, you should double those closing speeds and halve the time.

I have watched fast ships come within sight, and then disappear over the horizon, in far less than an hour.

catamount 03-30-2011 08:22 PM

Around 20 minutes is what many single-handed racers seem to set their sleep timers for....

scraph 03-30-2011 09:01 PM

Quote:

Originally Posted by JohnRPollard (Post 714830)
Can you cite a source for that interpretation? The language you cited clearly states otherwise.

"Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision."

The language does not say a man must be topside at all times and making constant sweeps ... It says 'proper lookout' ... by 'means appropriate' ... in 'prevailing circumstances' ... to make a 'full appraisal'. That is not a very specific statement ... it is, in fact, a rather vague statement with a lot of 'qualifiers'. The term 'lookout' isn't even defined by the rules. I would argue that the language does not clearly state otherwise ... but states just what it says.

The effects that you mentioned ... sea state, visibility, etc ... would fall into 'prevailing circumstances' category. A lookout who misses a mast on the horizon ... simply due to inattention ... is certainly not a sufficient lookout to satisfy the rule. 'Full appraisal' requires sufficient thought into your watch standing that no new data, within reason, can be expected between log sets. If vessels that you can reasonably expect to encounter can't become visible and then strike you in 10 minutes ... then you certainly don't need to be scanning every 2 minutes.

Post whatever sort of look-out you wish for whichever concerns you have. The point is ... continuous visual scanning of the horizon is not required by this rule and isn't certain to improve safety. I would argue that a purposeful and directed full scan of the horizon at a set interval is much more likely to fully appraise the situation than a man lounging in the cockpit 'keeping an eye on things' will. If your concern is with 40 knot freighters in the middle of the Atlantic ... then modify the look interval accordingly. If your concern is with a planing-hull power boat in the middle of the Atlantic planing along at 40 knots ... I mean, really? I would say expecting power boat traffic would put you into the nearshore category which is a situation I addressed. Or ... it can fall into the 'proper lookout by hearing' portion which I have not addressed.

Naval vessels ... we post constant lookouts and man 'bridge-to-bridge' radios. I seriously doubt you will be struck by a cruiser in the middle of the ocean in conditions of excellent visibility. The predominant risk of collision you have in the open ocean is with large, slow moving freighters ... they do not always post look-outs, monitor their radio, transmit AIS, or closely evaluate their own radar screens ... they are literally on autopilot. What we are talking about is reasonable, sustainable watch keeping.

This concept of 'look intervals' is used heavily in submarines (in high traffic areas). It works. My assumptions were for example. Use whatever assumptions you wish based on what concerns you. If you're never going to see a 100ft MHH, 30kt freighter where you're operating at though ... then you'll just be overworking yourself and risking collision due to complacency or fatigue. If you're in 20ft seas and are still assuming you can see the horizon ... then you're incorrect and the technique hasn't been implemented appropriately.

In conclusion, this isn't a set it and forget it strategy. It must be continuously reevaluated for its appropriateness. The objections you raise are certainly conditions which require evaluation as well.

JohnRPollard 03-30-2011 09:47 PM

Quote:

Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.
Scraph, that rule is compound.

First, the vessel must maintain a proper lookout at all times, using sight and sound.

Then, "as well as" refers to additional measures that should be taken, such as using radar if available.

In other words, the qualifiers you cited do not refer to the requirement to maintain a proper lookout at all times. They refer to the "as well as" measures.

I would not want any sailor to read your initial post and think that it is ever advisable to only scan the horizon every 45 minutes. I would not sail on a boat with a standing watch order to that effect.

sailortjk1 03-30-2011 10:05 PM

Good luck with all of that. Hope it all works out for you.

scraph 03-30-2011 10:20 PM

The 'as well as by all available means' is part a list of three ...
Maintain a proper lookout by:
1. sight, and
2. hearing, as well as
3. by all available means appropriate in the prevailing circumstances and conditions'

So, I agree that the 'by all available means appropriate in the prevailing circumstances and conditions' is its own item.

But 'so as to make a full appraisal of the situation and of the risk of collision' does not apply to each of the three individually. In other words, we aren't required three-fold redundancy where either of the methods alone will 'make a full appraisal...' and we just so happen to be executing all three. What is required is that the net effect of all three will 'make a full appraisal...' and not necessarily anymore.

For the purpose of preventing collision with a large, slow moving freighter or a 36 foot sailboat ... as were the examples given ... it is sufficient to prevent collision with those vessels given that the assumptions made are accurate. Visibility to the horizon and zero sea state in the middle of the open ocean where expected traffic is somewhat predictable is certainly not reasonable assumption in most cases.

I would be more than comfortable with a 20 minute look interval as mentioned above is most conditions. I agree that 45 minutes might seem a little long but my point was pointing out a method to evaluate what a 'proper lookout' is. Maintaining constant visual scanning is not necessary to maintain a proper visual lookout. Further, if equipped with radar and AIS I don't see a major safety concern with a 45 minute look interval, while short handed. It certainly is better to have thought and reason behind it than to simply shrug and say 'I can take a 2 hour nap, it's only 2 hours'.

scraph 03-30-2011 10:26 PM

Quote:

Originally Posted by sailortjk1 (Post 714885)
Good luck with all of that. Hope it all works out for you.

Right ... because I defined the zen and the art of all things seamanship with a single discussion of a single aspect of passage making. Okay ... I'll leave critical thought out of this forum and propose '20 minute egg timers' because 20 minutes seems good enough. :-/ How unsatisfying.

PaulfromNWOnt 03-30-2011 10:35 PM

I love "Rules of the Road" debates. It always seems that there are multiple interpretations for each little dot, clause, subclause, and article.

Personally, I abide by the PRIMARY rule of the road: In the event of a collision, the kid with the biggest toy wins.

Debate the other rules all you like, and while you're at it, you can try to figure out how to explain to King Neptune that YOU were the stand on vessel.

Lawyers belong in courtrooms, I'll happily watch my own arse at sea.


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