|
|
From: Rick Emerson (ssg.com!rick)
Date: Wed Oct 16 1996 - 10:27:03 EDT
Sean F. Holland writes:
> >How high (up a mast) can you practically mount a radome?
> >Because of my rig, I don't think that I can mount one below about 35
> >feet above the water-line.
> >I am looking at buying a unit like the Raytheon RL-9.
>
> Why do you wish to go to all the trouble & expense of mounting the Radar
> way up your mast. You will have to get an extension cable approximatly
> $50.00, and unless you wish to have the cable exposed will have the
> problem of how to secure the cable inside the mast. Also even a 16 LB
> radome up 35 feet will add 560 ft LBS when heeled, requiring the
> addition of 140 LBs of ballast 4' down.
I can't speak to the RL-9's cable set but most radars come with enough
cable for an installation without going to a longer cable. This was
certainly the case with my Raytheon R-20X which has its antenna just
below the spreaders (roughly 20+ feet above the water) and the display
in the nav station on a Baba 35. Fishing the cable through the mast
wasn't a walk in the park but it was accomplished without dropping the
spar.
As to the question of weight aloft, the real issues for a mast mount
are added inertial moment (the weight both resists initial motion and
then resists stopping that motion) and windage. Inertial moment is a
combination of weight and distance from the center of motion (a light
weight further out on the mast and a heavier weight closer in can have
the same moment). Adding more ballast as compensation for any loss of
righting moment only compounds the situation. Windage is a matter of
personal judgement. If a boat is unable to safely carry the added
drag, perhaps this is not the boat to go into a storm with?
> It might be far easier to install the Radar on a pole aft, sufficiently
> high enough to clear any Radar shadow from the boom. We did this with
> our RL-9 and accomplished the entire installation for less than the cost
> of the extension cable, plus might still have a Radar if we ever lost the
> rig. Good Sailing, sean
Your point about keeping the antenna (and the bulk of the RF system,
for that matter) off the stick in case it fails is well taken. A
separate radar mast, however, adds still more windage, complexity
(backing plates, etc.), and may have to go in a location not meant to
take the loading imposed by the radar mast (which may mean added
modifications to the transom skin).
Keep in mind that adding weight at the ends of the hull means not only
adding more weight forward to keep the boat in her lines but
increasing the polar inertial moment (making the boat more prone to
"hobby-horsing"). While this appears, at first glance, to be the
issue of inertial moment raised with mounting the antenna on the mast,
bear in mind the added weight from the radar mast, mounting hardware,
and possible hull modifications. When this is compared with the
weight of a mounting bracket attached to a mast already designed to
accept the load, the weight penalty is significant. And, of course,
this added hardware and installation isn't free although most of the
work can be done by the owner.
Having argued against a radar mast, I will say that if there is a need
for an additional utility mast for a wind generator, gin pole for
handling a large outboard engine, etc. then adding a radar antenna to
the mix is less of a problem. Bear in mind, however, that all of the
weight penalties associated with the added mast remain.
Finally, I have seen mounts which use the backstay, as a supporting
member along with a stress tube, as the mounting point. If there's
enough clearance between the leach and the backstay, this might be
worth considering, too, although my impression is these mounts run in
the $1500 range.
When the radar is in operation, walking on the cabin top becomes a
less than prudent notion if the antenna is mounted just above the
boom. It might be argued that nobody will walk on cabin top when the
radar's running but imagine working along in the fog at night with a
lookout forward, hunting for a mooring... A brief exposure probably
won't pose much risk but over an exteneded period, the risks
increase.
The truth is there are no best answers, only different solutions which
work best for individual boats and budgets.
Rick
|