I used the calculator to work out the sheet load and travel for the main sail to be 3.07kg and 19.8cm. This was using the existing deck tube/hole 14cm, Beaufort 5, Vertical C of E 75cm, no factor of safety.

 How will it work with a drum servo?

With a 15mm diameter drum (this was found by trial and error) + 1mm for cord, the calculator shows that 4.9 kg.cm torque is required for about 4 turns. The Hitec HS-785HB gives 13.2kg.cm at 6v. This gives a factor of safety of 2.7  

Drum type servo works

 How will it work with a sail arm servo?

I have always been sceptical of sail arms as they are inherently a poor mechanism and felt uneasy that all the servos are positioned fore and aft. My concerns were proved to be correct. I don’t think people understand the basic physics.

My starting point was to hand draw an arm at 10 deg angles and work out the vector diagrams, as this is tedious, I wrote another calculator. The results even surprised me.

 The mechanism multiplies the torque over certain angles.

The distance between the servo and first pulley/deck tube is important.

The arm angle and servo body have to be positioned in relation to a line drawn between the fixed point and servo shaft and not to the boat’s centreline.

 Go to Servo to see full explinations with graphs, calculators, servo comparisions.

Go to Sail to see worked examples.

 Factors to establish.

1). Most efficient distance of the primary fixed point (deck hole, pulley) from the servo shaft.

2). Arm angle giving the maximum sheet force.

3). Range of arm angles giving usable sheet force.

Looking at the chart theoretically, it would seem that there is little point in using servos that give over 90 deg of rotation. The grey area on the chart shows 90 deg range and the yellow shows 120 deg. The power loss is great at the extremes.

However, when searching the Net I came across the R/C Sail Forum. The following are edited extracts from one post on R/S Sailing

 "Most sail servos are mounted with the arm pointing across the boat (90 deg). The sheet is pulled tight at this point (90 deg) and let out when aft (180 deg). This means the adjustment is coarsest when the sheet is pulled in (90 deg) going to windward and finest when out (180 deg) on the run. This is the reverse of what is needed. To reverse this, put the arm forward (0 deg) when close hauled and at 90 deg when on the run."  (The post make no mention that this is also around the point of maximum servo torque and the multiplier effect).

 4). Range of usable sheet travel.

5). Forces on servo mountings.

6). What is the correct angle for the sheet leaving the fixed point?

 Arm type servo does not work in this situation

 Tony