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BOAT BUYING TIPS (3) – Boat building techniques

There are essentially two types of aluminium boat-building techniques used in Australia for recreational trailer boats…

Pressed sheet ‘tinnie’ construction – good for smaller boats that will be used in calmer waters such as rivers, lakes, estuaries and bays on calm days; and

‘Plate’ construction – great for bigger boats over 5m in length that will be used for bay, coastal and offshore boating.

TINNIE CONSTRUCTION – The tinnie method involves using an aluminium alloy with a softer temper that allows the aluminium to be easily pressed into shape. (Often cut from coils of aluminium rather than from flat plate.) Softer temper means lower tensile strength.

This softer material is typically pressed in a brake press to give the sides of the hull that distinctive ‘clinker’ look, which adds strength.

Some manufacturers now choose not to press the hull sides, to give the impression the boat is a plate boat. Flat sides don’t turn a tinnie into a plate boat. In fact, this process will significantly reduce the vessel’s overall strength if the framing is inadequate.

The next step involves inserting the bottom, side and deck material into slotted extrusions around the gunwale, chine and keel. The whole structure is then stitch-welded together or welded along the extrusions. A number of ribs are then welded between the extrusions (down the sides and under the floor), in an attempt to give the structure some strength.

This design often relies upon plywood flooring to brace the hull. Typically, a piece of marine ply, covered with carpet, is fitted as a floor and held in position with a few tabs welded to the ribs or a few self-tapping screws. Often, plastic mouldings are then used to fit-out the dashboard, side pockets and cabin areas.

The tinnie construction method is fine for smaller boats, which are to be used in calm conditions, but it simply can’t provide the durability of a true plate aluminium hull.

PLATE CONSTRUCTION – Plate boat builders usually use a much stiffer, harder, higher temper material cut from thick plates of high-tensile aluminium.

In aircraft construction, they also use high-tensile material as it withstands much higher forces before it fatigues and fails. Higher temper means a more ‘springy’ material that is harder and tends to spring back when deflected.

High-tensile 5083 DNV certified plate aluminium is the material of choice among reputable plate boat builders, whereas coils of softer alloy material is usually the choice of tinnie builders.

Good plate boats also draw great strength from a sub-floor frame that supports, braces and stiffens the hull. (Plate boats don’t have ribs around the inside of the hull. If you can see ribs or extrusion frames running down the sides under the side decks, it’s a dead giveaway the boat is built using the tinnie construction method, rather than true plate boat construction.)

There are two typical sub-floor systems used to build plate aluminium boats.

1. The way most other manufacturers build plate boats

2. The way Bar Crusher builds a plate boat using Rigideck® – the toughest sub-floor system in the world

Many plate boats use cross-frames and flat bar to strengthen the hull structure. This technique involves the placement of a number of frames running across the hull set at varying distances. Between the cross-frames, flat bar (typically 50mm x 6mm) is often stitch-welded to the bottom plate.

This system is commonly used in the construction of most other plate boats in Australia. If the cross-frames are situated too far apart, the bottom plate of the boat can deflect and deform. The flat bar provides some strength, but it needs to be braced properly to minimise deflection.

Bar Crusher boats are built using the true plate boat design and construction method, to produce a super-strong hull with almost zero flex. The Rigideck sub-floor system uses full-depth and full-length longitudinal frames so everything is boxed and triangulated. For even greater strength, Bar Crusher uses cross frames between the longitudinal frames to tie the whole sub-floor together.

If you think of an aeroplane wing or aerospace honeycomb structure you will be able to visualise the Rigideck system. All the sub-floor framing is braced in all directions to cope with tremendous stresses. (The reason every aluminium boat builder doesn’t use this system is because it’s more expensive, harder to do and requires better welding and fabrication skills.)

Bar Crusher’s unique Rigideck system provides a solid sub-floor frame that supports the entire hull structure of the boat.

Contact Bar Crusher for more information and advice about buying a boat.

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