As long as the compass is magnetic reference, everything else on Derreen
will be magnetic. Now with electronics and miniturization the way it is,
why can't we design a compass, that by adjustment, reads True?

-- Jim
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
JimAgue
M/V Derreen, Monk 36 Trawler
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-----Original Message-----
From: Hal Wyman [SMTP:twl@halwyman.com]
Sent: Tuesday, November 20, 2001 12:29 PM
To: Trawler World List
Subject: TWL: Navigational reference

At 06:52 PM 11/20/2001 -0700, Jim & Rita Ague wrote:

ITs already been done.
]
Check out KVH.

Arild

One thing that hasn't been mentioned in the discussion about true vs.
magnetic is raised dramatically by Jim & Rita's post. Unless you always
cruise in the same area, magnetic north changes from area to area
(referred to as magnetic variation) and it changes from year to year
(refer to any chart's compass rose for the rate of annual variation).
There is no way to keep a magnetic compass accurate as a result, and the
marine vendors could then sell us a different compass to be used for
each degree or two of variation!

On Sanderling, all the electronics (GPS and Loran) are set to give true
readings, and it is a simple matter to convert them to the magnetic
heading based on local magnetic variation, and steer the compass course
based on that. I do list my chart bearings in magnetic, just for easy
reference for a course to steer.

Jim & Rita Ague wrote:

--
Bob McLeran                              rmcleran@ix.netcom.com
M/V "Sanderling"                        Docked at Point Patience Marina
Hailing port: Wianno MA                      Solomons, MD
Hampton 35 Trawler

elnav@uniserve.com writes:

Can and where do you get one of these in Canada? What price range for
a binnacle mounted one? Why doesn't anyone mention push these in CPSS
courses?

Ciao _ George

From: "George Geist" scaramouche@tvo.org

Both the KVH and a similar product made by Ritchie are available through WM
or B/US.  Also, see websites at

http://www.ritchienavigation.com/

http://www.kvh.com/main/index.asp

Regards,

John Gaquin
m/v Brefnie Queen, 32' Luhrs
blocked and wrapped @
Bass River Marina, West Dennis,  MA

At 09:04 AM 11/21/2001 -0500, George Geist wrote:

REPLY
These  compasses are available in Canada. However  many dealers may not
stock them in quantity.
Last time I looked the price was about $400  for the Azimuth. which
automatically adjusts for deviation.

KVH  also has  a gyro stabilized  model which combines a fluxgate reference
with  stabilizing rate gyros.
This unit  corrects for both  deviation and variation and the gyros
stabilize the  unit during  violent  heave pitch and yaw movements.

They are considered a suitable  replacement for a normal north seeking gyro
on  smaller vessels ( under about 300 Tons)
As I recall the price for that one is in the $7000 range.

KVH also uses a  gyro compass reference  for their active satellite/TV
dish  tracking  mechanism.
I have installed a couple of them and they  work very well.

One reason  why these are not  promoted in the regular CPS courses  is that
it shortcuts the basic process and CPS believes that  stidents must know
the basics before moving on to  using equipment that  does the hard work
for you.

Cheers

Arild

PS. no financial  interest  other than as a satisfied user.  I use KVH
reference compasses as part of my compass adjusting work.

At 10:19 AM 11/21/2001 -0500, John Gaquin wrote:

REPLY

The Ritchie unit came along after  the KVH uni had already become a sucess
in the marketplace.

While fluxgate compasses have been around since WW2  it is only recently
that microprocessors  have been compined with  the fluxgate to give a truly
useful  product.

KVH developed their product  as a spin off from  military  products used
to control  automatic satellite communications tracking dishes and
automatic gunnery fire controllers.

By comparison the Ritchie  takes a low tech approach with a basic fluxgate
and  interface circuitry to give a digital  readout.

Both  compasses  have an auto compensating  feature.  Based on a constant
rate of turn  the  processor  compares the  actual  magnetic reading with a
calculated  value.
A standard rate turn is 360 degrees in  2 minutes.
So the  algorithm  starts the timer when the compass reads 000  and times
the  turn  until  it reads  000 again.    Assuming a constant turn rate  it
is easy to calculate  how many  degrees per second.

The actual magnetic  reading will vary and is a function of local
deviation.  This  data is now  stored in memory  as a deviation table.

Accuracy is  dependent on  how close to a perfect circle  you made during
the  calibration  run.

The one thing these compasses cannot  compensate for is  unexpected
magnetic fields such as you get from  elecrtic circuits  suddenly being
energized or  rom  magnetic  clips, tools and  such being placed in close
proximity.
These  were not  there during the calibaration run.

When I installed  autopilots  I always had to  calibrate the  fluxgate
compass.  I would leave the chart plotter running  to give me a track plot.
It is amazing how  distorted a circle can get  despite  every effort to
keep the rate of turn constant.

Carver boats have very little draft.  Even a 5 knot breeze will push on
the  superstructure  enough that  the rate of turn is slowed down for half
of the circle while being accelerated  on the other half.  The result is
obvious when you look at the track plot.

I also  did this on a trawler.  Deep hull, long keel and low
superstructure.  It was a fishing tug from Lake Erie.
The circles were perfect but  the track plot showed that each sucessive
circle was displaced slightly  from the previous one.
Since this was  done in Georgian Bay  with no current or tides to  contend
with I can only attribute this to wind.
However the rate of turn was much better than  what I  experienced with the
Carver boats.

My advice would be to strip off all canvas and biminit tops during the
calibration run if you want real accurate results.

FWIW  I found the KVH  was a better compass than the Ritchie.

Cheers

Arild