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Author Topic: Sail Arm Servo  (Read 5236 times)

Tugger

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Sail Arm Servo
« on: September 27, 2006, 10:11:08 PM »

Does anyone have any advice on how to increase the amount of pulling power on a sail arm servo. The servo in question is a Hitec 715BB. When sailing I can't get the sail to pull in if the wind is on the sail which is very annoying if I get a good blow and could get some speed up. The yacht I am sailing is a Weenip from the Model Boats Plan.
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martin-R

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Re: Sail Arm Servo
« Reply #1 on: September 27, 2006, 10:47:23 PM »

If you're using a 4xNiMH battery pack, it can be replaced by a 5 cell pack for a 25% gain in both torque and speed and a little loss of run time. Otherwise the only thing a can think of is  http://www.servocity.com/html/robotzone_servos.html for a gain of toque and a corresponding loss of speed at the same voltage.
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dougal99

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Re: Sail Arm Servo
« Reply #2 on: September 28, 2006, 10:52:07 AM »

Are you sure that the servo-sail cord can run free. I had a few problems initially with my We Nip that were down to snagging. You might also try shortening the servo arm slightly to provide a stronger pull. Failing that, as has previously been suggested, use a beefier servo.

HTH

Doug
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martno1fan

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Re: Sail Arm Servo
« Reply #3 on: September 28, 2006, 12:18:47 PM »

Does anyone have any advice on how to increase the amount of pulling power on a sail arm servo. The servo in question is a Hitec 715BB. When sailing I can't get the sail to pull in if the wind is on the sail which is very annoying if I get a good blow and could get some speed up. The yacht I am sailing is a Weenip from the Model Boats Plan.
normally you pull the sail in when the wind is off the sail ie turn the boat a bit ,not many servos will pull a sail in under load!!.however trying a larger battery pack may increase torque as stated but you still may not be able to pull the sail in under load.i find the best way to get the sail in is angle the boat out of the wind slightly then pull it in then return to course,i think this is normal practice !!.if you do it right you wont lose much momentum at all ive found.
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Doc

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Re: Sail Arm Servo
« Reply #4 on: September 28, 2006, 01:32:34 PM »

Simplest answer is to replace the servo with a stronger one.  I didn't say easiest, or cheapest, just the simplest answer.  Increasing the amount of voltage/current to the servo can increase strength slightly, just be careful you don't go to far with that idea, really upsets receivers.  Shortening the length of the servo's arm will also increase the amount of torque applied to the sail.  Then again, you won't have quite as much sail movement either, so not the 'best' answer.
 - 'Doc
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cbr900

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Re: Sail Arm Servo
« Reply #5 on: September 28, 2006, 02:11:58 PM »

Even simpler put in a winch problem solved...



Roy
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Tugger

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Re: Sail Arm Servo
« Reply #6 on: September 28, 2006, 10:20:55 PM »

Thanks for the replies unfortunately I already run it on 5 X 2500 AA cells. One suggestion from a club member is to fit a pulley system to increase the pull any ideas on how to do this? I looked at the specs on winches and they are the same pull as the sail arm which according to hitec should pull around 13kg or 30lb on 6 v.
 ???
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martin-R

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Re: Sail Arm Servo
« Reply #7 on: September 29, 2006, 07:11:13 AM »

I don't see how any pulley system is going to "pull" the sheets the same distance with more force from the same servo. However pulleys would allow freer movement than running the sheets through eyelets.
I've had a look at the Wee Nip and notice it only has a max of 2,000 sq cm of sail area. Your 715 servo should have about 10 kg/cm torque, more than enough to handle any wind in which you're likely to be sailing.
marginally more handles almost double the sail area of my Saphir
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cbr900

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Re: Sail Arm Servo
« Reply #8 on: September 29, 2006, 07:31:15 AM »

The spec sheet for winch servos may be similar, but a winch will actually pull a lot more than the arm servo, when you take into account the arm length as to the small circumference of the winch, a winch servo will pull an arm servo closed...


Roy
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Doc

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Re: Sail Arm Servo
« Reply #9 on: September 29, 2006, 02:56:33 PM »

The amount of 'pull' (torque) applied by a servo is determined by the strength of the motor in the servo and the length of the 'arm' or radius of the drum for the winch.  The smaller radius of the drum is just like having a shorter servo arm.  Shorten the arm by half and you double the amount of 'pull'.  'Course, you can't pull quite as far since the end of the arm travels a shorter distance.  A sail winch isn't limited in the number of turns it can make like the typical servo (only part of a complete circle).  A typical servo can be made into a 'winch' by removing the internal control that limits the motor's travel.  The 'catch' to that is that first you gotta remove it from the boat, disassemble it, remove the correct component, re-assemble it, make the @#$ drum, and fit the whole mess back into the boat.  (Just buy the silly thing, much easier/faster.)
Servo and arm or a winch, both have their 'pluses' and 'minuses'.  The two biggies are cost and amount of travel allowed.  You pays your money and takes your pick, as I'm told they used to say about something else that sounds like wench/winch.
Pulleys.  Or maybe 'block-n-tackle'?  It/they can and will multiply the amount of force exerted in pulling on the rope.  The draw back is that they will require a longer rope, or only pull for a shorter distance with the same length rope that you started with.
What's your -best- choice?  Beats me.  You decide...
 - 'Doc
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martin-R

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Re: Sail Arm Servo
« Reply #10 on: September 29, 2006, 05:00:16 PM »

When we talk of torque ratings, we are not talking an absolute no. of ounces or kgs. e.g. a 10 kg/cm servo will exert a force of 10kg with a 1 cm servo arm, 5 kg with a 2cm servo arm etc.
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tigertiger

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Re: Sail Arm Servo
« Reply #11 on: September 30, 2006, 01:47:45 AM »

Does anyone have any advice on how to increase the amount of pulling power on a sail arm servo. The servo in question is a Hitec 715BB. When sailing I can't get the sail to pull in if the wind is on the sail which is very annoying if I get a good blow and could get some speed up. The yacht I am sailing is a Weenip from the Model Boats Plan.
normally you pull the sail in when the wind is off the sail ie turn the boat a bit ,not many servos will pull a sail in under load!!.however trying a larger battery pack may increase torque as stated but you still may not be able to pull the sail in under load.i find the best way to get the sail in is angle the boat out of the wind slightly then pull it in then return to course,i think this is normal practice !!.if you do it right you wont lose much momentum at all ive found.

I agree with Martino1
If the sails are full of wind and you pull them in then the boat will go slower. It just looks faster coz the boat is heeled over. What you have achived is exchanging thrust along the direction of the keel (forward) for sideways thrust. For maximum forward propulsion the sails should be as far out as possible, without starting to flap loosley (luffing).

Least thats what I learnt sailing.
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cbr900

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Re: Sail Arm Servo
« Reply #12 on: October 02, 2006, 01:05:17 PM »

That really only applies basically to the downwind leg, if the wind is 90 degrees and the sails are wide open you will probably go sideways and not much forward, you have to close the sails almost totally to reach a good speed....



Roy
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tigertiger

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Re: Sail Arm Servo
« Reply #13 on: October 03, 2006, 12:47:48 PM »

That really only applies basically to the downwind leg, if the wind is 90 degrees and the sails are wide open you will probably go sideways and not much forward, you have to close the sails almost totally to reach a good speed....

I should have been clearer.
That is my comment about luffing, when sails start to flap at the edges.
When the wind is pushing the sails open, and you are just short of starting to flap,  then you are getting max forward thrust.
When sails start to flap you are loosing forward thrust, but by the same token you will not need max force (from servo) to pull sails in to a good setting.

Servo will obviously do most work if you are trying to pull the sails all the way in when on a beam reach (wind at 90 degrees to keel line). The boat will heel over, but will not be going at full sped.
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roycv

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Re: Sail Arm Servo
« Reply #14 on: October 07, 2006, 03:50:18 PM »

Hi all, I use a servo arm on a larger sail area than Weenip and agree with above about having a pulley system.  There is an alternative and that is to use hard plastic tubing to feed the cord around the boat.  Bend the tubing using hot water, well boiling actually, and cool when bent to shape.  No sharp radii!!.  1cm. radius works fine.  Thread the sheets through the tubing using fine piano wire.  Surprisingly there is very little wear, and very little friction.
I found that sail arm servos try to operate too fast and so I fitted a 'servo slow down' unit from ACTion kits and this works a treat.
regards Roy
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martno1fan

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Re: Sail Arm Servo
« Reply #15 on: October 07, 2006, 04:31:55 PM »

good tip mate i used that method for mine where it exits the deck i bent some and fastened it dowm to the deck and it runs very smooth.the tubing you get for running your ariels through work great too.they bend easy no need to use hot water and i usd a small plastic cable thingy!! the ones with a small pin to nail your telephone wires to the skirting boards !!.
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mike javelin

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Re: Sail Arm Servo
« Reply #16 on: October 10, 2006, 02:13:18 AM »

Another way of increasing the leverage is to move the sheeting point along the boom.
The closer to the end the more leverage.

A system used by the top ten boats in my big boat Class on the European circuit is the split tail mainsheet.
This allows the convenience of the center mainsheet but combines the leverage of the end boom system with the added benefit of being able to centerline the boom whilst sailing upwind without pulling down on the boom and tensioning and closing the leech.

A couple of added benefits are that the split tail will always centerline no matter how much rake you add and the extra leverage means that you do not need multi purchse so reduces the ammount of rope required.

There maybe some scope in using this system on models ?


and in action below.
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malcolmfrary

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Re: Sail Arm Servo
« Reply #17 on: October 11, 2006, 12:20:28 AM »

For any servo, there is just so much power available.  The pull exerted can only be increased at the expense of reducing the travel at the sail end of the linkage.  A normal servo will move its arm through 90 degrees, the longer the arm, the further this 90 deg will travel, but the less force will be applied.
Its also a good idea to make sure the sail servo is really firmly fixed - the force applied to the sails can pull it out by the roots.  Been there, done it, had to run/walk/limp round the lake....
My preference is for a winch with a continous loop with the lines controlling the sails taken off the loop.  This needs enough clear deck space to give a run that is long enogh  as well as straight and uniterrupted.
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