Replace mechanical rotary autopilot drive unit (ST4000Mk2) with linear drive unit?

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skunther

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Feb 16, 2021
439
Hunter Legend 35.5 Bellingham
Is it possible to replace my RayMarine ST4000Mk2 mechanical rotary unit with a linear drive unit? Is that even advisable?

The ST4000Mk2 I have is beyond repair, and rather than purchase a new unit, I’d like to look into the possibility of switching to a linear drive system without replacing the existing autopilot. It seems they are more reliable over the long term.

Is it even possible to convert drive units without also replacing my autopilot? Does anyone know if 1993 Hunter Legend 35.5 can even accommodate a linear drive unit?

Input is greatly appreciated.
 
jviss

jviss

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Feb 5, 2004
7,089
Tartan 3800 20 Westport, MA
What autopilot control unit have you? If it doesn't have a clutch output, then this is not a practical approach.
 
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dlochner

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Jan 11, 2014
13,052
Sabre 362 113 Fair Haven, NY
On a boat with wheel steering linear drive units are typically attached to the rudder post with a short tiller arm. They are then controlled by a computer that is sized to match the power requirements of the drive. All in all a much superior AP system for boats over about 32 feet, but not inexpensive.

While it may be possible to cob together some kind of device to use an above deck linear autopilot to work with wheel steering there will be numerous issues to overcome, off the top of my head, the travel distance for the drive unit, the power to required to move the wheel, and a suitable computer to control the drive unit.

Wish I had better news for you.
 
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skunther

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Feb 16, 2021
439
Hunter Legend 35.5 Bellingham
jviss said:
What autopilot control unit have you? If it doesn't have a clutch output, then this is not a practical approach.
Click to expand
I have the RayMarine ST6002 control head. From research online it seems to have linear drive capability, but I can’t definitively confirm it.
 
kloudie1

kloudie1

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Nov 6, 2006
10,104
Hunter 34 Mandeville Louisiana
Is your autopilot a ST 6002 or is it a 4000? I am thinking that both control heads have clutch connections .. Most clutches engage with 12Volts so if you measure the clutch connections, you should see 12 volts when the autopilot is on and not in standby. The biggest thing to check would be the amperage handling capability of the control head to the drive motor. A linear drive might pull more power than the old wheel drive and would burn out the drive motor switches. A high ampere rated (a little above the linear drive max power consumption) solid state reversing relay or two non-reversing relays could be used to drive a high amp linear using the low amp signals from the existing head/computer. If you have operable clutch power, the rest is pretty easy electrically.. the mechanical hookup to the rudder and mounting the linear drive might take some cogitating and calculating!
 
jviss

jviss

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Feb 5, 2004
7,089
Tartan 3800 20 Westport, MA
kloudie1 said:
Is your autopilot a ST 6002 or is it a 4000? I am thinking that both control heads have clutch connections ..
Click to expand
The only sure-fire way of figuring out if you can do this is to find out precisely what you have and then get the manuals.

That said, the ST4000+ head was a complete controller, with a clutch output, while the ST6002 was just the control head for a course computer. See these pics:

ST4000+:
ST4000+.jpeg


ST6002:
ST6002.jpeg
 
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S

skunther

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Feb 16, 2021
439
Hunter Legend 35.5 Bellingham
This from the manufacturer:
"The St6002 can be used, you would need an SPX10 course computer, fluxgate compass and Rudder reference unit with the ST4000 Mk2 wheeldrive, or if using a T1 linear drive, you would need the SPX30 course computer, fluxgate compass, rudder reference unit and ST6002 control unit.
I have checked our records and although the information is now quite old, it would appear the standard fit to a Hunter Legend 35.5, would be a T1 linear drive with EV200 electronics package"

It seems I could pick up an SPX30 to handle the load of a T1 and upgrade. But should I? Or would I be best off just getting a replacement ST4000Mk2 drive unit for $600?

Ultimately I'd just like to have a good autopilot that is reliable, adequate for my 13,000lb displacement, and doesn't soak up all my amp hours nor break the bank (nor be a PITA to install). The PO somehow caused the ST4000Mk2 drive unit to explode all over the cockpit, and it remains a mystery why. The owner before him had the same autopilot though (as indicated on the prior survey), so it must have been doing something right.

Should I skip the linear drive and just get the ST4000Mk2 replacement unit? Or is there a better option I'm not considering?
 
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D

dlochner

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Jan 11, 2014
13,052
Sabre 362 113 Fair Haven, NY
The bigger question is what are your sailing plans?

If your plans call for long passages and extended cruising then you will find a good below decks system more reliable and up to the task. On the other hand if your plans are for shorter passages (measured in hours not days) weekend cruises and a summer cruise, then you can replace the wheel AP 8 or 10 times for about the same cost as a below decks AP and carry a spare AP.
 
jviss

jviss

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Feb 5, 2004
7,089
Tartan 3800 20 Westport, MA
dlochner said:
you can replace the wheel AP 8 or 10 times for about the same cost as a below decks AP and carry a spare AP.
Click to expand
I'm sure you meant that as intentional hyperbole, no? I mean, at $600 to replace the wheel drive unit, you can buy six of them for the cost of the entire below decks kit ($3700). The entire wheel pilot kit is $1500.

But, it's really kinda apples and oranges, as the functionality and utility of the below decks setup far surpasses the wheel drive one.

(I recognize the below decks installation can cost quite a bit more than the wheel drive, too.)
 
D

dlochner

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Jan 11, 2014
13,052
Sabre 362 113 Fair Haven, NY
jviss said:
I'm sure you meant that as intentional hyperbole, no? I mean, at $600 to replace the wheel drive unit, you can buy six of them for the cost of the entire below decks kit ($3700). The entire wheel pilot kit is $1500.

But, it's really kinda apples and oranges, as the functionality and utility of the below decks setup far surpasses the wheel drive one.

(I recognize the below decks installation can cost quite a bit more than the wheel drive, too.)
Click to expand
It has been about 10 years since I priced out a below decks unit and more than that for a wheel drive. I used the OP's $600 figure and estimated the cost of a new installed below decks unity at ~$4500-5500. Rough back of the napkin calculations.

The main point is to install the unit that matches its intended use. If long passages, meaning 10-12 hours or more and in widely varying conditions is the intended use, then a wheel pilot will be overwhelmed and trashed pretty quickly and a below decks unit would be a much better choice.

The kind of sailing most of us do, shorter passages in predictable and comfortable conditions a wheel pilot will work, although a below decks pilot will also work and be more robust.
 
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jviss

jviss

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Feb 5, 2004
7,089
Tartan 3800 20 Westport, MA
skunther said:
Is it possible to replace my RayMarine ST4000Mk2 mechanical rotary unit with a linear drive unit? Is that even advisable?
Click to expand
I don't think so. I looked at Raymarine manuals, and even thought the ST4000+ has a clutch output, there's no reference from Raymarine of this being paired with anything other than a wheel drive or tiller drive. But, you should call them and ask.
 
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