Mini Gyro or Fluxgate any experiences.

[mocruising]

I am fitting a second auto pilot to my heavy displacement GRP sloop. I have purchased a Lewmar Mamba drive and Simrad AC course computer and AP 25 control unit and Simrad rudder indicator. The one remaining component is the compass either fluxgate or mini gyro. The fluxgate is £183 the mini gyro £411 but will it improve the downwind steering or will there be other advantages/disadvantages. The other unit has a fluxgate compass.

 

[AndrewB]

As no-one else has answered you ... I've found a fluxgate compass reasonably satisfactory with my CeeTrek autopilot. Fluxgate compasses respond to the earth's magnetic field whereas Gyro compasses follow true north. I don't see (from a position of ignorance) that either has any special advantage downwind, which is a tricky point of sailing for a powered autopilot. You would be better off fitting a wind sensor, but don't expect brilliant performance in strong winds, specially with the wind on the quarter. A true wind vane pilot would be better, and seems to me the logical choice for a 'second' autopilot rather than another electrical/hydraulic one.

(NB The only gyro compasses I've seen come from light aircraft and have the disadvantage of needing to be reset every few hours which makes them unsuitable for yacht use. Prresumably this weakness has been ironed out).

 

[Anonymous]

Compasses - fluxgate and traditional - suffer from errors due to acceleration/deceleration, heeling and yawing. When you are sailing downwind you are tending to yaw and heel quite badly, and depending on the wavelength you might be plunging into waves or being picked up and accelerated by waves from behind. All of these things will create errors in the output from a fluxgate. However, for the most part these errors can be smoothed out provided the autopilot can be set up to slow the response. There are two types of gyro - a rate gyro and a standard gyro. I imagine that your mini gyro is a solid state rate gyro - can you confirm and get back to us? Surely nobody is suggesting that you don't need a fluxgate in your system? You will still need a compass input even with a gyro.

The rate gyro is supposed to improve the autopilot response as it can compensate for the errors in the fluxgate and it also provides a much better output for MARPA on the radar. But you are going to pay for the privilege. By the way, Andrew mentioned the problems of resynching gyros in aircraft but this isn't the case for a rate gyro.

 

[sailinglegend420]

I fitted a rate gyro to my Raymarine T400 and it dramatically improved the autopilot in heavy weather. It is also a MUST if you have MARPA on your radar.

Probably the best £500 spent on the boat.

 

[roly_voya]

Dont know your particular setup but often problems with course keeping downwind are related to the response speed of the auto pilot, like many things the standard information is aimed at costal sailing in mod weather. A primary pilot for offshore is going to need to be at least one size up and preferably more then a lot of the problems disapear. Which is not to say rate giros are not the icing on the cake but they wont make an undersize pilot work.

 

[mocruising]

Both the units I have are well over rated they are designed for a 65' boat upto 40 tonne. We are 48' and 22 tonne. So will I need a rate Gyro and a Fluxgate compass Lemain. If it will improve steering downwind and in bad weather I will go a head with both.

 

[Anonymous]

As I said in my PM to you, in the Raymarine system they are different things - the rate gyro gives an output that is proportional to the rate of turn and the fluxgate gives an output which defines the actual magnetic heading but will all the errors we know magnetic compasses exhibit. What the rate gyro does is to allow the electronics to forecast better what is happening - for example, if a wave hits the bow which starts to accelerate in a given direction then the output of the rate gyro is large, immediately, long before the magnetic compass (fluxgate) has measured any change in ship's head. This allows the autopilot to start to apply helm immediately - just as a good helmsman does. It also helps the radar electronics to predict where the head is going to be when the antenna is again pointing at the target so MARPA performance is better.

Don't worry about the rating being for a bigger vessel than your own; a rate gyro will be fine for a smaller vessel.

We have heard from one poster in this thread that he considers it to be the best £500 he has spent on electronics and while we don't have a rate gyro (we haven't been doing any ocean crossings and we cope OK) I have only ever heard good things about them.

If you are in any doubt, maybe the distributor would lend you one for the weekend? It isn't like finding a spot for a magnetic compass; you could position it with Blu Tack and string, and use a flying lead.

 

[mocruising]

I have been advised that the Simrad RC 36 is a stand alone Gyro and not a rate gyro. It reports heading changes 10 times a second to the course computer as opposed to once a second with the normal fluxgate units. I think its the bit of kit for me.

 

[AndrewB]

I'm planning on fitting a new autopilot soon and have found this thread most informative - thanks guys. Apologies for the ignorant first posting but I wanted to keep the thread alive.

 

[Oldhand]

[ QUOTE ]
I have been advised that the Simrad RC 36 is a stand alone Gyro and not a rate gyro. It reports heading changes 10 times a second to the course computer as opposed to once a second with the normal fluxgate units. I think its the bit of kit for me.

[/ QUOTE ]

I replaced my original Simrad fluxgate compass with an RC25 rate gyro enhanced fluxgate compass. It improved autopilot performance no end and supplies decent 10Hz NMEA heading data via the control box for MARPA.