turning. High sided and fine-ended boats, such as dories, are affected by wind. Their trim can be altered by using a plastic container of water attached to a rope that can be moved to the bow or stern as need be. Long-distance rowers can keep up a steady 20 strokes per minute compared to a racing shell which can be rowed at 30–40 strokes per minute or more by fit athletes (depending on distance and racing circumstances). A rower can maintain high stroke-rates per minute for only a brief period. Longer, narrower rowing boats can reach but most rowing boats of can be rowed at . Many old rowing boats have very full ends (blunt ends); these may appear at first glance to be bad design as it looks slow, not fast. However a full-ended rowing boat will rise to a sea and not dig in as a finer hulled boat might do, thus a compromise needs to be made between the factors of speed and of seaworthiness. This style of rowing boat was designed to carry a bigger load and the full sections gave far more displacement. Also older boats were often very heavily constructed compared to their modern counterpart, hence weighed far more. A rowing boat designed as a tender carrying occupants to a boat on a mooring might tend to be short, whilst a boat for use on rivers and to travel long distances might be long and narrow. Oars Over time the design, of both the oars and the blades, has significantly changed. Typically, the part of the oars that are inboard of the rowlock have stayed the same length but the outboard part has gotten shorter. The different lengths of the oars affect both the energy that the rower has to put in as well as the performance, in terms of speed of the rowing boat. A short oar makes quick but short strokes possible. A short oar is easier to use in a narrow creek or a crowded anchorage. This is important in a small tender which may be heavily laden with passengers, limiting the swing of the oars. A short, quick stroke prevents the bow being driven under in choppy waters while heavily laden. Longer oars can be used to produce longer, slower strokes, which are easier to maintain over long distances. Designers may match oar length to the amount of space provided