davideboracay

Hi,
I am building a new sailboat. It is 44 inches long. I don't know how high the mast should be because it does not say on the plans.

What torque should the servos that control the two sails have (the front and the main sail)???

And how do i control the sail to have enough movement??

The rudder is under the hull. How do i waterproof the shaft coming in the hull?????

Guiri

Something like a Hitec 815bb should give enough to control both sails, although, depending on the sail area, a smaller servo might be sufficient. It's got 140ΒΊ of movement so there should be no problem with enough sail movement.
The rudder is just a question of ensuring that the tube for the shaft extends high enough inside the hull. You shouldn't have any trouble with water ingress.

davideboracay

thanks guiri.
How do i actually make the sail move with the servo?

DONS BOAT WORKS

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Like this.

davideboracay

thanks but the image is not really clear.
could you explain how it should work?

LtDoc

davideboracay,
As shown in the picture, the control arm is mounted on the servo at it's center point. A line is attached to each 'end' of the arm, one running to the foresail, the other to the mainsail attachment points through a hole in the deck (it's all mounted inside/under the deck). At full travel in one direction the servo pulls the sail in 'tight' (straight fore/aft), at full travel in the other direction the sails are full 'out' (90 degrees with the centerline of the boat, maybe).
The amount of 'travel' depends on the length of the servo's control 'arm', where the lines are connected to the 'arm', and also where they are connected to the sail/boom. Need less 'travel' on one sail than the other? Move that line's attachment point closer to the 'arms' center (slightly). Need lots of 'travel'? Attach the line to the end of the 'arm'. Lots of variation available.
The 'biggy' is that the servo be strong enough overcome the pressure of the wind on the sail. The more area of the sail and strength of the wind, the larger the servo. The stiffness/strength of the servo arm should be such that it doesn't flex when in use (or at least not very much ).
From there, it's just a matter of adjusting everything to suit your boat. Good luck!
- 'Doc

Oh, linning the 'hole' through the deck for the control lines with something to keep from making the lines snag/fray is a good idea too (metal grommet, as in shoe laces?).

davideboracay

thanks LTDoc.
what torque should the servo have?
where should the line be connected on the boom?

the further out the string is attached on the arm the less torque the servo has right???

what servo do you recommend?? my boat will be 44"long but i am not sure of the height of the mast and sail area because
the building plans don't say.

Thanks

Davide

Guiri

The torque rating needed from sail control servos depends very much on the sail area. As a guide, my Saphir, with 3,700 cm^2 sail area sails nicely in all but the strongest winds on about 12 kg/cm torque (a GWS S04 with 5xNiMh cells). The Hitec 815bb mentioned has about double that but weighs about 50% more than the GWS.

davideboracay

thanks guiri
How is the servo moving the sail on YOUR boat.
how many turns does th GWS S04 servo do?
what servo arm did you put on it?

Guiri

The GWS has one benefit - the price. Its only got a max movement of about 90ΒΊ, so I had to compensate with a longer arm than the standard one that came with the boat. The sail movement is also enlarged by anchoring a sheet at the stern, then forward to a pulley attached to the servo arm, back to the stern again where runs over another pulley and finally forward. The sheet is then divided in two: the first goes to the main sail and the second to the foresail.
One thing to make sure of is that your sail servo is very firmly fixed in the boat. The force of the sails will otherwise pull it out.

davideboracay

thanks guiri

where do you connect the rope on the boom of the sail ; close to the mast or close to the end of the boom???

Thanks

Guiri

On a 34 cm boom (Measured from pivot point - boom itself is a little shorter) it's 22.5 cm from the swivel point. Make sure the foresail distance from the swivel point is the same as for the mainsail.

davideboracay

thanks!!

LtDoc

Davide,
I don't have a boat of that size so can't really say exactly which servo to use. In general, the bigger the better, sort of. Depends a lot on the weight of the thing, and it's price!
There are a number of ways of 'multiplying' torque. 'Guiri's method certainly works. There are disadvantages, as in the amount of travel available, but that can be 'got around' in most cases.
Using a sail winch is a possibility. (Servo makes more than 360 degree turn.) Winchs have disadvantages too, a bit more complicated to rig, and again, cost. [Not sure which is the correct spelling, 'winch' or 'wench'. Same disadvantages apply to both, complicated and cost .]
The attachment point on the sail's boom.
Just depends on how much travel the sail should have, and the amount of line the servo can give to produce that amount of travel. The 'right' attachment point is where those two 'needs' balance each other, sort of, if that makes sense. Then again, you have the option of using other variables to get that amount of travel necessary (where the line is attached to the servo arm). As long as the boat isn't for a particular porpose (racing where design is 'fixed') none of this is carved in stone. If it ain't 'right', change it.
When you get to the point of totally hating the whole thing, beat it to death with a large rock. Very satisfying! Then build another...
- 'Doc

Guiri

I had to do some digging to find this link again. If you like playing with electronics (I don't), it shows a way of increasing the travel on larger aircraft type servos to make them more appropriate for sail use.
www.modelyacht.org/us1mpt4.pdf
The relevant article is on page 5

davideboracay

thanks Guiri
you are a lifesaver