The object is a float used for minesweeping operations, and the vessel is likely a minesweeper. (Note the wooden deck, to avoid the magnetic signature from a steel hull which triggers magnetic mines.
The minesweeper would stream a cable to one or both sides with floats like this to support the outer end, and angle it off to one (or both) side(s). The cable would be weighted to sink to a certain depth (something like 10-20 feet below the surface). The idea was to have the cable strike the mooring cables of moored or floating type mines. The sweep cable would have mechanical or cartridge actuated cable cutters every so far, and as it dragged against the mooring cable a cutter would cut the mooring cable and the mine would float to the surface where it could be destroyed by gunfire to set it off.
Note that the the object has something that looks like a basket at the nose, to which the towing cable was attached.
This cable sweeping technique was only effective against moored type mines, widely used during WW1. Other techniques were used against later acoustic, magnetic or pressure triggered mines.
Of course, any ship can be used as a minesweeper. Once.
European navies are highly respected for their minesweeping skills, much of it gained from cleaning up the vast WW1 minefields laid by many nations. Today the U.S. Navy has virtually no minesweeping capability left.