The smart buyer- Essential elements of powerboat design

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When you’re buying a boat, it goes without saying that you want to find one that best suits your needs. To fulfill that mission, you’ll need to know the characteristics that count.

So what are the elements of effective powerboat design?

I asked four well-respected designers to answer that question: Donald Blount, founder of Donald L. Blount and Associates (www.dlba-inc.com); John Canada, president of Ocean5 Naval Architects in Stuart, Fla. (www.ocean5.com); Bill Prince, president of Bill Prince Yacht Design in Port Washington, Wis. (www.billprinceyachtdesign.com); and Christopher Gratz, Pursuit Boats’ design and engineering manager (www.pursuitboats.com).

Prince’s first piece of advice: Avoid getting sidetracked by marketing fluff. “The Quadra-this, the FasTrip-whatever and the Delta-that — these silly monikers are largely superfluous marketing gimmicks,” he says. “A fundamental understanding of the role of a hull’s contours and their relation to a boat’s center of gravity opens the door to understanding which hull forms will perform well and which won’t.”

Planing hulls should be shaped to generate maximum lift and minimum drag, Prince says. “This lift is the only thing that keeps a planing hull up.”

An effective design balances the needs of the boat owner with safety, speed, comfort and efficiency. “For instance, a center console fishing boat needs to be stable at low speed and capable of handling rough seas at high speed,” Prince says. “To do this, it needs broad chine flats and a reasonably deep-vee, for starters.”

Most trailerable planing boats have a prismatic vee bottom aft, chine flats and strakes, Canada says. “Transom deadrise runs typically from 12 to 24 degrees, and the bottom may also include pads, steps, pockets or other appendages,” he says.

Although a higher deadrise angle will result in “lower impact pressures” and less slamming offshore, it also can make the boat tender when boarding or taking wakes or waves abeam, Canada says.

You’ll find warped bottoms (variable deadrise) on slower boats, such as semiplaning designs, while the “typical modified- and deep-vee are designs for higher-speed boats, like center consoles,” he says. Bay and flats boats aim for high speed and shallow draft, and power catamarans offer increased stability but lose storage below.

In addition to deadrise, the boat’s weight placement and power loom large, dictating the pressure distribution on the hull bottom and ultimately how the hull performs, Gratz says. “An optimal design will transition onto plane easily, operate at comfortable trim angles, and handle predictably and comfortably, all the while efficiently performing over the intended operating range,” he says.

An effective powerboat design will perform well in all loading conditions. Trim tabs should be unnecessary for planing, and the boat should come up with minimal bow rise, Canada says. “It is common for boats to have bow rise around 5 to 6 degrees, but visibility should remain, and the bow should come down quickly once on plane,” Canada says.

Blount agrees. Plus, the boat should plane relatively quickly, he says. “When you push the throttle forward, it should accelerate from dead in the water to planing in 20 seconds or less,” he says.

Once on plane, the boat should bank into high-speed turns, Blount says. “If it turns flat, the boat tends to want to throw you overboard,” he says. “A dynamically unstable boat will heel over so much that the gunwale touches the water, which is unsafe.”

The design also should deliver a dry, fairly quiet ride, he adds. “Some sportfishing boats can be quite noisy, with decibel levels of 85 to 90 or more at the helm station,” he says. “This fatiguing acoustic sound over time is going to make your pilot quite unhappy. You don’t want to wear out your ears out there.”

Canada urges boat buyers to examine the waterline, particularly the through-hull fittings near the waterline or the edge of the bottom paint. “A good design can be kept at the dock with 10 percent or 100 percent fuel and still sit at a good trim angle, allowing water to drain overboard,” Canada says.

On inboard boats, the center of the exhaust at the transom should be aligned with the waterline and the generator exhaust should be near the water but not submerged, unless it’s designed as an underwater exhaust, Canada says.

The experts agree that buyers should consider a sea trial mandatory. The top-end cruise speed should see the engines at about 80 percent of maximum rpm, and the boat should feel light and turn and handle well, Canada says. “The chines should not dry out at high speed [chine walking], and the ride should hold steady and smooth with no porpoising, which is up and down bow movement at constant speed without wave interaction,” he says.

Canada’s bottom line: An effective design will make the skipper’s job easier. And that’s how you’ll know whether the boat is right for you.

8 qualities that matter

• The design should maximize safety, comfort, speed and efficiency.

• The boat should handle predictably without chine walking or porpoising.

• The ride should be dry, with no pounding.

• The boat should sit at a good trim angle at the dock, allowing water to drain.

• Planing hulls must be shaped to create maximum lift and minimal drag.

• The boat should plane in less than 20 seconds without trim tabs and maintain optimal trim levels at all speeds.

• Engine noise should be low enough for conversation at the helm without shouting.

• Engines should run at about 80 percent of maximum rpm at the top-end cruising speed.

June 2013 issue