OK, here is what I came up with:
Download Attachment: servo1.jpg
This is a top view diagram with the front of the boat is at the top of the diagram. The servo shown in the middle of the diagram functions to trim both the squares and the jibs.
The servo horn is a 3 arm horn. The jib sheets are marked in red and the squares sheets are marked in blue (very patriotic and all).
There are 3 servo positions shown in the diagram.
Position 1 which has the solid lines for the sheets is the close hauled position for port tack. Notice how the starboard jib sheet is pulled tight to trim the jib to that side. The port jib sheet has some extra sllack in it to allow the sheet to lay lazily over the forestay of the jib behind it (overlapping jibs). The squares sheets are trimmed so that the starboard squares sheet is pulled somewhat taught and there is slack in the port sheet. This will cause the squares to be angled as if on a port tack (but if they are furled, this is really irrelevant - only the yards will be moving).
Position 2 is a reaching setting. Notice that the starboard jib sheet (dashed red line) has been eased. The starboard squares sheet has NOT been eased (this is important to note). At this point you would want to unfurl your squares. The port jib and squares sheets are not shown (the diagram is crowded enough as it is) but they are still slack.
Position 3 is a dead run position. The starboard jib sheet (red dotted line) has been further eased. The port jib sheet still shows some slack, but if the sail billows out this slack will be taken up. The key to the whole thing is that the port and starboard squares sheets are equal lengths at this point and there is no slack in the squares system so the squares will be held centered by both sheets.
Obviously, the system would work the same way for starboard tack with the servo continuin to turn further counter-clockwise.
The key to making this system work is to get the turning blocks or eyelets for the squares sheets mounted forward of the servo. The angle at which the servo truns to make the squares sheet pass directly over the servo axis is the point where the squares stop sheeting in and begin to ease. But the jib sheets will continue sheeting past this point.
I show both the port and starboard jib sheets exiting through a single fairlead in the deck you can have 2 fairleads if you want, or whatever. This is really just to show how the servo arrangement works.
You may want to hang small fishing weights from the squares sheets to take up the slack that is induced in the system as you suggested earlier.
If you have multiple jibs that each need a different amount of sheeting, then you can have multiple holes in the jib sheet arms at different radii to give the different amount of sheeting needed. The length of the squares arm may need to be longer or shorter than the jib arms depending on where the sheets are attached to the yards and how much sheet travel you need to trim the yards around.
What would be really neat would be to have an additional servo that could furl and unfurl the squares. Then you would really impress 'em at the pond!!!
Let me know if this diagram needs more explaining and I will fill in more of the details…