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Jeffa Autopilot Failure

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colinc View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote colinc Quote  Post ReplyReply Direct Link To This Post Posted: 15 January 2020 at 05:15
The manual for the Hypro is here

“Designed for under deck installation only”

“Avoid contact with sea water”

And as said, designed for much smaller boats and with much less thrust than the DD1.   

Frying pan to fire?
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ohthetrees View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ohthetrees Quote  Post ReplyReply Direct Link To This Post Posted: 15 January 2020 at 22:13
Heard back about a couple of colinc's questions. My assumption was that the DD1 would have less mechanical advantage at extremes of rudder angle, but that seems not to be the case.

Here are my Q's and answers inline:

1) Do I need to restrict the range of motion in the autopilot software? It seems to me with the link arm moved further out on the tiller arm, the DD1 arm must go through a greater range of motion. It seems that the DD1 would have very little leverage at extreme rudder angles.
The angle of motion should be the same on the quadrant and therefore the same on the autopilot software where the rudder feedback is mounted. The DD1 has to move further compared to your current setup which in turn, with more leverage, makes the drive stronger. At the outer positions the DD1 will have to move further to move the quadrant the same distance which again makes the DD1 stronger.

2) Surely I must shorten the link arm. Can I modify mine, or do I need to order a shorter one? 
The draglink can maybe be shortened a bit more. You should try it out, I think it will work out for you. After setting up the new position, please check the outer positions that the output lever isn’t close to camming over. 
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colinc View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote colinc Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2020 at 15:14
Thanks for getting the answers to those questions.

It confirms that the draglink ideally needs shortening whe nmoving holes. About 2cm in my boat which means getting the next size down draglink.  The correct length will have the autopilot and rudder drive arms in the centre of their arcs when steering straight ahead.   That "centre point" of the arcs has to take into account the different alignment of the drive and tiller.

It is also of note that the design relies heavily on the geometry between the two drive arms.  Jefa confirm that there is significantly more rudder torque at the extreme port and starboard due to the different arcs of movement.   My previous calculation of an extra 20% torque between the two holes was for the central position.  I think the difference will be much greater at the ends of the movement.  I'll try to work out the much more complex maths of those two positions.

In the meantime can you ask them one more question please: "how important is getting the correct offset between the centres of the drive and tiller shaft?   How much change in loading would there be if the offset was a lot less than that given on the Direct Drive Specification Form."

It would also be useful to know how the drive handles overload situations.  What happens if a wave is stronger than the drive can push against?





Edited by colinc - 16 January 2020 at 15:20
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