Milton Waddams wrote:
Depending on the size and type of boat, the range of visibility for
navigation lights is one or two miles. I'm a bit confused about the
range of visibility issue. Obviously the wattage of the bulb used in
the light is a factor but shouldn't the mounted height of the light
be a factor too? When I check out the navigation lights nowhere do I
see on the package that the light must be mounted x distance above the
water minimum for a two mile range. So do the Coast Guard regulations
mean a potential to be seen two miles and you can mount it below the
two mile visibility horizon or do they mean it has to be mounted above
the two mile visibility horizon? I don't see this addressed anywhere
and can anyone offer some insights or point me to where it is spelled
out? I would also appreciate it if someone could tell me the minimum
wattage bulbs required for one and two mile range. Thank you,

If you look in the annex of the ColRegs or Inland rules you'll find a
few specifications addressing the vertical height and the angles of
visibility. It also includes "luminosity" requirement, but all you
really have to do in this regard is read the label and buy a good brand.

However, to satisfy the one, two and three mile rules the height is
not terribly important. The "distance to the visible horizon" is:

D = 1.17 * SQRT(height) D in nautical miles, Height in feet

The calculation must be done twice, once for the observer, and once
for the observed object. Thus, if the observer is 3 feet above the
water, he can see the waterline of an object 2 miles away. Sidelights
mounting one foot above the waterline can be seen from over a mile
away, even with no elevation. You can add the two together, so an
observer three feet up can see lights one foot up from over three miles.

Powerboats should have a white light at least a meter above the
sidelights, so their visibility should be always be several miles.