ROV pressure under 100 mts


#1

Hello guys,

its me again, as said before I am trying to build an ROV for 100 meters

I want to build azimuth propellers, for that i am using a stepper motor with titanium Magnetic coupling, this is great for watertight but here I have a problem

if we go under 100 meters subsea, we have 11 atm of pressure if we go to formula:

P=pgh F=P*S

p=sea water 1030
g= gravity 9.8
h= 100 meters

P= 1,009,400 Pa

if F=P*S
S= my propeller lenght

F=1,009,400Pa * .2meters
F= 201,880 Newtons!!

so the sae is giving 201K newtons force to my building, how much Torque do i need on my stepper motor to move my azimuth propeller?? or am I doing anything wrong here??

please i need help


#2

I think you are looking at 2 very different things

Thrust - what you need to push something forward (simplistically) has nothing to do with the pressure rating of the vessel eg to push it forward at 1m depth takes (if you don’t consider tether drag) the same force as at 20 or 2000 meters its related to the hydrodynamics of the unit not the depth

The pressure rating or depth is related to the strength of the enclosure and is basically the crush depth of your dry sealed “stepper motor with titanium Magnetic coupling


#3

The hydrostatic pressure you get from depth is applied to both sides of the propeller so it cancels out.
A stepper is pretty inefficient for driving a prop. Look into using a servo or BLDC like the main OpenROV thrusters.


#4

Where did you find the second equation? I think you might be referring to the equation

Forwards Force = Pressure Drop * Area.

If we take a control volume around the propeller it would look like a wide, short, cylinder. The propeller creates a pressure drop across the control volume that acts along its area. Giving

Forwards Force = Pressure drop * pi * prop. radius^2

To answer you question, with any torque you can get some movement of the propeller. Are you asking what torque will get my vehicle to a certain steady state speed? If so you’ll need to do a hydrodynamic analysis of your vehicle as well as know more information about the performance of your propellers. I think this calculation will be more involved than you anticipated.