On the ROV hydrodynamics and static equilibrium


#1

Hi everybody

I am Boris Badurina from Croatia

I work in underwater construction and designing underwater tools, specialised equipment and machines

I agree with Ion discussion on rov hydrodynamic, but there is a little more to it.

rov need to be in hydrodynamic equilibrium but also in static equilibrium, or at least as near as possible.

proposed solution is on good way but drag caused by batteries is not included.

batteries will lover the center of head hydrodynamic resistance, which will cause torque moment because center of hydrodynamic resistance and center of thrust do not lay in same line.

this will induce forward pitch and it will increase with increased speed.

another thing is that in proposed solution center of mass is lower than center of propulsion which will also produce forward pitch, and it will increase with increased acceleration.

same consideration need to be checked for backward motion as well as up and down

this may or may not be important to someone.

Static equilibrium of the rov in this case is more important because it will have much bigger impact on rov behaviour. centers of gravity of motors and batteries are all on the back and center of buoyancy is forward which will result in wrong position of the rov.

this can be adjusted by pushing batteries forward.

it can be considered as simple balance (but underwater)


both issues have to be considered simultaneously and some compromises adopted in order to reach best possible solution.

I hope that Ion do not mind that I used his pictures.

I hope this will be useful to someone.

Boris


#2

Hi Boris ¡¡

Glad to read your comments and having an "hydro partner" on here.

OF COURSE YOU CAN USE MY PICS AS MUCH AS YOU WANT.

*** Your edxlanation HAS TO BE usefull for everybody with any interest in anykind of aquatic vehicles design *****

Some points:

Of course, preliminary Statics numbers have been made before any further analisys.

I didn't want to be too "technical", becouse I wanted everybody, with ot without ships building knowledge, to be able for understanding the explanation. That's why it looks simple to you.

Of course, I included the batts packs in the CFD analisys, and that's why leading and trailing cones(Spinners) have been fitted . The pic at my first post, is only a midship plane cut view, but the CFD analisys is a full 3D one.

I agree with all your points, but if building phylosophy has to be kept, approaching a real sub shape is quite complex.

I've tried fitting both battery tubes at the thrust plane, but it involves building problems that cannot be easily solved by means of flat plates and laser cutting while re-using as many parts from the original ROV as possible.

At the same time, that low CG weight, working as ballast, helps for preventing the ROV from rolling(Thats quite a little machine). Just by a slight angle change in the upper tail horizontal stabilizer compensates for the pitch due to batts dragg.

Anyway, in the building plans Im working on now, I've moved tubes up as much as possible and changed their possition from the outside to the inside of the hull (less dragg, better flow, simpler building).

About static equilibrium:

Once again, you're right. I didn't mind too much on it, becouse battery tubes can be easily slid ahead or astern, providing a simple way for static balance.

Actual problem:

The most critical point from my point of view, at present new desing stage, is providing a good dynamic balance when in vertical motion.

Best regards and thanks for your feed-back.

PD: Im also into professional ship building/management industry. Hope to keep in touch with you.