Autopilot gives up in following seas

[Stellar J]

Our 2 year old Moody DS45 has had autopilot challenges in 20 knots, broad reach conditions. Sometimes when a big wave passes under the stern, the clutch gives way, wheel spins to windward and the boat rounds up. We have replaced both the Jefa DD1 and the Raymarine ACU-400 with new units,with no change. Also her 6 month older sistership has the same problems.



Could it be the DD! is underpowered, or the ACU? Could there be a problem with the compass, tilt sensor or any other culprit? I don't see how the boat could cross the Atlantic with this configuration.



Any ideas here? Thanks Stellar J

[TheOffice]

" The drive can be used on small boats from 30 to 45 foot l.o.a. (or up to 150 Kgm rudder torque) equipped with a mechanical steering system that can be back driven."

Its too small!

[Scubaseas]

John Harries (morganscloud.com) just did a series of articles on autopilots and gain/sensitivity adjustments that explains things very well. I can barely walk upright without dragging my knuckles so read his stuff for a better explanation than I can do. You need to adjust the AP to conditions but more importantly you need to adjust sails for your weather. How much rudder angle do you have when this happens and how much weather helm? You might try reducing the size of the mainsail and see if that solves the problem. Or even dropping the main altogether might be necessary.

Some points of sail just don't work well with some autopilots/boat combos. Falling off or up a few degrees might help also.

[Scubaseas]

Originally Posted by TheOffice

" The drive can be used on small boats from 30 to 45 foot l.o.a. (or up to 150 Kgm rudder torque) equipped with a mechanical steering system that can be back driven."

Its too small!

That too. But the boat also has to be reasonably balanced for any size AP drive to work properly.

[Scubaseas]

Originally Posted by Scubaseas

That too. But the boat also has to be reasonably balanced for any size AP drive to work properly.

You be right, I missed the DD1 in the OP. I agree with you a DD2 would be a better choice for this boat. My brain saw DS45 and assumed it would be a DD2.

[alctel]

As others said, undersized unit and make sure your sails are balanced


I had the simrad branded version of the DD1 on my old 36' boat and it was *awesome*

[thomm225]

Take the main down or reef it down until the autopilot works, or sail with jib only.

The autopilot is being over powered by your mainsail.

[thomm225]

Autopilot works fine in this video with winds mid 20's and higher gusts with jib only

You can hear the autopilot correcting at the start of the video and getting the boat back on course.

https://www.youtube.com/watch?v=agtdrmAqojs

[thomm225]

Here I couldn't get the main down in time and the winds shot up to 34 knots or so.

Autopilot was overpowered by the main so I had to steer and couldn't go forward to lower.

After the seas and wind laid down a bit, I was able to record this short video but since I was looking through the camera I got out of sync and the boat tried to round up to windward in the video because I had too much mainsail up.

I should have had jib only of jib and totally reefed mainsail if I'd had 3 sets of reef points.

My new mainsail does have 3 sets of reef points

https://www.youtube.com/watch?v=-nXfjcCuiPk

[rslifkin]

In my mind, if it's possible to hand steer the boat, but the autopilot is giving up due to excessive rudder load, that means the autopilot drive is inadequate. The autopilot may give up because it's become impossible to keep the boat on course if you're massively overpowered, but it shouldn't give up just because the rudder loads got high.

[thomm225]

My point was if the autopilot is under powered or slow to react sailing downwind in heavy wind and seas, you have to adjust your sails to make it work.

Less main, more jib

Or no main and all jib.

[Kd9truck]

Hi, this has happened to me, on an Oyster and on my Jeanneau, to very different vessels.
It was explained to me that DD past a certain wind speed and wave swell the auto has to much to content with to stay on course. When this happened we reduced sail dropped speed and the auto regained control.

[SailingHarmonie]

Two things NEVER overheard in a sailor bar:

• "My anchor was too big."
• "My autopilot was too powerful."

Having many thousands of miles under autopilots that were well matched to the boats they were steering, I can tell you that it is NOT normal to lose control of the boat in when deep reaching or running even with a large quartering sea. Such behavior should not be expected or tolerated. Period. If you can steer the boat accurately by hand, the autopilot should be able to do at least as well.

Assuming, of course, the autopilot is properly tuned, the drive is sized appropriately, and the sails are trimmed correctly.

If you are sloppy on sail trim nothing will work... The more marginal your drive size is compared to the size of the boat, the more critical the tuning of the AP will be.

If you REALLY want to learn sail trim, use a mechanical wind vane where you can't power your way out of bad trim.

Or just balance the boat and sail without the autopilot. If the boat is balanced, it can be done. I've done a long passage without any form of automatic steering, just sail trim. A VERY educational experience.

[boatpoker]

You'd have better luck with a hydraulic autopilot

[SailingHarmonie]

Originally Posted by boatpoker

You'd have better luck with a hydraulic autopilot

Having a linear drive on my autopilot for a 53 foot boat that weighs 38,000 pounds and an unbalanced skeg hung rudder, I can assure you that hydraulics are not needed for the OP's boat. They are not bad, just not needed. And they are just as likely to fail if undersized for the boat.

That of course is NOT saying his current pilot is adequate...