I have an Irwin 1978 37 center cockpit. I am considering a wheel pilot, which I know is slightly underrated for my boat. I have seen the raymarine ST4000 for sale on ebay a number of times for a reasonable used price. I understand it is rated for a sailboat w/ a displacement of 18,000lbs. and no more than 20,000 lbs. The Mk II wheel-drive can be purchased to improve the ST4000. Then came the S1 w/ the Mk II wheel-drive which is the successor to the popular ST4000, but is rated at 16,500lbs displacement. Now the new X-5 Wheel autopilot which is also rated for 16500. Well, why did the displacement drop. If the Mk II wheel-drive can be used w/ the ST4000 then theoretically the S1 and X5 can handle the same displacement. Am I correct in my thinking or am I way off?