Longer answer: It depends upon the coefficient of friction of the surface and the vibration / acceleration environment. If the surface is glassy smooth, then the 20kg object will tend to slide down the surface at the slightest provocation.
Your question is more mechanical than electromagnetic. Magnets are often rated in pounds/kg of force, so the residual question depends on details you've not provided.
Also, if the surface is painted (I'm reading into your "Marine/Ocean"), then the wall will probably be coated in 147 layers of paint, and thus the magnet wouldn't have a chance.
All that said, large permanent magnet mounts are used to attach huge and complicated whip antennas to cars. The wind obviously applies sideways shear force just as with your application. With enough magnets, things can be made to stick.
Thanks for prompt reply.
I put 20 kilos but that is not fixed weight (probably it would be around 10 kg). I wonder how big and strong electromagnet should be? Surface is on ship so there is paint on it, though there is no 147 layers and thickness, I think is not so great to make some trouble for magnet.
Presume there is no wind and sea influence. So, what size is electromagnet to hold 10 or 20 kg on steel surface?
For example - If the object has a base diameter of 2 cm, and the 10 or 20 kg mass is concentrated on an arm at a distance of 1.5 m from the base, then there's no magnet in the world that would overcome that sort of mechanical leverage.
One can imagine other arrangements where a 20 kg mass could hung on the wall using a magnet with less than 20 kg pull.
By the time you sketch the mechanical diagram with all the required info, you'll be able to answer the question yourself.
Watch out for the known-false assumptions you've made.
By the way, to be clear, magnets "size" is often rated in force. Are you asking for dimensions and current?
It is box shaped object with base 50cm x 40cm. 60% of base can be used by magnet.
Dimensions and current is what I need.
Basically, I need to know if it possible to put electromagnet in that box to hold it on vertical steel wall.
Have you seen the switchable (permanent) magnets? With the handle in one position, they're a fairly strong magnet. With the handle in the other position, they're not.
