VIDEO: Market sees surge in stepped-hull designs

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A growing number of boatbuilders are inserting the stepped-hull design into their vessels, hoping to offer boats with greater speed and efficiency — a trend to watch in the industry and one you’ll see at the Fort Lauderdale International Boat Show.

Invincible, Contender, Regal, Beneteau Powerboats, Scout, Intrepid, Cutwater and SeaHunter use steps in one form or another.

With the technology comes some marketing hyperbole, however, said Ocean5 Naval Architects CEO Robert Kaidy. “The idea that the hull is operating a gigantic bubble and is supported by compressed air can only be described as complete baloney,” he said during a presentation in September at the International BoatBuilders’ Exhibition & Conference in Louisville, Ky. “The whole idea of a stepped hull having ‘air bearings’ or ‘air lubrications’ is unsupported.”

Click play for a look at some models.

Some stepped hulls lack the proper engineering and naval architecture, Kaidy said. “The reality is the market is filled with a lot of stepped-hull boats that don’t actually work,” he said in a follow-up phone interview. “Stepped hulls when applied properly are effective, however.”

How? “They lock the boat, regardless of speed, into its optimal running trim angle,” said Kaidy, whose company is working on seven stepped hulls. “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.”

Kaidy’s Ocean5 developed an analytical tool to determine this optimal trim angle. Its Virtual Seatrial computer software analyzes hull designs through a “digital sea trial.”

Kaidy predicts that during the next few years a “whole new stable of boats with stepped hulls will come to the market.”

I know that SeaVee is coming at us with several of them. The builder’s Facebook site says: “SeaVee announces the new Sea Vee Z High Performance model line featuring an innovative stepped hull design that will recalibrate the industry standard for performance, efficiency and handling. The new Z line will be unveiled on Oct. 31 at the Fort Lauderdale Boat Show, Booth No. 2023.”

I plan to put together an article about this design technique’s rise in popularity; its pros and cons; the characteristics of an effective — and ineffective — stepped hull; and the types of boats that benefit most from it.

— Chris Landry


7 comments on “VIDEO: Market sees surge in stepped-hull designs

  1. Ed Wiser

    This is very unfortunate. Boat buyers are generally unaware that it is usually very difficult to get a good transducer installation in a stepped hull. The ducer must be installed forward of any and all steps and this means it often comes out of the water. Manufacturers aggravate the problem by limiting access to the interior of the hull. Often, the best place to put the ducer is inaccessible because of a fuel tank. I work in an area where most boats are used for fishing exclusively. I tell my customers that fishing boats simply should not have stepped hulls, period.

  2. Robert Berg

    Chris, be sure to speak with Reggie Fountain when you write your article — as you know, he’s the proven master of the stepped hull. I run a 38 Fountain Sportfish Cruiser with a two-step hull. The boat is the most efficient, awesome hull I’ve ever had. I get nearly 2 mpg cruising at 41 mph, and burn only 24 gallons of diesel per hour. Not bad for a heavy 38 footer.

  3. Eric Sorensen

    Chris, great discussion, thanks. Rob makes a good point about the effect of trim on speed and efficiency, and observes correctly that stepped hulls, because the steps create localized pressure points spread out longitudinally, are limited in their trimability. Of course, this can also cause efficiency problems, say, when the optimum trim is not actually achieved by the steps, and handling problems when, for example, you can’t get the bow up enough when running downsea.

    There is also such a thing as a hull following a wave gradient too closely both longitudinally and transversely. In very rough water, I personally would take the well-designed un-stepped hull (a rare commodity, incidentally) any day precisely for its superior trimability, and its independence and predictability of reaction, if you will, relative to the waves it is encountering in a seaway. Then there is the matter of diminished dynamic resistance to roll (transverse stability) in a stepped hull at speed simply because the wetted chine area is reduced by the steps themselves.

    In a planing hull, once you’ve made the boat as light as possible, and given it the right shape and proportions, achieving greater efficiency is largely a matter of minimizing the hull’s combined resistance, of which there are two primary types. Frictional resistance is a function of the area and slipperiness of the hull’s wetted surface, while form resistance results largely from the volume and cross section of the immersed hull and appendages. Outboards and sterndrives are the trim champs since then can force the stern down as well as up. Assuming the boat’s LCG is where it ought to be, one simply “dials in” optimal trim, whether it’s Rob’s 3.5 or 4.2 degrees or somewhere in between. We hit the sweet spot by drying out the bow to reduce frictional drag, without unduly sinking the stern and adding form drag, or causing drag-inducing porpoising. Although it’s not predictive in the same sense as Rob’s software is designed to be, one only needs to look at the tachometer, speedometer and fuel gauge to figure out when you’ve hit the sweet spot.

    Now since you can optimize trim in your un-stepped boat just by using the drives and tabs, there must be more to relative efficiency and speed than just that. Even an optimally trimmed-out, non-stepped hull cannot, in my experience, achieve the performance of a well-designed stepped hull of similar displacement, size and proportions. That leaves us with resistance as the differentiator, and the biggest difference between the two boats is that one has a bubble-coated immersed running surface and the other does not.

    The bottom line is that stepped hulls are getting better and have clear advantages in a number of performance areas, but they are not always, or I would say even usually, the best solution for high-speed planing applications.

  4. Rob Kaidy

    Richard – Ha Ha – Good to see someone is actually reading this stuff and great to see you at IBEX.

    With regard to transducers, the naval architect has to plan ahead for the location of the transducer. Done right with good engineering ahead of time – like we have on the new Seavee’s, the transducer works great like a normal prismatic boat across the speed range.

    Eric – Good comments from the real world!

    Look forward to seeing you all at the show – (Richard, I’ll bring a mirror for myself!)

    Thanks Chris – Great teaser.

  5. Fil Berti

    Nice to see that stepped hulls are coming back to the right attention of boat market, we devoted many years of R&D to the matter, developed design methods, made trials, and specific software (SUPHAD). Good to see that some right concepts are now finally linked to stepped hulls: optimum trim possibility, higher aspect ratio of planing surfaces, so, better efficiency (no more “bubbles” or “air cushions”, or magical combinations of air and water…). I agree that a well designed stepped hull will always have better efficiency even over an extended range of performances. Sample, combining high deadrise angles with related best trim angles, good performances in rough waters are possible too, should be this the design target. On the other hand, it’s an old invention (more than 100 years…), during War II a number of efficient PT torpedo boats with steps were in service… Yes, it’s a bit more difficult the design phase, the “design tuning”, but if mission specifications are clear in mind, best global answer will be reached using the stepped hull concept. This is what emerges from our studies.

  6. Boater

    Best step hull ever made is the INVINCIBLE and knows exactly were to put the transducer for it to work at all times.

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