[alberg30] GPS mount

[Bob Johns]

From: Bob Johns <bobjns at nais.com>

Sanders and Russ,

>...swing away  mounting, for a loran.

I have a swing away mount for my loran, mounted as Russ describes. I use
elastic stop nuts to give a little tension on the pivoting screws,  and
this works fairly well, except the nuts do need to be adjusted every so
often. We tend to swing the loran out of the way when someone goes below.
It is a handy location as far as using the loran.

We have the depth sounder mounted in the bridge deck hinged cover. This
works well  for the sounder as we are not making frequent adjustments on
it, except to change the range scale. It might be a little awkward
operating the GPS in there but you should check this out yourself. We do
find ourselves flipping the cover closed when someone goes below.

>Also, you can't always expect pin point accuracy, like a loran,  so use
>caution , especially in fog.  My LORAN will bring me back within 30 ft of the
>waypoint I entered at  that location.

I was beginning to think that I was the only one that felt as Russ does on
accuracy of lorans vs GPS. This will be true until  the Department of
Defense stops reducing accuracy for civilians at random intervals. It is
true that the absolute accuracy of lorans is slightly less than for GPS
(600 feet vs 300 feet in my experience if DOD is playing games with
accuracies). Repeatable accuracy, where you have stored that location
previously, seems to be as good on our loran as a GPS operating with no DOD
error thrown in (but you can't tell when that error is absent so you better
assume no better than 300 foot accuracy).

We sailed to the Chesapeake several years ago with a friend that brought
his handheld GPS. Naturally we spent a lot of time comparing the two. On
the subject of determining boat speed, the loran was generally good for
checking the knotmeter - the differences that we saw were all within our
estimates for tidal currents. The reason the accuracy was good was that the
loran was set to average the computations over a 2 minute period. I had
tried less averaging time and found that the results were very erratic. The
2 minutes averaging time means that you cannot use the loran as a
replacement for the knotmeter because of  the lag caused by that response
time. We normally compare the knotmeter speed to the loran speed and assume
that the difference is due to current. If we find that the difference is
consistent in different current conditions we then start to consider that
the knotmeter may need an adjustment.

The GPS belonging to my friend had no adjustment for averaging time. It
apparently was set for very rapid averaging because the speed readings were
all over the map. With the knotmeter and loran reading constant speed, the
GPS would read as much as 1 knot on either side of the other readings. If
the averaging time could have been set to a longer value it would probably
have given good results. If you are buying a GPS, Sanders, make sure that
it can be set to average over at least a 2  minute interval and realize
that it will not be satisfactory as a replacement for a knotmeter, although
it should be fine  for checking the knotmeter accuracy with no current
present.

The other problem with my friend's GPS was that the  internal battery life
was limited and he was constantly switching it off to conserve the
batteries. It may be that newer GPS have better battery life and that this
is no longer a consideration but you should check it out. The best of both
worlds would be to have both internal batteries and to be able to operate
off ship's power. This would suggest that the GPS should be easily
removable from its mount on the boat.

We found that the loran gave erratic position reading under the Chesapeake
bridges. We were trying to save the locations as waypoints. It probably did
the same thing under other bridges, but it was the first time we had seen
the effect. The GPS seemed to be fine.

Russ, thanks for the comment about accuracy of lorans vs GPS. I had the
feeling that I was the only one who felt that way.

Sorry to be so wordy.

Regards,
	Bob Johns Wind Call #397




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