Both Pete and otn are right. On the one hand, most large vessels have
spare lights, at least for the major functions (stern, masthead,
sidelights, towing), so what might appear to be a pair of lights are
actually not used at the same time.

But, the US rules, at least, (I can't find an International Rules
citation for this) permit two lights to be used for one all around
light where there's a mast in the way, see 84CFR17 Annex I, paragraph
9 http://www.access.gpo.gov/nara/cfr/w...3cfr84_99.html

Basically, you have to ensure that the two lights appear as if they
are one at a distance of one mile or more by using screens or mounting
them close together.

Jim Woodward
www.mvfintry.com

otnmbrd wrote in message ink.net...
PeteAlbright wrote:
On ships with "all around" lights mounted close on the mast, it is common to
have two lights mounted on each side of the mast. Two lights mounted 12"
apart should not be confusing, and while a sail may block a light, it would
also be illuminated by it.

Pete Albright
Tampa, FL

I'd be careful on this. Although I won't disagree that on some ships,
this is true, due to considerations of construction, this requires
regulatory agreement and is not the norm.
Also, it is quite possible that some may see various signal light
"trees" on either side of a ship mast, which they may mistake for
duplication, wherein reality, they are separate signals for some
specific locale or function.

otn