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» Online Classroom   » Navigation Rules   » Public Discussion of the Nav Rules   » Nav Rules Problem 04111

Author Topic: Nav Rules Problem 04111
Dan Cline

 - posted January 07, 2006 12:42 PM      Profile for Dan Cline           Edit/Delete Post 
This problem asks about lights on a 50 meter vessel with a 100 meter stern tow. Is the answer "B" because Rule 24(a)(i) does not require the two masthead lights in a vertical line to be located at the forward mast, even though they are shown on the forward mast in the illustration on page 58 of the rules?
From: Midland, MI
David Burch

 - posted January 07, 2006 06:13 PM      Profile for David Burch           Edit/Delete Post 
For the benefit of those who do not have the question at hand, it is this:

BOTH INTERNATIONAL AND INLAND A 50-meter vessel is towing astern and the length of the tow is 100 meters. In addition to sidelights, which lights may she show to fully comply with the Rules?

A) Two masthead lights forward, a sternlight, and a towing light above the sternlight

B) A masthead light forward, two masthead lights aft, a sternlight, and a towing light above the sternlight

C) No masthead light forward, two masthead lights aft, a sternlight, and a towing light above the sternlight

D) Three masthead lights forward, one masthead light aft, and two towing lights in a vertical line at the stern


As is often the case in such questions, i am guessing that the word "may" is crucial here, as opposed to the alternative of "shall."

and i am guessing that we must think about the vessel length that says 50-meters (that is exactly 50 meters) since we know there is wording in the rules about vessel lights that change with 7 meters, 12 meters, 20 meters, 50 meters, 100 meters and 200 meters.

[ we leave it as an exercise to show that the number 150 meters can also affect vessel lights in several ways, but not related to this question. ]

So anytime we see one of these numbers in a question we have to remember that the wording varies as to how these limits are to be applied. They could be "less than" or "equal to and less than" or "more than" or "equal to or more than." i do not at this moment recall the precise wording in all cases, nor if all cases are used, but definitely more than one is used, so this is always some thing to keep in mind and a possible tripping point in choosing an answer.

Another key aspect of the questions is it applies to BOTH inland and international waters. that can be crucial as it might rule out some options that only apply to one or the other. Note this is not mentioned in our study guide since the whole set give applies to Both waterways, as mentioned in the header to the problem set.

Lights for power driven vessels are in rule 23, and here we learn a key factor. must carry two masthead lights, one forward (23ai) and one aft and higher (23aii), and only for a vessel LESS THAN 50 meters can you carry just one forward. hence we know so far we must (shall) have 2 masthead lights, one forward and one aft higher, plus sidelights and a sternlight. this is not enough to choose the answer, however.

next we move on to what changes when the vessel is towing, and again we note a key phrase in the question, the vessel is "towing astern" as opposed to side tows or pushing ahead and we note that the tow length is 100 meters. Light rules for towing vessels change for GREATER THAN 200m.

in rule 24ai we are told that when towing, instead of rules 23ai (light forward) and 23aii (higher light aft) do not apply and instead we must carry two masthead lights in a vertical line for tows of 200m or less (3 for longer tows).... but this rule does not say where these lights need to be, forward or aft, plus sidelights 24aii, sternlight 24aiii, and a towing light above the sternlight 24aiv

when we get to 24d, however, we are told that we must (despite what we were told in 24a) indeed obey 23aii, which is to carry a masthead light aft, higher than the forward one.

This is pretty convoluted wording, and i am sure there must be a reason for it. I have usually thought of this as 2 lights in a row for short tows, 3 in a row for long tows, and if the towing vessel is >50m, then you must be able to range on the masthead lights... ie see two sets, forward and aft, which from the side have the aft one or ones higher. so i am in principle going to see either 2 or 3 in a row in the front, with 1 higher in the back, or 2 or 3 in a row on the back, with 1 lower in the front (for 50m or more). and from the relative orientation of the front and back sets you can tell which way the vessel is headed. For vessels less than 50m, expect just the row of 2 or 3 either forward or aft or near the middle of the vessel.

We can now figure the answer.

A is not right since we are 50 m not less than 50m and thus are required to carry the aft light
D is also definitely wrong since it has 3 lights in a row, which is for tows > 200m
likewise C is out because we can't range on it and we are 50m

hence we are left with B = A masthead light forward, two masthead lights aft.....(they don't say but they could, that the aft set has to be higher than the forward one)

and this is a MAY rather than a SHALL because we could have alternatively shown two forward, and one aft that was higher.

sorry for the long answer, but these can be tricky sometimes, and i usually have to go through the reasoning myself to recall it. hope this helps. i will check tomorrow to see if this is right and edit as needed.

From: Starpath, Seattle, WA

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