Autopilot: Linear vs. Rotary Drive

[Rchrdm]

For a below-deck autopilot, such as made by Raymarine, what are the advantages/disadvantages of a liner versus rotary drive?

[Alan Wheeler]

First question is, what is your stearing?? Second is, what do you intend the unit to connect to on your stearing. That is the real difference between the two, as in what and how will the connect and drive.

[Rchrdm]

Thanks, Alan.

I have Edson wheel type steering with a quandrant on the rudder shaft. The current steering is by cable. I have heard of owners using either a chain (rotary) drive, or a linear drive. It seems the linear drive might be a little more difficult to install on my boat, though one owner has posted some detailed instructions.

[Rchrdm]

I currently have a SIMRAD Wheelpilot 30, which is mounted on the wheel. I am just assume that this pilot does not have the "smarts," power, response time, etc. for conditions offshore. I plan some long coastal passages. But, then again, I have not really tried it out under those conditions.

[sildene]

I have yet to install an autopilot and have to make the same choice. At a recent boat show I was told that one of the benefits of a Linear Drive (connected directly to the quadrant disc) to keep in mind, is that it should still be operational in the event of a breakage in the steering chain, whereas the chain drive connected autopilot would be disabled.

But that would be one of many considerations.

Martin

[Alan Wheeler]

The issue with mounting a drive on the quadrant is the fact that it is still connected to the wheel. And the cable to wheel is geared to give a good helm lock to lock respons. So any movement applied to the quadrant is multiplied to the wheel movement, which can easily be 7x. This can cause two issues, the fact that the wheel can spin at quite a high rate and catch someone unwares, and the other is that the effort is dramaticly increased at puts a lot of strain on the cable and gears.
A rotary drive is often chain linked to a chain to shaft driven system. The rotary drive has a clutch that disengages the rotary drive from the system when not in use.
A modern wheel pilot shoudl be able to steer the boat just fine. They do have issues as well. Like belts wearing etc. I don't know what a 30 would handle your boat like, but you need to get it out into something one day and see how it handles the stearing.
Sildene has a good point. Although provision needs to be made to ensure you can disconnect the pilot from the quadrant to ensure you can fit emergency stearing to the rudder.

[Antares]

I would be leery of the WP 30/32 offshore. We have tried it twice and it other than motoring in flat water, it is at best marginal. The real issue has been reliability. It basically is not water tight and over time the electonics get wet and then go away. After two mother boards, two compass units, and a set of new rollers I quit. After being into the WP30 for more than the purchase cost in parts and repairs, I was too gun shy to go with another wheel pilot. We are currently installing a NX2 underdeck unit with a hydraulic linear drive. I want an AP I have more confidence in. By spec our boat was within the capiblities of the the Simrad unit, but the overall design is not up to the task.

[camaraderie]

Antares:
We are currently installing a NX2 underdeck unit with a hydraulic linear drive

I think you bring up an important distinction...screw type linear drives vs. hydraulic.
I hav owned both and have had problems with the screw type and would second your choice of the hydraulic type as better suited to long term service in heavy conditions.

[Pblais]

Simrad has the better software. I had a Simrad AP11 and now use a Raymarine ST7000. The Simrad software is better by a LOT. RAymarine software is primitive and you have to adjust it manually. Simarad works much more automatic. It learns to add rudder over time so you can deal with alot more variables when the wind is just blowing hard. Linear drive is hydraulic so who cares if it is linear or not. For serious displacement boats it needs to be more than a wheel pilot. If your steering system is cable then there is not much choice at some point you can't get by with a wheel pilot. You want more than a wheel pilot unless you care to purchase two since you will trash them sooner or later. A tiny electric motor isn't a match. I have however heard great results with the newer Simrad wheel pilots.