Short versus Long Hulls

 από http://www.pocock.com

Boat Design – Short versus Long Hulls

One of the great debates of rowing is hull length. Even though it’s currently not quite the heated argument that it has been in the past, we found this article in our archives and thought it was worth a read. Originally written by Pocock owner Bill Tytus, this article is his answer to the surprisingly complicated question: “Does the length of a racing shell make a difference in speed?”
The short answer to this question is no. In order to understand why, you must first understand some basic hydrodynamics. Hydrodynamics is the study of things moving through water. It is not an intuitive science; it’s an empirical science based on painstaking measurement.
The research and study of hydrodynamics takes time and instrumentation, which is extremely expensive. The vast majority of what is known today has been developed by the navies of the world. In addition, America’s Cup sailing syndicates have also spent many millions of dollars on hydrodynamic research. Even after spending all this money, there has been huge inconsistencies in the performance of boats. This is an indication of how much there is yet to learn about how boats move through the water.
In addition to the expense of hydrodynamic research, racing shells have few dimensional similarities to other boats. So, although we might know something about aircraft carriers, destroyers, and submarines, this knowledge is not always applicable. This doesn’t mean that we can’t make meaningful hydrodynamic inferences for racing shells. It just means that we must be very careful with our hydrodynamic assumptions.
In regards to racing shells, we must be primarily concerned with the aspect of drag – which is what slows the boat down. The two most significant kinds of hydrodynamic drag are friction and wave drag. Friction drag is the wet area of the boat dragging against the molecules of water. Wave drag is the resistance due to waves being made where the air and water meet. For example, submarines do not experience wave drag when they are fully submerged.
It is widely believed that 80 to 90 percent of racing shells’ hydrodynamic drag is simple skin friction, and that wave drag is the major part of the remaining 10 to 20 percent. Certainly it makes sense to first try to reduce friction drag since it is such a large part of the total drag. At first glance, it would seem that a shorter boat might have less surface area in the water. Since friction drag is related to surface area, we would hope that the shorter boat would have less drag.
However, friction drag is not just a function of surface area only. It is actually a function of speed, surface area and a frictional co-efficient. Clearly, anything we can do to reduce surface area will reduce drag. Unfortunately, reducing the length of the boat also increases the co-efficient of the drag. What this means is that if we shorten the boat, even though the surface area is less, the drag per unit of area is greater.
Though wave drag amounts to a smaller proportion of total drag, it must not be ignored. Wave drag is a function of speed over length. It is easily understood that when speed increases, the size of the boat waves grow, which increases drag. Perhaps less obvious is that decreasing the length of the boat also makes the waves bigger. As crews push these boats faster and faster, the wave drag not only increases, but becomes a bigger factor in total drag. This indicates that al things being equal, longer boats might be faster.
Over the years, boat length has changed cyclically, just like fashion. History has never proven that shorter boats are more effective.