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operations, but with few exceptions, the explanations given here also apply to flying boats. 

A number of amphibious hull seaplanes have their engines mounted above the fuselage. These seaplanes have unique handling characteristics both on the water and in the air. Because the thrust line is well above the center of drag, these airplanes tend to nose down when power is applied and nose up as power is reduced. This response is the opposite of what pilots have come to expect in most other airplanes, and can lead to unex- pected pitch changes and dangerous situations if the pilot is not thoroughly familiar with these characteristics. Pilots transitioning to a seaplane with this configuration should have additional training. 

Many of the terms that describe seaplane hulls and floats come directly from the nomenclature of boats and ships. Some of these terms may already be familiar, but they have specific meanings when applied to seaplanes. Figures 2-2 and 2-3 describe basic terms, and the glossary at the end of this book defines additional terms. 

Other nautical terms are commonly used when operat- ing seaplanes, such as port and starboard for left and right, windward and leeward for the upwind and down- wind sides of objects, and bow and stern for the front and rear ends of objects. 

Research and experience have improved float and hull designs over the years. Construction and materials have changed, always favoring strength and light weight. Floats and hulls are carefully designed to optimize hydrodynamic and aerodynamic performance. 

Floats usually have bottoms, sides, and tops. A strong k eel runs the length of the float along the center of the bottom. Besides supporting the seaplane on land, the keel serves the same purpose as the keel of a boat when the seaplane is in the water. It guides the float in a straight line through the water and resists sideways motion. A short, strong extension of the keel directly behind the step is called the skeg . The chine is the seam where the sides of the float are joined to the bottom. The chine helps guide water out and away from the float, reducing spray and helping with hydrodynamic lift. Hydrodynamic forces are those that result from motion in fluids. 

On the front portion of the float, midway between the keel and chine, are the two sister keelsons . These lon- gitudinal members add strength to the structure and function as additional keels. The top of the float forms a deck that provides access for entering and leaving the cabin. Bilge pump openings, hand hole covers, and cleats for mooring the seaplane are typically located along the deck. The front of each float has a rubber bumper to cushion minor impacts with docks, etc. Many floats also have spray rails along the inboard forward portions of the chines. Since water spray is sur- prisingly destructive to propellers, especially at high r.p.m., these metal flanges are designed to reduce the amount of spray hitting the propeller. 

Floats are rated according to the amount of weight they can support, which is based on the weight of the actual volume of fresh water they displace. Fresh water is the standard because sea water is about 3 percent denser than fresh water and can therefore support more weight. If a particular float design displaces 2,500 pounds of fresh water when the float is pushed under the surface, the float can nominally support 2,500 

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