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Old 26-08-2007, 10:53   #1
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Autopilot: Linear vs. Rotary Drive

For a below-deck autopilot, such as made by Raymarine, what are the advantages/disadvantages of a liner versus rotary drive?
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Old 26-08-2007, 13:32   #2
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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.
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Old 26-08-2007, 14:43   #3
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Autopilot

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.
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Old 26-08-2007, 15:23   #4
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Let me also add...

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.
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Old 26-08-2007, 17:33   #5
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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
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Old 27-08-2007, 00:18   #6
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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.
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Old 19-09-2007, 10:36   #7
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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.
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Old 19-09-2007, 11:36   #8
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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.
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Old 19-09-2007, 18:47   #9
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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.
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Old 16-09-2013, 08:59   #10
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Re: Autopilot: Linear vs. Rotary Drive

Hate to rase an old thread but I have the same question. I can use either the raymarine mechanical Rotary or Linear drive in my boat. Rotary is less work to install. Are there advantages to either in reliability?
Any thougjts about the garmin linear drive?

Many thanks all
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Old 16-09-2013, 09:20   #11
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Re: Autopilot: Linear vs. Rotary Drive

Linear drives are more powerful and robust. Rotary are faster to install and cheaper to buy. Hydraulics are good, expensive and needful of careful installation and maintenance.
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Old 16-09-2013, 12:54   #12
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Re: Autopilot: Linear vs. Rotary Drive

Maybe terms are being confused? By rotary drive, I assumed a below deck chain drive to a sprocket directly connected to the steering system.

If so, these don't seem any faster to install or cheaper to buy. They are basically similar to electromechanical linear drives, only with different output shaft and gearing. They provide equal force, requiring strong mounting, and need equal care with installing them true to the force axis. Perhaps they would be quicker to install if a suitable mounting point already existed, or a rudder post/quadrant needed significant changes made to fit a linear drive.

We have both an electromechanical linear drive and a hydraulic linear drive. Both costs about the same. Both require the same careful installation, and neither require any maintenance. I once had to replace the brushes and seal in the hydraulic drive after 15yrs, but expect to need to do the same for the mechanical drive in that time.

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Old 16-09-2013, 17:21   #13
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Re: Autopilot: Linear vs. Rotary Drive

You're right, sorry. I assumed you meant steering wheel belt drive, not chain drive direct to the rudder shaft. A simple arm, strongly mounted to the rudder shaft and driven by a linear drive can be much cheaper than the chain drive to a rudder mounted sprocket.
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Old 16-09-2013, 17:26   #14
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Re: Autopilot: Linear vs. Rotary Drive

For us the rotary drive was a no brainer based on our rack and pinion steering and the amount of space a linear drive takes up. Mounting the rotary drive for me was also much easier as I would need to fabricate the attachment points for the linear and purchase a bronze tiller to connect to the rudder shaft.
The only downside I can see with the rotary drive is the inability for it to hold rudder angle like a linear can.....but it also forces you to keep an eye on sail balance.
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Old 16-09-2013, 18:02   #15
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Re: Autopilot: Linear vs. Rotary Drive

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Originally Posted by Sailmonkey View Post
The only downside I can see with the rotary drive is the inability for it to hold rudder angle like a linear can.....but it also forces you to keep an eye on sail balance.
I don't have any experience with rotary drives. Can you explain why they cannot hold a rudder angle? I just assumed that once they stopped moving, they were clutched someway to prevent back lash and held the rudder steady.

Are they constantly in motion, or can rudder force back-drive them?

Mark
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