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Old Jan 27, 2015, 08:01 AM
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Servo arm position

I'm resurrecting a 40 year old Dumas 30" Star with a Heathkit radio, also 40 years old. I've mounted a servo arm onto one of the servo's, I believe it has enough torque. Because of space limitations I have to position the arm so that it's movement will be from port to starboard instead of what, I think, is normally bow to aft. Any thoughts on this arrangement? Thanks in advance for all replies.
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Old Jan 27, 2015, 01:44 PM
If it floats....sail it!
FoamCrusher's Avatar
Elk Grove, CA
Joined Sep 2002
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John:

With the newer digital servos, while very strong at much lighter weight than the older analog servos, will not tolerate being under a load, like close hauled, for minutes at a time. They were designed for large planes where the loads are heavy, but of short duration. Put under a heavy load too long and digital servos overheat and die. I burned up a $150 digital on my US1M when I didn't have the sheets pulling over the center of the servo when close hauled

If you are going to be using the old analog servo in the port/starboard design, your servo will be under heavy load at close hauled - either on the port or starboard side. Analogs won't burn up, they just give up and move to whichever end they are being pulled. This will put the arm against the servo's internal stop - a pin or plastic bump on a gear, neither of which was designed for that kind of load. It may be fine on a light wind day, but my guess is that it will break something inside the servo when the wind picks up.

I would look at modifying your arm/sheet system, perhaps using blocks, so the the close hauled load is applied over the spline of the servo.
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Old Jan 28, 2015, 06:15 AM
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FoamCrusher, thanks for the info, I hadn't thought of that. A couple of questions for you. Wouldn't I have the same loading problem if the arm pulled bow/aft? What do you mean by spline of the servo? Currently I have the arm parallel with the body of the servo, (is that the spline?) and was thinking of mounting it parallel with the keel, that's why it would pull port/starboard. What if I mount the arm 90 to the servo? Then the arm would pull bow/aft but the servo would still be mounted parallel to the keel. As mentioned it's a boat I've had for a long time so I'm mainly getting it together for sentimental reasons so it will probably be mostly light wind sailing. I am thinking of buying a Marblehead if I get into racing.

Thanks again for the responce
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Old Jan 28, 2015, 07:23 AM
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Blackpool, Lancs
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Just set the run of the line so that at close hauled the line passes as nearly as possible over the servo spline. If you do that, it will not matter whether the travel is fore and aft or across the hull.
If the close hauled line crosses the spline, all the mechanical advantage is with the servo.
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Old Jan 28, 2015, 08:17 AM
RC Newbie with lots of ideas
Chile, Santiago Metropolitan Region, Stgo
Joined Dec 2012
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Quote:
Originally Posted by JohnL52 View Post
I'm resurrecting a 40 year old Dumas 30" Star with a Heathkit radio, also 40 years old. I've mounted a servo arm onto one of the servo's, I believe it has enough torque. Because of space limitations I have to position the arm so that it's movement will be from port to starboard instead of what, I think, is normally bow to aft. Any thoughts on this arrangement? Thanks in advance for all replies.
This setup would need an additional pulley at one side of the boat, which may stress the hull in strange ways...

Have you thought instead about putting the sail servo sideways, so that the arm moves vertically aft-forward? Look what this guy did in the picture below: the servo sits sideways just in front of the mast.

In my opinion, putting the servo down by the hull is risky: water inside the hull can easily reach the servo. Maybe it's a good idea to put the servo sideways just below deck and let the arm move below it?

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Old Jan 28, 2015, 10:00 AM
Si2
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United Kingdom, England, Portsmouth
Joined May 2014
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waterproof servo?
I like the idea of it being low down.
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Old Jan 28, 2015, 12:12 PM
If it floats....sail it!
FoamCrusher's Avatar
Elk Grove, CA
Joined Sep 2002
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John:

It is what mfr said.

By splines I meant the exit shaft of the servo upon which you place the arm. The shaft has splines (little ridges for the arm to grip) but a common misuse of the term is to call the splined drive shaft "the splines". Sorry for the incorrect use of the terminology

Using the servo as in the picture above will work, as long as you get the 1:2 reduction in leverage (and the increase in take up) by putting a block on the end of the arm. The arm in the photo is clearly too short for a 30" LOA boat. Some hull designs are deeper than they are wider so mounting so the arm swings along the centerline, either sitting in the bilge or bolted under the deck, is the best way to go. If in the bilge, I would either use a waterproof servo or do the waterproofing myself. Electronics and water don't mix well, especially salt water.

My US1M's at 9" longer than the 30" Star need about 9.5" minimum of sheet take up for the main so a single arm system needs half of that, 4.75" of swing. Allowing for the inward taper of the hull, the depth of the servo and positioning of the blocks and fairleads, there is just enough room in a 7" beam boat to make that work. I am working on a Frank Russell design right now that has a very narrow hull and had to go to a winch system because there was not enough room to swing the servo arm. Alternative block systems added too much friction so it was open my wallet for an RMG winch

However you set up the servo, laterally or parallel to the centerline of the boat, when close hauled the sheets must pass almost directly over the shaft of the servo so the shaft bearings take the load and so the servo is not using too much power to keep the arm in that position.

I would search the forums looking for how the Star 45's are set up. Although they are 1/3 larger, everything scales and the rigging should be close. The Star 45 has a class forum you can access from the AMYA webpage.
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Old Jan 28, 2015, 07:32 PM
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Thanks everyone for the great info.
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