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Old 17-01-2022, 13:59   #1
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'Notched' zero point not reading zero on rudder angle sender.

Simrad RF300 rudder feedback unit feeding AC42 Autopilot Computer and AC24 Autopilot. Maxum 4100 SCA, twin Cummins 6BTA 5.9 M-3 330HP diesels.

Had the boat for almost five years now - and finally decided to tackle one of my 'too lazy' to do concerns. Even with the engines' RMPs 'match' by tach (close) and ear (no weird harmonic type noise) I've always had to have a little left rudder on to maintain a straight heading.

So the other day, in the slip, I put the admiral on the helm and had her go hard port/hard stbd and also rudder amidships.

Hard port indicates ~39 degrees. Hard stbd ~42 decrees. At rudder amidship (as indicated on the autopilot (amd MFD which is fed on the NMEA bus) the RF300 feedback unit's 'alignment notches' are not lined up. Stupid me forgot to take a pic, but I think most of you can visualize how the notch on the 'movable' part of the feedback unit is designed to be lined up with the 'non-movable' base of the unit when the unit is 'sending' 0 decrees.

Has anyone else ever had this sort of problem and know of a solution?
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Old 17-01-2022, 14:03   #2
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Re: 'Notched' zero point not reading zero on rudder angle sender.

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the RF300 feedback unit's 'alignment notches' are not lined up.
is there a way to adjust the length of the RF feedback connecting rod (the connection to the rudder tiller/quadrant)? Often there is a barrel adjuster for length, which can potentially zero the alignment (just make sure the actually rudder hits its stops before the RF arm does).

edit from bad pictures on the internet - it looks like ends of the connecting rod can be screwed in and out to adjust length and fine-tune center point.
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Old 17-01-2022, 20:30   #3
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Re: 'Notched' zero point not reading zero on rudder angle sender.

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edit from bad pictures on the internet - it looks like ends of the connecting rod can be screwed in and out to adjust length and fine-tune center point.
Thanks BW ... yes, there is a way to change the length of the connecting rod ... but that won't solve the problem I described.

I'm wondering why the feedback sender itself isn't sending the 'rudder amidship' signal when the 'notices' are aligned on the movable, and fixed, pieces of the transmitter. The installation guide says to have those two aligned and then attach to the rudder stock (cross bar in my case since I have two rudders) with the rudders centered/amidship.

What I'm planning on doing is simply getting the rudders as close to 'centered' as I can and then adjust the length of the connecting rod/rotation of the fixed portion of the transmitter (it screws down/is mounted) with four screws that have 'slides' that will allow me to easily rotate it until I read zero on the helm. The notches won't be aligned, but I'll know where the rudders actually at.
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Old 18-01-2022, 01:08   #4
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Re: 'Notched' zero point not reading zero on rudder angle sender.

I hope you have RTFM, both for installation and operation.

The rudder zero and full travel points are adjusted during the dockside commissioning. BTW, the rudder stops may not be actually symmetric. One of the main purposes of the alignment is to make sure that your autopilot does not try to push the rudder past the stops.

The RFU alignment notch is a general guide, but does not have to be exact. In fact, when the center area of my RF300 wore out in the middle of the Indian Ocean, I rotated it about 15 degrees away from the alignment notch so that a straight course operated in an area of less wear. I then redid the dockside alignment to compensate, and the autopilot worked for another 800 miles.

I'm not sure about your boat, but if you don't have one right hand prop and one left hand prop, you will experience pull to the left or right under power. The autopilot will of course compensate for that.
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Old 19-01-2022, 09:27   #5
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Re: 'Notched' zero point not reading zero on rudder angle sender.

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I hope you have RTFM, both for installation and operation.
Yes, I have - both manuals, and the autopilot manual also.


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The rudder zero and full travel points are adjusted during the dockside commissioning.
I'm doing a short haul on Friday 21 JAN for a bottom scrape and zinc replacement. I'll 'sight' the rudders to zero as best I can (will look up how best to do this - but if anyone has a good way, please let me know). When I splash the boat I'll check what the indicated rudder angle at the helm for this new 'mechanical/physical' zero and use that as a temporary 'electrical' zero in my mind. After getting back in the slip, I'll adjust the connecting rod's length (or rotate the fixed 'base' of the transmitter) to make this the new 'electrical' zero also.

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The RFU alignment notch is a general guide, but does not have to be exact.
Thanks ... good to know.

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I'm not sure about your boat, but if you don't have one right hand prop and one left hand prop, you will experience pull to the left or right under power.
I do have one of each ... and here dummy me thought that by having one of each, the 'walk' would cancel each other out - learn something every day - thanks.
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Old 19-01-2022, 10:32   #6
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Re: 'Notched' zero point not reading zero on rudder angle sender.

At the risk of being flamed by the engineering advocates, I’d point out that the "rudder amidship" description in the AP manual doesn’t really have any concrete relationship with the actual physical position of the rudder. From the view of the AP, "rudder amidship" is the position of the rudder that results in a straight track over water in the absence of any external effects like wind, current or mechanical asymmetries in the underwater shape of the boat, the rudders or their linkages.

If I remember my Simrad manual, it suggested that the "rudder amidship" calibration was done by ranging some on-shore landmarks, manually adjusting the steering until the track was straight and then setting that point as "rudder amidship."

In theory, you might be able to adjust the steering system so that it appears to be mechanically in the center. But I’d bet in the real world, that won’t work.
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Old 19-01-2022, 12:48   #7
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Re: 'Notched' zero point not reading zero on rudder angle sender.

To further confound the adjustment, rudders are “usually” adjusted to be not exactly parallel with each other. This reduces chatter, by loading them slightly.
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Old 19-01-2022, 21:56   #8
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Re: 'Notched' zero point not reading zero on rudder angle sender.

the notches may not be the center. I certainly would not trust that. when installing it you get it close. then you adjust the offset in the pilot to correct it. there should be a rudder calibration in the pilot menu to adjust it.
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Old 19-01-2022, 22:43   #9
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Re: 'Notched' zero point not reading zero on rudder angle sender.

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Originally Posted by Bycrick View Post
At the risk of being flamed by the engineering advocates, I’d point out that the "rudder amidship" description in the AP manual doesn’t really have any concrete relationship with the actual physical position of the rudder. From the view of the AP, "rudder amidship" is the position of the rudder that results in a straight track over water in the absence of any external effects like wind, current or mechanical asymmetries in the underwater shape of the boat, the rudders or their linkages.

If I remember my Simrad manual, it suggested that the "rudder amidship" calibration was done by ranging some on-shore landmarks, manually adjusting the steering until the track was straight and then setting that point as "rudder amidship."

In theory, you might be able to adjust the steering system so that it appears to be mechanically in the center. But I’d bet in the real world, that won’t work.

Right on!
Steer the boat by hand to make it go straight.
Rotate the RFU sender til meter reads zero. / Len
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Old 20-01-2022, 07:28   #10
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Re: 'Notched' zero point not reading zero on rudder angle sender.

With the newer RPI units and APs, the actual physical orientation of the RPU has NO connection to how the AP control unit interprets the data. During the calibration procedure, you manually steer the boat in a straight line, then tell the AP "this position of the RPU is rudder amidship." There’s no need to rotate or align the RPU itself.

The same is true about the adjustment for hard-over position. You manually turn the rudder to the position you think is hard over, then tell the AP that it should stop at that point. It makes no difference to the AP how many degrees that angle is.

Purely for convenience, that arc is displayed as "degrees." But that’s really more information than one needs to steer the boat. When was the last time you heard a recreational sailor give the command "Ten degrees left rudder" and really mean it literally.
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Old 20-01-2022, 15:36   #11
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Re: 'Notched' zero point not reading zero on rudder angle sender.

What IS important, too; is make sure the sender stays in its mechanical range, without bottoming out.
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Old 21-01-2022, 22:28   #12
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Re: 'Notched' zero point not reading zero on rudder angle sender.

Thanks to all for the input. I range a visual range - no current in the area, and only a light wind, but the canal is fairly well protected, so minimal input. Rudder indicated Right 1 decree, so I'm simply gonna call it good with no further adjustments. As many pointed out, the key is simply finding the spot where the boat goes straight ... since I'm all engines - at minimum RPM with rudder centered when handling in the slip/marina/docking/locks, etc, anything that's +/- a couple degrees aren't going to hurt anything.

Again, thanks to all for the input - good knowledge/discussion. I will double check the AP knowing about the 'limits' so that it doesn't get near the hard stops.
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