The stepped hull has come of age
Posted on 29 November 2011
Written by Eric Sorensen
The quest for speed has long been a sort of Holy Grail in powerboating, and the stepped hull has played a role in that pursuit. Stepped hulls have been around since 1910 or so, when powerboat pioneers Gar Wood and Christopher Columbus Smith (founder of Chris-Craft), among others, used them on hydroplanes. The hydroplanes featured a transverse notch, or step, in the bottom amidships that reduced wetted surface at high speeds, thereby reducing frictional resistance.
The same amount of dynamic pressure was present to support the weight of the boat, but it was distributed over two or three small patches of the hull bottom. Each of these patches was wider and shorter than the single hull section in the water on an unstepped bottom, which further decreased frictional drag.
Rarely do you get something for nothing, though, and it turned out that speed came with a price. Indeed, the stepped boats were fast, but they could be unpredictable and hard to handle at high speeds, with physics exacting its revenge in the form of spinouts and other potentially dangerous antics. That’s the fundamental reason they remain relatively uncommon today. The good news is that much has been done to address the longstanding problems with stepped hulls, making them a viable option for the right skipper.
Comparing apples to oranges
The stepped hull’s performance differs greatly from more traditional hull designs. A conventional vee-bottom planing hull has comparatively predictable resistance along its surface at a given speed and displacement in calm water. However, the stepped hull can be unstable when moving at high speed while being supported by the dynamic lift of water flow, rather than just the buoyancy of the hull. The conventional unstepped bottom dries out sequentially from the bow aft as speed increases and the hull rises vertically. As the boat’s speed increases, the center of resistance (created by the wetted patch) along the hull bottom incrementally and predictably shifts aft.
That isn’t so with the stepped hull, which has a forward step that stays in contact with the water. As a result, the stepped hull runs naturally with a more bow-down trim. With the props abaft the transom and the longitudinal center of resistance farther forward in the stepped hull — combined with the aerated, slippery aft sections — the bottom will be itching for a chance to slide out from under you.
In other words, the quality of step-induced slipperiness aft that makes the boat go faster also makes it more prone to thrilling everyone on board with a sudden 180-degree turn at 50 or 60 knots. Let’s take a look at why stepped hulls behave this way.
The stepped bottom reduces drag by airing out the hull immediately abaft the step. (Most of these boats have one or two steps.) The amount of hull no longer in direct contact with the water depends primarily on the boat’s speed and loading as well as the height of the step. This is where the unpredictability can come in, depending on how well the hull is designed. Essentially, the longitudinal center of resistance shifts forward since it is the aft portion of the hull that is ventilated. The prop thrust is fixed in the middle of the stern, and the ventilated (aerated) stern is as slippery as a greased eel. If you get the center of hull resistance forward a tad out of column directionally with the wetted hull patch forward, stand by.
In short, unrefined stepped hulls at high speed turn almost as easily as they are driven straight ahead, if by turning you mean the stern suddenly overtakes the bow. It’s like slamming on the front brakes of a bicycle rather than letting the rear brake do some of the work. This happened with some frequency with stepped hulls from several production boatbuilders, especially in the 1970s and ’80s. Boats that didn’t quite have all the bugs worked out were sold to unwitting consumers who were in for a mighty unpleasant surprise when operating at high speeds. That’s why a stepped hull that doesn’t incorporate the right design elements requires constant attention at the wheel — and a skipper with a high level of skill.
The size and shape of the aperture or pocket at the chine at the end of the step is critical in determining whether the hull will perform and handle well. A stepped bottom works by creating a low-pressure area as water flows past the step. As the velocity of the water flow past the hull increases, the pressure immediately behind each step decreases, which is what draws in air and distributes it along the bottom of the hull, allowing for increased speed.
If the pocket at the chine that feeds air to the step is too small so that airflow is easily blocked, say when the boat is overloaded or in a hard turn, then a partial vacuum will be created as suddenly as airflow is stopped. The result is akin to a very large and powerful hand grabbing hold of the gunwale on one side when the boat is running along at 40 knots. The side being held will stop, of course, and the other side will keep going. If you and your passengers are still in the boat and it hasn’t capsized, you’ll be staring at your own wake right off your bow. Man, what a rush! I, for one, would pass on it.
Fortunately, designers like Formula’s John Adams and Michael Peters Yacht Design have made great strides in overcoming the stepped bottom’s performance problems. The recent improvements are partly because of the increased deadrise found in modern designs. The more deadrise a hull has aft, the more it resists turning and the greater the directional stability. That’s one of the things that makes the deep-vee hull, stepped or not, a great rough-water boat. Designers are also adding aggressive strakes and vertical surfaces aft that act like small keels to give the boat more directional stability.
One challenge for the designer is getting the hull to heel just the right amount in a turn. If the hull is too flat in a turn, it will spin out more easily. If the hull heels too much, it can capsize. Finding the right ground involves balancing the centrifugal forces acting on the boat in a turn with the hull’s center of lateral resistance, which isn’t easy to do, especially in a smaller boat where varying passenger loads have more of an impact dynamically.
Establishing that ground is the essential balancing act of superior planing hull design — getting the hull to heel at an angle commensurate with its rate of turn so centrifugal force goes right through your feet, rather than throwing you outboard or making you fall inboard in a turn. The designer has to juggle a lot of factors: the vertical center of gravity, distribution of weight, the lateral resistance created by the deadrise of the immersed hull, keel shape, strake and chine geometry, appendages and the steps themselves, which can act to further destabilize the boat once it’s in a hard turn.
Stepping up the pace
Despite the fact that the stepped hull can present a challenge from a boat-handling perspective, it’s definitely an intriguing design. As the boat picks up speed and starts to plane, water flow past the step increases until it separates altogether from the hull immediately abaft the step. Something has to take the place of the water no longer behind the step, and this something is the air drawn in by the step openings, or apertures at the chines, as a result of the low pressure created by the passing water.
The area behind the steps is under partial vacuum, which is just a pressure lower than atmospheric. This draws in ambient air, which is then distributed aft along the hull bottom (due to the boat’s forward motion) and out (due to the hull’s deadrise). Since air is more slippery than water, the boat creates less drag and goes faster for a given thrust. The result is a hull that typically goes 6 to 10 knots faster for a given thrust, or that can go the same speed with less power and fuel consumption. So the stepped hull has less bottom in contact with the water, and the bottom that is in contact abaft the steps creates less drag and allows for more speed.
In addition to the smaller hull surface in contact with the water, each wetted area is wider and shorter than the wetted surface of a conventional hull. This matters because the longer the patch of hull that’s in contact with the water, the greater the resistance, since the boundary layer of water increases in thickness as it gets longer. It’s much better to break up the hull contact points into wider, shorter segments to further reduce frictional drag.
As I’ve noted, stepped bottoms are not for everyone, largely because a fully loaded boat traveling much below 30 knots simply isn’t going to go fast enough to get up and ride on its steps and transom (the third contact point in a two-step hull). The steps will actually create extra resistance when cruising at less than 30 knots since they increase form drag and turbulence without the benefit of reduced wetted surface.
We all know that it often gets rough on open water and that big waves will keep you cruising well below 30 knots much of the time, making the stepped hull a much less viable choice for a family boat. In fact, if you see steps on a boat that cruises below 25 knots, especially if it has a high CG and narrow beam, then absent extremely light bottom loading, you are almost certainly in the presence of a marketing gimmick. The ultimate, and most transparent, fraud is the chine that’s carved out to look like the entrance to a step, while the hull bottom stays as straight as a Garrison Keillor Lutheran.
Like a catamaran that only performs well when it is light enough for the tunnel to rise out of reach of the waves, a stepped hull must not be overloaded. Putting too much weight aboard will make it harder to achieve the speed required for the stepped bottom to work properly. Too much weight will also make it easier to block airflow to the steps, as the hull sits deeper in the water, increasing the chance of a spinout, capsize or sudden stop. So watch your weight. That’s one thing I like about a well-designed conventional monohull over a cat or a stepped hull; it is much more forgiving of excessive weight and operator inattention.
Stepped hulls ride on pressure points located just forward of each step and at the transom. Since the hydrodynamic pressure at each of these small patches of hull is very high and localized, it is difficult to change the hull’s trim, (its bow-down attitude). If the hull is well-balanced in terms of its longitudinal center of gravity and its center of dynamic lift, then all is mostly well. But even in the best planing boats, it helps to be able to raise the bow for improved control down-sea, and to drop the bow for a smoother ride (if slower and wetter) up-sea.
The stepped bottom also reacts more quickly to a wave gradient, so the hull will be at a more acute angle to the next wave when it impacts it than the slower-acting (in trim) conventional bottom. For all its efficiency and speed, the stepped hull is harder to fIy, or control in trim and heel, than the slower conventional hull. The same pressure points that lock in trim also give the hull a tendency to precisely follow the wave gradient, which is often exactly what you don’t want. For instance, you want to keep the bow up when running down the back of a wave. If you want to be able to run at 15 to 25 knots, which you will have to do anyway when it gets rough, then a stepped bottom will do you no good, and at slower speeds it will actually be less efficient than a conventional hull.
There is also the effect of the steps on transverse, or lateral, stability. All is well when the boat is running along at high speed in fairly calm water. However, if the boat suddenly slows — the driver chops the throttle, for example — and abruptly changes direction, roll can be accentuated. Also consider that the hull at the chines directly abaft the step generates no lift, and this is not what you want when trying to recover from a deep roll, whether that roll was induced by heel or listing. And, naturally, you can expect a stepped hull to heel more in a turn for the same reasons, though this can be offset by chine geometry (chine flat width and down angle).
Taking the right steps
Along with the issue of stalling out the steps and creating a momentary vacuum, high-speed hard turns should be undertaken with care and only by those with the requisite skill and judgment to do them safely. The bottom line is that high-speed hard turns have been a bugaboo of stepped hulls.
I was at the Multi-Agency Craft Conference in June at the Navy’s base in Little Creek, Va., and I ran and rode on several of the most advanced step-bottom boats around, including one by Michael Peters. In calm Chesapeake Bay conditions and with moderate loading (half fuel and modest passengers), the Metal Shark Fearless 40, a Peters design, did fine in a hard turn, as the builder pointed out. As one would expect, the people driving the boats were experts, but this was the one potentially poor handling characteristic they all pointed to as a non-issue.
Operator inexperience is one reason stepped hulls spin out in a turn, along with inattention. If you are used to a conventional deep-vee, like I am, you are likely accustomed to running with the drives trimmed out to minimize wetted surface and increase speed. I only use the tabs to correct for my single outboard’s prop torque or if there is a lot of weight aft. If I am going into a hard turn at high speed, I drop the engine down, tucking it in to get the prop deeper in the water to prevent it from ventilating. The boat slows and the bow drops, but it’s otherwise a non-event. I can also chop the throttle part way into a turn without incident, other than slowing the boat down. Not so with the stepped hull, which would be a thrilling place to be under those conditions.
If the stepped hull’s drives are tucked in, you accentuate the problem of the center of frictional (and form) resistance being farther forward, which leads to coursekeeping instabilities. In this case, the bottom is wet much farther forward than it should be, essentially creating a longer lever arm transversely. With the bow immersed, there is a greater distance longitudinally from the props to the center of resistance, so the same turning force is working over a greater distance, which is what makes the boat spin out so easily. And remember that a stepped hull tends to run flatter than a conventional hull to start with. Before starting a hard turn at high speed, the drives must be trimmed to get the bow up and out of the water, and you can’t chop power going into the turn.
There is also the issue of vertical center of gravity. A boat that has less transverse stability or whose transverse stability is subject to change considerably depending on speed, loading, attitude and wave conditions won’t tolerate a higher vertical center of gravity. In other words, you can’t add a lot of weight up high, like installing a flybridge or tuna tower, after taking delivery. Expect the boat to be more reactive to strong relative wind, especially from the beam, when the boat has more sail area up high.
Finally, you can’t have waterjet power with conventional steps, since the water filling the jet intakes will be full of entrained air. Just choose one, waterjet power or conventional steps, and leave it at that.
A stepped hull is a very different animal than a conventional planing hull. It’s not necessarily a bad animal; it’s just different from what most people are looking for. It’s a little like owning a poodle and a Doberman. Both make fine pets, but you don’t have to be as nice to the poodle.
Be sure to keep these factors in mind when choosing a boat for casual family use. Conventional planing hulls are more forgiving of operator error and inexperience, can tolerate heavier loads without ill effect and can operate efficiently from below 12 or 13 knots on up. Stepped hulls work their magic when everything is dialed in and the boat has a focused mission in moderate sea conditions involving cruise speeds starting at a minimum of 25 knots. Dialed in means the boat’s balance (dynamic lift vs. center of gravity) is spot on and not subject to much change. Running trim is optimized for speed, handling, distribution of wetted surface and minimal resistance. Displacement is under control, and the hull’s basic geometry (shape in three dimensions) is optimized, such as deadrise distribution, chine flats, strakes and buttocks aft. The steps are the correct transverse angle and height — the buttocks ahead of the step have to be lower than the buttocks abaft the step — and they should be reliably supplied with air at the chines under extremes of loading, heel and speed. When all of these conditions are met, the step- bottomed boat’s performance and utility can be a joy to behold.
What all this means is that the stepped boat is for the specialist in a niche market. Invincible, Intrepid and Contender are all popular among this niche segment in part because their boats can go faster for the power and farther on a tank of fuel. Based on what I have seen, owners of these boats tend to be more experienced, so they likely know how to drive properly and safely and have screwed up often enough in the past to know they must be careful and pay attention.
If you want to cruise at 30 to 60 knots, go farther on a tank of fuel and faster for the horsepower, and you have enough experience in high-speed planing craft to have equal parts ability and humility, then by all means there could be a stepped hull out there with your name on it. Some stepped bottoms are predictable and handle very well, while a few are just plain dangerous. Make sure you can tell the difference by doing your homework, but don’t put too much stock in what you read and hear from any single source. Dig around, diligently acquire expertise, talk to people who own (or, better yet, used to own) one of these boats, and run them to decide if one is right for you. You’ll be glad you did.
Eric Sorensen is a consultant to boat- and shipbuilders and to the government. He was founding director of the J.D. Power and Associates marine practice and is the authoir of "Sorense';s Guide to Powerboats: How to Evaluate Design, Construction and Performance." A longtime licensed captain, he can be reached through his website at www.ericllc.com.
See related article:
- Passing the stepped-hull test
This article originally appeared in the December 2011 issue.