Hi,
to make a long story short, I’m doing a project where I need to control and adjust the angle of an axel, much like controlling a rudder on a boat. The drive unit that will perform this will be a linear actuator that I will design myself due to functions that normal linear actuators on the market don’t have. To design this I need to find a suitable motor and gear, and that is where I need some help from the internet 
the controlling of the linear actuator will be from a PID-regulator, and the way of adjusting the actuator to keep the error between the Setpoint and Input will mostly be by short, small adjustments at around 2-4 seconds each time, in both directions. One important aspect of this is; when the Dc motor is shut down, I need the actuator of mine to stay at the position where the motor left it, i.e, it should be some what self sustained.
I don’t have much practical experience of small DC motors and gears and such, I have been scanning the internet for information, but its kind of a jungle out there when it comes to motors in this kind of applications 
So, What is the best option of 12Vdc motor- and gearbox choice to get as much torque as possible, I can deliver max 30A continues, and I need about 20rpm on the output shaft from the gear.
After searching around for days I have found a few interesting gadgets;
Motor: this motor kind of has the output effect for it and with this Gear that reduces the speed to around 20rpm. this would give me an MAX torque of about** 220 Nm**.(126oz-inch*256:1 = 32 000 oz-inch) That is okey, I kind of want more though 
. It would do the job for me with this numbers, maybe not at a perfect way, but it will manage.
Is there other options out there that is better for this stuff? I mean is there any motor typ out there that suits better for this kind of job where the torque is all that matters? I have been reading that “pancake motors” should be something else, but I have not found any that is applicable to this. but tips on where to find a 30A pancake motor with gear is more then welcome, ANY tips and guidance is much welcome!
Due to the ratio of the gearbox, will it be somewhat self sustained? if so, roughly, how much force do I need to apply to make it turn?
As a side note,
I found this after further investigation in the internet, Raymarine Drive unit. The typ 2, is capable of an MAX torque of >1000Nm!!!. they say that it consumes between 40-70W, but as long as they have not invented a way to bend our physical laws that cant be accurate(they are most likely averaging the power consumption over time, and the drive unit is also at standstill between the corrections and thus it will lower the consumption readings over time). But I know for a fact that the controller of that drive unit is capable of continues deliver 30A, and that the drive unit is fixed 250mm out from the rudder axel.
the travelling speed between mine and Raymarine is almost the same. So one would think our numbers should be pretty close to one another, but it really is not. Raymarine is around 750Nm if the 250mm leverage is taken i to the consideration? and mine are at** 220Nm**. It makes my brain hurt a little bit this:)
Thanks in advance,
Best regards//