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May 03, 2012

The Perfect Boat Hull

Decoding the mysteries of hull shapes in fishing boats

Boats, like cars, often feature a shape that defines them, that makes their purpose expressly obvious. The shapes of a Maserati car and a Yellowfin boat, for instance, say — what else? — speed.

But hull shapes also carry subtler messages that help boat buyers home in on just the right choice for their fishing style. The width of the chines, the deadrise angle, and the presence of a pad and steps — among other facets — change the comfort and speed of the craft.

What’s in a Shape?
“The general philosophy for any planing boat (the category for most trailerable fishing boats) is that the shape of the hull should match its weight,” says Darron Roop, a naval architect and yacht designer based in Virginia Beach, Virginia (­shorebreakmarine.com). “You want the hull to ride at the right attitude so it handles and tracks better. The second aspect is: Where’s your impact zone? Where’s the boat hitting the water?”

Factors besides hull shape obviously enter the above equations, and Roop is quick to point out that no single design element determines the overall package. In addition, different boats are made for different purposes and different buyers in different locations. But — in general — when tinkering with the running surface of a monohull boat, Roop and others often play with the deadrise angle.

In the fishing world, monohulls comprise flat-bottom skiffs, moderate-V and deep-V boats. Multihull boats have also gained a foothold among anglers because of their deck space and stability at rest. In fact, some designers say power-catamaran hulls are underutilized as fishing boats. The most common error made in building cat hulls, they say, is making the sponsons too wide.

Flat hulls might be easiest to build and have more initial stability per weight, according to the book The Nature of Boats by New York naval architect Dave Gerr (www.gerrmarine​.com), but they also generally flex more and handle waves rather harshly.

As designers increase the angle of the hull to form a V, they see the vessel’s rough-water ride improve. A standard moderate-V hull carries a deadrise angle of 15 to 20 degrees at the transom. Deep-V’s generally start at 21 degrees and go up to about 26.
“At high speed in rough water, deep-V’s pound less than most, and are more stable,” Gerr says. “But as you slow down, they start to flop back and forth more; they’re not as comfortable at slow speed or at anchor.”

A final category is the variable-deadrise hull, wherein the lowest or deepest section of the hull forms a steep angle, and each subsequent section between the longitudinal strakes (the ridges that run fore and aft) angles less and less. The theory is that such a graduated deadrise puts the deepest angle at the keel so that it lessens impact. Flattening the deadrise as it approaches the chines (where the hull bottom meets the sides) should then offer some of the stability benefits of a flatter hull.

Tricks of the Trade
With some moderate- and deep-V vessels, the deadrise remains more or less constant from amidships to the transom, though it narrows at the bow. But in many cases, designers create what’s called a modified-V or “warped plane” hull — wherein the deadrise decreases from a high of perhaps 22 degrees amidships to 15 degrees at the transom. A flatter transom allows the boat to travel faster than a deeper hull with the same horsepower. Stated differently: The more deadrise you have, the more power you need to get the same speed.

Lou Codega, a Virginia-based naval architect (www.loucodegana​.com) who has designed vessels of all sizes — including current Regulator hulls — used a modified deep-V for those popular fishing boats. At the transom, Regulators measure 24 degrees; the deadrise increases almost all the way forward to about 48 or 49 degrees.

“Where you really feel the most impact is at the place where the water hits the boat. That can be anywhere from the bow to 50 percent or 70 percent back toward the stern. That’s the deadrise that the water sees,” he says. “And that’s really where the magic happens. There’s no formula or textbook that says, ‘At this specific point on the hull, you should have 36 degrees deadrise.’ The difference between a good- and bad-riding boat is how you’ve gone from whatever number you have at the transom to the number at the bow.”