In this chapter the technology relevant to pre-industrial ships and shipping is described. This is concerned with the force and power required to propel hulls through the water, whether propelled by sails or by rowing. Resistance to motion is caused by friction and by the waves that are necessarily generated. Both these increase as the speed increases but the wave making resistance increases particularly quickly with the result that, for a given displacement and propulsion, long ships are faster than short ships. On the other hand, the longitudinal bending moment exerted by the water on a hull is proportional to the product of displacement and length. For a given strength, a long ship must have less displacement than a short one and is less able to carry cargo. Merchant ships, which must carry a heavy cargo, tend to be short and slow, while warships, which need to be fast, tend to be long and light. Pre-industrial ships were usually of wood and this restricted the length, speed, and displacement that could be attained. Some attention, therefore, is given to the strength and deflection of hulls.
In the pre-industrial age, the two principal means of ship propulsion were by muscle power (rowing) or by wind power (sail). Rowing had the advantage that the resultant force acted in the direction of the keel, and the boat, unless there were crosswinds or crosscurrents, travelled in the intended direction without any leeway. It was possible to travel into the wind or into a current so long as they were not too strong. The disadvantage of rowing was that for large ships, it required many rowers and it was, therefore, too expensive for most cargo boats but was commonly used for warships, particularly in the safer waters of the Mediterranean. Here, and along coastal waters and rivers, rowing was always important and thus an analysis of the mechanics of rowing is included. Usually rowing efficiency was quite high but Aristotle, as mentioned in the previous chapter, thought