Sail servo angles

I may be reinventing the wheel in Footys (and I know that Lester Gibert has looked at the matter on his site) but it seems to me that the layout of most Footy sail servos is wrong.

As I understand it, most sail servos are mounted so that the servo arm points across the boat (more or less) when the sheets are pulled in tight and aft when the sheets are out. This means that adjustment is coarsest when the sheets are in and finest when they are out - which is the wrong way round. A small movement of the sheet will have little effect on the run but a big effect going to windward.

Answer: the servo arm should point FORWARD with the sheet in and across the boat with it out.

What’s the snag?

Hi Angus,
I would think that with the minimal load imparted by a ‘Footys’ rig, then any alignment would do. I have set up mine so that the arm is at ‘sheeted full in’ position when the arm is over the steering servo. Yes, it is causing the load to act at a bad angle, as the ‘perfect’ angle is meant to be longitudinal to the servo case, but I have had no problems so far. It also depends where the first lead of the sheet line is installed. Mine is set up so that at full in, then the arm is just about at the ‘toggle over’ point, ie the sheet is in line with the arm.

Point isn’t to be nice to the servo - but it looks like we basically agree.

What the stick controls is the angular movement of the servo. So if the arm is at 90 degrees to the sheet, we get a lot of movement of the sheet for a little movement of the servo - fine on the run but not so good on the beat where we want very fine adjustment.

The nearer the sheet is to being in line with the arm, the less movement of the sheet there is for a given movement of the stick - so fine adjstment.

Incidentally, your bit about the location of the primary lead is very much to the point. Lester gives examples with different primary leads for main and jib to give different ‘final’ tightening of the sheet.

Hope this explains it better

No arguing with that Angus, you are right as I see it. There are limitations within the space of a footy of course but the key is the position of the primary lead.

I would add that in all of these things… and from many years of teaching people to fly R/C. Always remember that modern R/C equipment is ‘proportional’… learn to use it.


Yes, I call this rotary/linear differential & is essentially represented by a cosine curve. It happens in any type of lever arm mechanism, eg; piston & crankshaft.

Another result of your placement is that the effective power on the sheet will increase when more closely sheeted in, which is desirable. The reason being that the “moment arm” length of the servo arm becomes proportionately shorter as one approaches 180 degrees. This reduces strain on the servo. If the arm is oriented to go “just past” 180 deg, it will be essentially locked in place when fully sheeted & use very little power.

If one wants to negate this effect, the servo must power a disc (drum winch)
shape, where the moment arm length remains the same regardless of winch rotation angle.

Very well put Bill and absolutely right. I had not thought about using the ‘inline’ position of the servo arm and the sheet to reduce the effort on the servo. At the ‘inline’ position the force to hold the sails at close hauled could effectively be zero… and hence zero currect drain. Now that is what I call efficient.


It’s also possible to provide a limited differential movement between the main and jib by the use of a servo arm with an angle other than 180 deg between the two sides. The arm for my jib is about 30 deg ahead of the arm for the main, such that the jib is “locked” at full in (“back” of the servo arm arc) while the mainsheet still has some travel. Thus, if I ease off a little, the jib doesn’t go out much, but the main opens up to get the boat back on its feet.

This wasn’t my invention. I copied it some time back. My only regret is that I don’t have a long string of wins to prove any expertise in the matter.

Mike Biggs

Just as a side line. There are a number of steam locomotive buffs around here. Has anyone applied a knowledge of locomotive valve gears (some of which have very clever kinematics) to rig control problems. Another fruitful source of inspiration might be pre-electronic textile machinery.

One of the more interesting applications of the rotary/linear differential is the “Ackerman” principle used in steering geometry of vehicles. When turning a corner, the radius of the outer wheel is greater than that of the inner wheel causing the steering angles to be different to avoid scuffing of the tires. Ackerman allows this to happen auutomatically by the design of the angle of the Pitman arms:

Another useful application can be found in the “pull-pull” steering used by the model airplane guys:
I use this in the steering of my rc iceboats & landyachts.

Below is an shot of my Footy servo layout. Since it is forward sheeting, the arm points a bit beyond straight aft when fully sheeted