The Von Karman - Gabrielli diagram
Development of WIG boats has mainly been technology driven so far. Therefore the question is justified whether WIG boats can be commercially viable, especially since no WIG boats have been built in big numbers so far, although a few models have been commercially available.
A new product must have distinct advantages over existing alternatives in order to find its way to the market.
The potential benefits of WIG boats are:
On the other hand WIG boats do have their limitations:
For certain applications the benefits far outweigh the limitations so that WIG boats are a viable alternative to competing means of transport. An operator will only seriously consider to operate WIG boats when some conditions and requirements are met (e.g. ref.ref.924):
Some of the above issues will be considered in more detail below.
A well designed WIG boat will have a relatively high L/D and a low speed as compared to a short-haul aircraft of similar size, but it is a lot faster and more fuel efficient than a fast ship. Different transport vehicles can be compared with the Von Karman-Gabrielli diagram. In this diagram the L/D is given as a function of the speed. Different vehicles have been indicated in the diagram ranging from a bicycle to the concorde. The L/D of a bicycle may seem awkward, but the lift is just equal to the weight. A very remarkable feature in the Von Karman-Gabrieli diagram is the technology line, it is pushed towards the upper right corner with advancing technology, consequently all current forms of transport are below that line. The technology line represents a certain value of the so-called transport efficiency, the product of L/D and speed.
The Von Karman - Gabrielli diagram
Another remarkable thing in the diagram is the triangle inside which no conventional means of transportation appears to exist. This is just the area where a WIG would be located, with a cruise speed of 100 to 400 km/h and a L/D of 15 to 30. So WIG boats do fill the speed and efficiency gap between marine and air transport.
Currently applications for fast (passenger) ships are limited to a range of about 250 km. A longer range would give an inacceptable trip duration because of the limited cruise speed (well below 100 km/h). For the same duration a longer range can be realised with WIG boats cruising at 2 to 5 times the speed of a high speed marine craft.
Fuel efficiency, the amount of fuel used per passenger per km, is proportional to the inverse of the transport efficiency, which is a measure for the efficiency that is especially suitable for comparing different types of transport vehicles operating at different speeds. Generally a more efficient vehicle has a higher transport efficiency, independent of its speed and L/D. The red line in the Von Karman-Gabrielli diagram representing the current maximum transport efficiency shows that WIG boats are efficient, because they are potentially close to it.
The cruise power per unit weight, expressed as the P/W ratio is very low for WIG boats as compared to other forms of transport as explained in the graph below. Full advantage of this very low power requirement in cruise flight can only be obtained when the installed power comes down towards the cruise power so that the engines run at their optimum rating at cruise setting.
The power to weight ratio of different modes of transport
The potential high fuel efficiency of WIG boats will be achieved with very large boats only. Smaller boats must cruise at higher relative heights in order to clear the waves, consequently the fuel efficiency of a small WIG boat will be very similar to that of a regional aircraft. Fortunately the operational cost of WIG boats do not consist of fuel cost only. Other contributions are maintenance, capital, crew and insurance. This is where the real advantage of small WIG boats over short-haul aircraft lies. Due to the marine nature of WIG boats they can be built and maintained much cheaper than aircraft which have to comply with FAA regulations. Also crew training is much less demanding, therefore crew cost will also be less.
The only competitors for some areas where WIG boats could be applied are short-haul turboprop aircraft, they are close in specifications, although somewhat more expensive to run. An obvious advantage of WIG boats in this case could be its flexibility, there is no need for a runway, any existing port would do. Of course WIG boats would be restricted to operation in coastal areas or large lakes where the wave height is limited. This is especially true for relatively small WIG boats.
Although it may be argued that the safety of a WIG is excellent since it is always above the runway, this very same "runway" also presents some potential problems. A WIG needs a certain minimum flying height to be fuel efficient so it will always fly at the lowest possible height, dependent on the actual wave height. The trouble is that waves are not equal in height along the route. Measurements in the past showed that the wave height is normally distributed. This means that most of the waves have a height around the average height, but that sometimes a very high wave can occur.
These very high waves can be three to four times higher than the average wave height. These so called rogue waves are caused by interference of different wave patterns and can occur very suddenly. A WIG must either be allowed to strike a wave every now and then (rigid construction) or fly at a height where it will never meet one and thus be less efficient than it could be. This problem will of course be smaller for larger WIG boats.
Another safety aspect of WIG operation are obstacles such as other traffic, islands and bridges. A good navigation and radar system must be present to warn the pilot of obstructions. Maneouvrability of the WIG boat should be sufficient to safely navigate around the object. Finally it is needless to remark that the (longitudinal) stability of the WIG boat must of course be excellent, so that no dangerous situation can arise during normal operation.