Another Linear Actuator Thread

[Sully]

Hi everyone,

I'm planning to build a below deck autopilot for my boat (L=9.5m, d=4000kg), so I'm searching for a nonexpensive linear actuator. I've been reading the other threads about the topic but I still have a lot of doubts.

Usually the other threads are focused on tiller pilots, so I don't know if I can use them as a reference. For example, some people in the forum have tested Hurd actuators. I saw that they have one that could fit speed and thrust general requirements: Hurd ELA45 with 500N max. load, 300mm stroke and 33.8-93.5 mm/s speed. (Info: I contacted them and it is sold for 119,9$ + shipping 70$).

Would this actuator (for example) fit as a below deck actuator for my boat characteristics? Or I have to search for something very different as long as I don't have a tiller?

Thank you so much, as always.

Sully

[sebba]

a below deck autopilot motor need a clutch for disconnecting it from the rudder.
for hydraulic actuators there is a bypass valve
for electro-mechanical transmission you need a clutch
how do you intend to take the actuator out of "circuit" when manual steering?

[Sully]

Oh, that's definitely a thing. So is it necessary for the linear actuator to include a clutch? Isn't there any other option available to disengage the system?

Thank you!

[sebba]

unless hydraulic, I don't know any solution for proper disengage actuators.
for tiller, on deck, you can remove it by hand but under deck is a different story

[seandepagnier]

A few ideas:

1) hydraulic steering. The piston is always engaged and either manually controlled or by pump. This is a single point of failure though, but usually it can be manually removed and emergency tiller used.

2) electromechanical with clutch. These are expensive but use a ball-screw with clutch so they can be backdriven The clutch disengages the motor and belt that drives it to reduce friction but the ball screw itself is backdriven and adds a little bit of fricction

3) morse cable with pin. This arrangement the rudder must be aligned with the actuator manually and a cable inserts a pin locking them. I dont really like this system because it is could be difficult to align if below decks.

Other systems might include cable steering with hydraulic autopilot and bypass valves, but this still has significant friction to backdrive. There really is no universally ideal system that I know of but I have seen a lot of different arrangements. I came to the conclusion so far that I just produce electronics and let the users figure out the drive system... unfortunately.

If you do find a clever arrangement or anyone else has alternate systems it would be great to hear about it.

[chris]

@Sully, those specs sound about right and what I used (10m/6000kg) fits your spec. The travel time and force depends on your setup. Sean didn't mention it here but he recommended getting one with (rudder) feedback (potentiometer).

1. I dismissed #1 as I don't know hydraulics, don't want another system on board and the power draw seems high.
   
2. I had looked into #2 and found the least expensive one to be around $1,600. Rather pricey indeed.
   
3. I'll try to implement in May. I'll post an update when I have one.
   

Here's a rough outline:

• Actuator permanently attached to steering arm below deck
  
• The other end of the actuator is attached to a car on a track (think genoa track & car)
  
• On the car the pin with a round tip is spring loaded so it fits into the one and only hole on the track
  
• If the morse cable is taut the car will slide around and you're manually steering
  
• If the morse cable is slack the pin will hopefully find its hole and engage.
  
• I'm hoping to just smash +10deg while manually holding course until the pin engages as I can't see below deck (this is working on the bench)
  

The morse cable is a quite simple mechanical solution so I'm not real concerned. However, with the spring the default is to engage when the cable is free to move. I'd prefer the default to be disengaged should there be a failure but that makes the engagement process quite error prone. In my case the track will be right next to a hatch so I could pop that open and pull the pin that mounts the actuator to the car or apply percussive maintenance.

I got the initial idea from a post by Stan Honey where there was a photo of a morse cable implementation on his Cal40. In his setup the cable appears to disengage the actuator's gearbox but I couldn't find any suitable actuators that weren't $$. 

One could maybe fancy that up by replacing the cable with a latching bistable solenoid driven by the clutch function on the pypilot. But, that's more electronics in a marine environment. I prefer the cable for simplicity.

Any other suggestions welcome.