Autopilot: Linear vs. Rotary Drive

[Badsanta]

My thinking is to connect the rotary to my steering shaft that goes to the rack and pinion gear. That some of the water pressure on the rudder would be contained by the rack and pinion. also I would get some benefit of the gearing along with the rotary power. I have not explained that well, but that's my thinking. The ram would have to handle all the pressure and the rotary would be offset buy the gears.

[Sailmonkey]

Originally Posted by colemj

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

The rotary drive is just a gear reduced motor attached to the sprocket through a magnetic clutch. When the rudder is turned to an angle as called for by the course computer the motor power is cut. if the rudder angle on our boat is more than a roughly 6-8 degrees the force of the water tries to straighten out the rudder, as the motor runs through a high reduction gear it is difficult to turn, but possible. As the rudder is moved by the water, the course computer sends current back to the motor to move the rudder back to the set position.
Boats with a worm gear steerer won't suffer this, but all others will to some degree or another. It's not been a problem, just something to be aware of. I usually set up the sails like I would with the windvane steering to reduce this tendency.

[Sailmonkey]

Originally Posted by Badsanta

My thinking is to connect the rotary to my steering shaft that goes to the rack and pinion gear. That some of the water pressure on the rudder would be contained by the rack and pinion. also I would get some benefit of the gearing along with the rotary power. I have not explained that well, but that's my thinking. The ram would have to handle all the pressure and the rotary would be offset buy the gears.

If you have a rack and pinion you can swing the rudder by hand and turn the wheel, if you have a worm gear you cannot. I went through great lengths to eliminate as much friction as possible in ours.

[colemj]

Originally Posted by Sailmonkey

The rotary drive is just a gear reduced motor attached to the sprocket through a magnetic clutch. When the rudder is turned to an angle as called for by the course computer the motor power is cut. if the rudder angle on our boat is more than a roughly 6-8 degrees the force of the water tries to straighten out the rudder, as the motor runs through a high reduction gear it is difficult to turn, but possible.

I'm a bit dense on this, but wouldn't that be the same for a linear drive attached to the quadrant/rudder post? Our linear drive doesn't back-drive if the clutch is on. Maybe the gearing is higher on the linear drive?

Mark

[Sailmonkey]

Originally Posted by colemj

I'm a bit dense on this, but wouldn't that be the same for a linear drive attached to the quadrant/rudder post? Our linear drive doesn't back-drive if the clutch is on. Maybe the gearing is higher on the linear drive?

Mark

The linear is essentially a screw jack. You can't compress against a screw, but you can turn a motor by spinning it.

[Amnesia II]

No one has mentioned the noise factor yet. I've used both systems and currently have the Raymarine linear which to my ear is silent as opposed to a rotary/chain drive which I heard every move....on off on off and so on!

So far I've had no problem with the linear....across the Pacific in all weather conditions, worked very well and no noise!!

[Sailmonkey]

Our rotary makes no noise at all, other than when the clutch engages/disengages. But I did make sure the sprockets were aligned and the chain properly tensioned.

[colemj]

Originally Posted by Sailmonkey

The linear is essentially a screw jack. You can't compress against a screw, but you can turn a motor by spinning it.

Got it now - thanks!

Mark

[Badsanta]

Originally Posted by Sailmonkey

If you have a rack and pinion you can swing the rudder by hand and turn the wheel, if you have a worm gear you cannot. I went through great lengths to eliminate as much friction as possible in ours.

Your right. But it seems to be easer to steer than a tiller. Seems to be some advantage. But yes I understand

[Rick Williams]

since I already have a rotary drive that has died, I'd like to replace it with another but I was on a friends boat with a rotary drive that growled under the captains bunk like a caged bear for the entire trip up the East coast. Are the new rotary drives quiet, reliable and durable?

[ColdEH]

When you speak of a rotary drive , is this what you mean ?

http://www.p2marine.com/catalog/imag...tary-drive.jpg

These are robust and mounted below deck .

Regards


Sent from my iPhone using Cruisers Sailing Forum

[ColdEH]

I will just add to this.

I have had both systems , my last boat was 48 feet and had a linear hydraulic drive , it worked just fine, the only concern I have with them is the way they are installed . The end of the ram takes the full load of the rudder , if you are getting into some nasty weather the point were that ram is attached has to be enormously strong ! This was not a problem on my last boat it was aluminum , I can see issues if you are doing laminations to the inside of the hull . This has to be very strong!!! I have seen many rip right off the hull and take everything they are attached to go with them, IE bulkheads and stringers .

My present boat has a below deck Mamba drive , which turns via a chain a rack and pinion gear set up that is attached directly to bottom of the rudder post , This is completely separate from the cable system that I steer with . The solid bronze gears take all the rudder load , the Mamba drive sees no load at all , the mounting of the drive is not crucial other then the alignment of the chain .

Of the two which one do I prefer ? They both work great but to tell you the truth I look at my rotary drive much less then I looked at the linear drive . The stresses on that ram always gave me concern .

I like the rotary drive for its simplicity . Just the electric to worry about , not the electric over the hydraulic . One less system to break or maintain .

Also the Mamba drive is whisper quiet . If it makes a noise it is usually just in need of a chain lube .

Regards

[Sailmonkey]

Originally Posted by Rick Williams

since I already have a rotary drive that has died, I'd like to replace it with another but I was on a friends boat with a rotary drive that growled under the captains bunk like a caged bear for the entire trip up the East coast. Are the new rotary drives quiet, reliable and durable?

I can't hear ours. It's a raymarine rotary drive, type 1.


Sent from my iPhone using Cruisers Sailing Forum

[Frankly]

On a well traveled boat the AP can get a pretty good workout and any electro mechanical drive can eventually wear out. Worn systems often make a lot of noise on their way to failure.

As a general concept an AP that drives from the steering wheel shaft will consume less electricity than one coupled directly to the rudder shaft. The reason for this is that the polar (rotational) moment of inertia is a function of the square of the gear ratio that it is reflected through. This is the reason when you try to turn your rudder by grabbing the blade it takes so much force to start the process. On a well balanced boat (and depending on sailing conditions) much of the energy is consumed simply overcoming inertia of the rudder/steering system. Hydrodynamic forces are small for small rudder deflections. Modern AP systems with rate sensors are even more sensitive to this effect as the AP is so quick to respond to heading errors.

On Cbreeze with the Edson Rack and Pinion drive system and an old Benmar rotary drive (mechanical clutch) coupled to the back of the steering shaft, driven with Raymarine X-5 electronics, my stereo uses more electricity playing a Eagles CD than keeping the ship on heading (nominal sailing conditions).

[Rick Williams]

thank you all for your feedback!
Based on the information, I'm using the sprocket that's already installed and replacing the rotary drive.