Autopilot drive mounting question

[jviss]

The orthodoxy of below-decks autopilots is that the linear drive be connected to a separate tiller arm that is attached to the rudder post, and not to the existing radial drive.

For those of you not familiar with this setup, I am referring to boat that have vertical rudder posts that are steered with Edson wheel steering, using a circular, cable-driven fixture called a radial drive - like a quadrant that simply goes all 360 degrees.

Edson's recommended bronze tiller arm is a whopping $417! That's the direct price, and for the C-36 unit with 2.860" bore for rudder post, there is no machining charge.

Edson warns against attaching to the radial drive, but gives no compelling reason.

"In no case should the autopilot be attached directly to the quadrant or radial drive wheel!"

I think it's just because they want to sell tiller arms! One purported reason for the tiller arm is that in the event of a steering system failure, the autopilot provides a backup. But, with the linear drive on the radial drive, which is directly connected to the rudder post, there seems to be nothing in the path to break except the radial drive itself, which seems quite robust.

I can only imagine that the radial drive is strong enough to be driven by the linear drive, especially since Edson puts a rudder stop right on the top outside edge of the drive, which can presumably absorb the rudder force in that condition.

As a matter of fact, that rudder stop might be a good place to attach the linear drive, and then make an alternate rudder stop solution.

So, my question is, what are your thoughts on just connecting to the radial drive, and skipping the separate tiller arm?

Thanks,

jv

 

[RichH]

Connecting directly to the 'quadrant' is chancy due to the load that can be imparted from the linear drive.
When you are steering 'by-hand' the quadrant and the balance of the steering system can take the 'impacts' ... because your hands and arms can 'easily be moved' by the system and 'soften the impact'; not so with a quick acting linear drive firmly connected. If you do connect directly to the quadrant, etc. be sure to regularly check the quadrant, etc. connection to the rudder shaft for 'cracks', ... if you dont be sure that you can quickly connect 'the emergency tiller'.

 

[Joseph Shirley]

In MHO don't connect to the quadrant. It is designed to transmit the rudder turning torque through the tension in a wire in each direction, which is applied at the outside, or aft frame of the quadrant. That securing spoke on each edge of the quadrant is designed to resist the tension in the one direction only. The rest of the quadrant mearly holds the wires at a predetermined radius from the rudder post, and provides very little strength to resist the reversing load applied by the linear drive.

Good luck

Joe S

 

[BobD17833]

Rudder arm

The quadrant or radial wheel is a casting not a machined part. It is designed to be loaded on the periphery of the wheel where the cable for steeringis attached. Drilling a hole at a 10" radius from the center of the rudder post to connect the linear drive will futher weaken the disk, which is thin already. The linear drive is capable of exerting up to 750 lb on the disk.
Any faillure of the disk due to the AP connection, will also make your primary steering (wheel) inoperable and you are defeating one of the adavantages of the AP, a backup to a steering system failure, in addtion to the emergency tiller.

How much risk are you willing to incur?

IMHO, Buy the tiller arm and do it correctly.

BobD
Boston

 

[jviss]

additional technical thoughts...

So here are a few more things I thought of:

- the linear drive should be installed so that it does not reach it's own end stops, it should hit the boat's rudder position stops first. So, if the autopilot drives it to the stop, what is the load and stress situation at this point?

- does anyone know, offhand, the diameter of the radial drive wheel on the Catalina 36? Does 10" sound right?

- the tiller arm and linear drive stroke recommendations from both Raymarine and Simrad result in rudder operating ranges of nominally +/- 35 degrees. Ray is 12" stroke, 10" radius arm; Simrad is 8" stroke, 7" arm. I don't think these are etched in stone, I think they are recommendations based on the pilot operating parameters: if you get close, it will work, as long as there is sufficient rudder travel.

- I confess this is a lot about the money, the work, and maybe even the space available to install this thing. On the space side, I don't know how many have done it, but I've upgraded my original steering system. In 1984, at least, Catalina were using an Edson pull/pull conduit system. The design and installation of this was far outside of Edson's design guidelines, particularly for min. bend radius. Mine failed. The correct fix, according to Edson, is to remove the conduit and replace with an idler assembly, which I did. This requries moving the radial drive wheel on the rudder post, for proper cable alignment. I wrote about this on this site, I think, years ago. Anyway, I'm not sure I will have the clearance for the tiller arm without cutting stuff up.

 

[Tim R.]

So here are a few more things I thought of:

- the linear drive should be installed so that it does not reach it's own end stops, it should hit the boat's rudder position stops first. So, if the autopilot drives it to the stop, what is the load and stress situation at this point?

In most AP computers there is a setting for you to limit the drive from hitting the stops. You can set it a few degrees shy of the stops. You are basically enabling an electrical stop instead of a mechanical one and is only active when using the AP.

 

[jviss]

In most AP computers there is a setting for you to limit the drive from hitting the stops. You can set it a few degrees shy of the stops. You are basically enabling an electrical stop instead of a mechanical one and is only active when using the AP.

That's interesting, and right you are, regarding the ST6001+.

That said, it's interesting to me that Raymarine specifies a 10" radius tiller arm, and that the Type 1 linear drive has a stroke of 12"; and, they require that the rudder hit it's physical stops before the drive reaches the end of it's travel, else damage could occur. This set of dimensions and conditions will require a maximum rudder travel of < +/-37°. The ST6001+ commisioning guide calls for setting the AP rudder limits to 5° less than the max travel noted on the AP head. This means that the AP will only drive the rudder +/- 30°, in a fairly well set-up system.

For the Simrad drive, with their recommended arm, I can get to about the same.

I will then plan on making mechanical stops at +/- 35°, and set the ST6001+ for +/- 30°.

But this is a bit off the point....

 

[jviss]

In most AP computers there is a setting for you to limit the drive from hitting the stops. You can set it a few degrees shy of the stops. You are basically enabling an electrical stop instead of a mechanical one and is only active when using the AP.

Sorry, Tim,

I lost my mind writing my last reply. To address your comment, yes, the AP shouldn't drive the rudder onto the rudder stops. During operation, this feature will prevent that. In addition, the linear drive should never be driven to it's limits, even when the AP is off, and this is the reason for the physical rudder stops.

 

[Paul Cossman]

Autopilot drive attached to quadrant

My Raytheon Type I linear drive is attached directly to the forward center of my Edson steering quadrant by means of a hole that was drilled and a through-bolt with a quick-release pin on my 24,000 pound 1991 Hunter Passage 42. This set-up has taken me over 35,000 miles, on two coconut milk runs and back (plus a bunch of inter-island cruising). While my drive unit has required replacement twice, the quadrant connection remains intact. (One replacement was due to a faulty installation which allowed salt water to get into the drive arm and rust. The second replacement was due to the drive arm deteriorating and eventually seizing--cause unknown). Both failures placed forces on the quadrant which held up without failure or obvious stress or distortion. I say obvious since I have done nothing more than a visual inspection, after removing and replacing the through-bolt (which was supplied with the drive unit).

Like many of the posts I see on this site, the prior responders in this thread make very good points, and if I were now initially installing this unit I might do it differently. I just thought I'd let you know my experience. We use our autopilot (with an ST 7000 control head) virtually all of the time, including in seas/weather up to 15-20 feet and 40-45 knots.

 

[BobD17833]

Rudder stops

Don't reset your rudder stops until you've completed the drive installation and alignment. You may find you'll need a stop at a different angle than 35, in your example.

The orientation of the drive as it is mounted and connected to the tiller arm is the determinate. i.e. With the rudder centered the linear drive MUST BE 90 DEGREES PERPENDICULAR to the tiller arm for that rudder that position (center).
With the rudder centered and the new tiller arm coming off the rudder post inline with the rudder (6 or 12 o'clock) AND you can mount your linear drive (perpendicular) to a soilid bulkhead, then the the drive stroke (6") and rudder (35) limits will be equal, extended or retracted.
If your conditions do not allow the drive to mounted as described, the alternative will be that with the rudder centered, the tiller arm will be coming off the rudder post at something other than 6 or 12 o'clock, to accomodate the solid mounting surface for the linear drive. If you do the trig., the drive fully extended and retracted will be different when the rudder hits the stops, but they both must be l<6". The only way to get there is to reduce the rudder stop to something less than, in your example, 35 degrees.

The take away is, determine how and where the drive will be mounted so that it meets the conditions: rudder centered = drive stroke centered, drive perpendicular to centered rudder, stoke extended and retracted both = <6".

BobD
Boston

 

[RichH]

... all make good points.

Another consideration in my decision making process was with an auxiliary tiller arm, if 'anything' failed in the pedestal system (chain, sheeves, quadrant, cables, etc.) I could still use the AP (named 'Otto) to steer the boat (and without having to tireless use an 'emergency tiller').

 

[jviss]

Paul, thanks, that's a lot of miles. What kind of Edson quadrant was it, a bronze quadrant, or the cast radial drive wheel?

BobD, I don't completely get what you are suggesting. Would you agree that if the stops were dependent on the tiller arm, that tiller arm or not, they would be in the same place?

I think the design goal is to have rudder stops that restrict rudder movement to less than the range of motion of the linear drive; to mount the linear drive such that mid stroke is rudder centered, and movement left and right is symmetrical respecting the drive, i.e., tiller arm and drive centerline at 90; and, range of motion that the AP drives is less than range of rudder stops.

In summary, if:
LD[°] is the angular range of motion that the linear drive can exert;
RS[°] is the hard rudder stop range of motion;
AP[°] is the range of motion that the autopilot is limited to; then the requirement is:

AP° < RS° < LD°

So, as a design example, for 1/2 the range, Raymarine Type 1 on a 10" tiller arm:

LD = 37° (given)
RS = 35°
AP = 30°

Similarly, or Simrad HLD350 w/ 7" tiller arm:

LD = 34° (given)
RS = 32°
AP = 27°

(One might consider shortening the tiller arm on the Simrad design, assuming there is sufficient power reserve, to get up to +/- 30° available for the AP).


So, you want to hit the rudder stops before the LD hits it's limits, and restrict the AP to less than the rudder stops so you never drive against them.

 

[Paul Cossman]

JVISS, it's a bronze quadrant: a true quadrant, not a complete radial. And my linear drive is set up as you suggest: rudder stops hit before linear drive hits its own limits; drive is centered (and movement is therefore symmetrical). BTW, while I've been remembering this, I remembered why the second drive failure occurred. It wasn't a seizure. What I remembered "seizing" were just bolts--either the bolts anchoring the drive unit motor base or the bolt attaching to the drive unit to the quadrant--when I tried to remove the unit; I can't really recall which bolts anymore. I recall now that the gears had just worn out in the drive unit. When I got my new one (sent to NZ), I just installed the new one as it was from the manufacturer, circa 2003. But I also bought an upgraded set of gears that Raytheon sells, a set made out of metal rather than plastic. I think it cost me about $150. I remember asking them to install them for me, but they wouldn't do that for some reason. So now I just have the metal gears as spares, and will install them if the plastic gears ever give out again.

After experiencing two autopilot failures, one of them about 500 miles offshore, I now have a complete autopilot backup: drive unit, computer, control head. Hated to spend the bucks, but since we only double-hand, I NEVER want to go through that again.

 

[jviss]

... all make good points.

Another consideration in my decision making process was with an auxiliary tiller arm, if 'anything' failed in the pedestal system (chain, sheeves, quadrant, cables, etc.) I could still use the AP (named 'Otto) to steer the boat (and without having to tireless use an 'emergency tiller').

Thanks for your input, Rich. I don't think there is anything in the path between the autopilot and rudder, other than the drive wheel (functionally identical to the quadrant), so failures upstream (towards the hand steering wheel) would be the same as for the auxiliary tiller arm. I think the only concern with the proposed approach is the strength of the radial drive wheel, and the connection between that and the drive ram. My intuition says it is strong enough, assuming the connection between the drive ram and the drive wheel is well engineered and executed.

If you look at the Edson catalog you'll see the "standard" rudder stop. It is a flat bracket that bolts to the radial drive wheel with a vertical stainless steel pin, with a piece of hose slipped over it (as a cushion). I may just take mine and turn the pin down to the correct diameter (12mm; interestingly, Simrad dimensions this as 12mm (1/2") on their drawings), drill for a hair-clip cotter pin, and drop the ram's ball end onto it. I might look into solidifying the connection between the rudder stop bracket and the wheel, perhaps by fitting it precisely, or bedding it with epoxy (to increase the contact surface area, and therefore lateral (static) friction; and also prevent "working" due to flex). This assumes initially that it is at the correct radius, which it probably isn't!

 

[BobD17833]

Skewed tiller arm

JV

Re: "Would you agree that if the stops were dependent on the tiller arm, that tiller arm or not, they would be in the same place?"

No -
The example I suggest is that when the tiller arm must be attached to the rudder post Not parallel to the rudder when the rudder is centered - To Accomodate The Mounting Of The Linear Drive.
Consider that the you may not have a strongback mounting surface that will allow the drive to be mounted perpendicular to the tiller arm when the tiller arm is parallel to the rudder when rudder is centered.
Therefore the tiller arm must be rotated on the tiller post so that the drive is perpendicular with rudder cenetered.
This condition will satisfy the drive alignment conditions, but creates a longer (or shorter) arc when moving to one end stop than to the opposite end stop. The arc difference translates to a longer drive throw from center on one side and a shorter throw on the opposite side. So....
Do you set the stops unequal and favor the full 6" drive throw on each side of center, or do you set the stops equal and have less than 6" drive throw on one side and full 6" throw on the opposite side?
I'd favor the latter and decrease the stop position angle but retain equal stops side to side.

I agree with the remainder of your post - "I think the design goal is..."

My point is, don't set the stops untill you determine the mounting position of the drive.

BTW - you can change the stop angle by simply increasing the rubber bushing diameter on the stop pin.

BobD
Boston

 

[jviss]

JV

Re: "Would you agree that if the stops were independent of the tiller arm, that tiller arm or not, they would be in the same place?"

No -
The example I suggest is that when the tiller arm must be attached to the rudder post Not parallel to the rudder when the rudder is centered - To Accomodate The Mounting Of The Linear Drive.
Consider that the you may not have a strongback mounting surface that will allow the drive to be mounted perpendicular to the tiller arm when the tiller arm is parallel to the rudder when rudder is centered.
Therefore the tiller arm must be rotated on the tiller post so that the drive is perpendicular with rudder cenetered.
This condition will satisfy the drive alignment conditions, but creates a longer (or shorter) arc when moving to one end stop than to the opposite end stop. The arc difference translates to a longer drive throw from center on one side and a shorter throw on the opposite side. So....
Do you set the stops unequal and favor the full 6" drive throw on each side of center, or do you set the stops equal and have less than 6" drive throw on one side and full 6" throw on the opposite side?
I'd favor the latter and decrease the stop position angle but retain equal stops side to side.

Hey BobD,

Thanks again for your thoughts. I quoted above, but corrected a minor typo in what you quoted from me, which may have led to a misunderstanding, which is "independent of" rather than "dependent on." Sorry about that.

In any event, I disagree with what your analysis.

Regardless of how I mount the linear drive, the rudder stops will have to be installed so that the rudder really swings the same amount, port or starboard. Really! Even if I don't install an autopilot.

If for some reason I must mount a tiller arm such that it is not aligned fore and aft, I will mount the linear drive so that it is perpendicular to the arm and at mid-travel when the rudder is centered. In this condition, regardless of the angle of rotation around the rudder post of the tiller arm when the rudder is centered, the drive will drive it symmetrically about that position throughout its travel.

I don't agree with your statement that "This condition will satisfy the drive alignment conditions, but creates a longer (or shorter) arc when moving to one end stop than to the opposite end stop."

 

[sail361]

I have a ST6001 autopilot my boat, but it is not connected to an Edson setup, but looking at mine, it has some sort of attachment comnnected to the quadrent. I took a crude photo of it. Hope it helps

 

[jviss]

I have a ST6001 autopilot my boat, but it is not connected to an Edson setup, but looking at mine, it has some sort of attachment comnnected to the quadrent. I took a crude photo of it. Hope it helps

Thanks for the pic. Looks like a bracket bolted to the Edson radial drive wheel - is it?

 

[Rick Sylvester]

Facts

I'm willing to bet that the folks who state with such certainty that the radial/quadrant is weaker than an AP tiller arm have never done any destructive testing to support that. I think Edson is full of crap when they tell you not to mount the ram to the quadrant. After all, you COULD do a poor job and it could fail but it sounds more like "we don't get to sell you a $417 part." That's just my uneducated opinion.

What I know for a fact is that my best friends' had their tiller arm fail while on a blue water passage a couple weeks ago. The two of them had to hand steer for two days.

My ram is bolted directly to the quadrant. Nearly 10,000 miles and no problems whatsoever.

Should this info be what you base your decision on? Of course not. But it does provide some creditable evidence.

 

[jviss]

I'm willing to bet that the folks who state with such certainty that the radial/quadrant is weaker than an AP tiller arm have never done any destructive testing to support that. I think Edson is full of crap when they tell you not to mount the ram to the quadrant. After all, you COULD do a poor job and it could fail but it sounds more like "we don't get to sell you a $417 part." That's just my uneducated opinion.

What I know for a fact is that my best friends' had their tiller arm fail while on a blue water passage a couple weeks ago. The two of them had to hand steer for two days.

My ram is bolted directly to the quadrant. Nearly 10,000 miles and no problems whatsoever.

Should this info be what you base your decision on? Of course not. But it does provide some creditable evidence.

Great post, thanks. Doesn't look like the current tiller arm, nor does your quadrant look like my radial drive wheel. But I think your point is still valid. I believe that if the bracket is designed such that the load is well distributed onto the wheel, it should be fine. "Well distributed" means that the loads are transmitted broadly, so there are not small stress points; and, that the bracket is secured firmly enough that it won't "work," or move and wear out it's bed and bolt holes.

I've been turning this over in my head a lot. We've all hand steered these boats, and even steered bigger boats with a tiller. Mechanical advantage is amazing stuff, and I can't imagine steering it by hand with only 10" of leverage on the rudder post; however, that radial drive wheel is pretty sturdy. I would love to be able to make a bracket that would place the axis of the linear drive in the same plane as the steering wires, so I won't have the twisting load from a cantilevered pin. Probably not an issue, though.