Sometimes three or four dredges are attached to a wheeled stiff axle. A number of these dredges can be towed from a heavy spreading bar. Usually there is one on each side of the ship. The length of the bar and the number of dredges towed is dependent on the power of the ship and the room on the side of the boat for working the dredges. There could be 3 on each side on a small 10 metre boat. There could be up to 20 on each side for a 30 metre ship with 1500 hp. The weight and strength of this can dig up the ground. It overturns rocks and can crush animals and plants in its path.