Just by looking at it, the hull of a motor boat can tell us a great deal about its nature. And a good boat starts right from its hull. Planing boats can have a classic V-shaped hull or with steps, which are steps that create space in the boats’ living work. They are seen on models of all lengths from the Axopar 22 (7 meters) to the Cigarette Tirranna 59 (18 meters) and beyond. But what are the steps in the hull for? And how do they work?
We asked three gurus of this type of hull: Jarkko Jämsen and the Navia Design team, Harry Miesbauer of /H/arry Miesbauer Yacht Design, and Ted Mannefelt of the Mannerfelt Design Team.
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What we will talk about in this article
- Steps in the hull: what boats are they for?
- Power and step boats: how do they work?
- What are the advantages of a step hull
- What are the disadvantages of a step hull
- Steps in motor boats: how many are needed and why
Steps in the hull: what boats are they for?
They are designed for fast planing boats, that is, those boats that “glide” over the water instead of moving the water mass in front of them.
Planing boats and lift angle:
Speaking of planing boats, the concept of lift cannot be ignored. What is it? A force that, due to the effect of speed, carries our boat upward above the surface of the water.
Lift is the vertical reaction to the resistance to motion of flat or nearly flat surfaces as they move with minimal incidence on the water surface. Or when they are immersed in it. Everything then depends on the pressure exerted by the water on the surface of the immersed body and the angle of incidence .
What is the angle of incidence? It is an angle between the direction of the moving body (relative to the fluid) and the direction of the velocity of the body relative to the fluid. When the angle of incidence decreases, lift increases and thus improves the boat’s trim.
“All planing boats,” Navia Design explains, “can benefit from this stepped hull. The final design can then vary slightly depending on requirements. In other words, a stepped hull can be optimized for high speed or to be more fuel efficient at cruising speed.”
Basically they are good for any type of planing boat. And which ones are not good for?
“They are not good,” Ted Mannerfelt tells Motor Boats, “for boats with IPS, hydrojet or shaftline transmissions.”
What about in terms of length? One of the smallest boats around with two steps is the Axopar 22 designed by JarkkoJämsen and his team.
“For a production boat,” they speculate from Navia Design, “we could start from a length of 5 meters and up. This is also because the steps involve weight increases that are felt more on a small boat. For a bespoke, light-displacement boat that uses special, lightweight materials in its construction, we might start at 3 meters and up.”
Power and step boats: how do they work?
Well, now after seeing what boats they are designed for, let’s see how the steps work on the hull.
“They basically create a cushion of air,” Harry Miesbauer explains, “which allows for less wetted surface area and therefore less friction. This creates softness in impact, but also stability and a higher top speed.”
We experienced these effects, for example, by sailing aboard the Frauscher 1212 Ghost(read the test here), one of the latest motor models designed by Harry Miesbauer.
In this diagram provided to us by Ted Mannerfelt, it is possible to understand how air passes under the hull reducing friction.
Basically, due to the effect of velocity, the water meets the step and slips underneath it, finding the void. Thus, an air pocket is created between the surface of the water and the hull itself, acting as a cushion. This supports the hull. Depending on the steps present, this effect will recur each time the water encounters one. The air pressure thus generates lift and at the same time reduces the wetted surface of the hull. Therefore, here we have less hydrodynamic friction.
What are the advantages of a step hull
Now that we have seen how such a hull works let’s try to understand what the main advantages might be. These hulls are known to have excellent high-speed handling and efficiency, good trim control, and good seaworthiness even in waves. But in reality there is much more.
“Our team,” Ted Mannerfelt explains, “has been studying this type of fast powerboat design for more than 30 years in the racing world. Our race boats in history have brought home as many as 25 world championships. So from experience and good results in competitions we converted our knowledge to pleasure boats. The main advantage as far as we’re concerned is that it reduces drag by about 5-12% on a normal standard boat.”
Therefore, this allows for faster, better-performing, more fuel-efficient boats, even with less horsepower. In the case of outboard engines, this also means having fewer engines as confirmed to us by Navia Design.
“The advantages of this type of design,” they continue from Navia, “with a step hull can be seen starting at speeds of 15 knots. The main advantage is a reduction in drag due to a reduction in wetted area. Step hulls have a better lift/wet area ratio. This reduction in drag allows higher top speed and smaller engines combined with better fuel economy. A good example would be the Axopar 37, which offers a good performance/fuel consumption trade-off from two engines. It is not uncommon to see boats of similar length in the United States that with three or even four outboard motors have a marginal performance advantage.”
If you are wondering whether stability is affected in any way, the answer is no.
“This shape of the hull,” Harry Miesbauer confirmed to us, “does not penalize the stability of the hull, either when sailing or at anchor. And in my experience there are also pitching advantages. What increases, however, is the attainable top speed and the possibility of being able to make wider and therefore more comfortable boats while maintaining a high standard of performance. In fact, despite the beam, the boat still has low friction.”
What are the disadvantages of a step hull
A question arises at this point: if they are so advantageous, why aren’t all motor boats made this way?
“The disadvantages of this design,” Navia explains, “begin with the increased design complexity. There is a greater risk of making mistakes, and then the construction is also more complex. There is then a weight penalty that is a bit more, and at low speeds (below ten knots) there is less efficiency. If we then also want to go to a cost analysis for the shipyard, having a stepped hull design only makes economic sense on fiberglass. If an equivalent design were made, for example, of aluminum, it would be significantly more expensive than fiberglass.
“At low speeds you struggle more,” continues Harry Miesbauer, “and the boat at low speeds takes longer to get on plane. But the boats for which this design is chosen are not designed to remain in displacement trim for long periods of time.”
The issue of “penalty” in terms of weight that has been discussed so far refers mainly to fiberglass recreational boats. This is because since they are fast boats, they are subjected to greater stresses, for example, when sailing on waves, and therefore need a stronger hull. At the same time, a racing step boat made entirely of carbon can weigh even less.
Another factor not to be overlooked on a 6-7 meter boat, for example, is then its ability to be wheeled. And the steps in this case can be an obstacle.
“In addition to the certainly more expensive construction,” notes Ted Mannerfet, “it could be difficult to get the boat out of a dolly if the rollers turn out to be too small in diameter. And then in the design phase placing them badly on the hull undoubtedly brings disadvantages.”
This solution in any case does not prevent one from “carting” one’s boat. A practical demonstration we have in this rendering below of a Cigarette 42, with double step, wheeled.
Steps in motor boats: how many are needed and why
At sea we see boats with one, two, three or even four steps. But is there a right number and if so what is it? A good design must take into account a great many factors. It all depends on the length of the boat, the length at the waterline, not forgetting the weight, center of gravity, etc..
“For boats between 5 and 15 meters,” Navia Design argues, “the best compromise is to have 2 or 3 steps. But for even longer projects with a narrow beam we may have even more. At some point, however, the benefits begin to diminish because of the penalty of weight addition. For example, some very high-speed RIBs and racing hulls are known to have five or more steps.”
“Under 10 meters it is a matter of feeling,” according to Harry Miesbauer, “On this size, that is, boats that are all in all small, a good solution can be given by a single, but more pronounced step. In this case we are talking about a boat of about 6-7 meters.”
“With our experience,” explains Ted Mannerfelt, “we prefer two steps, but in some cases three steps. That depends on the type of boat.”
Article by Gregorio Ferrari
