I just glanced at Raymarine's website and it seems to me that their optimum below-deck autopilot for our boats would be the ST70 SmartPilot X-10.

It's interesting that this autopilot model has eliminated the need for a rudder position sensor, although how they do that I have no idea...perhaps some kind of position feedback via the drive voltage connections to the linear actuator. I suppose that the linear actuator, once calibrated with a rudder center datum, would of course know where the rudder was at all times, the issue being a way to communicate that back to the A/P computer.

Another interesting difference versus my A/P is that this one is stated to be applicable to vessels up to 24,000 lbs, whereas the model I installed was applicable to vessels up to 19,000 lbs.

Hi Al,
We installed a Raymarine ST6000 Smartpilot with a linear drive on Wind Star, and it is one of THE BEST things we have bought!

The only difficulties we have experienced are our SSB knocking out the unit temporarily when transmitting - we got a "rudder sensor lost" kind of message so we turned the auto pilot off and restarted it and it was OK. Re-routing the SSB aerial cable further from the control head seems to have fixed that one.

I also got the Raymarine 10-X linear drive. It works like a champ. One thing that does piss me off a bit is the lack of force information. I spent 20 years as an engineer designing control systems, so I know for a fact that the internal servo control system has data on motor current. This means it knows how much torque (force) is required to stay on course, but they don't bring that info out to the operator.

I've taken over manual steering at times only to be shocked at how forces had grown due to a change in wind. It's the sort of feedback a human helmsman has which leads to changing sail trim. The autopilot muscles through without any sort of warning message. It tells me when I'm off course but not when I need to trim. My engineering experience leads me to believe that they have overlooked an important feature that would be easy to impliment in software.

Greg, I see your point (I'm an old aircraft autopilot tech...emphasis on the 'old'), but the Raymarine A/P does give you that information, of a sort, via the rudder position indicator on the control head. If the rudder displacement from center is small, then the load - ie, torque generated by the linear actuator - ought to be small, and vice versa. Wouldn't that be the case?

No, maybe not. Rudder displacement might be the result of boat trim, not torque??? Let me chew this one over a while.

I would think that when holding a heading at a given STW (speed thru the water), the hydrodynamic forces on the rudder would increase predictably with rudder angle. I can't see that it matters why the rudder is at that angle.

I just purchased a Garmin GHP12 autopilot with a class A linear drive. It is rated for boats up to 28000 pounds so it shouldn't have to work too hard on my C36. The linear drive has a large range of travel, about 11,5 inches or so. With that length of travel I should have no problem installing it so I can get full lock to lock travel on the wheel with the system in standby. I noted that some others who have the Raymarine have had to limit their wheel travel or disconnect the drive in order to get full lock to lock travel at slow speeds.

Lock travel on our Raymarine is restricted, which worried me a bit when I installed it. I checked with Raymarine and it's OK to fit the linear drive to the second hole from the end of the tiller arm, increasing travel a little. In practise I myself don't really notice the loss of travel...

Tom, there are two concerns to balance here:

1) the lock to lock range of the rudder is limited by the snubber. Without the snubber acting as the limit, the rudder would smack the fiberglass hull structure and cause damage.

2) Raymarine's installation manual requires that the autopilot linear actuator be installed such that the snubber limit is reached BEFORE the linear actuator reaches its mechanical extension limit. In my installation, I was conservative. In order to allow for eventual deterioration of the rubber, I shortened the snubber cable a very small amount. This does affect manual wheel steering lock to lock movement by, maybe, a fifth of a spoke, maybe less. Practically speaking, this reduced extreme wheel motion is not noticeable at the helm.

Rarely if ever will the autopilot drive the rudder to its snubber limit. In the years since my installation I can't imagine that this has ever occurred. When maneuvering in close quarters the A/P is off, or should be. At least until someone invents an Automated Docking* capability as new automobiles nowadays have an optional parallel parking capability.

*patent pending

One reason I decided to go with the Garmin unit is that I would be able to maintain my existing stop to stop rudder travel with the AP in standby without bottoming the actuator in either direction. This helps with tight docking manouevers. Of course to do this, requires the autopilot tiller connection be short. With the 28000 pound capacity of the Garmin unit, the loss of mechanical advantage would be accomodated by the additional thrust capability of the Garmin unit.

Tom, I understand the point you're making, but personally I think it's not as significant as stated. I'll wager that the difference in lock to lock between your C36 and mine is negligible. Incidentally, when I purchased my Raymarine system the only one available for the the C36-class market was a 19,000 lb system. Apparently today they have a heavier duty actuator available. A big factor in my selection of Raymarine was integration with the existing Raymarine suite (ST-60s, GPS chartplotter, etc) that was on the boat.

My point isn't to promote Raymarine versus Garmin, but to have a dialogue for the benefit of our readers. Also, I enjoy the discussion because it taxes my grey matter in an interesting and enjoyable way. It's been many decades since I got paid for working on autopilots for aircraft (hint: it was the '60s) and so the mental exercise is pleasantly challenging.

Al, are you planning to install the autopilot yourself? If so, we would be very grateful if you could photograph the process and write up your experiences when done. We would love to have another below-decks autopilot article in the C36IA Technical - Upgrades library.

My Raymarine AP is being installed at the moment.
I am using an Edson rudder arm to connect the drive to the rudder shaft.
The installer recommended the feedback unit, while Raymarine said it was unnecessary. But we can always add it later.

I will get some pictures and feedback shortly..

Sent from my iPad using Tapatalk HD

[QUOTE=LCBrandt;12078]Greg, I see your point (I'm an old aircraft autopilot tech...emphasis on the 'old'), but the Raymarine A/P does give you that information, of a sort, via the rudder position indicator on the control head. If the rudder displacement from center is small, then the load - ie, torque generated by the linear actuator - ought to be small, and vice versa. Wouldn't that be the case?

No, maybe not. Rudder displacement might be the result of boat trim, not torque??? Let me chew this one over a while.[/QUOTE]

Well you're right, indirectly. The linear system does not have rudder position feedback but I do have a centerline leather tag on the wheel. What I'll do is notice the position of the wheel when I turn on the pilot and realize how far off it's gotten when there is too much force. Of course we all get sensitive to our boats after a while and the wind noise, heel, and other factors make an overpowered situation pretty evident. I've also noticed a very distinct transition to turburlence in the water behind the transom as the boat starts to near or exceed hull speed. In smooth water I sometime hear a difference in the gurgling behind the boat. When the wheel is over the the point where I'm almost dragging the rudder sideways through the water, lots of related factors are evident.