RC Model Yachting Servos

RC Model Yachting Servos
RC Model Yachting Servos (for the Soling 1 Meter)
(rev 9/13- revisions in blue)
One of the amazing things about RC sailing are servos, and the fact that servos are, in general, so reliable.
They may cost as little as seven bucks, and get worked like crazy in our little boats, yet still last 3, 5 or
even more years. But, sooner or later, all servos pack it in. So, what can we do to make them last longer,
and then to be ready for the inevitable servo failure?? Also- what do we do so that failure doesn't cost us
a day- or worse a whole weekend of sailing??
What causes servos to fail sooner than they might??
1. Excess heat buildup
a. Wrong servo- not enough torque.
b. Poorly setup mechanicals that the servo has to overcome.
c. Too much voltage going to the servo.
d. No provision left for cooling air to circulate around the servo.
2. Water intrusion
Here are a few things to do:
1. Buy servos with adequate torque (power). For a Soling 1 Meter, you need at least 150 oz/ in of
torque for a 6-7” sail arm and about 40 oz./in. torque for the rudder. (See the chart- Appendix to
this article)
2. Run a higher-voltage 5-cell NiMH or LiFE battery pack = 6 to 6.6 volts- no more.
3. Set up the mechanical operation of your boat to minimize "servo stress"- set your sail arm such
that when the maximum load is on the servo (close-hauled), the arm is parallel to the load. This
means pointing toward the mainsheet exit. Then the servo doesn't have to keep working to keep
the sails sheeted in.
4. Mount the servos off the hull floor, near the center of the boat, so air can get around them.
5. Store your boat with the hatch removed- give it a chance to dry out between sailing dates. This is
more for the electrical connections to your servos than for the servos themselves.
Water Intrusion
Water in a servo will cause an electrical failure or short. “Waterproof” servos have extra gaskets making
them more resistant to water intrusion. But IF you mount the servos properly, and then maintain a dry
boat, water will not be an issue even with a standard sevo.
Connector corrosion
Moisture causes corrosion of the connectors. A “green” copper corrosion appears on the male and female
plugs, and will reduce voltage. Keep the plugs off the bottom of the boat, and periodically clean the plugs
using a plastic brush on a Dremel.
Fit
Different brands of servos use different shaft and spline configurations- the arms are not inter-changeable.
Voltage
Most servos are designed to operate on minimum 3 volts, up to a maximum 7.2 volts. Any voltage higher
than that will shorten the life of the servo. (BUT- LiPo batteries often peak at higher voltages than 7.2V so it is recommended that you use a voltage reducer IF you are using LiPO batteries.) Some
manufacturers are starting to add “high voltage” servos in response to some of the newer types of battery
packs.
We recommend a 6 volt, 5-cell/ 6 volt NiMH battery pack, or one of the newer LiFE (lithium iron) battery
packs- each cell is 3.3 volts, so a 2-cell pack is 6.6 volts. Be sure the battery pack and charger you use are
compatible- the LiFE batteries use a special charger – as well, older battery chargers designed for NiCD
(Nicad) rechargable batteries may not work for NiMH.
Be prepared for a servo replacement! Since servo wear-out IS inevitable, be ready to replace a bad
servo in the shortest time. If sailing as part of a club, the other sailors’ experience has told them which
servos are up to the job. So, buy and install the same servos everyone else is using. Come to an
agreement with the others so someone has a replacement servo, ready to sell, in their tool box. If you can
afford it- have a set of new servos- sail and rudder- in your own toolbox. This will get you back on the
water in 5 minutes or less should a servo fail.
Other issues:
Servo travel: Servo travel is an issue- since most servos only travel 90 degrees (about 45 per direction),
and that is often not enough to get the sail plan out far enough for running.
a. Radio- some transmitters have an adjustable travel output- say an extra 25% that you can set.
b. Sail Arm- the longer the sail arm, the greater the travel, BUT the less effective torque. A 6"
double arm (3” per side) cuts the effective torque to 1/3 of the rated servo torque. Compared to a
6” arm a 7” arm increases travel, by about an inch, but reduces torque.
c. Travel of the servo itself. While MOST servos travel a total of 90 degrees, some have greater
travel: the HiTec 815 and 700BB rotate 180 degrees; the Futaba S 3801 rotates a total of 145
degrees and the HiTec 765 HB rotates 140 degrees. All of these are manufacturer’s specs- you
may get more or less travel using your radio.
d. Another variable in sail travel is the boom pivot length which is the length from the jib club
pivot to the sheet connection, and likewise on the main.
i. You can get more travel on either one of them by moving them forward, or…
ii. By moving one of them forward relative to the other, effectively shortening the pivot
length on the one you didn’t move. So- if the jib doesn’t go out far enough- move its
pivot point forward.
iii. IF you have a digital servo, programmed for max travel, you can usually reach near 180
degrees travel- you can program it yourself using a servo programmer, or have someone
else do it for you.
e. Or use a servo stretcher. These are electronic devices that can double the travel of the servo by
modifying the signal. They do NOT reduce torque in the process. See the note below.
f. It is also possible to modify the servo to add travel by adding a resister internally. See internet
articles on the “how to” of this procedure.
Servo Gears: For most applications, nylon gears are fine, especially for the rudder. Nylon gears are selflubricating as are metal-impregnated nylon gears. Metal gears are really only helpful where there is some
shock to the servo- like on RC model cars used in off-road racing, for example. So- either nylon or metalimpregnated nylon is best for our sailing.
Ball-bearing servos are an advantage in terms of wear, although seldom do servos fail from shaft wearit’s usually the circuit boards in the servos, the gears or water-related failures that do the servo in.
Digital servos vs. analog: digital servos are becoming more popular all the time. The main advantage is
their programmability- but you will need a programmer to get that advantage. They cost considerably
more than analog servos, and have more power for a given size and weight. A HiTec HS7955TG Digital
Servo has 333 oz/in. or torque, yet is the same physical size as an HS-311 (49 oz./in) normally used in
rudder applications. But digitals also tend to be more sensitive to overload, and tend to run hot- so install
carefully leaving room for air to reach them.
Installation: Mount your servos securely. I mount them through a hole cut in the radio board, using
¼” basswood blocks glued to the radio board for servo mounts- not just screwing them to the plywood. I
add 3/16” square pieces of wood on their sides, to support the twisting motion as the servo works.
Also- be sure to mount your servos with space around them to allow cooling air to get to them- off the
floor of the boat.
Note on Servo Stretchers: servo stretchers work by modifying the signal between your
receiver and the servo. They essentially “stretch out” the signal, making the “steps” in your
servos movement twice as long, and doubling the total travel.
However, on a servo that already has (say) 140 degrees travel- like the HiTec HS-765 (140
degrees travel) or the HS-815 (160+ degrees) they will not work properly.
Servo Applications for the Soling One Meter
These are the widely-available brands of analog and digital servos, within the 150+ oz./in. torque range
most consider minimal for S1M sail plan (600 sq. in). The servos listed are also all approximately the
same physical size- so therefore largely interchangeable.
ApproxServo Size
2.30 X 1.10 X 2.00”
Sail Servo Type
Approx
Torque
Approx
Notes
Rotation –
total
degrees
145
Incl. single sail arm
Futaba S3801/3802
Analog Weight 3.77 oz.
Futaba S5050 Digital
4.48 oz.
HitecHS-705MG
Analog 4.72 oz
Hitec HS-715BB
Analog 3.59 oz
Hitec HS-725BB
Analog 3.90 oz
6V
156.0 oz/in
6V
264.0 oz/in
Adj.
6V
201.4 oz/in
90
6V
201.4 oz/in
90
6V
201.4 oz/in
3.5 turns
Hitec HS-755HB
Analog 3.88 oz
6V
183.3 oz/in
90
Hitec HS-765HB
Analog 3.88 oz
6V
183.3 oz/in
140
Hitec HS-785HB
Analog 3.88 oz
Hitec HS-5745MG
Digital 5.68 oz
Hitec HS-5735MG
Digital 5.10 oz
Hobbico CS-73 Analog
3.88 oz.
Hitec HS-5755MG6V
Digital 6.08 oz
6V
HiTec 815 BB
Analog 5.36 oz.
6V
183.3 oz/in
90
Karbonite gears
6V
249.9 oz/in
Adj
Metal gears
6V
254.0 oz/ in
Adj
Metal gears
6V
182.0 oz/in
90
347 oz/in
Adj
343.0 oz/in
160
500 oz./in.
Adj.
500 oz./ in.
Adj.
Savox SA1230SG
Digital 2.8 oz.
HiTec HS7980TH
Digital 2.7 oz.(!)
Incl. winch pulley; for
trolley sail control system
Widely used.
Metal-impregnated nylon
(Karbonite) gears, ball
bearings, Incl. single sail
arm. 140 degree rotation.
Will NOT work with a
Servo Stretcher.
Rudder Servos All approx. 1.57 L ×0.78 W×1.43 in H
Hitec HS-311 Analog
Hitec HS-322/ 322HD Analog
Hitec HS-325 Analog
Futaba S- 3003 Analog
1.52 oz
1.52 oz
1.52 oz
1.30 oz.
51.4 oz./in nylon gears
51.4 oz./in.
51.4 oz./in. ball brng Karbonite gears
56.8 oz. nylon gears
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