Expert Oliver Franzius demonstrating the portable examination device. Here the specific testing knob scans the rudder blade.

 

Experts expect a new method to examine glss-fibre yachts to reveal damages and osmosis endangerment. (Article from: "Yacht" 09/97)

A shocking incident and it's results. The gaff schooner, ready to go alongside, badly miscarries tha landing and despite frantic attempts to put an brake by fullstop and return engine, several tons of wood rush still high speeded straight into the other party's glass-fibre yacht. There is a terrible groaning and creaking to be heard throughout in the whole harbour, but in that incident luckily nobody got hurt.

After the shock while both yachtsmen check the hull of their yachts, they surprisingly would not find anything else but small and seemingly insignificant chinks.

The crew of the rammed yacht however still can't reasure it's doubts and has the yacht been checked in an nearby shipyard. The opinions of the experts differ extremely. While one of them only speaks of harmless chinks, the other predicts a write-off. The shipyard's experts propose various test drilling through the hull, the deck and the buildings and others declare the freeboard surface must be sanded down to the lamina.

Aren't there any other methods of checking the laminate from possible cuts inside which would avoid the destruction of the examined surface? Until now the answer was no. But recently Oliver Franzius, a ship builder and sports boat expert from Hamburg, had the idea to use ultrasonic method on the a.m. purpose the same way it is used for a long time for material checks on metal, windmill wings and aircraft pieces made of glass-fibre compound material.

Testing knob of the ultrasonic dev.

Franzius was the first to invest into a portable device to "x-ray" the substance of fobre-made hulls without damaging the laminate or gel coat. This is said to enable a much more effective evaluation of the damage and the costs of repair assessment than complicated test drilling would allow. The examination costs are not charged according to the yacht's value, but the time and work spent an damage check.

Graphic of the checking device:
the ultrasonic waves are reflected by the backside of the intact laminate (2. Oscillation lefty/from left). Here the echo of the air bubbles can already been seen (to the left). The intact laminate shows a measured diameter of 5,9 mm, indicated on the right above.

For examination of the glass-fibre compound (the so called GFK-examination) the expert uses the qualities of ultrasonic waves which only can percolate homogenous material but is reflected by layers of air. Pressing the GFK-testing-knob agoinst the fibre hull of the ship, the sonic permeates the laminate and is reflected at the point, where layers of air limit the laminate. Generally this is the backside. Ist also may happen that the sonic waves meet laminated wells, small bubbles inside or hidden delaminations. In that very moment the amplitudes shown on the portable monitor change abruptly and indicate only a flat echo. In this depth the laminate does not perform it's necessary homogeneity any more.

But to interpret correctly the dancing oscillations of the amplitudes on the monitor, it is inalienable to know the exact structure of the laminate and to know a great deal of the glass-fibre compound technology.

Oliver Franzius acquired this special knowledge during a couple of years, studying at diffrent shipyards as for example in New Zealand. He trained his interpretation skills with artificially faulty laminate material and now he is able to tell even indistinct and smooth oscillations of the amplitudes indicating intact "sandwich"-lamina from the signs of defectively glued parts of the nucleus. His first big success was to discover in a 6 years old GFK-yacht a couple of bubbles together with a defective resin percolation inside the "sandwich"-laminate, which led to a complete renewal of the exterior laminate-installations.

The damaged part of the laminate is revealed by moving the testing knob along the surface.

It's not only those spectacular damages, that Franzius is after but also the microscopic bubbles inside those first layers of lamina behind the gel coat in the submerged parts of the hull, which are exposed to the water's physical influence that would lead to the frequent and feared osmosis. This above described examination method can be performed also in examinations of rig parts, aluminium and carbon-fibre mast to show microscopically small chinks. Last but not least ist must be mentioned that quality safeguarding on the shipbuilding sector is of utmost importance.

Here, the delamination is fully visible. The clear ultrasonic echo shows the remaining laminate diameter up to the air bubble inside the laminate. The material above the bubble only shows a diameter of 2,5 mm.

Furthermore, another possibility could be to certify those second hand yachts coming from third parties i.e. countries, which will not be allowed to enter the EU without the requested "CE"-mark as per June 1998 (s. for ref. Yacht 2/97). In the future through sonic measuring methods it can be flwlessly proced that the ship had been build according to the technical standart.

(Author and Photos: Kai Greiser)