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Old 04-06-2015, 06:12   #1
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Linear drive AP - elec or hyd drive motor?

Which type of linear drive AP "motor" is better - electric or hydraulic?

Looks like some Raymarine drive units are powerd with electric motor only and Simrads are powered with electric hydraulic pump. This will be robustly installed on Irwin 43 (26,000 to 30,000 disp).

Just to be clear - the boat has mechanical, cable, quadrant steering - not hydraulic steering - and I'm comparing below deck, linear drive APs that will connect directly to the quadrant/rudder post.

Some AP linear drive units are powered by electric motors and others are powered by elec-hydraulic pumps/motors....opinions which are best for offshore sailing?

Thanks
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Old 04-06-2015, 06:40   #2
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Re: Linear drive AP - elec or hyd drive motor?

We have both electromechanical and hydraulic linear drives. The electromechanical drive is quieter, uses less power and produces less drag on the steering system when not engaged.

For equal size, hydraulic drives typically provide greater thrust but lower torque while using more power than electromechanical. The hard-over times on some hydraulic units are quite long in comparison to electromechanical, but equal or even shorter for others. This seems to be manufacturer-specific.

Hydraulic linear drives are available for larger boat sizes than electromechanical. Your boat falls in the high end of electromechanical, but not over the specs of the larger ones. Whatever drive you get, you want your boat to be lower in the drive's recommended spec range rather than higher.

Hydraulic linear drives are typically perceived as more robust, but this is mostly just perception and not reality. Raymarine has had their electromechanical drive out there for many decades with very few failure complaints - and most of those on 20-30yr old units. For all drives, the key to robustness is bullet-proof mounting, keeping them dry and providing good electrical power.

Mark
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Old 04-06-2015, 06:44   #3
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Re: Linear drive AP - elec or hyd drive motor?

Great reply Mark - clear, concise - thanks!

Others?
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Old 06-06-2015, 01:16   #4
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Re: Linear drive AP - elec or hyd drive motor?

I installed my AP myself and opted for a Simrad AP coupled with a JEFA linear drive (Danish fabrication). Lots of couple and low power consumption. I later found out that apparently this linear drive is installed by most singlehanded sailors racing in the France to Brazil race, only coupled with a NKE unit. But NKE is way beyond my budget.
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Old 07-06-2015, 19:36   #5
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Re: Linear drive AP - elec or hyd drive motor?

FWIW we destroyed a Jefa electromechanical linear drive unit off of the Oregon coast in 30-40kts winds and large following seas. When we replaced it, we went with a hydraulic ram with a substantially higher load rating.

The hydraulic is noisier and uses quite a bit more power than the electromechanical unit, which was slick and inaudible over the wind and wave noise.
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Old 07-06-2015, 19:54   #6
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Re: Linear drive AP - elec or hyd drive motor?

What was the failure mode? Your boat would probably be on the high end of that unit's specs and recommendation. Which hydraulic ram did you replace it with?

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Old 07-06-2015, 20:51   #7
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Re: Linear drive AP - elec or hyd drive motor?

The Jefa drive came as part of Garmin's GHP12 Autopilot system and is what they call their "Class A" drive. Garmin rates it for boats up to 28,500 lbs. of displacement, far below what my boat displaces. I would argue, with the benefit of hindsight, that this is probably an excessively simplistic way of looking at these things.

re: failure mode at some point after steering faithfully for a couple of days straight in building following seas, it suddenly stopped moving at all. I noticed this when despite the AP still being engaged, the boat suddenly swung around beam to large breaking waves. Not fun! After we pulled in and removed the drive unit you could hear broken bits inside jangling around so something physically broke.

The new unit is Garmin's class B compact drive, made by HyPro in the UK.

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Originally Posted by colemj View Post
What was the failure mode? Your boat would probably be on the high end of that unit's specs and recommendation. Which hydraulic ram did you replace it with?

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