Seld Steering devices for steel hulled boats

My cousin went to buy a self steering device for his 28ft stell yacht
last week. The mechanical type, an arm that expands and contracts and
it attached to the tiller, uses 1 amp approx.

Price was about $600 Aust ($400 US)... good so far.


When he asked if it would work on a steel yacht they said no and
refused to sell it to him, presumably the steel would affect the
electronic comapss in the unit.

Has anyone with a steel boat found this to be so?

The other alternative is a unit with a separate compass, where the
compass would be placed high on the mast, but these are many times
more expensive.

Comments anyone?

N. Peter Evans

Will it work acceptably? Maybe. I'll even risk "probably". Will it
work well and hold a really good course? Probably in easy conditions,
possibly in heavy conditions. (Probably OK in Port Philip, possibly
not in the Bass Strait.)

At the most basic level, the fact that it's a steel boat doesn't
matter. The autopilot doesn't need to be able to steer, say, 287
degrees, but just be able to hold a course when you set it on that
course (autopilots 30 years ago did just that -- you aimed the boat
and turned them on).

On a given course, at a given angle of heel, the magnetic field will
be constant over long distances (tens of miles), so deviation from the
steel won't matter.

The second order problems, however, are real. When the boat yaws, you
want the AP to sense it immediately and begin to counter steer, but
not over steer. Modern flux gate compasses (which the cited AP almost
certainly has) are not too good with yawing -- they have acceleration
errors -- and the steel makes it worse.

Bottom line -- you might not get great results with the cited AP in
heavy going, but I'd think it would be pretty good at holding a course
in easy weather -- just don't expect to be able to punch in a
particular course (ie "287") and have it steer that -- you might have
to set it to steer 276 in order for it to steer 287.

The other possible solution is to steer to GPS data. The Raymarine
ST1000 tiller autopilot (retail US$430 here or GB£215 ex VAT in the
UK) will accept NMEA data, so you could steer to the GPS and avoid the
compass/steel problems altogether. This could be either BTW (bearing
to the waypoint) or COG (course over ground) -- I prefer the latter as
it takes current into account and doesn't get squirrelly hear the
waypoints.

BTW, the solution to this is not simply mounting the compass higher.
First you try using a rate sensitive flux gate compass that has
accelerometers in it to try to deal with the yaw. However, none of
Raymarine, Furuno, or Simrad will guarantee that that will solve the
problem for good autopilot perfomance or ARPA radar (automatic
plotting). You usually will have to go to a gyrocompass, or, more
recently, a GPS compass (which has to be the neatest new technology in
the last two or three years -- too bad the cheapest of them is
US$3,300).

Jim Woodward
<a style='text-decoration: underline;' href="http://www.mvfintry.com" target="_blank">www.mvfintry.com</a> (Fintry is around 125 tons of steel)

nicholas.evans RemoveThis @students.vu.edu.au (peterMelbourneAustralia) wrote in message news:<81aba80f.0307212235.2c8a6269 RemoveThis @posting.google.com>...
 > My cousin went to buy a self steering device for his 28ft stell yacht
 > last week. The mechanical type, an arm that expands and contracts and
 > it attached to the tiller, uses 1 amp approx.
 >
 > Price was about $600 Aust ($400 US)... good so far.
 >
 >
 > When he asked if it would work on a steel yacht they said no and
 > refused to sell it to him, presumably the steel would affect the
 > electronic comapss in the unit.
 >
 > Has anyone with a steel boat found this to be so?
 >
 > The other alternative is a unit with a separate compass, where the
 > compass would be placed high on the mast, but these are many times
 > more expensive.
 >
 > Comments anyone?
 >
 > N. Peter Evans<!-- ~MESSAGE_AFTER~ -->