While searching for a definition for the word "DEADRISE", I found the below detailed explanation, which I consider a perfect one. Hence, I just thought of sharing it with you. BTW, as per Aquasport site, the Osprey 170' has a deadrise of 12 degree.
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"Planing hull— Achieving high speeds on the water requires a hull that easily transitions into the planing mode. Characteristics of a planing hull include flat bottom surfaces from amidships aft (from the middle to the back of the bottom) and flat transom (back of the hull). The transom must meet the bottom at a sharp angle. At certain speeds that vary with each bottom, the boat is supported by the same dynamic forces that keep a water skier on top of the water. Put simply, it planes.
The sharp angle (or "break") between the transom and the bottom is critical to a planing hull. It allows water flowing beneath the boat to come free of the hull without climbing up the transom and where it would create speed-robbing suction. The smooth, rounded shape of a displacement hull does not allow the water to break free in this manner.
Absolutely flat bottoms, a sub-species of planing hulls can be extremely fast, but are not easy to control and tend to bull their way through waves like a pile driver. Giving the bow a pointed shape allows a hull to slice through waves instead of slam into them. For a boat to operate above displacement speed, the V-shape of the bow must "warp" into flatter planing surfaces near the stern.
Stand at the back of a planing hull boat and you will notice that the bottom angles upward from the keel. This angle is known as the "deadrise." Prior to the 1950s, designs had little or no deadrise to the bottom at the transom in order to achieve acceptable performance with the smaller engines of the era. As more power became available, designers were able to increase the deadrise angle to give boats a softer ride and better control.
Here are a few more terms you're likely to encounter in bottom design:
Round bottom— These are used mostly on vessels intended for displacement speeds. Round hulls provide a gentle ride with little fuel expenditure, but tend to roll from side to side in certain sea conditions.
Low deadrise bottom— Sometimes called shallow-V bottoms, these running surfaces have a deadrise angle of 20 to 25 degrees amidships which reduces to 5 degrees or less at the transom. (Deadrise is the angle measure of the "V" in the bottom. It is generally greater at the bow, or "entry" of the boat, and shallower at the transom, or stern of the boat.) This provides a smoother ride, but it still retains the quick ability to plane of a flat-bottomed boat.
Deep-V bottom— This type of hull has a deadrise of 25 degrees amidships with reduces only slightly to 20 degrees at the transom. This shape allows high-speed operation in rough water.
With few exceptions, planing hulls have chines and strakes. Chines are the angles on the hull where the vertical topsides meet the bottom. Older designs often had rounded or "soft" chines. Today's hulls almost universally have a sharp corner or a "hard" chine. Round-chine boats tend to roll a bit more, so they're not favorites with anglers who prefer the stiffer characteristics of a hard-chine boat. (A boat that resists rolling is said to be "stiff.")
Strakes are those little "ridges" you see below the chines. Their primary job is to provide lift, which is important in the planing process and overall running efficiency.
Now and Then
Hull shapes evolved slowly during the era of wooden boats because wood only bends and twists so much without splintering. Designers became free to explore their imaginations when fiberglass became the dominant material for boats. Not all of their ideas were good. Many early attempts at cathedral hulls (more on those at the end of this story) resulted in hard-riding, sluggish hulls that required inordinate amounts of fuel. Other designs like the deep-V proved their worth and remain popular. A couple of hull forms are particularly popular today
Ski boats— Hulls of competition water-ski boats are designed to minimize wakes at certain speeds so that slalom skiers can slice through them more easily. Ski-boat builders spent years creating running surfaces that produce the flattest, softest wake. Funny twist? Those same designers have revisited those designs in their wakeboard boats, where the goal is to provide big wakes at certain speeds.
Stepped hulls— One of the earliest attempts to increase boat speed involved putting small "steps" in the hull. Steps are, essentially, elevation breaks that create multiple running surfaces. As the boat speed increases, it rides on these surfaces, with area of contact with the water moving progressively further aft. The results? Less hull in contact with the water, less friction and more speed with less power than a conventional deep-V hull. The knock, at least against some stepped-bottom boats? Less wetted surface in the water, less friction and reduced lateral stability and control.
Beam, or the width of the hull at its widest point, is important in determining the boat's fuel efficiency and ride through choppy waters. The trend in recent years has been toward wider boats because increased beam (width) allows more room for interior accommodations. Wider boats also feel more stable, which makes them popular for dockside entertaining. In addition, they are less likely to roll from side to side while underway.
These benefits come at a price. A wider a boat is more likely to pound (slam into waves) as it moves through choppy water. Plus, it can take more fuel to move a wide boat at a given speed than a narrow hull of similar length.
Narrow deep-V hulls, on the other hand, tend to slice through waves, and because they have less wetted surface and less friction, they can be more fuel efficient than wide-body boats of the same length. On the downside, they tend to roll from side to side more while underway and offer less interior space.
Most boat builders are aware of the relationship of beam to length when it comes to performance. High-speed powerboats tend to have narrow hulls with relatively large deadrise angles that often approach a traditional deep-V design.
Cruising boats are much wider. Today it's not unusual to see the hulls of a cruiser with a beam approaching one-third of the boat's overall lengths. Modern, high-horsepower engines allow these wider boats to move at speeds faster than the so-called "speedboats" of a generation ago. The penalty, of course, is more fuel consumption.
Compromise a Constant
While this series isn't devoted to fishing boats, offshore anglers face a dilemma that illustrates why all boat purchases require accepting compromises between speed, comfort and function.
Offshore fishing often requires long runs through rough seas. For that job, a relatively narrow, high deadrise hull is best. But, once on at fishing grounds, a stable platform with a large cockpit for both anglers and their gear is a must. While drift fishing, a wide boat with hard chines and a relatively flat deadrise is desirable. Obviously, one hull can't meet both demands. Fishing skippers have to choose a mixture of deadrise, beam, length and hull shape that is best for their needs.
With cruising boats there are fewer conflicting demands on the hull. Maximum interior accommodation is the overriding consideration. Most of today's cabin boats have wide beams to allow for extensive galleys, large beds and comfortable saloons. A rougher ride is less of a consideration because family cruising is primarily done during fair weather and gentle sea conditions.
Until powerboats were mass-marketed, hull shapes reflected the types of conditions faced on waters surrounding individual boat builder's shops. Boats for shallow water areas where choppy conditions predominate tended to be narrow, with fine entries (narrow bows) and small planing surfaces aft. Contrast that with the wide-beamed, relatively flat-bottomed boats built in areas where crossing sandbars with following sea (running in the same directions as the prevailing swells) was the major problem."
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