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Author Topic: What servo should I use?  (Read 2190 times)

walktheplank

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What servo should I use?
« on: November 12, 2008, 09:03:19 PM »

Hello All,  This is the first sail boat I have bought, I think I got it at a very good price. But I was wondering which servo I should get,
and is there a lot of difference between a sail arm, and a sail winch servo.
All I know is that one is a leaver as the other is a drum. In the compartment there is a arm, see photos,
Any help and comments would be gratefully received. Sonic.
ps The teddys was no hlep.
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dougal99

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Re: What servo should I use?
« Reply #1 on: November 12, 2008, 09:24:40 PM »

Your boat is already equipped with a servo arm so I would go with that. I have used standard servos in two of my yachts for the sail arm with no problems. Futaba (and, I suspect, others) do an uprated servo designed for yachts but I have not found the need for one, although I fitted on in the first yacht I built, which, ironically, is the smallest of the three  ;)

HTH

Doug
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Stavros

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Re: What servo should I use?
« Reply #2 on: November 12, 2008, 11:39:01 PM »

Futaba 765 sail arm winch Futaba 3003 for the rudder



Stavros
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tigertiger

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Re: What servo should I use?
« Reply #3 on: November 13, 2008, 05:12:52 AM »

If it was fitted with a sail arm before then it is obvoisly suited to that.

Sail arms are less problematic than sail winches. Winches have a tendancy for the sheets to come off the drum, this can be overcome but arms are easier.

And as has been stated, you do need to buy a Sail Arm Servo. A standard servo will not have enough torque to pull in the booms.
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Welsh_Druid

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Re: What servo should I use?
« Reply #4 on: November 13, 2008, 08:17:53 AM »

Stavros

Don't you mean the HiTec HS 765 sail arm servo ?
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roycv

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Re: What servo should I use?
« Reply #5 on: November 13, 2008, 12:10:16 PM »

Hi all, I would look at the Howes web site as they have standard size servos with very high output torque and cheap too.  I have used them  for similar size sails.
Roy
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tigertiger

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Re: What servo should I use?
« Reply #6 on: November 13, 2008, 03:53:59 PM »

Hi all, I would look at the Howes web site as they have standard size servos with very high output torque and cheap too.  I have used them  for similar size sails.
Roy

Roy does raise an interesting point about size. Check the size of the hole in your radio tray and check dimensions.

Many sail servos are the same size (dimensions) as standard servos.
But they are not standard servos.

Als Hobbies website  servo pages contain all the dimensions for Futaba, Hitec and others. Even it you don't buy from Als you can check sizes. Altough I have always found service from Als telephone mail order extremely helpful, and they will tell you if it is in stock, and if not sugge4st an alternative to avoid the backorder scenario, or an unwanted substitution.

no connection, just a happy customer.
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walktheplank

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Re: What servo should I use?
« Reply #7 on: November 14, 2008, 11:24:50 AM »

Thank you all,
“Doug”  “Stavros”  “tiger tiger” “Roy”
I have noticed that we all have our own options, which is great info, so I will try different servos and see how it goes, but i am still not sure the different jobs the two servos do, so if you have a bit of time to explain i would be grateful.
 thanks again Sonic 
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roycv

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Re: What servo should I use?
« Reply #8 on: November 14, 2008, 01:30:20 PM »

Hi WTP, Bit of large subject is servos.  In general a standard servo converts the angle movement on the transmitter stick to a push / pull action to use for say a rudder.  But I will just refer to sailing boats.

The power of the servo can be assessed by how much torque or power there is for the push pull action.  It is measured in kgrms / cm.  The further away from the centre turning point of the servo arm the less power is available.
For a 5kgrm / cm. servo you have 5Kgrms pull or push available at 1 cm. from the servo arm centre.  If you extend the arm to 5cms from the centre you have a force of 1 Kgrm available.  2.5 cms is 2 kgrm and so on.

There are also very small servos for doing small jobs on small models.

So for a dedicated sail arm servo it needs to have a bigger or more powerful motor to give a reasonable pull at the end of the arm.  This usually involves more substantial or metal gears inside. So the box is a little bigger.

This might be quoted as 13.8Kgrms at 1 cm (Hitec 725B)  or by arithmetic  10 cms (4 inches) 1.38 kgrms. 

The other option is a drum winch, the difference here is that the pull is fixed by the drum radius.  I think the standard drum supplied (Hitec) is 1.9 cms radius.  The output power from the motor is a little lower giving 11.5kgrm / cm.  so you have a pull of 6.05 kgrms pull (11.5 / 1.9).
 
The arm winch will have a transition of about 0.8 seconds through about 90 degrees movement so it is quick.
There were sail servos from Sanwa with 180 degrees of movement.

The drum winch will have several rotations from one end to the other so to speak (Hitec 3.5 revs).  This is at a speed of 1 rev in 1.3 seconds, about 4.5 seconds.  This is enough power to work the sails of most non-competitive sailing boats.

So you sacrifice some speed for power.

If you look at some of the cheap high power standard size servos form Howes, I am sure there are others, you will find a 10 kgrm servo which could well be made into a sail winch for a small yacht.

Take into account that if there is a lot of work to be done, then you really need a rugged servo to do it.

I would suggest that a 36 inch loa yacht needs a proper sail servo BUT there is an alternative.

You can use 2 servos in parallel from one socket on the receiver by putting in a Y connector. The Y connector single end goes in the receiver and this leave 2 ends for servos.  Then you can have one servo for the jib and one for the mainsail.
They do not have to be the same size / power servos.

It all comes down to what you might have in the spares box or whether its  cheaper to go one way or the other.

I use a sail arm servo in one of my yachts and decided to slow it down by using an Action kit servo slow down module and this works very well, in general you do not want a very fast  action from a sail servo.  The slow down module is placed between the receiver and the servo.

Hope this helps, Roy





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malcolmfrary

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Re: What servo should I use?
« Reply #9 on: November 21, 2008, 11:30:58 PM »

The boat already has a cut out for a sail arm servo.  Just find one to fit the hole - it will be man enough for the job, and much easier to rig than a drum type.   Having a bigger motor and being faster acting, it will be happier with a higher capacity battery, but there are AA rechargables nowadays that are quite respectable - look for 2000mAH types.
Measure the distance that the arm covers between fully one way and fully the other.  This is the max travel you can impart to the sails.  Then measure from a fixed point on the deck to a point on the main boom directly above, then swing the boom out as far as you think it needs, measure again.  Subtract one from the other, and this is the travel you will need.  You might have to move these points in or out a bit to get a match.  The required swing will probably be less on the jib - the easy way to get this is to fix it to the servo arm a bit further in.  This way, both sails swing the same angle and take the same setting.
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tigertiger

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Re: What servo should I use?
« Reply #10 on: November 26, 2008, 12:18:10 AM »

Not all sail arm servos are bigger than a standard servo.

I have used two different Futaba Sail arm servos and they are exactly the same size. But the motor is more powerful. I have these up to 11kg/cm torque.

The term 'standard' is a bit fickle. My understanding of 'standard' is, the one you get in the box wiith a radio set. Low torque and really suitable for pushing and pulling control rods/lines to rudders and (for aircraft) control surfaces.
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