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Old Jan 28, 2011, 11:58 AM
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United States, MN, Brainerd
Joined Oct 2004
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Brace Geometry and Slack Control with Sliding Winch Servos

The brig Syren (from the SC&H Brig kit) is rigged with semi-prototypical braces. That introduces the problem of differential brace slack between port and stbd as the yard is rotated.

On Syren, the slack is compensated by using elastic cord for the brace pendants and by rigging the brace blocks at the masts to "trolley" back-and-forth (yielding extra length to the higher tension pulling brace). The system works fine, but is a bit complex and non-prototypical.

A more prototypical approach would provide brace haul and slack control from crew on the deck (or in the R/C case, from servos below deck).

By mounting the winch servos on a spring or bungee tensioned sliding base, automatic differential tension compensation occurs. I tried this on a test rig set up with the exact dimensions measured for the Syren main course rigging ("yard" and "braces" were set up in precise geometry on a flat surface - the family room floor...). See the pictures. The brace run simulated is from yardarms to mast to pinrail.

Pic 1: Yard at 0 deg rotation. Braces taught (geometry of this position is the longest run for the braces, so they are at the tightest). The drum is centered, but can be moved side-to-side as necessary as the yard is rotated.

Pic 2: The "yard" is rotated 45 deg. by turning the drum but keeping it centered, and the loop of slack in the yielding brace is seen. The slack is about 3".

Pic 3: The yard is rotated 45 deg., but the drum is "trollied" laterally as it's rotated to take up the slack. To reach 45 deg yard rotation without slack, the drum moves from center about 5.5 inches. No slack in the braces, but since the hull is only about 12" wide, there is not enough width to take the 5 to 6 inch travel from center to both port and starboard. And that's only for 45 deg rotation. Measured rotation on Syren is over 50 degrees.

At this point, playing with the model paid off. I saw that if the drum stayed on centerline, but was moved fore-aft instead of port-stbd, compensation would occur with even less servo travel length.

Pic 4: The yard was rotated until the hauling brace was 13" long, based on the actual measurement from Syren. That gave a measured rotation on the floor rig of 54 deg, which agrees with the "slightly over 50 deg" measured on Syren. The drum was pulled a total of 4.5 inches on centerline to prevent slack.

The geometry seems to work. Because of limited hull width, fore-aft servo movement is the better option than port-stbd in this model. To make room for the movement, the servos can be mounted transversely instead of in-line.
The drum diameter can be kept small - the large flanges currently on Syren shouldn't be necessary with the tension compensation at the drums themselves.
A simple sliding mounting bracket to allow the 5" of servo travel should be simple to make. Tension could be provided by spring or bungee. I prefer bungee. With a sping, you have to choose the right spring tension and length to get the desired tension profile over full travel. With bungee, both length and tension are more easily changed by securing the bungee at different lengths and by varying the number of strands used.

Added a diagram ofa possible way to build the trolley winches.

I am going to re-rig the foremast to test all this out this coming season.

Comments, discussion????
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Last edited by DanL; Jan 28, 2011 at 12:31 PM.
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