Hull stepping can be a contentious subject. Some boatbuilders avoid it simply because of costing, some claim it produces a twitchy ride at speed – which sometimes can be the case – and others embrace the concept and do their homework. However you look at it, a ‘good’ stepped hull will yield dividends in terms of performance, efficiency and ride.
Technohull of Athens are one of those that appear to have done quite a bit of homework. They are hardly new to the idea, having already built a wide range of high-performance stepped-hull RIBs. Now they have taken their hull technology one step further in creating a 100-knot, 38ft RIB. This will be the first of several boats from this Greek yard that will use their newly patented DynaStream technology. Technohull are not reinventing the wheel in planing hull terms, but they are claiming to have achieved a 15% improvement in efficiency from their variable-deadrise multistep hull.
Built with no less than five steps, the hull appears slightly staggered in appearance. Interestingly, it has two conventionally deep steps and three smaller steps. In channelling air under the hull through the step channels, the effect is to create six separate planing sections, the leading edges of which produce proportionally greater hydrodynamic lift than the aft planing sections. I will elaborate no further, as to fully appreciate the science behind this you need to read Dave Marsh’s definitive feature on hull stepping in Issue 146. The overall effect is to increase hydrodynamic lift and reduce drag in the process, with an emphasis on the stern quarters of the hull. In doing so, bow attitude is lowered, with a consequent improvement in ride.
Technohull claim that this new hull design has many other benefits, in particular improved handling and seakeeping at speed. I am speculating that in using two different sizes of step, they are aiming to achieve a staged system of stepping where the larger steps become effective before the smaller ones. Again I speculate that this is primarily for handling benefits in producing more predictable user-friendly handling. At high speed, when the smaller steps start coming into their own, the hull utilises the full lift benefit of six separate planing sections.
In using a variable-deadrise hull, as Fairline, for example, have done for many years to great effect, they aim to have the best of both worlds. In simplistic terms, this enables a sharper hull midsection, while using a slightly flatter profile for the stern quarters. The argument for this design is that it maximises lift and increases stability from the milder deadrise angle at the stern, while sharpening the midsection to deal more effectively with the sea state. Technohull have also created special ‘spray control chines’ for ‘an incredibly dry ride’.
Not surprisingly, a resin-infused carbon-fibre composite is used for maximum strength and minimum weight. Given the extra intricacies of stepped-hull construction, this boat will have a pretty generous price tag. To date, the only prototypes have been a scaled and full-size model, so we wait with bated breath for the real McCoy.