Explosion Hull On Show

Sydney Morning Herald

Friday July 27, 1990

By PETER FORBES

One of the stars of the Sydney Boat Show which opens to the public today is the new Gelignite 35 aluminium cruiser-racer yacht designed by the late Ben Lexcen.

Lexcen's name is always a crowd-puller, but what is attracting most attention is the yacht's aluminium hull, which was literally blown into its final form by a short, sharp shock from a high explosive.

Sounds weird and destructive, doesn't it, but not so.

It is a high-tech production-line method developed by Sydney man Don Anderson to reduce the price of large aluminium boats, just as GRP(fibreglass) is used for repetitive production of large hulls.

As with fibreglass, a mould is used. In this case, a precisely built steel female mould is let into a concrete-lined "breathing bed" in the ground.

Into the mould are laid 5 mm sheets of aluminium that are then welded together in the basic shape of the mould. This shell is then filled with water and PETN high-explosive is carefully positioned inside it.

Next a big bang, a jet of water and a smooth, attractive, round-bilged aluminium hull is formed in a microsecond.

This explosive technique is known as the HERF (high-energy rate forming)method, a technique Don Anderson first saw in the United States, where it is has been confined to the forming of nose cones for missiles and aircraft in the aerospace industry.

Anderson is a chemical and mechanical engineer.

The HERF method, he says, reverses the traditional painstaking, one-off method of building aluminium boats by working from the inside out, first laying up and welding the frame, then putting on the skin and later fairing the resultant distortions with fillers.

This is the first time HERF has been used anywhere in the world on such a large scale and for the construction of a boat.

By the high-energy method, the hull comes out smooth and the frames are put in later to strengthen it. Indeed, it is here that that another technology revolution cuts in.

Welding in of hull-strengthening longitudinals would distort the smoothness of Anderson's hull, so he turned to the 3M company for its Scotch VHB (very high bond) tape.

With this tape, he glued in 10 longitudinals. Not only was distortion avoided, but the method was fast and easy. (For more on the tape, see below.)

After the stringers go in, seven cross frames are spot-welded to them.

Anderson says he is aiming at the Australian market and then overseas. He has been advised that there could be good markets in Europe and the US, and ultimately for licensed manufacture there.

His marketing adviser has been Balgowlah consultant Bruce Ritchie. In a commissioned report, Ritchie, who is no stranger to yacht design and production himself, gave the yacht good wraps after "months of exposure to the boat".

He reported that the Gelignite 35 came second in a Sydney fleet of mainly North Shore 38s and would have won "if there had been a protest flag on board".

He said he had found the yacht "stiff, seaworthy, easily handled by two people, quiet, attractive to the eye, lightweight, fast, spacious and, I believe, quite safe at sea". Other benefits were its freedom from rot from osmosis.

And the price? Purchasers will be looking at $166,000 (after sales tax and brokerage provision), which compares well with about $250,000 for an imported yacht of equivalent size.

Anderson has applied for international patents and plans to begin commercial production after the Boat Show, producing boats in the 6-15 metres range - five boats in the first year, 10 in the next.

DETAILS

Length overall: 10.71 metres (35ft).

Beam: 9.78m. Waterline: 9.78m.

Draft: 1.96m.

Displacement: 4.4 tonnes.

Ballast: 1.6 tonnes.

Rig: three-quarter fractional.

Sail area: main, 57.6sq m.

Keel: Ellipsoidal lead bulb.

Deck: Triaxial GRP with alloy honeycomb core.

Rudder: Elliptical foam-filled GRP on stainless-steel shaft.

Engine: Nanni 11hp twin-cylinder freshwater-cooled diesel.