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Given that a boat traveling at, say, 30
nautical miles per hour takes two minutes to
traverse that mile or, worst case, two boats of
similar size and speed headed directly at each
other take one minute to close that gap, you
may decide the answer is no. You’ll have to
decide exactly how much range is enough,
and how much height is therefore enough.
Remember, radar is most often needed for
collision avoidance, when visibility is restricted. For radar to operate well in close, how
you mount your radome matters.
Remember the “cone of silence” from the TV
show, “Get Smart”? Radar suffers from its own
cone of silence, which is affected by the angle
at which your antenna is mounted. Once you
know how high you’ll mount your antenna,
you need to determine the angle at which
you’ll mount it. There’s another big difference here between sailboats and powerboats.
Most modern planing powerboats run with
a bow-up attitude while sailboats cruise at a
more or less flat angle. Sailors (and owners of
displacement powerboats) can skip this part
and simply mount the dome at zero degrees.
The rest of us will want to pick up a
clinometer (an inclination
meter), which will tell you
the angle at which your boat
runs. These cost less than
$20, can be found at any
West Marine, and are used
by sailors to determine angle of heel. You
can also get an app for Android and iPhone
smartphones for a couple of dollars that will
do this accurately. Even though you should
travel slowly in poor visibility, and therefore
off plane, this angle may still have a noticeable
effect on radar performance and it’s worth
figuring out what it is.
Powerboaters will orient the clinometers
fore and aft in the boat, making sure it reads
zero while the boat is at rest, then run the
boat at a moderate cruising speed, on calm
water. The clinometer will tell you the running angle of your boat, which is usually
somewhere between three and six degrees.
Now you know what angle wedge you’ll need
to keep that radar antenna at the ideal angle
when you’re running. Note, in the case of
some large boats and custom-built boats, the
manufacturer may predetermine the mount-
ing angle and build it right into the hardtop
or mounting area.
These few degrees may not seem like a big
deal, but radar beams are functionally narrow,
vertically, offering around 25 degrees ( 12. 5
degrees above and below the centerline) of
Radar antennas come in
both small, closed-arrays
(radome) or larger open-arrays. larger open-arrays give better target
the first consideration
in mounting a radome
is to get it above the
heads of the crew to
avoid blasting them
with microwave energy.