HMWPE – the perfect material for search and rescue boats?

When human lives are at stake, rescuers need to be able to rely on their boats 100%, even under the worst kinds of marine and arctic conditions. Sturdiness and resistance to damage are therefore the most important requirements for materials used to build search-and-rescue boats, and other boats used in harsh environments and challenging conditions, such as military or oil-recovery boats. HMWPE (High Molecular Weight Polyethylene) is not only very resistant to abrasion, puncturing and tearing, and retains hardly any permanent deformation, it also has other very useful properties, explained in the following comparison between HMWPE and other common boat-building materials.

The materials

HMWPE

HMWPE = High Molecular Weight Polyethylene, also called “PE500 grade”. It’s very abrasion-resistant and therefore often used in conveyor belts or kitchen cutting boards. Very often it is also (not entirely correctly) referred to as HDPE.

As a uniform sheet material without fibres, HMWPE is isotropic (= it transmits forces equally in all directions), which means no special design considerations regarding fibre direction are necessary.

Pros: ► Buoyant on its own, floats in water. ► Very high abrasion resistance ► Very high puncture resistance ► Low friction (on beach, ice, …) ► Hardly any permanent deformation ► Easy to repair (weldable) ► Easy to repair on site ► PE boats are silent. PE attenuates engine noise ► PE Boats over 6 meters are lighter than other boats

Cons: ► PE welding is a slow process, leading to more effort and costs. ► Big thermal expansion factor, assembly of boats difficult. ► Only black PE is ultraviolet-stabilized.

Aluminium

Sheet aluminium for boat construction is usually an aluminium-magnesium-alloy. As a uniform metal, Aluminium is also isotropic.

Pros: ► Lightweight ► High structural stability in larger boats

Cons: ► Non-buoyant on its own, sinks in water. ► Quickly deforms permanently when hitting obstacles (rocks, …), making the material brittle. Aluminium, once deformed, cannot easily be “bent back” without a high risk of further damaging the material. ► Risk of tearing, especially when catching on sharp obstacles like rocks or underwater structures ► Risk of galvanic corrosion /electric corrosion/bi-metallic corrosion in marine environment (salt-water), depending on which aluminium alloy is used ► Excellent heat conductor, very cold surfaces in arctic conditions ► Higher friction than PE ► Aluminium boats are very noisy

GRP

GRP (Glass-Reinforced-Plastic, “Fibreglass”) is used for RHIBs (Rigid Hull Inflatable Boats).
GRP is anisotropic, which means that, like wood, it has different properties depending on fibre direction. While this could be considered a disadvantage in flat material, this allows for optimized designs, re-enforcing structures “along the load paths”, making it a very interesting material in yacht design, where it allows for good weight/strength-ratios, while wear-and-tear resistance is of lesser importance there.

Pros: ► Lightweight. Good weight/strength ratio (if processed correctly)

Cons: ► In smaller boats needs air-filled tubes for both stability and buoyancy (risk of catastrophic failure). ► Less resistant to abrasion (rocks, ice) ► Less resistant to puncturing

The comparison

HMWPE Boat or Aluminium Boat?

Aluminium has its place as a boat building material where conditions are less challenging, and more consideration can be given to “treating the boat well”, for instance in private fishing or leisure boats. Because of its high structural stability, aluminium is also suitable for larger boats.

Where the boat has to perform flawlessly, even under very harsh conditions, HMWPE is a much more reliable material. Operators need to be able to focus on the task at hand, like saving human life, without having to worry that their vessel might get damaged or even fail them. No matter what “abuse” you put your HMWPE boat to, it will come through. Superior resistance to puncture, tear, abrasion (making some aluminium boats vulnerable to galvanic corrosion as a side effect), and permanent deformation (which aluminium is also very prone to) allow search and rescue crews to forget about integrity and performance of their boats when they have more important things on their minds.

HMWPE Boats or GRP-Boat?

The comparison between HMWPE-Boats (sometimes also called HDPE boats or PE boats) and GRP-Boats is more complex, as it is not just the material that differs, but the entire construction:

GRP-based boats are constructed as Rigid Hull Inflatable Boats” (RHIBs). They are similar to the traditional inflatable boats from which they were derived, in that Rigid Hull Inflatable boats get a part of their structural stability from their pressurized tubes, re-enforced by a rigid bottom/hull.

HMWPE-Boats, on the other hand, are designed as Rigid Buoyancy Boats” (RBBs). While appearing similar on the outside, they follow a fundamentally different design principle. The entire boat is constructed structurally stable, with buoyancy provided both by the hull itself and the boat’s rigid D-shaped pontoons.

The summary

All three materials have their unique strengths, and their place in boat construction.

► Aluminium boats can shine in “fair-weather conditions”, for instance as well-treated private boats, especially larger boats or yachts, as it is easily robust enough for the low demands in that area.

► GRP is an interesting material in yacht-building, allowing for optimized designs, a wide spectrum of shapes and a great weight/stability ratio. When used as rigid hulls/bottoms (to compensate for the inherent problems of inflatable boats), GRP fails to play to its strengths, though.

► When compared to both Aluminium and GRP, HMWPE comes out furthest ahead where conditions are most difficult. Rigid Buoyancy Boats made from HMWPE can take all kinds of abuse before even deforming, let alone puncturing or tearing. If they do get damaged/punctured, they still keep their stability and buoyancy, never endangering the mission, while being easily repaired later.

Where conditions are harshest, challenges hardest and demands most uncompromising, Rigid Buoyancy boats made from HMWPE allow crews to focus all their attention on their missions, while they can be certain that, no matter what they are faced with, their boat is up for it.
Simply put: high stakes under relentless conditions call for HMWPE boats.