Hi, the main difference with the arm sail servos and a standard servo is the angle turned through. The sail winch arm servo goes through about 180 degrees and the standard servo just 90 degrees.
However if you measure the distance the sheet travels it is very small for the last 20 degrees of the extreme movement. So 140 degrees has the most effect. There are two options, the easy one is to use a small pulley with the sheet tethered to the inside somewhere and this doubles the movement of the sheet.
If you are happy with small electrical work put a very small 1K ohm resister on each side of the potentiometer in the servo and that will extend its movement to roughly 180 degrees, but you may have to experiment with resistor values.
There is an electronic way using a servo extender, they are very small and plug into the rx and the servo plugs into the extender. A servo morph, will have the facility to slow the movement down and save a too fast adjustment to the sails. The basic extenders vary a lot in price £10+ in the U.K. but the last purchase I made was for 4 of them for £10. via ebay and China.
Remember that the torque quoted for servos is given per cm so halves at 2 cm and at 3kgrms/ cm which is a standard servo an arm winch with a 12cm throw has only 0.25 kgms pull. If you go down this route with a standard plastic geared servo the gears will strip their teeth off at the first gust of wind.
This is why we use the metal geared 15Kgrm servos where there is still 1.25 kgrms available at 12cms.
The position of the sail servo arm winch is usually at the side of the hull giving the hull width for the arm. If you need more pull then a second similar servo could be connected via a Y connector lead to the receiver. The signal from the rx is all that is needed and splits easily with no loss, there is no current used at this point.
It is worth checking that when the sails are pulled in that the sheet lies across the centre of the servo rotation point this means that if there is a gust of wind pushing the sail boat over the pull is not through the gears but straight across the servo, which is a mechanical connection. The same applies to when the sails are fully out. This will determine the length of the extension arm.
The point of exit of the sheet from the hull is crucial, the sheet exit point should be directly under the attachment point to the sail boom otherwise the equal angle movement of the booms will be compromised.
It goes without saying that you should never angle a sheet through a metal eye the losses can halve the pull available. There were tiny pulleys available from Grapner but they may have gone now. However there is a neat and cheap solution.
At the point of exit from the hull use a hard plastic tube. I use a small length of solder wire slipped inside the tube to keep it open while I pour boiling water over it and bend it through a small arc. When you have it right, cool it in cold water and remove the solder wire and cut to length and you have a nice smooth low friction outlet.
Hope this helps, it was all gathered over several years of operation.
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