The Stepped hull equation

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Increased speed and efficiency are helping fuel the popularity of stepped-bottom boats

The next time you’re at a boat show, you might be surprised by the number of boats with steps built into their bottoms. A growing number of builders have turned to the stepped-hull design to offer boats that are faster and more fuel-efficient.

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And these are not go-fasts. They’re boats from such familiar names as Intrepid, Cutwater, Regal, Formula, Beneteau, Scout and Yellowfin. All use steps in one form or another. You’ll find them on the bottom of sportboats, bowriders, express cruisers, convertibles and center consoles.

“The stepped hull has really exploded with these big center consoles because they have gone from 30- to 40-knot boats to 50- to 70-knot boats, so they really have encroached into the performance-boat category,” says John Cosker, owner and founder of Mystic Powerboats in DeLand, Fla., and the designer of stepped-hull boats, including the newer center consoles from Contender. “You can buy a triple- or quad-outboard step-bottom center console that can carry 10 or 12 people and go 70 mph.”

Stepped hulls have been around for more than 100 years, says Michael Peters, the owner of Michael Peters Yacht Design in Sarasota, Fla. In fact, Peters has a model of the first stepped-hull raceboat — a 1910 John Thornycroft 26-footer with a single step.

“For a long time nobody paid attention to [stepped hulls], and then about 15 years ago they started to become popular, and there’s been a steady climb since then,” says Peters, known as one of the early pioneers of stepped hulls in recreational boats. “What we are seeing is outboard center consoles that never had steps now have them.”

Contender and SeaVee both recently added stepped hulls to their fleet. In fact, Contender since 2009 has introduced five stepped-hull boats — a 25, 30, 32, 35 and 39 — and a sixth one under 25 feet is on its way. The company still offers conventional deep-vee monohulls, but the stepped hulls are outselling the non-stepped boats by a wide margin, says Contender marketing director Les Stewart Jr.

SeaVee recently added three stepped models to its lineup. The Z-390 pictured here is the flagship.

The recession prompted Contender to venture into the design. “In order to stay relevant and fresh and on people’s minds, we wanted to come out with a boat that people couldn’t ignore,” says Stewart, whose father, Les Stewart Sr., is a partner in the company. “There were indicators the stepped hull was becoming more popular, but we wanted to take our time with it and produce something we knew was going to be a hit and keep us on the map.”

In 2009 Contender sank some money into the development of three stepped hulls “at a time when money was tight and fuel efficiency was of paramount importance,” Stewart says. “And it has worked. It has kept us relevant.”

Stepped hulls bring a lot to the table, including increased speed, a fuel efficiency improvement of about 15 percent, quicker planing and better sightlines because of a flatter running angle. The Contender 35 ST (stepped hull) tops out at 66 mph, compared with 59 mph for the Contender 35 T (tournament), a stepless deep-vee. At 43 mph, the 35 ST gets 1.6 mpg and the 35 T 1.3 mpg. Consuming 90 percent of the 400 gallons in its fuel tank — 360 gallons — the stepped-hull boat can travel 576 miles, compared with 468 miles for the conventional boat. That’s quite a difference in range.

The fuel savings is even more attractive to tournament anglers in Florida, who often buy non-ethanol fuel that can go for more than $6 a gallon, Stewart says. “These guys don’t want to end up spending more on fuel than they can win in prize earnings.”

The steps reduce a hull's wetted surface and, therefore, drag.

The stepped hull, of course, is not a good fit for all boats and boating. The fuel economy benefits, for instance, aren’t realized at speeds less than 35 mph. Other variables factor into the equation, too, says Boston Whaler senior naval architect Bobby Garza.

“I can‘t deny that a well-designed stepped hull can yield higher top speeds with equal horsepower, potentially reduce power requirements, and the result usually includes an increase in fuel economy,” Garza says. “But this usually occurs at faster-than-normal cruising speeds at a very specific weight, longitudinal center of gravity and running trim angle that is determined by the placement of the steps.” Boston Whaler’s lineup does not include any hulls with steps.

Boston Whaler must satisfy a broad customer base, one that believes “our boats are fast enough,” Garza says, adding that other drawbacks such as sensitivity to weight distribution have kept the builder away from steps.

“Young people buy our boats, and older people buy our boats,” Garza says. “Some people want to go faster. Some want to just cruise around with the family. Some hardcore-fish our boats, and some just use them as dayboats for entertaining and water sports. For these reasons, we have to give them a hull that will appeal to the masses and satisfy the majority.”

One characteristic of stepped hulls that certainly would appeal to many boaters is the ability to plane quickly with little bow rise, says John Livingston, president of Fluid Motion, the builder of Ranger Tugs and Cutwater Boats in Kent, Wash. Cutwater’s 26, 28 and 30 pilothouse boats ride stepped hulls. “You don’t lose sight of the horizon,” says Livingston, who before establishing Cutwater designed stepped boats for Regal. “When you take off in a conventional boat, you build up a bow wave in front of you. The nose comes up as you climb the bow wave. With a stepped boat, you eliminate that rise and get up flatter.”

The stepped hull maintains its flatter angle at all speeds, he says. In fact, the stepped hull’s angle of attack helps it run through rough seas, says Ken Clinton, president of Intrepid Powerboats in Largo, Fla., builder of a 14-model fleet of stepped-hull center console, cuddy cabin and walkaround outboard boats. “It’s different from driving a conventional boat,” he says. “In rough water, you need to give it more throttle and get up on top of the water. That’s where a stepped hull likes to run.”

A stepped hull can be used to downsize power, Peters adds. “Not everyone needs to go fast,” he says. “You can achieve the same performance with less horsepower in a stepped boat.”

That’s what Livingston was after when Regal called on him to design the Regal 21 about 17 years ago. “We wanted a design that would allow a 21-foot boat to have the same performance numbers with a V-6 [sterndrive] as a V-8,” Livingston says. “The only way to accomplish that was to put a step in the bottom, and we did that. By running a boat with a smaller engine, you have quite a price edge over the competition.”

So how does the stepped hull produce all of these benefits? The configuration gives the boat the ability to maintain its most efficient running attitude, says naval architect Rob Kaidy, former CEO of Ocean5 Naval Architects, the firm that designed three new stepped boats (32-, 35- and 39-footers) for SeaVee. Kaidy is now vice president of engineering and chief naval architect of SeaVee Boats.

“They lock the boat, regardless of speed, into its optimal running trim angle,” Kaidy says. “Every planing hull has an optimal trim angle that is typically between 3.5 and 4.2 degrees. That is where the lift is greatest and the drag is lowest. It’s called the drag bucket.”

In simple terms, steps “force the boat to stay level or close to level,” Peters adds. Ocean5 developed an analytical tool to determine this optimal trim angle. Its Virtual Seatrial software analyzes hull designs through a “digital sea trial.”

Kaidy predicts that in the next few years a “whole new stable of boats with stepped hulls will come to the market.” Bonadeo Boatworks soon will introduce a 26-foot center console bay boat, says company owner Larry Bonadeo. “We will see how it plays with our warped bottom, which gives our boats that good ride,” Bonadeo says. “If it picks up some performance and does not denigrate ride quality, we will build the first bay boat with a longitudinal step.”

A longitudinal step runs fore and aft; a transverse step runs from one side to the other. Both reduce the bottom’s wetted surface. Transverse steps are more prevalent. Smaller boats usually incorporate one step and larger boats two. Such is the case with Wylie Nagler’s fleet of open fishing boats — models bigger than 30 feet have two steps, and those under 30 feet have a single.

“The longer the boat, the more wetted surface you have,” says Nagler, founder and owner of Yellowfin in Sarasota, Fla., who introduced his first stepped hull fishboat in 2000, a 31-footer. “So with a bigger boat that has more surface area, you want to break the friction and suction with the water to introduce air in two places instead of one.”

Yellowfin's fishing boats larger than 30 feet have two steps; the smaller boats have a single step.

It may sound complicated, but it all comes down to simple science, says Steve French, owner of the industrial design firm Applied Concepts Unleashed in Stuart, Fla. “Steps are an absolute math principle,” says French, who has built several stepped boats, including the F22 center console and F&S 54. “It’s simple math and not subjective. The black art is the many potential forms and configurations that steps can take.”

As stepped hulls become more prevalent, so have their marketing and promotion. Kaidy, Peters, Cosker and other have seen plenty of boats promoted as stepped but with only notches in the chines. “A few brands have what I call chicken steps,” says Cosker, who has designed all of Deep Impact’s powerboats. “The companies are trying to show something for marketing but are afraid to go into a full step-bottom design.”

Some stepped hulls lack proper engineering and naval architecture, Kaidy says. “The reality is that [there are] a lot of stepped-hull boats that don‘t actually work,” he says. “The idea that the hull is operating on a gigantic bubble and is supported by compressed air can only be described as complete baloney. The whole idea of a stepped hull having ‘air bearings’ or ‘air lubrication’ is unsupported.”

SeaVee and Ocean5 were careful to get it right, says SeaVee president Ariel Pared. “We have a lot of customers looking for high performance,” Pared says. “We wanted to give the boater a stepped hull that really works the way it is supposed to work.”

SeaVee introduced the 390-Z last fall at the Fort Lauderdale International Boat Show, and 32 and 34 models are coming. All three can be fished hard or used for day boating.

Pared says steps introduce air to the bottom that can interfere with live well pickups and transducers. But SeaVee designed a keel pad in the 390-Z’s bottom to allow these components to do their jobs at high speeds. The pad also guides seawater directly to the bait well intakes.

Design and construction of stepped hulls calls for careful engineering, builders and designers say. “You can’t just take a conventional vee bottom and throw a few steps in and think it will run great,” Nagler says. “There are things that need to change with the design. Balancing the boat at speed is critical — finding the center of gravity and getting it right.”

Adds Cosker: “Part of it is poor step geometry, not laying up the steps properly in the first place. Some of the steps that are hand-built need a little more accuracy to get all the angles right.”

Poorly designed boats can actually be dangerous, causing bow steering, especially in following seas, designers and builders say. They can also become unsafe in high-speed turns, where the stern becomes unglued and the boat spins out. These boats suffer from a high level of directional instability, Peters says. “You could be running along, and the bow could catch, and the boat could spin out on you. Or in a turn the bow could drop and the stern whips around,” he says. “Some of these spinouts can be so severe they can roll a boat.”

To mitigate directional instability, Peters added a shallow tunnel abaft the last step in his designs about 12 years ago, which helps the hull hold its course in turns. “When it leans over and wants to slide, [the tunnel] presents enough lateral area to catch the hull but not so abruptly that it will trip it,” he says.

Poorly executed stepped hulls can have trouble with shifting loads, too, says French. “If someone moves on the boat or there is a shift in weight, it’ll cause the boat to either heel or trim dramatically one way or the other.”

Some are unable to stay on plane at medium speeds, he says. “I know several stepped boats that do well at 8 knots or 40 knots, but they can’t do 24 knots.”

Engineered and executed correctly, stepped hulls have plenty of pluses, says Livingston, the Cutwater builder. “Fuel is always an issue. Everyone wants a more efficient-running boat, and steps are an easy way to get there if you know what you’re doing.”

BENEFITS

• reduced frictional drag

• quicker planing

• reduced bow rise / better sightlines

• speed improvement of 15 percent or more

• power reduction of as much as 25 percent

• reduced fuel burn

• improved ride in rough water

POORLY DESIGNED STEPPED HULL

• stern sliding

• directional instability and bow steering

• decreased trim movement and control

• diminished dynamic resistance to roll (transverse stability)

See related article:

- Quotable

April 2014 issue