Repair of metal boat hulls with riveting
The vast majority of pleasure craft produced in our country have metal hulls. They are made either of duralumin by riveting, or of aluminum-magnesium alloys by welding.
If the production of such vessels with established serial production in the factory is not very difficult, then at home repair of boat hulls, especially from duralumin, is a problem for many amateurs.
Recall that duralumin is an alloy of aluminum with copper, magnesium, manganese, silicon and some other elements, the total content of which in the alloy does not exceed 6-8%. Its most important feature is the ability to harden after quenching, a combination of low density with sufficiently high strength. Moreover, unlike steels, the maximum strength of aluminum alloys is achieved with «age».
For the construction of recreational metal vessels, sheets of duralumin of the D16AT brand are most often used. For example, motor-boat hulls are made of them «Ob», «Kazanka», «Progress», «Dnipro», «Amur» motorboats with riveted construction.
As a rule, the thickness of the lining of the bottom of motor boats is 1.5-1.8 mm, sides and deck — 1.2-1.5 mm, transom — 1.5-2.0 mm. For boats, the thickness of the bottom lining is 2.0-2.5 mm, the sides and deck decking are 1.2-1.5 mm.
During operation, the hull may receive various damages: dents and cracks in the skin sheets, knits and profiles of the kit; weakening of rivet seams and violation of their water resistance, especially in areas of increased vibration of the hull at the transom and at the beginning of the running waterline; holes in the skin and deck, etc.
The most dangerous damages are cracks, holes and violation of water resistance when the rivet seams are loosened. It is almost impossible to eliminate such damage by welding, since duralumin has a tendency to crack and reduce the strength of welded joints by 40-60% compared to the base metal.
Holes and cracks in the sheathing sheets can be sealed, for example, with a thermal adhesive (see the article «Repair of metal hulls of boats with thermoplastics»), but it is not always available at hand. Such defects can be eliminated in the old and reliable way — by riveting. However, it is necessary to know some features of repair and a number of technological techniques.
Small scratches and shallow nicks are eliminated by stripping with sandpaper and restoring the paintwork.
Repair of a through crack in the skin will require a little more time. First, carefully set its boundaries visually or with a magnifying glass. Then, holes with a diameter of 2.5-3.0 mm are drilled at both ends of the crack to prevent its further spread. Then a reinforcing pad is placed inside the case. In terms of area, it should cover the crack from all sides by about 25 mm. It is better to choose the material and thickness of the lining the same as they are on the repaired skin.
From the edge of the cut-out lining, the burrs and chamfer are removed with a file, the rivet holes are marked and drilled. The diameter of the hole should be slightly larger than the diameter of the rivet.
For riveted connections of duralumin housings, anodized rivets made of D18P and B65 alloy are used; for aluminum-magnesium alloy housings, they are made of AMg5P alloy. The distance of the centers of the rivet holes from the edges of the lining should be at least 1.5 diameters; in places to be stamped, 1.75 diameters. When repairing in inconvenient places, it is allowed to reduce this distance to 1/2 of the diameter. The cover plate is temporarily installed in place and holes are drilled through it in the skin. Then the cover plate is removed, the surface of the case is cleaned of chips and burrs, degreased with any solvent, such as acetone, white spirit, etc. Before the final installation of the lining, it is necessary to lay a sealing thiokol tape or mastic under it.
|Rivet diameter depending on the thickness of the sheets to be joined|
|Total thickness of sheets, mm||1,6||1,8-2,3||2,5||3,0|
|Rivet diameter d, mm||2,6-3,0||3,0||3,0-3,5||3,5|
Deep dents in the skin can be straightened by heating the metal with a blowtorch or a gas burner and knocking out the bulge with a hammer. The straightened place is cleaned to a shine with sandpaper, degreased and leveled with putty, and then, after the putty dries, it is tinted.
The hole is sealed like this. First, the exact area of the hole is determined. If the hole has torn bent edges, it is better not to straighten them, but to cut them out, giving the hole the correct shape of a circle. From the inside of the case, an overlay is installed, the diameter of which should be 50-60 mm larger than the diameter of the cutout, in order to provide the entire perimeter with the overlap necessary for riveting with a dense double-row checkerboard seam with a pitch of 15-20 mm rivets. A round insert is placed flush with the skin in the cutout so that the gap between the insert and the edge of the cutout does not exceed 0.5-1.0 mm. The insert is riveted with a single-row seam with a 20 mm pitch. To avoid the appearance of crevice corrosion, it is necessary to lay a sealing gasket between them or lubricate the surfaces to be joined with an epoxy compound before riveting.
Larger holes are sealed using the same technology, but the lining inside the case is not made solid, but ring-shaped, with an inner diameter 50-60 mm smaller than the diameter of the liner.
In all cases, it is recommended to rivet all overlays and seals with the same rivets on which the main body is assembled.
To seal the resulting joint, thiocol sealants of the U-30M brand (according to GOST 13489-68) and UT-37 (according to TU 51-38-14-179-67) can be used. The basis of these sealants is a liquid low molecular weight polysulfide polymer — thiocol, which, when vulcanizing agents and an accelerator are introduced, turns at normal temperature into an elastic rubber-like material that practically does not shrink. U-30M sealant protects metal surfaces in contact with fuel and water from corrosion. Due to its low adhesion to aluminum surfaces, it is applied to special adhesive sublayers. U-37 sealant protects surfaces to a greater extent from the effects of high humidity, sea and fresh water. It does not require adhesive sublayers.
In case of significant damage to the skin, it is necessary to replace the entire sheet or most of it. In this case, the thickness of the sheet, the amount of re-cutting along its edges and all the elements of the connection are assumed to be the same as at the nearest junction of the sheathing sheets. A new sheet is applied from the outside to the body and drawn in place, then cut to a clean size. If necessary, by tapping it into the desired shape, preheating the sheet to 350 °C and slowly cooling it in air. Temporarily, the sheet can be attached to the set with assembly bolts, installing them through 200-300 mm. If necessary, leveling pads are installed between the set and the sheet — strips of duralumin D16AT. All contacting surfaces must be pre-primed. At the end of the fitting, rivet holes are already drilled in the sheet, the sheet is removed, the touching surfaces are cleaned and degreased. A sealing tape (sealant) is laid over all connections, then the sheet is put in place and tightly compressed according to the set using assembly bolts. In the future, the assembly bolts are removed and replaced with rivets.
Defects of rivet joints
1 — excessively riveted closing head; 2 — skewed head; 3 — not fully riveted head; 4 — displacement of the head; 5 — the diameter of the rivet hole is too large; 6 — undercut of the embedded head; 7 — the rivet is not sufficiently seated; 8 — the head is broken in the center; 9 — riveted sheets are not compressed (not settled): 10 — the rivet metal is too hard; 11 — metal undercut at the embedded head; 12 — the rivet is skewed due to the mismatch of the holes in the sheets; 13 — the countersink is too deep for the countersunk head; 14 — the casing is too strong; 15 — the countersink is too small; 16 — the rivet casing is too small; 17 — the misalignment of the embedded head; 18 — poor-quality casing of the sheets.
If it is necessary to replace several damaged parts, then it should be done gradually — one at a time and carefully so that the repaired housing does not lose rigidity.
When installing new parts, old rivet holes can be used only if there are no cracks or crumpling of metal next to them. It is best to drill holes for rivets of the nearest larger diameter.
It must be remembered that the length of the rivet rod should be equal to the total thickness of the riveted sheets, combined with the amount necessary to form the closing head. For a semicircular head, this value is 1.5d, for a conical one — 1.3d, for a semi-secret one — 1.1d, for a secret one — 0.9d. Rivets with a semicircular and conical embedded head are used for riveting the joints of the set. The bottom lining in the stern of the planing boats to reduce water resistance is advisable to rivet with rivets with secret embedded heads, provided that the thickness of the sheet allows you to make a countersink. The rest of the repair of the bottom lining, sides or decks can be riveted with rivets with flat-convex semi-secret heads.
The sockets for the embedded heads of countersunk and semi-secret rivets are countersunk at an angle of 90°. The depth of the sockets for the countersunk embedded head with the reverse method of riveting should be 0.1 mm less than the height of the rivet head, and with the direct method — equal to the height of the head.
With the direct method of riveting, hammer blows are applied from the side of the closing, i.e. the riveted head, with the reverse — from the side of the embedded head. The reverse method is used when, for example, it is inconvenient to strike from the inside of the case (as a rule, rivets are laid from the outside). If the embedded head is not flat, then blows are applied to it through a crimping with a hole in the shape of the head. The formed head is formed by flattening the end of the rivet rod on a massive support.
Riveting in a direct way is performed somewhat differently. A support is installed under the embedded rivet head, a casing is put on the rivet rod, the connection is sealed with several hammer blows, after which a closing head is formed by hitting the rivet rod protruding from the connection with a hammer. If necessary, the closing head is formed by crimping.
If there are cracks in the frames and stringers, it is mandatory to install duplicate linings. If the stringer is destroyed at a relatively long length, the damaged area should be cut out and replaced with a new insert. It is fitted to both ends of the remaining parts of the stringer as tightly as possible (the gap should not exceed 0.2 mm) and is connected to it with the help of butt pads-shorties.
The seams fastening the corners of the set to the skin are made in single row with a step from 8 to 10 rivet diameters. Waterproof, durable and dense seams are made in two rows (rivets are staggered) with a step in each row from 3 to 5 diameters. The distance between the rows of rivets, depending on the type of connection, is usually assumed to be from 2 to 5 diameters.
In places where leaks appear at the seams, traces of corrosion most often appear near weakened rivets. With a light tapping with a hammer, weakened rivets can be detected by a rattling sound. As a rule, the holes under the weakened rivets turn out to be highly developed, have an oval shape. Such rivets cannot be tightened, it is better to replace them with new ones of a larger diameter. Loose and dropped rivets, rivets with heads disconnected to the hidden part flush with the sheet, and with a bare countersink of the sheet are also subject to replacement. The rivets are most often damaged near the transom, where strong vibration from the running motor affects, and in the middle part of the bottom, which accounts for the greatest shock loads during the course of the wave.
It is impossible to remove damaged rivets by cutting off the heads with a chisel, since the edges of the holes are inevitably damaged, cracks and tears form in the sheathing sheets and shelves of the set. Rivets are recommended to be drilled. To do this, first you need to tilt the center on the embedded head, substituting support from the closing head. Then, with a drill bit whose diameter is equal to the diameter of the rivet rod, a hole is drilled to a depth equal to the height of the head. After that, the embedded head is easily broken off, and the rest of the rivet is knocked out with a beard, the diameter of which should correspond to the diameter of the rivet being removed. After the repair, the quality control of the riveting is carried out, as well as the water resistance of the rivet seams in the outer skin sheets. The parts to be connected must fit snugly together. The rivet heads should be tightly tightened to the surface of the patches or the surface of the skin without nicks and tightening.
Removing rivets from the repaired structure
a — the correct way of drilling to a depth equal to the height of the head; b — the wrong way of drilling; c — removing the rivet with a punch and ring support.
After the repair is completed, the surface is cleaned with sandpaper and degreased with any solvent, preferably gasoline B-70. The hulls of repaired duralumin vessels are primed with phosphating soils VL-08j or VL-02j (yellow), while an acidic diluent is introduced before using these grades of soils (approximately 30 minutes). For the primer, you can also use zinc whitewash, zinc yellow or green crown, as well as EP-51 white epoxy paint. GF-031zh and FL-OZZH soils have high operational properties. The second of them is suitable for applying subsequent layers of paints on almost any basis — oil, pentaphthalium, etc.
After priming, the body can be painted with oil paints based on zinc whitewash, for example, ochre, ultramarine, cobalt, etc. It is better to use not oil paints, but oil enamels, which have greater strength and give a good gloss. However, in any case, the paint must be selected in accordance with the previously applied one. Both the aesthetic appearance and the safety of the case depend on the correct selection of paint.
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