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Cylinder Molding

This is a system promoted by West Coast designer Kurt Hughes and friends since the 1980s and is often confused with the Constant Camber (CC) of Jim Brown, John Marples et al, even though there are significant differences.

It is a system for laminating 2–3 layers of thin, full-sheet plywood into a skin of greater thickness with a preset curved shape, created by rolling the thin plies (typically 3 mm) over a form with identical 'aerofoil-shaped' sections of changing cylindrical shape, but with no lengthwise camber as for CC. As the whole panel is then vacuum-bagged, Kurt actually prefers to call it 'vacuum-formed plywood' (VFP) but the name 'cylinder-mold' (CM) is far more known, despite the confusion it inevitably creates with CC.

As Kurt describes the system in detail on his website giving sketches and photos, I'll not repeat all details of the method here. For this, go to: — Cylinder Mold Multihull Construction

However, I will independently comment on the system and highlight differences with some alternatives. (I also have a somewhat different take on some of the options that Kurt mentions and suggest you read about these in other chapters on this site.)

While the Constant Camber system uses a fairly complex mould of ¼ to ½ boat length to create molded-ply panels with curvature in both directions, the Cylinder Mold system more typically creates ply panels of full boat length. Both systems give more curvature along one edge than the other. Once cut to a pre-designed profile, they pull together to form a very good boat shape for the narrow hulls required for multihulls. The VFP (CM) system is also constantly under refinement and presently uses greater camber than is the norm for Constant Camber. This has permitted more modern hull forms of nearly semi-circular hull shape as shown in this section through a KH-23.

The main difference with 'Constant Camber' is that the VFP-CM system uses full sheets of ply to build up cylindrically curved panels, whereas the CC system builds up panels with some compound curvature through the more elaborate system of diagonal planking with pre-shaped strips (see the Method 4: CC system article for more details on this).

Kurt claims that there is no faster system for building a multihull, though naturally there are some who challenge that. But it's certainly 'one of the faster methods', though that could depend very much on whether the lay-up and vacuum-bagging of the plies all goes as planned. Starting with very flat sheets will help, though thin plywood can be hard to find without some deformation. Minor bulges should settle down IF the vacuum bag is well sealed and the vacuum sufficient. (See article on Vacuum-Bagging in the Construction Section of this website.)

One thing seems sure though: the system cannot produce totally identical hulls each time it is used. Very small differences in both the plywood itself as well as the profile cut and joint, will result in slightly different sectional shape. This is not a critical issue for most one-off builders but if a series were to be undertaken, they would not be exactly 'one-design'. Fitting bulkheads in place as the hull is formed might help achieve the required shape, however this risks to create flatter areas in-between unless the bulkhead bears against a good stringer that keeps the ply fair. Using bulkheads alone is not something Kurt recommends.

Earlier examples of both CM and CC seemed to have a sectional shape that was more elliptical with rather more 'rise of floor' (vee'd at keel as in photo) than the more semi-circular sections of later designs, and while this still works ok for the design of amas, it could be a significant limitation for a main hull seeking more volume. As the shape required for trimaran amas might well be slightly different from that required for the main hull, one might either need a different mould for them, or the same mould could be used with more wastage of the panel, cut away at the keel edge. Because the building mold requires that the panels are held at the edge of the mould, there could be a fair amount of scrap in this case.

I have already mentioned some concerns about using sheets that are not flat and I've seen or heard of applications of this method that, if not assembled carefully and also properly well sealed for the vacuum, have left panels with slight bumps and even areas of non-bonding between the plywood sheets, so care is required.   This highlights one of the challenges of the CM system .... making certain that the layers of plywood are truly bonded together, as if they are not, water will certainly find its way in and then the hull could soon be scrap.

Once cured, the panels generally still need some mild 'torturing' to take up the shape designed but the use of plywood rather than strips, helps to make a nice fair hull once it's all in place. The mould is quite simple and no backing sheet is required for the vacuum, as a large plastic sheet laid behind the first plywood, serves for that purpose. The mould might actually be either male or female, though Kurt has till now always worked over a male mold. (see photo, with kind permission of Kurt Hughes)

Both hulls sides can be built in the same mould and as previously mentioned, the mould is typically made full length of the boat. So once ready, the perimeter of the two full length panels are cut to a profile outlined on the plans by the designer and are then 'stitched' together with copper wire along the keel—as for a 'stitch and glue' dinghy.

A critical part of the final shaping comes from clamping the sheer edge (gunwale) to a pre-made deck jig that fits external to the final boat. (Also note that this also forms a useful shelf for tools while working ;-).

This deck clamping opens up the two sides and really creates and controls the final hull shape as well as holding it in place, while the hull is sealed and sheathed with glass and bulkheads fitted to retain the shape.

Advantages of the system are these:
  1. The use of large sheets covers area very quickly.
  2. The same use of large sheets also helps to assure a fair hull surface.
  3. The exterior sheet can be pre-coated and sanded while on the mold, to reduce finishing later.
  4. The mould required is simple and quick to build, and both sides can use the same mould without change.
  5. The system lends itself to a simple application of vacuum-bagging, with a large plastic sheet laid over the mold first and then sealed around the assembled sheets.

Disadvantages of the system might be:
  1. The assembly of large, full-length panels requires a large space for the mould as well as for panel storage and handling.
  2. Difficulty to construct a boat to exactly the body shape required, in order to achieve the buoyancy and performance originally designed in.
  3. Limitation of what hull shapes can be built, ie: no reverse flares, knuckles etc.
  4. Requires particular attention to the making and assembly of the very long scarf joints and their alignment. A good team of 3 or 4 can help at this stage.
  5. Potential difficulties to control the vacuum process as the panel is so large with lots of perimeter. Any failure at this time could be expensive as the whole panel could be scrapped.
  6. Extra work to measure and outfit the interior for bulkheads and floors etc, as this cannot be done from plans, due to the inherent variations caused by the basic system. See coming article on 'Fitting bulkheads'.
  7. The system limited to narrow hulls with a L/B ratio of say 10 or more.

Although it may seem like there are a lot of disadvantages listed, most of these can be readily worked around if you have the space, work carefully and take care with the scarfs, sheet alignment and vacuum-bagging. Overall, it's an interesting, economic system that will continue to develop within the limit of shapes that can be accommodated and personally, I tend to prefer CM to the Constant Camber which tends to result in a rather narrow, excessively vee'd shape, even if CC hulls can look good and ride comfortably.


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