Sail Servo

Hello guys,

I was wondering if the HS 225 will be enought to handle the sail of my Victor V 12 or if I need to invest in the HS 225 MG ?

The GWS Micro 2BB MG has plenty of torque in a small size (5.4 kg/cm; 28 gms) and is a bit cheaper than the HS225 MG. A lot use it for the RG65 class.

This brings up the broader question of just what range of servo strength is needed for sail and for rudder in a sailboat of Footy size?

Starting point for discussion:

Rudder 10 - 12 ounce inches

Sail 20 ounce inches

Are these about right???

Go too weak and a higher than expected wind can cause loss of control.
Also if you strain the mechanism, the current draw can be excessive if the servo is underrated.

How light? there are 5 gram servos available. Are the gears too fragile. I have heard of servo death in the middle of a race. Is it better to go to 7-9 gm for rudder and 12-17 gm range for sail, or all the way up to the very heavy standard servos? I don’t think so, but I have only been on the pond a few times.

Nate " I’m used to 10 - 30 pound warships" Graham

Nate - I use the Bluebird 380MG for the winch servo at 13 grams with 35 oz/in. at 4.8v or 54 oz/in. at 6v. It seems strong enough to handle Footy loads in high wind conditions.

Niel, have you had any issues with Bluebird cases breaking on you? I had one give in light air at the Region 1 regatta last spring… Major disappointment considering the circumstances…

Nate - sorry it isn’t that simple. Servo torque isn’t like cubic inches in a motor car engine: plug some in and set off with smoking tyres.

The performance of any servo system depends on the layout of the mechanism connecting the servo to the sail.

This is nothing to do with yacht, model yacht or Footy black magic. It is plain, simple boring trignometry for 14 year olds. Admitedly there are quite a lot of trignometrical sub-problems between the axle of the servo and the point of application on the sail and I have never had the energy to put together a general purpose spreadshhet - but it isn’t actually very difficult in principle.

Let’s look at it from a purely geometric point of view. Start above deck and assume a basically conventional system.

If I have the sail right out, the load on the sheet is low. This not because sails don’t work hard down wind, it’s because your pull on the boom is roughly horizontal. As the boom comes in, the horozontal component (useful work) becomes less and the non-useful component (pulling the boom down) decreases. The ultimate position (no matter what the rrelative heights of boom and sheeting point) is that sheeting the boom to the centreline. This requires infinite force.

Now let’s look below deck. Even if we have done the right thing and got the sheeting point fairly exactly on the plane of rotation of the boom, we will still probably have an increase in sheet loading as we pull the sail in. Apart from the purely geometric effects, the fact that we have a higher apparent wind speed (we are sailing into the wind, not away from it) will increase sheet forces going to windward. So we have to look at the layout of the servo arm so that it produces maximum mechanical advantage as the sheet comes home. In practical terms, this means that the arm should be in line with the sheet at the ‘tightest’ sheeting position.

Now over to you lot!

There are several other issues:

  1. The accuracy of the vendor’s data sheet. I was very unpleasantly surprised by some testing on a pair of HS-81MG servos, which were rated at 36 oz-in at 4.8V. Actual measurements showed only 20 oz-in was available at 6V. I tested 2 units to make sure that I wasn’t being misled by a defective unit.

  2. Additional losses. Each turn that the sheet takes will also cause significant loss, so it is desirable to minimize the number of turning points. I had done some testing a few years ago, and found a friction loss of about 25% at a glass bead turning point.

  3. Many Footy sailors are now using the Lithium AA cells. Four of these will put out a very stiff and constant 7 volts. The maximum allowable voltage is generally not specified on most servo motors. This would relate to two issues internal to the servo:
    A) Overheating of the amplifier chip
    B) Voltage breakdown of the integrated circuit
    Either of these issues could cause permanent damage to the servo. I would be interested to hear if anyone has encountered these issues.


I’ve read your comments about the HS 81 servo before, although my only experience has been with stripping of the gear teeth with the HS81 Nylon gears. Mind you, perhaps both these problems are why the HS 81 has been discontinued and replaced with the HS 82. I wouldn’t expect Hitec to admit why they made this change, but I’ hoping for the best as its the HS 82MG I am going to try next.

I did some crude tests w.r.t. sheeting guides. I just clamped a series of guides in a small table vice and ran the sheet material though them with a small weight on the end hanging vertically down. I then added weights to the other end - also in the same plane - and measured what weight was needed to move the first one. In the end my conclusion was that it was the radius the sheet material was turning over that mattered and not the material it was made out of. It seemed to me that once you had got to a 1/16" radius there was very little improvement. In fact, the worst of all those I tested were the glass beads. I think it is too easy to be impressed with the overall smoothness and curvature of the outside of the bead that it diguises a rather poor transition from the main bore.

Finally, I have just started my second set of Energiser Lithium AAs. The first set lasted about 24 hours actual sailing time, including some quite rough conditions here in the U.K. I am now into the 4th hour on the second set so all I can say is that the servos have survived at least 28 hours with no apparent ill effects. My only complaint is that when measuring no load voltages there is no warning before they give up completely. My new cold votages were 6.98V. The morning of the final day they measured 6.54V, but after 2 more hours they died, and when measured immediately gave only 3.5V. I think it might be a good idea to arrange some way of testing the voltages while operating the servos which would, perhaps, be more realistic. I shall have to put that on my rapidly extending job list.

I hope this helps.



I’ve been using the Spektrum mini servors for the rudder, you know the tiny ones that come with the TX/rx… wich has 13.2 oz/in at 4.8v
and for the sail servo I am using futaba 3115 micro for the sails 38 oz/in @ 4.8v

I am also using this servo as my rudder servo for a Victoria and a Soling 1 Meter with no problems…

the 3115 will become my servo of choice for the rudder on the footy as well if the Spektrums ever give out.

OK so let’s optimize the geometry and minimize the angle changes and feedthroughs. Place the servo under the deck with the servo arm above deck ( like someone we know). Make only one angle change and that is a post with a screw eye elevated so that the boom just passes over it at the sheet’s point of attachment - thus as close to no download as possible while letting the boom pass over the point. The servo arm is at 90 degrees to the centerline of the ship when the sheet is out and in line and away from the boom attachment with sheet in.

Now #1 is there a better setup to minimize unwanted forces?

#2 Though the most elegant way to proceed would be with some trig and physics equations to determine the expected range of loads with different sized common rigs, but I would think that those with more experience have an idea of what is needed on paper for large and small rigs of the 3 most common types - unarig, swing rig, and bermuda. With this, we could arrive at a recommended base power level. line geometry that deviates from the Best Case Geometry would require more power and toughness of parts.

#3 On paper requirements meet real world as others in the group have mentioned servo failures. Your comments and my experience in RC suggest that gear stripping and motor failure are the most common failure modes. So were the failures due to poor products or products undervalued for the task?

Better stop here,

[QUOTE=Walt H;48013]There are several other issues:

  1. The accuracy of the vendor’s data sheet. I was very unpleasantly surprised by some testing on a pair of HS-81MG servos, which were rated at 36 oz-in at 4.8V. Actual measurements showed only 20 oz-in was available at 6V. I tested 2 units to make sure that I wasn’t being misled by a defective unit.

Was 20 oz-in still sufficient to handle sail loads?

I bought one of those and the 360 to test. I think the 2 or 3 lighter Bluebirds below the 380MG may be enough. It will be interesting to see.

Has anyone found a better material for turning points? I was considering a small length of polyethylene tubing but since friction is proportional to length travelled in the item, i think this will be worse. Gary’s good old brass screw eye is the best idea I have seen so far.



I use fishing guides #6 or #7 single foot. with the right peice of carbon tube, an a touch of epoxy they glue right in place. for a servo arm.

and withthe single foot I make a small hole in a blockof wood drop in some epoxy let it set up and then glue the block to the hull. I use this as my dead man point for the double purchase in hull as well as a turning block to keep the line near the gunwales (sp) so if they do go slack they aren’t likely to foul on something in side the hull. (vic and S1M)

now on my footy I do not use a turning block I just go straight to the thrudeck which is the plastic antenna tubing stuff. with a touch of heat the ends soften and round off nicely once you cut it.

not a very hitech aproach with the fishing guides, but simple works…and they are not very heavy.

No, it wasn’t, which is why I started the testing. In strong winds, when running, the sail actually pulled the servo beyond the stops on the pots. It was not possible to jibe, because the sail couldn’t be pulled in. It returned to proper operation after the wind weakened.

I’ve been using a Futaba 3102 hi-torque mini servo (mfr claims 56 inch ounces of torque) for my Footy’s sail control. I have had it stall when the boat was on a run a couple of times in heavy winds, to the point where I had to head up slightly to ease pressure on the sails so it could start sheeting in, but once it started, it came in OK. This was with only a twelve inch high rig with about 95 square inches of sail, and dry batteries (4 AA). I’m not so sure I would trust anything with less torque, especially with larger sails, if the breeze was moderate or stronger. I used a cheapie $8.00 micro servo on the rudder, because I already had 3 of them on hand, and while it seems to work OK, it’s as noisey as a coffee grinder. The little Futaba has a nice, tight sounding whir as it operates.

Bill Nielsen
Oakland Park, FL USA

Thanks Walt for the info. That is what I needed to know. I do have a correction, I had the numbers on the BMS servo wrong - I was looking at 380 not MG and 373. Bill, that is a lot of torque to need for a small sail. I really didn’t expect to need more than 35 - 40 oz-in of torque, i.e., an equiv to a STD servo. I was hoping to use sl less.

Oh well. time to put them on the water and see. when the snow clears :slight_smile:

Hi Nate,

Yes, it seems like a lot, but I had been sailing for several heats in very strong winds, and the high torque servos draw more current, so the batteries were probably not at top voltage. Also, I’m not sure how accurate the manufacturer’s torque claims are because I haven’t actually tested them myself, but based on Walt’s experience, I wouldn’t be surprised but what they might be a bit optimistic. If you are going to use a standard size servo (my Futaba is a Mini), both JR & Spektrum standard sized servos claim 72 inch ounces, and at 42.3 grams, I don’t think they’re significantly heavier than most other standard servos. Of course, if you already own a bunch of standard sized servos(and who doesn’t?), go ahead and use one of them, if it’s not up to the task, you can always replace it later.

Bill Nielsen
Oakland Park, FL USA

Love it. simple is good. I particularly enjoy finding items made for other pursuits that are adapted to modeling. Especially when they are well made commonly available and relatively cheap. I ordered some from cabela’s.

Thanks for the idea,

If you go to your local fishing rod repair shop you can usually walk away with an arm load of tapered CF tubing (rods)complete with eyes for nada.