Every Skipper that has scratch built a boat or assembled one of the many kit boats, all have pondered the geometrical question,"What size Sail Arm do I need for my boat?".
There are many formulas for doing so, such as marking the sheet lines when they are at full out and full in, or even measuring from the sheet attachment points on the boom to the boom pivot. I found that my Sail Arm Gauge took out all the guesswork of determining the actual lengths I needed while setting up the Fairwind with a Double Arm Winch.
Shaped straight like a tongue depressor, it consists of nothing more than a scrap piece of birch made from the aircraft plywood found at Hobby Stores. Nothing pretty, nothing high tech. It is graduated in ¼" increments from the arm's center, with marks and drilled on center at each mark with a 1/6" bit to accommodate the eyelet screw. Any wood thickness over 1/8" is sufficient for this task as you will not be loading the sails enough to bend the arm. Cut it wide enough to fit your servo horn, and as long as your boat is wide. The one pictured below is 7 inches long. Mount the servo horn on center. I chose the round horn for support and extra holes to position the arm properly when the servo is parked (i.e neutral).
Instructions for use:
  1. Charge your batteries. Never tune your sheets with weak batteries. Fully charged batteries will let the servo travel to its end points. Repeatability is the key to long lasting enjoyment of every Hobby Sport.
  2. Radio on, sticks centered at neutral, and servos traveling in the direction you want (i.e. stick out = sheet out). Cycle servo from end point to end point and leave centered by releasing the stick. Program your end points if you have a computer radio. Cycle the servo with the stick periodically to make sure the servo is properly centered. I like knowing that my servos are perfectly centered without the use of the radio's trim tabs. Do not turn off radio or servos during this block of instruction until you are finished.
  3. Anchor the sheets to the hull with tangs or eyelet mounts (such as Pekabe) off center and opposite side of the arm's travel. If the Main arm is traveling on port-side then the anchor should be on the starboard side of center for better geometry.
  4. Position 2 eyelet screws, one on each arm, and run sheet lines through them. The eyelet screws have enough gap to pop out 30 and 50 lb. Spectra or Deneema lines without having to cut lines.
  5. Mount the arm onto the servo in the best manner so that sheeting full in or close hauled has the arms away from the sheet exits and full out or running will not allow the arm to pass through center. If the bow is 12 o'clock and the stern is 6 o'clock and assuming that your main sheet exit is aft of the winch, then the main arm should point towards 12 o'clock when sheeted in fully and not pass through 6 o'clock when running or you will be pulling your lines back in when you think you should be at full out.
  6. Bowsies need to be constantly re-adjusted so that the booms will not travel beyond 90° when running. Most Skippers will not let the sails out that far.
  7. Re-position the screws on the arm to determine the proper length arm(s) for your boat.
  8. Re-position the screws on the arm again to determine the proper length arm(s) for your boat.
  9. Measure from the arm's center to each screw, transfer your findings onto your new arm, and cut to size.
This gauge will also help you decide if you need a non-symmetrical or curved arm instead of a straight one such as that found in the US1M Construction Guide, and if you need extra travel by modifying the servo with a signal amplifier, commonly known as a $15 Servo Stretcher (Robot Zone .com). Enjoy.