Why The Hell... (random rant)

"Mxsmanic" wrote .

Just out of curiosity, what are the usual navigation methods for boats and
ships these days? I suppose at sea you don't really have any equivalent
of
VORs, so that's not an option, although I seem to recall that Loran and
(formerly) Omega were developed specifically for ships at sea (as well as
GPS,
in part). It sounds like you don't navigate much with a magnetic compass.
And how do you find your way around with charts when you have no
landmarks?

OK, this becomes partly OT but bear with me.

Basics are as always: Chart, compass, floating navaids (navigation buoys)
and fixed navaids (beacons and lighthouses).

Boats nowadays use fancy GPS chartplotters. In addition, ships and big boats
use radar.
There used to be radio beacons (corresponding to NDBs) but they are being
phased out. Before GPS there was Loran-C and Decca, which could provide a
fix with about 300 metres accuracy in coastal areas.

For me, the mag compass is still the primary steering reference at sea. GPS
is a very nice addition as it provides cross-track (i.e. course deviation)
indication as well as the distance to next waypoint. In coastal waters,
there are usually visual landmarks so in those areas only the chart is
needed.

Big ships seem to rely extensively on GPS, more so than in aviation. But
that has to be put in context. They also have excellent radar, which is a)
not relying on external sources and b) shows not only navigational features
(beacons etc) but also other traffic. In addition, unlike aircraft, in case
of a GPS navigation system failure, they always have the backup options of
using the sextant or, as a last resort, to stop and anchor.

Dual antenna GPS systems are quite common on ships, but as far as I
understand, not so much to give a heading reference (the compass is still
excellent for that) but more to aid maneuvering in cramped harbours. A big
ship has a lot of inertia, so once it starts yawing it will keep going for a
minute or so even with full opposite rudder. Therefore modern ships have
smart computers that compute each "wheel-over-point" where a turn has to be
initiated, in order to exit the turn at the right position. With dual GPS,
the yaw rate as well as the position of bow and stern can be precisely
monitored.

GPS on boats is a very nice thing, but it has its drawbacks. For example,
everyone tends to program their routes in exactly the same way from buoy to
buoy, with the result that traffic going both ways concentrates in a narrow
area. A dangerous thing, well known also in VFR aviation. Another danger is
that people venture out in IMC conditions (i.e. fog) much more than
previously. They can navigate OK, but without radar they have no way to see
and avoid other traffic until it's dangerously close. And of course Coast
Guard gets lots of work from boaters who become utterly lost when their
fancy GPS plotter fails.