I have read some where that if you have a pulley on the end of your servo arm you can increase the effiecent of your servo. I have been looking for some type of pulley of roller bearing to try this. Have any of you guys tried this?
Increases the amount of sheeting travel, but certainly not the ‘efficiency’ (because of increased friction losses). Some details at
Here is a design for a pulley on the end of a sail arm that I put on my Footies I make the pulley on my lathe, takes a few minutes.
Attached is a simple block I make from 1/16 aluminum tubing and a 3/8 inch dowel. The tubing is bent around the dowel, then epoxied to a disk cut off the end of the dowel. I saw a similar design somewhere on the Internet that used Teflon tubing, but the Teflon had to be stitched to the disk because epoxy will not bond to it. I find that the friction in the aluminum tubing is so low that there almost isn’t any. (The aluminum tubing can be smaller in diameter, just so long as it’s larger than the line you hope to put through it. Larger diameter tubing, though, is difficult to bend without kinking.)
It helps to leave the “arms” of the tubing extended a little more than indicated in the sketch. Lengthening the arms causes the block to turn to maintain alignment with the sail sheet better.
Bearings for the blocks can be made from short lengths of brass tubing, with the inner tube being slightly longer and the outer tube being epoxied into the disk. (The hole in the disk does not have to be in the center. If offset from the center, you will get the same effect as lengthening the arms - the block will better maintain alignment with the sheet.)
The longer inner tube allows a small screw to fasten the bearing tightly to the servo arm.
Otherwise, many skippers just use nylon or other plastic bushings epoxied into the servo arms. This works well and is simple, but the friction might get up a little with the higher loads on some of the larger models.
I wonder about how much a pulley would help on the Footy. the winch arm on my Footy has a simple hole, chamfered well & smoothed (keeping it simple!). Even though I’m using a standard (or “weak”) servo for the sails, I don’t think it’ll matter that much to have a pulley, as it would on a bigger boat, with more sail area, sailied in greater pressure.
I like to ask myself this question when making Footy parts: “Could a cub Scout do this or make this?”
The coefficient of friction for a ball-bearing block (coeff fric = 0.025) yields about a 7% friction loss with a 180 degree turn of the line, and the coefficient for a polished stainless steel fairlead (coeff fric = 0.1) yields about a 37% loss with a 180 degree turn. The estimated friction loss is an exponential function of the friction coefficient and of the angle of the turn:
Loss = EXP (coeff * turn), where the ‘turn’ is measured in radians.
These numbers apply when the line is under an adequate tension load, and represent a ‘standard’ engineering approach to estimating friction losses in lines. If under a light or negligible load, then certainly the friction coefficient can reduce to around zero.
Current issue of AMYA which is doing a repeat of past articles - has a comparison or friction loss of various methods of sheeting through/to the winch arm.
Watch for it in this issue - you are an AMYA member - right???
To bend aluminum tubing without kinking, use a piece of teflon rod or copper wire (called a mandrel I think) slightly smaller than the tubing to be bent. Oil or silicone lube on the outside of the mandrel, place in tubing, bend tubing and mandrel, some heat may be necessary to remove the mandrel from the tubing. Try out on scrap first. Used sand as the mandrel at work a few years back for 2" tubing and it worked great. Clyde
Thanks Guys some great ideas. As always useful information here…
I use a small glass bead in the end of my servo arms. They are about 1/8 inch thick and the hole in the center is nicely radiused. I thing that Spectra fishing line is pretty slick and the friction on the glass seems pretty small. A pack the size of my fist is about $3 at the crafts store.
I have just lashed and taped the small “fishing swivals” on each end of my arm, both smoother and extends arm length so I have more tuning control of sheet lenghts.