Autopilots--basic question

[Omatako]

I have an ST6001 and it is the best, most reliable piece of kit on my boat. It steers a course under sail, under power, to a course or to the wind. There are two things about AP's that are important to remember.

The first is that they tend, depending on the gain and sensitivity settings, to correct more than necessary when first engaged. But then they start to "learn" repetitive adjustments and begin to ignore them. The frequency of adjustment on a long voyage goes down to such an extent that on a Pacific crossing, I used to often wonder whether it was still working because the wheel wouldn't move for minutes at a time and then almost imperceptibly.

Why is this matter important? Because often people are critical of the holding capacity of the AP immediately after it has been engaged but they all do that for a while. Also the perception that they eat up power is erroneous on a longer voyage because with the adjustments becoming so small and infrequent, the AP uses very little electricity.

The second is as SD has already said, the wind vane setting needs time to settle and will not successfully adjust to the frequent wind shifts caused by buildings, hills and so on. But once at sea away from the influences of land, the boat will hold a relative course to the wind unerringly. We sailed a course to the wind on a 45nm trip across open water and the AP held the course with absolute precision. I do not believe that a skilled helmsman could have done it better.

There was one question above about how many members have actually got wind vanes on their boats. Well, the Raymarine and I suspect most other AP's will interface with the NMEA signal coming from the standard wind instrument and don't need a dedicated vane. The down side to that is that with a high degree of heel the wind vane at the mast-head provides a reading that may not be as accurate as one would wish for. However, for long distance cruising where most of the sailing is done off the wind, this is of little concern.

These are just my own observations, YMMV. One thing I will say with absolute certainty, I will not go on a long trip without my AP working. Imagine a crew member who never sleeps, never needs lunch breaks, never goes to the head, never gets tired? That's your AP.

[sailingdog]

I'd also point out that more advanced autopilots will have a yaw sensor, and can tell the rate at which the boat is turning, in addition to having a compass. This can make it far more usable.

As for having a windvane... most of the boats out there don't. Some don't due to issues with having a windvane work properly on them. This is often the case with multihulls and high-performance monohulls.

[pdqaltair]

Originally Posted by sailingdog

I'd also point out that more advanced autopilots will have a yaw sensor, and can tell the rate at which the boat is turning, in addition to having a compass. This can make it far more usable.

As for having a windvane... most of the boats out there don't. Some don't due to issues with having a windvane work properly on them. This is often the case with multihulls and high-performance monohulls.

To be sure, there are many variables. I've found that looking the vane when the AP is strugling (my boat or others) will often reveal the problem; if the vane is swinging all over due to motion at the mast head, it will strugle. There is damping in the circutry, but only with limits.

How does mine perform?
* Flat water. Perfect, any point of sail, compass or wind. Sometimes I think it steers better than I can.
* Swell. Good up wind, OK off the wind but higher power consumption (hand steering is more work too). I can point better hand steering. I can fly the chute a bit better hand steering. Set sails more conservatily/over trim a bit if hard on the weather or running chute. Other courses, set normally.

It would be nice to have a simple button to increase the gain in swells, to help it anticipate the way a helmsman can.

But I have never found conditions, other than wing-and-wing in a blow, where I couldn't trust it for hours.


That said, my last boat was a performance cat (Stiletto 27). Would I trust the AP under sail? Nope. The boat was just too tender and the AP couldn't keep up unless the water was flat and the wind very steady. Of course, lashing the tiller would have been pittiful also. It was VERY dependable under power, though, in any conditions.