hi roger,

if you want to be sure things will be ok you have some options. one of
those solutions is ferrite rods. you basically just get a ferrite rod,
it's a rod like they used to use for antenna in the old days, and get
whatever cable you are using and wrap it around the rod like a coil and
just keep wrapping it around. the more wraps, the more rf protection
it gives you. another thing you can try is ferrite beads, or their
equivilent, that you can get at the "radio shack" store. these are
basically ferrite beads that are broken in half with plastic around
them, you just snap them on to the outside of the cable. when i use
them i usually run the cable through them, then bring it back on the
outside and run them back through a second time, making a loop around
the bead so that the bead won't slip, and also giving a little more rf
protection in the process. a third option is toroids, which "radio
shack" again has something you can use, they sell these toroids that
are square in shape and are again broken in half and have plastic on
the outside. the great thing about these is that the plastic housing
is made in such a way that you can attach these things together and
make like a block of them that is as many units long as you like. this
came in really handy for me with noise coming from the heater fan in my
personal vehicle where i have an hf radio, i just made a block of these
and got some wire and wrapped a lot of turns through the block making
in effect an rf choke. the flexibility of this solution was really
important because i needed more blocks because i couldn't do as many
wraps of wire, because the wire gauge had to be so high because the
amps the fan draws is high. it is the same for a radio you need to
protect, like an hf amateur radio, you need more blocks and higher
gauge wire because the radio requires so much power to transmit at 100
watts or whatever it's rated for, you can't use a really thin gauge
wire to make the rf filter because the thin wire won't carry enough
power without getting hot and eventually melting the insulation off of
the wires.

even if you are not having any problems i would get some of those
little bead type rf chokes from the "radio shack" store, or somewhere
else, and keep them handy just in case you need them. i have had to
use them on data cables before. one example is my hf radio - it has a
remotely mounted head display that you put somewhere and then the main
body of the radio resides some place hidden away, and they are
connected together by a data cable similar to the one you are using for
your gps unit. well, things worked ok in my setup until i put in the
antenna tuner at which point when i hit the tune button on the radio
the tuner would start cracking relays until it had the right inductance
and i would get a huge amount of rf noise on my data cable and it was
shutting off the radio. so basically i'd hit the tune button at
certain frequencies and my radio would suddenly cut off, rendering it
completely useless at those frequencies. as stated, the problem was
that the data cable between the head unit and the radio's main body was
acting as an antenna and picking up rf from the antenna and tuner and
doing crazy things. the solution was to put one of those beads at each
end of the data cable, one at the head unit, the other at the radio,
and that solved the problem with minimal expense and hassle, took 5
minutes to install.

when dealing with rf and radios and data cables and things it never
hurts to have a bunch of toroids and ferrite rods around, an
assortment, and some extra wire. because you can always when faced
with some interference just get a rod, wrap some wire around it, and
try it .. and if it doesn't work, try it somewhere else, or wrap more
wire, etc, until it chokes off the rf signal enough that it stops
causing you problems. though i have not yet faced it, i imagine that
you can run into all kinds of interference in various anchorages
because so many radios and things are being used there. yachts nowdays
have so much expensive electronics on them that they cause all kinds of
interference to even basic necessary systems, and also you can get some
weird electrical going on in the water too because everyone is using
the water as a ground, so your boat is basically electrically part of a
circuit with everyone else's boats.

in the end it rarely hurts to put an rf choke near anything that's
either causing or receiving noise. an rf choke basically traps rf
energy and won't let it pass on the wire, so putting it near something
that is being troubled will stop the rf from getting in, and putting it
near something causing troubles will keep the device from radiating rf
on the line. ham radio people are great folks to get to know when you
are having rf problems because they tend to have solutions to all kinds
of troubles because they have a lot of troubles of their own to deal
with. they also often have ferrite rods and things laying around and
know how to use them, and they can look at what you are doing and
instantly "see" what is causing troubles, long power wires running
along side sources of rf, electric motor noise, etc, they can help you
isolate and fix problems.

do a web search on "ferrite rod", "toroid", and "rf choke" for more
info. cheers.