Last update, December 12, 2009
The RC radio manufacturers seem to change the servo model numbers every year. So, the concentration here will be in torque values and speed of the servo. 60 degree of throw is adequate. This will give you a clue of what you are looking for while cruising the servo market.
Included in the search for a servo is the interior quality of the unit. Metal gears and ball bearing top the list. Secondly, is a drop in footprint of the servo. There is a standard to this at 20x40mm and the drop depths are around 28mm.
Digital versus analog servos is not a big deal really. If you want the best they are worth the money. It is found the digital units are a bit smoother and more to the interior quality. The key contribution is that the servo will exert max power to small movements. Meaning, that when a selected position is reached it will go all in to hold it. It will also do this with four times the pulses per second. You can test this on a powered up analog servo by pushing on it with your finger. You are likely to note a bit of movement and a push back at you. The digital acts like it is locked till you over power the torque rating. This pro makes it the servo of choice for the rudder.
While they draw a bit more power (not that you will notice with battery capacities these days), they seem to operate cooler. This can be major in the humid oven of the bilge. They are more expensive and maybe the adage is valid.
Torque values are very important. It is not recommended to go below 100 and there is no need above 160 and then get the slowest one you can find in this range.
Serious competitors want a strong rudder servo downwind where the EC12 can become easily overpowered. When this happens the speed through the water can place pressures on a deflected rudder that will overpower the servo trying to steer against a tendency for the boat to bear off. When the servo gives up the rudder fairs with the passing water and the boat goes out of control. Not good if are leading the fleet or collide with another boat because control was lost.
Skill at the helm aside, more power was needed. We like a hefty rudder compared to the jib trim servo. And we are partial to Futaba servos. This is December of 2009 on the market that would be nice for the rudder:
S9350 Digital 139 torque, .12 seconds speed and 60 degree throw. $100.
Then there is a big jump:
S9351 Digital 192 torque, .13 seconds and 60 Degree throw. $110.
The latter is a new model and would be our next choice.
Interestingly enough, not a lot of power has been needed for this servo. As you will note in the sheetline schematic, the only thing holding the jib against the wind pressure is the servo and the extended arm. We have been using 100 torque servos for years without a line slip. So, save your money here.
We have not used it:
S3305 124 torque, .20 speed and 60 degree throw. $33.
S9405 100 torque, .11 speed and 60 degrees throw. $70.
The latter is our choice for what is in the case and on the board you will see.
Servos are competitive from big RC suppliers. We are partial to Servo City, as they specialize in the hardcore necessities and provide good documentation online. Their servo chart is well worth browsing.
Twitcher? Dont use them here anymore. They do not need to be powerful and a standard 44 torque servo will work.
When you have a servo that is doing the job for you and there have been no issues for at least two years, live with it till it dies. If you buy good servos from the start there is no need to change any time often. If you have a servo failure in a sailing season you introduced it, go elsewhere when you have made sure it was not your fault. However, it is important to note that you need to have a 3 amp wiring plan and batteries that are up to snuff and checked. It is the details and Vern has had it pounded into him.
Most of the electronics failures, other than unit heat generated issue, points to the skippers electrical design and process; serious. Batteries and wiring design and health are the major cause for system failures.
The bilge is not a pressure cooker. It is though a very hot and humid world.