I think you have already gotten some great advice reading the other responses, but being a Marine Electronics tech I had to of course through in my two cents worth... First off, Jeff is very correct, and autohelm 4000 is not rated anywhere near 30,000 pound displacements. For that size of vessel you are into Autohelm 5000 or 6000 series. With these systems you have several options of drive actuators (the part that actually moves the steering quadrant) depending on your vessel. There are a couple hydraulic pumps set ups, as well as linear push pull "ram" arms and a rotary system as well. The second issue that was brought up was initialization. Jeff mentioned a sensing unit, this is actually an electro/magnetic compass unit called a fluxgate. IF its too close to items such as engines or large masses of metal it will cause a massive magnetic deviation which will make the unit not steer correctly, however it should still "initialize" and power up, but when you do the sea trial and swing the compass the controler will tell you it has too much deviation. Before installing fluxgate compasses it is usually good to test the area you plan to use with a hand held magnetic compass first. Avoid lockers in which tools may be stored, within 3 ft of the engine, and within 3 ft of areas of large elctrical fields such as distribution panels. Another reason that autohelms will not function is due to voltage drop, if the continous voltage reaching an autohelm drops below 10VDC it will reset itself and not operate. This could be due to underperforming batteries or even undersized cables. Be sure to size all power cables large enough for the wiring distance, the autohelm manuals themselves give you a chart to use in sizing cable runs. Finally, give Raytheon a call, I will if you are interested get you more numbers but I believe a few have already been provided in other replies. Raymarine is very helpful and will give you good advice on sizing and installing the systems. One last consideration as to size, IF the boat is going offshore rather than coastal it is usual to size one step larger than normal to meet the added stress on the system, redundant systems are recommended (or windvane plus an autopilot) and certainly a below decks unit to protect it from rough seas is advisable.