Planing Hulls

Most powerboats of somewhat ordinary size are designed and built for planing.

Almost anyone can create such a boat.
It will require a flat bottom surface aft. A kitchen table will do just fine.

However, planing boats will be more useful if there are two bottom surfaces instead of one, angled into a V-shape, and if these two flat sides are bent into shape, forming a bow. This would be an example of a good planing hull:

Still, making a V-shaped bottom will increase the power requirements.

 

What the Planing Hull Cannot Do

The planing hull has two major limitations:

One is that the G-forces may be damaging to both boats and people when going fast in a chop.
That is, unless the V-shape has a very deep angle in which case it will need even more engine power.

The other and more serious drawback is that these hulls are generally inefficient, on the verge of unusable, when trying to go more slowly – at intermediate, sub-planing speeds.

This is how it may look. At some speeds, the stern wave is so huge that it can be used for surfing – note that you don’t need a rope, you just go surfing downhill. I have never seen this practiced in Scandinavian waters but it appears to be a popular pastime in the US – one example:

 

Other Kinds of Powerboats

But there are other hull shapes. A well designed long and slender semi-displacement hull will be able to go at speeds higher than displacement speed or ‘hull speed’, but lower than planing. Take for example ‘Raket’ by CG Pettersson, built 1919, originally capable of doing 12 knots powered by a 30 hp Buffalo engine:

So why don’t we just increase the engine power of such a boat and it will go even faster?

The problem is, with its round bilge hull shape, it is essentially a displacement type of hull. If you increase the speed, it will sink deeper and deeper in the water. This has been tried in the 1920s by businessmen going from their summer houses on Long Island to their offices and banks on Manhattan. Trying to speed up their big, fast commuter yachts by installing bigger steam engines and turbines, at speeds around 40 knots the yachts travelled so deep in the water that there was a risk of sinking.

So, there we are. Over the years, 3 distinctively different powerboat hull types have been developed, tested and refined:

Which kind of hull shape should you go for?

First, determine how fast you want to travel; then it is a simple matter to decide of which kind of hull will make you happy:
* The displacement hull can only go slow, and its speed is limited by the length of the boat.
* The semi-displacement hull can go faster, but not very fast.
* The planing hull can, but it will struggle at intermediate speeds.

But isn’t there any way to provide a bigger range of usable speeds?

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Perfecting a Powerboat for a Greater Range of Speeds

Making a boat able to go at all speeds – both displacement, semi-planing and planing speeds – is still a great challenge.

Over the years, there has been some research, development and inventions in this field. A few of the aspects which will determine whether such a boat will be successful or not:

• the basic proportions, length to beam ratio, volume distribution, weight
• concepts, like round bilges or sharp chines
• the actual shape of the bow area, possible lifting strakes
• the shape of the running surfaces aft
• devices like trim tabs and interceptors, in different positions

But when setting out to design a new powerboat able to behave well at all speeds, the process is delicate and the outcome is still difficult to know at the design stage. We have done this a couple of times, and to some extent I guess that we have been lucky – but you cannot rely on luck alone. In order to assess the behaviour of our designs we have chosen different routes.

With the Celeste 37, we collaborated with specialists SSPA to analyse her performance before the build started and also with Volvo Penta for a performance estimation regarding resistance, speed, and trim in relation to power and propeller. The results helped us to finely adjust trim by tweaking the hull shape by a few centimetres in the stern area but more than anything to confirm that her basic hull shape would work fine.

The Celeste 37 is designed for high planing speeds. Her hull may not be radical but is different from most powerboats in the shape of her bow, which is sharper and slightly less buoyant. There are reasons for this, of course.

One of the intentions with this design was to avoid a ‘hump’ in her resistance curve in the difficult transition from low speed to planing speeds. She had to run fine also at intermediate speeds between 10 and 15 knots, without drawing a big wave.

Her stern is also different in not having a typical transom, but a rounded stern, much like a double-ender. This rounding of the stern continues under water and was another of the reasons that we wanted an analysis before build started.

She has turned out rather different from most powerboats in offering a smoother ride, with less slamming in waves. She has very little increase of trim and resistance in the transition zone, when going from low displacement speed to planing. And, when just going at speed (in this instance, around 23 knots), she goes lightly, with little fuss and low fuel consumption. Again, maybe not radical, but a very good behaviour – sorry for the shaky video quality:

 

Pushing the Boundaries – ‘Crossover’ Powerboats

With the Nautus 7-50, we wanted to take these characteristics one step further. The hull was designed to a great extent based on a somewhat intuitive idea and the end result is different from most existing both planing and semi-planing hull types. It may not be new, but it is unusual, without doubt.

This new hull is basically a planing hull but it has a sharper bow than almost any such hull, in order to ensure a smooth motion without slamming

It also has similarities with semi-planing hulls but the sections are more typical for a planing hull

The images show the hull in early stages of the project.

Analysing the hull and its parameters, it looked good. But we could not stop there because, apart from the hull shape being a little odd, the numbers, too, were unusual and we found no good references to existing hulls.

This is tricky, because in this situation you will not know how the actual boat will behave in real life. In particular, we added a shallow tunnel aft to allow space for a more horizontal propeller shaft, and a small fin to improve manoeuvring and these could affect the behaviour as well.

It was clear to us we had to evaluate the hull in a serious way so we made a complete CFD analysis, at different speeds. This analysis is computer heavy and very time-consuming. Testing one speed takes a full night but the computer runs by itself and the results can be viewed in the morning. The output shows the wave pattern created by the boat, how it lifts a little out of the water, raises its bow (called trim), the pressures along slices of the underwater body and the resistance in the same areas.

This is an example of the output we would get from a simulation at 12 knots (which is one of the notoriously difficult speeds for a planing powerboat of this size):

From these tests, one for each speed studied, we could assemble a series of graphs showing drag, trim, heave (lift of the hull out of the water) and power requirements:

The results looked fine so far and we felt more confident that the real boat would work well. Still, with some reservation regarding effects of the propeller tunnel and fin, and a few other aspects.

A year later the first boat was finished and tested, and all involved have been overwhelmed by how well she runs at all speeds, how easily driven it is, with a power consumption of approx 0,40 – 0,45 litres per nautical mile, more or less regardless of speed – always silent and smooth, without any slamming in head seas even at high speeds.

She does indeed seem to be one of the relatively few crossover boats, good at any speed. What is perhaps more important is that she shows this kind of civilised behaviour while offering much lower emissions, less noise and less wake. This is further underlined by the fact that she has the most modern type of common-rail engine – a type of engine which is much more power efficient also at lower speeds than previous generations.

The finished Nautus 7-50 can be viewed here