Re: [Amel Yacht Owners] Re: Steering Failure on 1992 SM72 "Aloha"

Craig Briggs

I posted my last before I read Bill Kinney quoting the Linear Drive's thrust as 480 kg. Raymarine shows both the linear drive and rotary drive handling a 20000 kg boat and they both have about the same power consumption, that is, essentially the same motors. 

The difference seems that the linear drive has the mechanical advantage built in by way of its rotary-to-linear mechanism whereas the rotary unit has that mechanical advantage supplied externally by the rack and pinion and the sizing of the chain sprockets. And, as Bill notes, with normally small turns of the rudder, overcoming the back force of turning the wheel isn't a problem, and may not be a lot as compared with the 480 kg of thrust available.
Interesting footnote to the Raymarine spec is that with the linear unit one must be able to back drive the steering system from the rudder.

Sounds like a fun project, Herb, to add a linear drive to your Santorin, and I should think it will work just fine. 

Cheers, Craig

---In amelyachtowners@..., <sangaris@...> wrote :

Excellent question, Herbert, and one I've wondered about, too.
My wheel goes about 3 turns from stop to stop. That's 3 X 360° = 1080°. My rudder moves about 30° each way or 60° stop to stop. That's an 18:1 mechanical advantage, which is why Herb has to push so hard on the rudder to spin the wheel and move the rudder. (to say nothing of wheel diameter, rudder center of effort and friction)  
Granted the linear drive has a lever arm at the quadrant giving some mechanical advantage, but it seems it still must take much more force than the rotary drive.
Interesting discussion.
Cheers, Craig

There is no need to disconnect the cables when using the linear drive.  The cables and wheel actually move quite freely, the force you need to move the rudder while on the hard manually is mostly just the inertia of the mass of the rudder. The inertia of the steering wheel adds some, but is not large.

The linear drive generates 480 kg of force, and has no trouble turning the rudder even under very heavy loads.

The cables and rack will see much reduced load and will wear LESS using the linear drive than they will using the chain drive.

Most of the time, if the boat is trimmed well, the autopilot movements of the rudder should be very small.

Bill Kinney
SM160, Harmonie 
Annapolis, MD, USA

---In amelyachtowners@..., <herbert@...> wrote :

Hi Bill, I think I could not express myself very clear, try to say it again, sorry for my English: 

when the rudder is moved by the rotary drive (or by the steering wheel) it moves very smooth, you can turn it with the "little finger".  If I try to move the rudder (on the hard) by pushing or pulling it I need quite a lot of power until it moves (and of course the steering wheel turns also). So I thought, that the system is designed in a way that it runs very smooth when the power transmission goes from wheel to rudder but not the other way round (eg like a worm gear...).

If the linear drive turns the rudder by moving the quadrant I thought that there might be a lot of friction until the steering wheel gets turned by the ultraflex cables.  Is it true that the linear drive also turns the steering wheel when working - I assume yes?

This is not a question how the Autopilot works but how the Amel Steering works

thx, herbert


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