servo torque

[bwells]

I need a new steering servo but was wondering what torque would be appropriate for a boat. I've seen them from16 oz/in to 65 oz/in. Any help would be appreciated, Thanks

[Diegoboy]

How big is the boat and how fast do you expect it to run? Lipos or NImh?

[bwells]

32 inches with a OSE 700 sc on NiMH 2 8.4 in series top speed 30 MPH. It's the yellow one in my avatar. Good question as speed seems like the determinating (sp) factor.

[Diegoboy]

The HS-311 should be more than adequate!

Control System: +Pulse Width Control 1500usec Neutral
Required Pulse: 3-5 Volt Peak to Peak Square Wave
Operating Voltage: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C
Operating Speed (4.8V): 0.19sec/60° at no load
Operating Speed (6.0V): 0.15sec/60° at no load
Stall Torque (4.8V): 42 oz/in (3.0 kg/cm)
Stall Torque (6.0V): 49 oz/in (4.5 kg/cm)
Current Drain (4.8V): 7.4mA/idle, 160mA no load operating
Current Drain (6.0V): 7.7mA/idle, 180mA no load operating
Dead Band Width: 5usec
Operating Angle: 45° one side pulse traveling 450usec
Direction: Multi-directional
Motor Type: Cored Metal Brush
Potentiometer Drive: 4 Slider/Direct Drive
Bearing Type: Top Resin Bushing
Gear Type: Nylon
360 Modifiable: Yes
Connector Wire Length: 11.81" (300mm)
Weight: 1.52oz (43g)

[bwells]

WOW, that is a lot of info but what controls the voltage going to the servo and do I have any control over that? Doesn't the receiver take care of the voltage going to the servo? I take it that servos are a stall motor but don't understand how it could go from zero to full steer in steps, a rheostat maybe and what brings it back to zero, a spring?

[Diegoboy]

Yes the voltage is controlled by the ESC which provides power to the servo from the main battery. (unless it's an Opto ESC)
The servo is controlled by a series of pulses through the signal wire, where the length of the pulse indicates the position to take.

[bwells]

The receiver or the ESC controls the servo? okay, I'm liking the pulse thing as how the servo works and this gives me something else to ponder as I have a 1952 hard drive. Thanks

[bwells]

I'm going to make the assumption that the speed requires more torque to turn so I think a good rule of thumb would be speed plus 50% for size of servo. I.E. 30 mph plus 15 =45 in/oz and for you fast dudes that do 70, I would say 70+35= 105 in/oz servo. Just a guess but can anyone come up with a formula or an idea on how to rate servos or am I way off base here?

[Diegoboy]

So many numbers... too much math, my head hurts!

The speed does play a part, but to what degree to derive a formula, I don't know. My renegade's servo was fine for the speeds I was getting, then I put on a custom made V932/3 prop, and now the servo doesn't have the torque to turn her anymore (full throttle). I'd like to know the formula, but it would have to factor in the weight of the boat, depth of the rudder, width of the rudder, speed, hull type, etc...

[bwells]

I think speed is the key ingredient here as it will be harder to turn the rudder the faster you go. I would eliminate rudder size, hull shape and weight of boat in this formula and stick to the key factors. I love math and this is getting a little too heavy for me. OKAY I'll go with hull size as an issue and weight of boat too! You're right! What about my formula?

[jcald2000]

For any boat speed over 40 mph, I only use 100 oz + servos, you only have to loose 1 boat to prove this theory in!
It's worked for 14 years, so far. LOL

[Fluid]

I think speed is the key ingredient here as it will be harder to turn the rudder the faster you go.

Don't try to "over-engineer" this as there are too many important variables you have chosen to ignore. Rudder size is almost as important as speed, but the centrifugal force of the boat in a turn is just as vital. Too, where in relation to the front edge of the rudder is the pivot axis? How long is the steering arm on the rudder? You simply cannot ignore all the variables - that would be like assuming that a certain speed requires the same voltage regardless of the boat....

Go by experience, not some unproven, flawed math theory.



.

[Diegoboy]

For any boat speed over 40 mph, I only use 100 oz + servos, you only have to loose 1 boat to prove this theory in!
It's worked for 14 years, so far. LOL

You must only run big heavy boats then, because that's overkill to a boat like the Renegade.