i did a couple of quick searches and looked up a few links to add ...

http://www.radioshack.com/product/in...tId=2103979&cp

that is a link to the snap together type. you basically can snap them
together like stacking a pile of doughnut pastries making a longer
"block" of them. then you wrap the wire as shown. like i said, for
higher power things you need heavier gauge of wire, so you have to make
a longer block to get the same amount of protection. it's all about
the number of turns around the toroid, the more the merrier.

http://www.radioshack.com/product/in...tId=2103222&cp

here are the "bead type" that i use on data cables. you just snap them
on, though like i said i tend to make a loop through them so that they
don't slip on the cable and remain in the position where you put them.

http://www.aa5au.com/gettingstarted/rtty_rfi.htm

that is a ham page showing how to use toroids and ferrite rods, i just
did a google search on "ferrite rod" and "rfi" to find it. in those
pictures you can see the rod is just that, a rod, and you just wrap
whatever cable you are trying to affect around the rod and it chokes
rf. it also shows a round toroid and how to wrap it. this guy doesn't
like the snap on beads, he says they are not effective. i can say with
certainty that they did affect my radio's data cable ... the radio was
cutting off before, and after installation it wasn't cutting off,
that's my definition of effective. haha. i did do one thing that i
hadn't read as being necessary, however, and that was to put a dab of
that greasy conductive paste on the surfaces of the ferrite where it
snaps together so that it was electrically connected to it's other
half. i don't know if that matters or not, but it seemed prudent to
me, i imagined the loop needing to be electrically connected like
inside a transformer, and i didn't trust the plastic housing to press
hard enough on the two halves to keep them squished together enough to
be electrically connected.

the other thing you need with all of these solutions, of course, is
generous amounts of electrical tape. even the snap together plastic
ones i find don't stay snapped. well, that's not entirely true, the
little bead type stay snapped without tape, but the bigger block type
don't, they are just too big and clunky. the wire wraps does help to
hold them together, but a little tape doesn't hurt either, and i wrap
the wire after i've made my loops to keep it looped also.

in the end, the longer a cable is, the more it acts like an antenna
affecting whatever is attached at either end. so if you have a super
long data cable it's going to be acting as an antenna and picking up
signals from your radios, your engine alternator, fans, compressors (if
you have a refrigerator), the neighbors amateur radio, all the vhf
radios in the harbor, etc, etc, it's just a big antenna. your
equipment is going to have some rf suppression built in ... but how
much ? when will you run into problems ? how much noise is too much
noise ? you just never know until it happens. so the easiest thing to
do is to keep some rods and things around, some toroids, just an
assortment of things, just like you'd keep extra screws and bolts
around for handling problems and fixing things. then as you run into
problems you can create solutions for them. and i would imagine even
if you yourself don't have problems eventually someone at your
anchorage will, and you can be their hero haha. maybe the person you
help has a wine cellar on board! lol.

cheers