The effect of oil is greatest on free waves - that is, waves in deep water. The effect of oil is least, or at best uncertain, in surf breaking on a bar or beach. Nothing can prevent the larger waves from breaking under such circumstances. The most effective oils are the heaviest and thickest. Animal and vegetable oils, and waste oil from the engine, are the best.
Crude petroleum is better than nothing.
Kerosene is of little use. Very cold water will cause some oils to become too thick, which will reduce their wave-dampening characteristics. Oil is as useful for ships as it is for boats. To be effective, the oil must be spread to windward. The best method for dispersing the oil is to hang small canvas bags, pricked with a needle, over the windward side of the vessel.