An object is set into motion when it is pushed or pulled, i.e.
subjected to a force.
Force is measured in Newtons, which expresses the amount of energy being applied over a given distance and time. To pull the boat through the jelly required a certain amount of force to keep it moving. The greater the force applied, the greater the spring stretched within the spring scale.
Each boat would have had a different angle on it's bow to cut through the jelly. If the boat had an acute angled bow (i.e. 'sharp'), the boat would have neatly cut through the jelly and would have required less force to keep it moving. If the the boat was shaped like a barge, with a blunt front end, a larger force would need to have been applied to keep the boat moving at the same speed.
Ice breakers in polar regions are designed to slowly rise up onto ice, thereby allowing the weight of the boat and the shape of the bow (an inclined plane) to break the ice. This process isn't performed at great speed as the ship can still be damaged. Dense pack ice cannot be negotiated as the ice floes are too thick, however thin ice of a few metres thick can be broken. Commercial shipping lanes within the Arctic circle are regularly cleared by ice breakers.
Specifications on the Yamal, a nuclear powered Russian ice breaker.
Reference:
Kane, J. W. Sternheim, M. M. (1988). Physics. 3rd ed. John Wiley & Sons, New York.
