Hull of a design


Clement David, Senior Naval Architect of Navia Design talks to ONBOARD about his industry changing stepped hull designs for superyacht tenders and world renowned marine vessels

Which kind of boat and hull needs a stepped hull?
The benefits of stepped-hull designs kick in from speeds of 15knots upwards, so all planing boats can benefit from a stepped hull design. The final design will be slightly different according to the required brief. In other words, it can be optimised either for a high top speed, or for being more fuel-efficient at cruising speeds.

What are the benefits?
The primary benefit is a reduction of drag due to a reduction of the wetted area. Stepped hulls have a better lift coefficient coming from the multiplication of the stagnation lines (the area of high pressure at the start of the planning area), this reduction of drag allows for higher top speed and smaller engines/ meaning better fuel consumption.

A good example would be the Axopar 37, which offers a good performance / fuel consumption compromise from one, or max two engines, whereupon it’s not unusual to see similar length boats in the USA, for example, with three or even four outboard engines, although with marginal performance gain.

Are comfort or stability at anchor compromised with this kind of hull?
Step hull designs allow more freedom in terms of design. We can choose the width/beam at the waterline, while keeping efficiency, that still allows us to tweak its behavior at anchor. A hull that is too wide, for example, is stable in low height waves at anchor, but as soon as the wave height increases it becomes more uncomfortable and moves (rolls) very fast. If the hull beam is too narrow then the motion at anchor is slower, but it will still react to any waves. For better seakeeping ability a narrow beam is better.

Does the difference between one or two steps depend on the length of the boat?
Yes, it depends on the length, but also width at the waterline, displacement and location of centre of gravity. All four design parameters influence the number of steps. A good design is about getting all four of these parameters correct at the same time!

What is the smallest boat length that can have a stepped hull?
One of the drawbacks of a stepped hull is a small increase of weight and, most importantly, it requires a more complex construction. These weight increases can be significant for a small boat, but less so for a larger boat. For a serial production boat, we could start from 5m length upwards, but for a custom made, light displacement boat using unique weight saving materials in its construction, we could start from 3m upwards.

Hull Types 1 - Classic efficient planing hull
Hull Types 1 – Classic efficient planing hull
Hull types 2 - Classic comfortable planing hull
Hull types 2 – Classic comfortable planing hull
Hull types 3 - Stepped Hull
Hull types 3 – Stepped Hull


Are there any differences between a RIB boat and normal boat?
There are ‘pluses and minuses’ between RIBs and normal boats, which can be compared and contrasted across both construction and stability.

Construction: Tubes are lighter than GRP, but it is difficult to integrate features such as handrails, cleats, cupholders etc on tubes. More importantly, the width of the tubes greatly reduce the available space on deck. Stability: GRP hulls provide good stability and the rules are equally favorable for RIBs. An Axopar, for example, is designed like a RIB, with the second step not used when planing, but becoming really useful for stability.

From a planing point of view, there is no difference since the design of the hull is independent of whether it’s a RIB or not a RIB.

RIBs benefit from being light and easy to transport, and have high people carrying capacity, but there is limited space for equipment on deck. The tube is like an empty space which we cannot use which can impact greatly on available deck space.

What are the disadvantages or contradictions?
The disadvantages of a stepped hull design are that it is more complex in terms of design and there’s a higher risk of getting it wrong! It’s a more complex construction as well, and there is a small weight penalty and at really low speeds (<10kts), the efficiency is marginally lower.

By comparison, the benefits are many, such as much better handling and efficiency at high speed, good control of the trim and limited wetted area at low trim giving good seaworthiness / good sea kindliness (vertical acceleration on waves) while keeping high efficiency.

From a profit-making perspective, having a stepped hull design only makes commercial sense in GRP. If an equivalent design was made in, say, aluminum, for example, it would be significantly more expensive than GRP.

Are you supposed to use more than two steps for a particular kind of hull?
For boats in the 5 to 15m size range, the best compromise is to have two to three steps. But for even longer and narrow beam designs we could have even more, but at some point the benefits start to become reduced through the penalty of adding more weight.

For example, some ultra-high-speed RIBs and racing hulls have been known to have five or more steps. But these are unique one off designs with one purpose in mind.

With more development will it be possible to go under a 5m boat?
Yes, it is possible for sure. But as discussed before, there is an increase in weight and therefore construction is more complicated, so there is a decision for the buyer to make first.

Which technology do you use to design this kind of hull? How does augmented reality help you in this?
The basic design parameters of a stepped hull come together from a variety of sources that includes our own in-house software (Savitsky, wake simulation…), which uses literature that gives us a good idea of the basic step position and parameters in a very short space of time.

After that, we use CFD (Computational Fluid Dynamic) software to fine tune the steps and to confirm the precise position of the spray rails in order to maximise the final design.

Volume of fluid CFD allows us to track the different free surface (limit between the water the air) and then have an accurate picture of the air/water mix and spray. After this, we use Virtual Reality for accommodation layout and fittings.

Is it true that a stepped hull is more expensive to be built? Or does it depend upon the mold?
In serial production GRP, yes the cost to build is more expensive than a normal hull, but the difference is relatively marginal and it’s still a very workable possibility. In the case of metal or wooden hull construction, the extra cost for a stepped hull production would be quite significantly higher than a normal hull design.

This kind of hull has a peculiar shape, correct? Do you think that you will use 3D technology in the next few years, maybe to build the mold?
We already use 3D modelisation and CNC milling but, currently, we don’t use 3D printing for the global boat as the material is not as efficient as GRP, but Navia is constantly looking out for the next best opportunities to recommend to our clients.

We do, however, use 3D printing for prototyping of small components and for mock-ups of certain areas such as cabin spaces and helm areas, steering consoles and such like.

When we designed the new Axopar 22, for example, we used 3D printing for creating prototypes of all the knobs and switches placed on the dashboard.

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Axopar 28 T-Top

Axopar 28 T-Top