Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Yachts and Sail => Topic started by: durhambargeman on March 15, 2007, 03:53:02 pm
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Hell all, As a complete novice to model boat building, what are the differences between a servo with a sail arm and a sail winch?
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Hell(o) Halcyon
About £50!! ;D
Basically, a sail arm depends upon its length to get travel to control the booms, thus to get a long travel you need a long(ish) arm. Not all hulls have the space for the required length. Further, the longer the arm the more force the servo needs to move it against the resistance of the sails. Sail winch servos are available which have more torque than the standard servo. I've used both and not noticed a marked difference (small yachts).
A winch is designed for the job in hand and may provide more travel than can be effectively provided by a servo. They also tend to be a lot more expensive.
You pays yer money etc
HTH
Doug
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Hi,
a sail erm servo has an arm fitted, not unlike a servo arm you would see on a steering servo.
A winch has a rotary drum that the cord winds onto as you winch in/out.
The arm type operates between set angles, not sure what they are, but less than 180 degrees, so the range of operation is limited. The drum type rotates I think 2 to 2.5 rotations, so the range of operation depends on the diameter of the drum.
Clear as mud!
Ian
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Hiya
Only thing to add to the above is that the sheeting speed on the latest sail arm winches is faster than an economy sail winch, but as mentioned the torque is less and of insufficient 'grunt' to power anything above a 1M yacht - (IMHO)
MikeK
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Hiya
Only thing to add to the above is that the sheeting speed on the latest sail arm winches is faster than an economy sail winch, but as mentioned the torque is less and of insufficient 'grunt' to power anything above a 1M yacht - (IMHO)
MikeK
Except perhaps the Hitec 815BB which has 19.6kg of torque, almost doulbe that of the beefiest futaba sail arm servo (11kg). However it is considerable bigger than the standard servo.
It is recomended for the Victor Models Valkyrie which is a 50in bermuda rigged sail boat with 800sq in of sail.
I got mine for under thirty quid from Als Hobbies.
The sail winches are much slower, about 2.5 seconds, but for normal sailing not a problem.
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http://www.servocity.com/html/servo_power_gearboxes.html is a bit pricier option but can provide much increased torque from standard Futaba/Hitec arm servos with significantly better speed than sail winches. As a rough guide 150 oz/inches is about 10 kg/cm. There is also an option for 180º arm movement.
The Cirrus P-CS704/MG with 13 kg of torque is also much lighter than the beefier Futaba/Hitec offerings.
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The disadvantage I find with drum winches is when the line falls off the drum and tangles up,
dead control's very fast boat, going towards the other bank.
Arms for me everytime now.
(trouble with elastic tensioning is the elastic has a life span, cracks and snaps when you need it most,
(what do you mean REGULAR maintainance))
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I had forgotten about the line coming off the drum. I have only seen this if the line is too fat.
A disadvantage of the sail arm is it requires more space. If the hull is very narrow(less than 100mm say), then a winchmay be required.
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Hi
When using a winch you should have either a elastic tensioner to "pull" the cord off the winch drum, and hold it snug, so it doesn't tangle, or use a double drum with an "endless" cord and attach your sail sheets to that. The general idea is shown on this site
http://www.onemetre.net/Build/Drums/Drums.htm
Cheers Telstar
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I have never seen a sail arm servo strong enough to close sails in the wind, but you will not have the same problem with a drum type winch, which are available from sevrohut.com at about $18.00 American, which is very cheap......
Roy