Comparisons of Hulls
Hovercrafts are typically manufactured either from Composite Fibre Reinforced Plastic (FRP) and Polyvinyl Chloride (PVC) foam sandwich system or an Aluminium based construction system. In this article we have compared the hull structural systems as relevant to hovercraft, in particular commercial hovercraft in the weight range of 600kg to 8,000kg payload capacity.
A comparison of hull structural systems as relevant to hovercraft, in particular commercial hovercraft in the weight range of 600kg to 8,000kg payload capacity.
The Composite Fibre Reinforced Plastic (FRP) and Polyvinyl Chloride (PVC) foam sandwich system of construction can have many variations. The basis of comparison used here is based upon typical structures and construction systems as used in commercially produced hovercraft today. This is a balance between good structural performance, practical production techniques and to a lesser extent commercial considerations causing some downgrading of the structure and materials compared with the ultra high construction techniques and materials as used in the aerospace industry today. As a comparison if the most basic FRP boat was rated at 0 and the highest tech composite aircraft was rated at 100, then we consider that the structural performance of the hovercraft construction systems described here would rate at about 85 with a cost factor of about 60. This is a good balance between performance and durability in a marine environment while still using commercially viable construction techniques.
It is not appropriate for us to assess where the Aluminium based construction systems lie on a similar continuum. The last hovercraft hulls constructed in Aluminium according to the style of aircraft construction finished with the Saunders Roe series of hovercraft. All hovercraft built in Aluminium today use an advanced version of the typical Aluminium boat building style of construction. What is compared here is a direct relationship of the various properties of the described construction systems as they relate to hovercraft built today and in particular commercial/military hovercraft in the stated weight range.
By making this comparison a prospective client will gain a better understanding as to why the construction systems are so radically different and the many advantage the FRP/Sandwich system has over Aluminium when used for building hovercraft. It is also thought that the perseverance of some manufacturers with Aluminium may be related more to habit, ease of controlling their sub-contractors and certain complacency in the market place than to the technical reasons described below.
|Composite PVC Sandwich||Aluminium|
Damage caused by impacting a PVC foam sandwich panel results only in localised crushing of the core and some tearing of the laminate, usually only the outer skin. Even very severe damage does not cause damage to the overall structure outside of the immediate damaged area.
Limited Energy absorption
Accidental impact will result in distortion or puncturing of the panel and often distortion of the supporting framework requiring expensive and time-consuming repairs. A severe impact can distort the whole structure causing severe misalignment of machinery and fittings.
Small punctures seldom result in leaking due to the foam crushing. When the outer skin is pierced the foam core will be crushed and forms a cushion to protect the inner skin. Additionally as the outer skin is ruptured, the localised panel stiffness is destroyed and the inner skin now acts more as a flexible membrane that is easily deflected from the intrusion and springs back into place after the intrusion is removed.
While the panel will require repair due to structural damage, it is very seldom that this is of an urgent nature and can be performed at a later convenient time. Even with relatively large damaged areas, leakage is seldom enough to endanger the hovercraft.
Smaller punctures can easily result in leaks of sufficient size to endanger the hovercraft by flooding. Aluminium is much more likely to suffer damage requiring repair than FRP/Sandwich construction system.
All of the materials used in the manufacture of a FRP sandwich hull construction are supplied to the manufacturer in the form of liquid (resin in drums), rolled fabric or cut PVC sheeting. Because the construction is a moulding process that becomes hard (cures) as a whole part already formed there are practically no joints or high stresses built into the hull.
By good design it is possible to have the few unavoidable joints located in non-critical low stress areas. The materials used do not suffer from fatigue as metals do and therefore there is absolutely no risk of structural failure due to poorly maintained (out of balance) machinery or cyclic loading of any kind. (The energy absorbing nature of the materials helps here also).
Aluminium is supplied to the manufacturer in sheet, plate and extrusion form. Any construction requires that joints are made and the material is distorted (bent) to conform to the construction shape. This causes localised high stress which when coupled with cyclic loading (vibration from machinery and loads on the hull from normal operation) can result in fatigue cracking or failure of the joints. Welded joints result in localised high stress and at the same time reduction in strength. This needs to be taken into account by the designer and is usually compensated for by the addition of more material (= cost and weight).
Most structures are a combination of rolled sheet and extruded sections. It is well know that extruded sections have a typically low fatigue resistance due to their chemical composition.
Damaged FRP Sandwich structures are easily repaired by semi skilled workers and trained crew. Repair kits for minor repairs may be carried onboard and used in the field without any requirements for power or other repair facilities.
Aluminium structures require skilled tradesmen and are seldom able to be effected in the field (other than small temporary type repairs). Major damage causing the overall misalignment of the structure would require the complete stripping down of the hovercraft and ealignment or replacement of the hull.
High Panel Stiffness
Due to the thicker sections a sandwich structure provides there is a much-reduced requirement for internal framing in the hull. This results in cleaner and smoother internal surfaces thus providing more space and easier maintenance (cleaning).
Low Panel Stiffness
Due to the thinner nature of aluminium sheeting the panel stiffness is very low and considerable internal framing or local stiffening is required. This results in a loss of internal space and considerable internal clutter. Maintenance is harder both from a cleaning point of view and the need to repair or replace the internal stiffening in the event of damage.
This is not an issue with the materials used in the Airlift- Hovercraft’s hovercraft. All materials in the hull do not corrode All water exposed surfaces are manufactured from Epoxy or Vinylester based resin systems which do not suffer from Osmosis. In addition the exterior surfaces are painted in 2 pack Polyurethane paint which is the toughest and most durable paint available.
Even marine grade Aluminium is subject to corrosion and must be monitored carefully throughout it’s lifetime for signs of corrosion. Aluminium hovercraft typically use very thin sheet and it does not take long for corrosion to have a debilitating effect on the whole hovercraft. Typical problems are:
High Tech Laminates
It is possible to include high tech fabrics into the basic laminate design to impart special features to the laminate in select areas. This can be important in special areas where unusual loads are expected such as panels around personnel and engines that may be subjected to small arms fire. Kevlar is one such fabric, which can be used judiciously to provide a reasonable degree of armour plating.Other special areas which get high shock loading such as engine mounts and gun mounting points can make use of these materials so that much of the energy is absorbed by the designed flexing of the material thereby relieving the overall structure of much of the shock loading. This will result in a lighter weight and a more resilient structure, which translates into higher payload and less maintenance.
Low Tech Approach
Aluminium sheeting is manufactured in specific thickness’ and cannot be locally thickened without welding or riveting a doubler plate in position. It is very difficult to design in special reinforcing or energy absorbing materials into an Aluminium structure without incurring a significant weight or cost penalty.Engine mounts and gun mounts and the like will need to be over-designed in order to ensure good life when subjected to impact loading as their resilience and fatigue resistance are well below that of a composite structure. This weight penalty will reduce affective payload.
Composite FRP is more expensive than Aluminium. It is possible to build cheap composite hulls but the materials that would be selected for this criteria would be low strength and heavy. There is a world of difference between good quality composite design and construction and the cheap mass produced pleasure boats that are readily available.Quality control can an issue as the raw material (resin and fabric) essentially arrive in a drum or on a roll and are converted into a useful product by workers who may or may not have a good work ethic every day. This problem tends to arise in the factories where lowcost mas produced products are made. All good fiberglass shops and especially those working with the high cost high technology composite materials as we use in our hovercrafts have a quality control system in place that prevents rpoblems from arising.
Cost of materials is less than good quality composites. Also there is less need for stringent quality control in the construction process as the aluminium quality is controlled by the metal manufacturer. Note that low technique FRP structures such as in production type pleasure boats are not to be compared and a good Aluminium design which will generally be better than this type of FRP construction system (such as gun sprayed chopped roving and polyester resin systems).
Again the natural flexibility and energy absorbing characteristics of the FRP/Sandwich materials used ensure minimum noise is transmitted through the hull. This structure actually deadens engine noise.
Low energy absorption characteristics of metals (Aluminium) transmit all noises very effectively. This can cause additional crew fatigue. Additionally any noise control measures (insulation) required will add weight to the overall structure and reduce payload. In tropical conditions these materials can also soak up moisture causing added weight and deterioration of the material as well as hiding corrosion behind a damp material.
Freedom of Design
The homogenous nature of the FRP/Sandwich system allows greater design freedom both in the shaping of the structure and the placement of the reinforcing and stiffening materials. This means that a single panel may have a graduated strength and stiffness designed to match exactly the anticipated loading conditions. This results in a much more efficient structure (no useless material included) providing maximum structural performance for minimum weight thereby maximising payload and the overall hovercraft performance.
Because Aluminium is available only in standard sizes and any single panel must have sufficient thickness to comply with the strength requirements of the most highly loaded part of that panel it follows that there will be excess thickness in less loaded parts of the panel. Additionally overall panel stiffness requirements may dictate a material thicker than is needed to satisfy the panel strength requirements. This again leads to compromises of design and overall structure efficiency. Generally speaking a well designed Aluminium construction (particularly with large panel areas as in a hovercraft) cannot compete for stiffness against a well-designed FRP/ Sandwich structure such as used in the Airlift-Hovercraft hovercraft.
The PVC sandwich core is a natural insulator. This results in considerably reduced heat transfer c.f. Aluminium. Generally speaking this is a good thing for hovercraft hulls as cabin heating and cooling requirements are greatly reduced. A smaller air-conditioner is required. This weighs less and has a lower power requirement from the engines thereby having a compound improving effect on overall hovercraft performance.
Aluminium is one of the best thermal conductors available. In hovercraft hull terms this usually means that additional thermal insulating materials are required or alternatively the air-conditioning unit is upsized to cope with the additional heat gain through the cabin roof and from the engine compartment. All this adds weight and reduces engine power available to drive the hovercraft. Generally speaking this is very bad for hovercraft and passenger comfort.
Designers of performance craft of many types have long been aware of the many advantages of FRP/Sandwich construction systems. Although the system is generally gaining popularity it has been slow to be accepted mainly due to the requirement for greater quality control of production and a slow learning curve of production techniques by commercial builders.Many of the advantages of this system are not visible in the show room and therefore are not easily sold to a customer who has little appreciation of anything other than the nice shine on the outside. On the other hand one has only to look at any high tech racing boat or yacht and one will realise that this is the preferred system for performance and durability.Material, labour and quality control costs are high with this system, however when the overall cost of the product during its life cycle is investigated then the advantages of lightweight and durability far outweigh many other construction systems. For hovercraft this is particularly important as increased payload and reduced engine power requirements both have a compound effect on capital cost, operating cost and overall craft performance. A kilogram saved in construction is a kilogram of payload that can be sold over and over again.We have been building with this construction system since 1979 and have not had a single structural failure in the many thousands of hovercraft hours accumulated since then, many in very arduous conditions and heavy commercial service.
Aluminium is a good material; it has many advantages in commercial vessels that cannot be competed with by other materials. However each material and construction system has it’s place and unless the hovercraft constructor is prepared to employ the full high tech aircraft materials and construction techniques then Aluminium cannot compete with FRP/Sandwich systems for structural performance, efficiency or weight saving.Unfortunately, aircraft type construction systems are not corrosion durable in a marine environment and the resulting maintenance overhead is very high. Medium technology Aluminium construction systems as used by commercial passenger ferry builders are well refined and produce good quality, strong and fast passenger ferry boats c.f. the heavier steel vessels of yesteryear. It is a known fact that the British have pioneered hovercraft and prefer Aluminium construction systems.The Griffon and BHC AP1-88 hovercraft both employ medium technology Aluminium construction systems. It is also a known fact that these hovercrafts suffer from reduced performance mainly as a result of their heavy hull weight and the compound added on weight due to this construction system.The older SRN 4 and SRN6 types of hovercraft were built using an aviation approach to their structure. While the weight is low the cost of construction and maintenance was very high. Interestingly the new and independently designed British hovercraft ABS-M10 now uses the FRP/Sandwich construction system. This hovercraft is one of the best in its class.