Just by looking at it, the hull of a motorboat can tell us a great deal about its nature. And a good boat starts with its hull. Planing boats can have a classic V-shaped hull or a stepped one with space in the boat’s hull. They can be seen on models of all lengths from the Axopar 22 (7 metres) to the Cigarette Tirranna 59 (18 metres) and beyond. But what are hull steps used 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
What we will talk about in this article
They are designed for fast planing boats, i.e. those boats that ‘glide’ over the water rather than moving the water mass in front of them.
Planing boats and lift angle:
When talking about planing boats, the concept of lift cannot be ignored. What is it? A force that, due to the effect of speed, brings our boat upwards above the surface of the water.
Lift is the vertical reaction to the resistance to motion of flat or nearly flat surfaces while they are moving with minimal incidence on the surface of the water. Or when they are immersed in it. It all 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 body’s velocity relative to the fluid. When the angle of incidence decreases, lift increases and thus improves the trim of the boat.
“All planing boats,” Navia Design explain, “can benefit from this stepped hull. The final design can then vary slightly depending on requirements. In other words, a stepped hull can be optimised for high speed or to be more fuel efficient at cruising speed.”
Basically, they are good for every type of planing boat. And which ones are they not good for?
“They’re not good,” Ted Mannerfelt tells Motor Boats, “for boats with IPS, hydrojet or shaftline transmissions.”
And in terms of length? One of the smallest boats around with two steps is the Axopar 22 designed by Jarkko Jämsen and his team.
“For a production boat,” Navia Design suggests, “we could start from a length of 5 metres upwards. This is also because the steps involve an increase in weight that is felt more on a small boat. For a made-to-measure, light displacement boat that uses special materials and reduced weight in its construction, we could start from 3 metres upwards.”
” Basically they create an air cushion,” explains Harry Miesbauer, “which means there is less wetted surface and therefore less friction. This creates softness on impact, but also stability and a higher top speed.
We have seen these effects for example aboard the Frauscher 1212 Ghost (read the test here), one of the latest motor yachts designed by Harry Miesbauer.
In this diagram provided by Ted Mannerfelt you can see how the air passes under the hull and reduces friction..
Basically, due to the effect of speed, the water encounters the step and slides underneath it, finding a vacuum. 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. As a result, we have less hydrodynamic friction.
These hulls are known for their excellent handling and efficiency at high speed, good trim control and good sea-keeping even in waves. But there is much more.
“Our team,” explains Ted Mannerfelt, “has been studying this type of fast powerboat design for over 30 years in the racing world. Our racing boats in history have brought home 25 world championships. So from the experience and good results in racing we have converted our knowledge to pleasure boats. The main advantage as far as we are concerned is that you reduce the drag by about 5-12% on a normal production boat.”
This makes it possible to have faster, better-performing, more fuel-efficient boats, even with less horsepower. In the case of outboard engines, this also means having fewer engines, as Navia Design confirms.
“The advantages of this type of design,” continue Navia, “with a stepped hull can be seen from a speed of 15 knots upwards. The main advantage is a reduction in drag due to the reduction in wetted area. Step hulls have a better lift/wet area ratio. This reduction in drag allows a higher top speed and smaller engines combined with better fuel consumption. A good example would be the Axopar 37 which offers a good performance/fuel consumption compromise from two engines. It’s not uncommon to see boats of similar length in the US that with three or even four outboard engines have a marginal performance advantage.”
If you are wondering whether stability is affected in any way, the answer is no.
Harry Miesbauer confirms: “This hull shape does not affect the stability of the hull, either at sea or at anchor. And in my experience there are also advantages in terms of pitching. What does increase, however, is the top speed that can be reached and the possibility of building boats that are wider and therefore more comfortable, while maintaining a high standard of performance. In fact, despite the beam, the boat still has low friction.”
This begs the question: if they are so advantageous, why aren’t all powerboats made like this?
“The cons of this design,” Navia explains, “start with the increased design complexity. There’s a greater risk of making mistakes and the construction is also more complex. There’s also a slightly higher weight penalty and at low speeds (below ten knots) the efficiency is lower. If we are also going to do a cost analysis for the yard, having a stepped hull design only makes economic sense on GRP. If an equivalent design were made of, say, aluminium, it would be significantly more expensive than GRP.
“At low speeds you struggle more,” continues Harry Miesbauer, “and the boat takes longer to plane at low speeds. But the boats for which this design is chosen are not designed to remain in displacement trim for long”.
The issue of the “penalty” in terms of weight that has been discussed so far relates mainly to fibreglass pleasure boats. This is because as they are fast boats, they are subjected to greater stresses when sailing over waves, for example, and therefore need a stronger hull. At the same time, a racing step boat made entirely of carbon can weigh even less.
Another factor that should not be overlooked on a 6-7 metre boat, for example, is the fact that it can 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 are too small in diameter. And then in the design phase, placing them wrongly on the hull undoubtedly brings disadvantages.”
In any case, this solution does not prevent you from “trolling” your boat. We have a practical demonstration of this in this rendering below of a Cigarette 42, with double step, wheeled.
Boat can have 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 many factors. It all depends on the length of the boat, the length at the waterline, not forgetting the weight, the centre of gravity, etc..
“For boats between 5 and 15 metres,” says Navia Design, “the best compromise is to have 2 or 3 steps. But for even longer designs with a narrow beam we could have even more. At a certain point, however, the benefits start to diminish because of the penalty of adding weight. For example, some very high-speed RIBs and racing hulls are known to have five or more steps.”
“Below 10 metres it’s a question of feeling,” according to Harry Miesbauer, “On this size, i.e. small boats, a good solution can be one step, but more pronounced. In this case we are talking about a boat of around 6-7 metres.”
“With our experience,” explains Ted Mannerfelt, “we prefer two steps, but in some cases also three steps. This depends on the type of boat.”
by Gregorio Ferrari