Hovercraft and the environment

There is a common erroneous perception that hovercraft are not environment friendly, but that is not the case hovercraft are, in fact, much more environment friendly than a conventional vessel.

The misconception arises from memories of older types of craft which tended to generate an excessive level of noise. So let us deal with noise first.

Noise:

Much of the noise a hovercraft makes does not in fact originate from the engines but from the propellers. As with all propellers, the speed of the blades and the airflow around them is the critical aspect here. Think of the rotor blades on a helicopter compared to the propeller of an aircraft; you can easily see that the larger blades of the helicopter rotate at a slower rate.

This is because the tip of the blades must never exceed Mach 1 otherwise the blade tip will generate a small sonic boom. Thus the maximum speed at which a propeller may rotate is determined to some extent by the diameter of the propeller. Exceed this speed and you get excessive noise and vibration.

Airflow over the blade tip is also critical because turbulent airflow generates noise. Compare an old style jet aircraft with a modern one fitted with high ratio bypass engines. The older aircraft generates a very loud roar which can be heard for several minutes and is often accompanied by a bass rumbling vibration felt in the chest. The newer aircraft does not have either effect.

So why is there this difference? The answer is in airflow at the back of the engines. In the older designs, the hot exhaust gases are directly mixing with the surrounding atmosphere; it is the turbulence of this mixing which generates the noises. In the modern high ratio bypass engine, a cone of unheated air is passed through the engine and out around the exhaust; this keeps the exhaust smoother and prevents mixing taking place until the gases have cooled down somewhat; this is where the reduction in noise levels comes from.

In the case of hovercraft propellers, the older types tend to have exposed blades and it is the turbulence off the ends of the blades which causes noise as it hits the surrounding atmosphere. Modern hovercraft designs have the blades contained within a short tunnel called a duct; this not only avoids the turbulence spilling off the blade tips but it also helps to control the air passing through the propeller allowing the thrust to be made more directional, so this is a double boost to efficiency as well as reducing noise levels.

Wash:

The next thing to look at is the wash generated by the craft in the water as it moves along.

A hovercraft will always generate considerably less wash than a conventional vessel of the same size or displacement. This happens because the hovercraft travels above the water, not through it.

Rear ¾ angle photographs of the SRN4s taken in calm water conditions will show a very small bow wave ( one such picture will be added here in due course ) but this is otherwise undetectable.

But even a casual glance at a conventional vessel of similar size or displacement moving at a much lower speed in rougher conditions will reveal a considerable bow wave. This is because the hull of the conventional vessel is in the water and it pushes the water aside as it moves through it whereas with the hovercraft the effect is from the very low pressure of the crafts weight being distributed over a large area ( see below ).

Excessive wash can, of course, cause erosion to and stir up the bed of shallow water; it therefore follows that any vessel which can minimise this effect is doing less environmental damage. Hovercraft do not just minimise the effects of wash, they virtually totally eliminate it.

Downwards pressure:

This is a subject which becomes relevant when a hovercraft needs to travel over land, in particular a beach, in order to reach a hoverport. And it can be quite easily demonstrated that the hovercraft will, in fact, cause less disturbance than the high tide or a person walking across the beach meaning that hovercraft are able to cross areas which are environmentally sensitive without damaging the environment or disturbing the wildlife living there.

Let us start with a large craft like the SRN4. Just how much pressure is it exerting on the surface?

Well, the larger Mk3 craft are 56.38m long and 23.16m wide. This gives them a surface area covered by the craft of 1,306m² and their gross weight of 320,000kg results in a weight distribution of almost exactly 245kg per m², which is very roughly the same pressure as water to a depth of 24.5cm.

Or in other words, the hovercraft is putting a lot less pressure on the surface of the beach than the high tide does!

But what about the pressure exerted by a smaller hovercraft? How does this compare?

Let us look at the AP1-88 series 400, which is a typical mid sized hovercraft.

These craft are 28.5m long by 12m wide ( surface area 342m² ) and the gross weight is 69,000kg. This gives a weight distribution of about 202kg / m² which is lower than the figure for the SRN4, but not hugely different. In this case it is about equal to a 20cm depth of water on the beach.

To give a complete picture, we need to look at small craft as well, and a very popular example is the Hov Pod. In this case, the standard dimensions are 3.63m by 1.86m and a typical maximum gross weight is 560kg ( note this varies according to the actual specifications of each individual craft ).

The mathematics here work out as 5.49m² for the area, giving 102kg / m² , a remarkably low pressure figure equal to only 10cm of water.

So we can see a clear trend here; the bigger the craft, the more pressure it exerts on the surface, but this is not a linear trend; indeed with the very large craft there is little increase as the craft size increases this is because the weight increases roughly proportionately with the size.

But there is one thing in common with all these craft even the largest craft, the SRN4 Mk3, clearly exerts a great deal less pressure on the beach than a person walking across it does! You don’t even need to do the mathematics to see this; consider a reasonably light person of around 75kg in weight walking across a beach. At any one time only a few cm² of feet are in contact with the beach and this tny area is taking the pressure that an enormous SRN4 Mk3 spreads out over nearly half a square metre

Port facilities:

A hovercraft can land on any simple flat surface which is reasonably level, it does not even need to be prepared. But let us consider a full scale hoverport. What do you need? Just a flat area that is big enough for the craft to land on and turn around on, plus a ramp to the water or beach, really. You don’t need to dredge anything or excavate huge docks.

Compare that with the huge effort you have to go to in order to create somewhere for a conventional vessel even only a small one to berth. It is quite clear that a hoverport can be created with minimal disturbance to the local environment, and as we have seen above the operation of the hoverport will cause no environmental problems either.

But it is not just the immediate locality that benefits. With smaller facilities, there is a considerably lower consumption of materials like concrete in order to build the facilities, which means not only are far lower volumes of raw materials required to build the facilities but also there will be a lot less energy and therefore CO² emissions involved in the construction work for the facilities.

So again, the hovercraft is a clear winner in this respect.

Craft utilisation:

Hovercraft are much faster than conventional vessels to the extent that they will typically make three crossings of a given length than a conventional vessel. You can therefore make the same number of crossings on a given route with one hovercraft as you can with three conventional vessels.

Even if the hovercraft has a capacity of only half that of the conventional vessel it is being compared to, you can still do the work of the three conventional vessels with only two hovercraft.

Clearly, then, if you need only two hovercraft to cover the work of three conventional vessels then the hovercraft is again ahead on the environmental aspects because with only two craft then far fewer natural resources ( which again translates into a reduction in energy and CO² ) have been used to build the two hovercraft than the three conventional vessels. Further, a hovercraft is far lighter than a conventional vessel of the same dimensions or capacity and therefore requires fewer resources to build each craft as well.

Summary:

It is quite clear from all the details given above that hovercraft, by far, represent the most environment friendly method of transportation for ferry services and similar uses, especially where there is little land available for port facilities.

There is no reason why hovercraft cannot be used in the vicinity of environmentally sensitive areas; they will not damage the land or disturb the wildlife at those locations.

Construction of both hoverports and the hovercraft themselves consumes far less in the way of natural resources and therefore energy with a consequent obvious reduction in CO² emissions than the facilities and vessels required for conventional ferry operations.

Given the concerns about the environment these days, it is of considerable surprise that there are not more hovercraft operations than there are at present.