Repair of metal hulls of boats using thermoplastics
The repair of metal hulls of boats and yachts has always been a difficult matter, especially if they are made of alloys based on aluminum, magnesium, copper. Brew a crack (see the article «Repair of metal hulls by welding») or a hole in such a hull at home, and even more so in field conditions, sometimes it's just impossible.
It is especially difficult to eliminate holes when using welding; due to thermal and shrinkage phenomena, deformation of the housing at the welding site is almost inevitable.
A simple and affordable way can be to repair the hull of a motor boat or boats polymer thermoplastic composition, which consists of fiberglass and a thermoplastic material applied to it — polyvinyl butyral. Instead of fiberglass, metal plates of the desired thickness can be used.
The most preferred are fiberglass grades having high physical and mechanical properties, for example, ASTT(b) — C2-0, T-11-DHW-9, etc.
Polyvinyl butyral is a product of the interaction of polyvinyl alcohol and oil aldehyde. It is produced by industry according to GOST 9439-85 of various brands: PP, PSH-1 (PSH-2), NK, etc., in the form of white powder. It has good adhesive properties both to fiberglass and to metals (steel and aluminum), is frost-resistant, has high mechanical strength, is sufficiently water-resistant, insoluble in gasoline, kerosene, fats, etc.
Polyvinyl butyral can be used of any brand, but as practice has shown, the most preferred brands for repair are PSH-1 (PSH-2).
The polymer thermoplastic composition is made in the form of patches of various sizes. If the area of the hole is not more than 500 mm, it is better to use a composition based on fiberglass, if more than the specified value, it is desirable to use metal plates.
The patch is made in the following sequence. First, the fiberglass is marked and cut. The edges of the workpiece are coated with glue No. 88, «Moment» or others. and they are kept until the glue dries, this prevents the edges from spreading. The workpiece is placed on the surface of the heating plate (preferably electric) and pressed with a frame with a lid. Instead of fiberglass, you can use metal plates of the required sizes, having previously degreased them.
For uniform and high-quality melting of polyvinyl butyral, the working surface of the plate is preheated to a temperature of 270-300 °C. Polyvinyl butyral powder is poured into a fiberglass blank with an even layer 5-7 mm thick and continued heating until the powder melts completely. Work on the preparation of the patch must be carried out under the hood, as the vapors of polyvinyl butyral are toxic.
Plasters of the required sizes can be made using an electric furnace (thermal furnace). To do this, a blank of the required dimensions is cut out of sheet steel. Then it is degreased with any solvent and after drying it is heated in a thermal furnace to the required temperature. Fiberglass is laid on a steel sheet, a layer of polyvinyl butyral powder is poured on it and placed back in the oven until it melts completely. If the patch is made using metal, then after heating it in a thermal furnace, the metal billet is pulled out and placed in a heated state on a layer of polyvinyl butyral powder with a thickness of about 7 mm. After 3-5 seconds, the workpiece is again placed in the oven and kept there for about 1 min, until the powder adhering to it completely melts. The cooled patch is cut to the desired size. It can be stored for a long time in a plastic bag at any ambient temperature.
The technological process of repairing holes in the hull of a boat or yacht is as follows: the defective places are cleaned of dirt and oil, washed with a solvent (acetone, white spirit, aviation gasoline, etc.) and wiped dry with a rag. It is advisable to remove the paint around the hole at a distance of up to 200 mm from its edge. The edges of the hole must be aligned. The edit begins with the edge closest to the dent, on which a number of hammer blows are applied within the convex area. Thin sheets of the case are ruled by light wooden hammers (mallets) or hammers with copper or brass tips.
Under the hammer blows, the material around the convex place is gradually leveled. It is desirable to cut off the burrs. If there are cracks in the hole, then holes with a diameter of 3-5 mm should be drilled at their ends to prevent their further spread.
Then along the perimeter of the hole, retreating from its edge at least 50-70 mm, the surface is cleaned with a metal brush, re-degreased with any solvent.
After preparing the defective place, a patch of the required shape and size is marked and cut out of the patch. It is made in accordance with the shape of the hole, and the overlap of the hole lining should be within 20-40 mm; if there are holes limiting the spread of the crack, they should also be blocked.
The prepared defective place and the pad are heated with a blowtorch for 30-60 seconds, during which time the polyvinyl butyral has time to melt. After that, the patch pad is pressed with an adhesive layer of polyvinyl butyral to the surface of the hole, taking into account its overlap. Through the asbestos cloth, in order not to get burned, the lining is rolled with a metal roller or evenly pressed with a rag. A polyvinyl butyral melt should protrude along the perimeter of the lining.
The curing time of the patch corresponds to the cooling time of the case at the site of the hole to a temperature of 20-30 °C. If the patch is prepared in advance, it takes no more than 5 minutes to seal the hole. If necessary, the patch can be applied on both sides of the hole or in several layers, reheating this place.
When repairing, it must be remembered that the roughness of any metal surface improves adhesion. Therefore, before sealing the hole, cleaning the surface with a metal brush is mandatory. Moreover, a thick layer of polyvinyl butyral on the surface of the patch leads to the appearance of additional defects: pores, discontinuities, etc. Therefore, the thickness of the layer after curing should not exceed 0.5-1.5 mm.
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