"Zero deviation" said the autohelm

[jonjo]

\"Zero deviation\" said the autohelm

While doing some Autohelm training circles yesterday the Raymarine brain eventually concluded it had detected zero deviation. I supose this only indicates the remote electronic compass is well sited and I should still check the main steering compass for magnetic deviation?

 

[ParaHandy]

Re: \"Zero deviation\" said the autohelm

bluidy 'ell ... must be an all time record or the boat's 100% non-metallic ... erm ... hing on ... if you'd previously set it up and zeroed it then the deviation could be zero now ... ?

 

[peterb]

Re: \"Zero deviation\" said the autohelm

But it doesn't necessarily mean that the Autohelm compass is accurate. It means that the boat's magnetism has no effect (or has been automatically taken into account). But if the fluxgate head is not properly aligned with the boat's fore-and-aft line, then there will be a constant error. For this reason the final stage is to correct this constant error when on a known heading, either against shore marks or against a properly corrected magnetic compass.

We always have our binnacle compass properly swung (and if necessary corrected) each year. When setting course we punch in the required course to the autopilot, wait for the boat to settle, then check the course against the binnacle. If the binnacle course isn't quite right, then we make the necessary correction to the autopilot.

 

[Rob_Webb]

Re: \"Zero deviation\" said the autohelm

All you guys - I've just installed a new ST4000+ and whilst the basic function is fine it still wanders a bit too much in a seaway. Which of the following 2 options should I focus on:

1. Continue to experiment with fine-tuning the multitude of variable settings that the unit has?

2. Bite the bullet and fit the optional rudder position sensor - the instructions claim this is not mandatory (only with hydraulic wheel systems) but mine is direct link so hoped it wouldn't affect performance too much. This will be a bit of a mission and I only want to do it if I'm pretty sure it will solve the problem.

Any thoughts?

 

[bonny]

Re: \"Zero deviation\" said the autohelm

Regards autopilot deviation. I'm under the impression that the self deviation is based on the assumption that the boat has a constant rate of turn, this should in theory result in a constant rate of change of magnetic flux across the fluxgate compass. Duiring the "deviation calibration" the AP monitors the rate of change of the resulting magnetic flux, and if this is constant then will record zero deviation. This does not necessarily mean that the rate of turn of the boat was constant, but if the water is flat and there is no wind in relation to the water then this is likely to be the case. If you have a gyro compass then this will see the true rate of turn.

Well that's my theory! /forums/images/graemlins/tongue.gif

 

[ParaHandy]

Re: \"Zero deviation\" said the autohelm

Rob

there isn't a true feedback loop between the rudder position and the AP because the AP is rigidly attached to the rudder therefore there is no advantage to be gained by fitting a rudder position sensor?

I've had a fiddle with the settings some time ago and it isn't perfect (think the boat is too heavy for it) but it works. In rough weather, it is a bit much for it ...

Douglas

 

[tcm]

Re: \"Zero deviation\" said the autohelm

nah. If the nice expensive elctronics works i reckon you can chuck away all that crappy manual wavy-around compass thing, together with the ancient paper charts if you have a nice chart plotter and (cos the gps shows the time near enuf) you can skip the windy-up cukoo clock as well!

 

[jonjo]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
chuck away all that crappy manual wavy-around compass thing, together with the ancient paper charts if you have a nice chart plotter

[/ QUOTE ]
Don't think I need those big flappy white things either. Got 7.3 knots at max throttle (3100 rpm) from the 19hp Volvo 2020, which is pleasing given that most modern yacht buyers would opt for a 30hp engine in a 5.7 ton 35 ft hull. The 3 blade folding prop seems to provide a lot of punch.

 

[jonjo]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
If you have a gyro compass then this will see the true rate of turn.

[/ QUOTE ]
The circuits were on flat water and constant due to the constraints of the Caen Cannal. Yes a gyro is fitted as well, the performance of which impressed the local Bavaria Agent. During some 90 degree course changes there was no hint of overshoot or counter helm as the boat arrived on the new course.

 

[jonjo]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
if you'd previously set it up and zeroed it then the deviation could be zero now ... ?

[/ QUOTE ]
I believe the boat had only travelled 20 meters in water prior to the auto pilot training run.

 

[Talbot]

Re: \"Zero deviation\" said the autohelm

Presume from what you are saying that you have one of the new S1 systems with the rate aided gyro add-ons. from all reports these are the dogs whatsits (sorry Ship'swoofy) /forums/images/graemlins/laugh.gif

 

[jonjo]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
the final stage is to correct this constant error when on a known heading, either against shore marks or against a properly corrected magnetic compass.

[/ QUOTE ]
I will do this once I get the boat to Portsmouth. Guess the spinnaker tower will provide many new bearings.

 

[jonjo]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
the new S1 systems with the rate aided gyro add-ons. from all reports these are the dogs whatsits

[/ QUOTE ]
Yes I have the G variant of the S1 AutoHelm controller.

I got mixed advice about the benefit of the Gyro. The Raymarine sales folks at the SIBS 2004 said the benefit is marginal because the new software in the S1 controller is so smart but the technical support phone line contradicted this advice.

 

[peterb]

Re: \"Zero deviation\" said the autohelm

[ QUOTE ]
I'm under the impression that the self deviation is based on the assumption that the boat has a constant rate of turn, this should in theory result in a constant rate of change of magnetic flux across the fluxgate compass.

[/ QUOTE ]

Not so. Fluxgate compasses work by measuring the field strength in two directions, normally fore-and-aft and athwartships. Comparing the two enables the direction of the field to be calculated.

Since the compass has the two perpendicular components of the field, it can calculate the total field stength. If there were no deviation, then the field strength would be constant irrespective of the boat's heading. If there is a deviating field coming from the boat, then as the boat rotates the deviating field will first aid and then oppose the Earth's field; the result will be a field strenght that varies as the boat rotates. The compass uses this change in total field strength to calculate the required correction.

 

[Gludy]

Re: \"Zero deviation\" said the autohelm

I was clearly told by Raymarine that there is no point in having the gyro fluxgate because the g series has a built in gyro chip - much like stablising binoculars do.

My dealer told me different but conceded after speaking to Raymarine - they had been fitting gyro fluxgates for a while without realising they made no difference.

Certainly the unit has performed very well in all weathers.