Difference between revisions of "Constant camber"

Latest revision as of 16:00, 3 December 2009

This technique was pioneered by the late Jim Brown together with input from Dick Newick and John Marples, as a system to achieve the advantages of a cold molded boat but with less labor, and specifically developed for narrow multihull hulls. Designer Chris White also once trumpeted the system when he was creating his personal trimaran, Juniper, as well as for others of his design. [Footnote: Chris now leans more towards using an accurate mold and building with strip foam and glass … mostly to be able to precut bulkheads and other interior work and know it will fit.]

In constant camber (CC) construction, one creates identical panels of curved plywood sheets, which you then join to create the two sides of the hull. The plywood is typically formed by laminating together diagonal layers of pre-cut strips of wood.veneer or thin plywood. (The panel can also optionally be vacuum-bagged to the mold surface for improved bonding).

Once moulded, this formed sheet has the same curvature over its width – a somewhat aerofoil section with a deeper curve along the keel edge than the other. The mould typically has a slight (large-radius) curve over it’s length as well, so that the final form of each sheet has a slight compound curvature built in. This can be seen in this photo of a panel being lifted off its mould.

One advantage of keeping the “camber constant ” is that once you determine the right shape for the strips of veneer, you can use the same shape for all of them instead of having to individually trim and fit (spile) each one to fit together with the previous strip. Another is that by varying which part of the mold you work over, one can generally use the same mold for all curved parts of the boat.

By butt-joining the cambered sheets together, cutting to the required profile and further bending the corners, a generally fair hull shape can be created without the need for much internal framing other than bulkheads.

The typical elliptical-vee form is somewhat low in displacement however, so the resulting shape is best suited to relatively narrow, lightweight designs. The result can be attractive though (see photos of Chris White’s Juniper and Discovery 20) and as for any multi-directional laminated wood system, it is certainly strong and tough. A number of charter boats have also been built using this system – particular for day charters where the payload is light.

Although the hull shape can be somewhat controlled with guide frames and bulkheads, this system is still hard to build exactly to the dimensions called for by designer plans, so any question of building a strict one-design this way is out. Bulkheads are best shaped to fit after the hulls are assembled, as forcing the skin to fit a bulkhead, risks to cause unfairness and hard spots. (This latter comment also applies to the VFP (CM) system though the latter requires less fairing work as larger sheets are created with less joints). (See Method 5: Cylinder Molding)

The first step is to make the mould. This is then covered with a thin layer of ply in order to give an air-tight backing for vacuum-bagging the strips in place as one proceeds. Excess filler (phenolic micro-balloons or thickened epoxy) is cleaned off between each layer in order to assure good contact for the next layup. Once complete, the multiple units (whose number will vary with the design) are sanded and commonly coated with 2 coats of epoxy before assembly. The moulded sheets are joined with either ply butt-straps or the butts can be made with tapered layers of fiberglass tape after first preparing the joint.

The above sketch comes from Chris White’s fine book about ‘The Cruising Multihull’, and is reproduced here with his kind permission, along with the photo of ‘Juniper’ (below).

The exterior cutting profile is then transferred from the plans to the assembled side panels and once cut, the panels are brought together around temporary frames. The lower edge of the side panel is attached to a keelson and the deck edge spread for the deck.

Here on the right is Chris White’s elegant 52ft ‘Juniper’ hulls built using the Constant Camber system.

By varying the mould dimension and curvature, the system can be used for all different sizes of boats and CC was also used for Chris White’s very neat Discovery 20 design pictured underneath.

Advantages[edit]

The positive things about the system are that the mould does not need a huge space* and that the process uses wood which many builders are familiar and comfortable with. Also, the interior of the finished boat is relatively smooth as there is less need from internal framing. The mold can also be reused over and over again for each and all curved panels. Generally, two persons can create one panel per day if the ply strips are all prepared beforehand.

• (I should mention that some builders have chosen to make longer (even full length) moulds that reduce the need for butt joints … borrowing a feature of Cylinder Molding (CM) in that respect).

Disadvantages[edit]

The negative thing is that the system is labor intensive, particularly as the surfaces require the same degree of surface finishing as does a double-diagonal construction – with many hard-edged joints to grind in order to get the surface ready for glass sheathing and subsequent painting. All interior bulkheads and frames need to be custom fitted as it’s virtually impossible to build to precise offsets …. an issue this build system shares with several others. The mould, with its double curvature, is also relatively complex and also requires covering with a surface ply to act as an air-tight backsheet when vacuum-bagging is used.

Despite the above, as for all boats, it is important to remind readers that the hull or hulls are only a relatively small part of the whole project … depending a lot on how the interior is finished and equipped. To appreciate this, take a glimpse into the work on a homebuilt Constant Camber cruising trimaran, by going to this website and prepare to be impressed.

Rikki-Tikki-Tavi is a beautifully built CC40 to a design by John Marples, developed from an earlier Searunner 40 design by the noted trimaran pioneer, Jim Brown. Thankfully, a small trimaran for daysailing will be a lot less work than this beauty. http://www.svrikki.net/RTT/FrameSet.html

The boatshop with perhaps the most experience worldwide in building with the Constant Camber system, is located on Cebu Is. in the Philippines. Called ‘Boatshop Philippines’, they have reportedly built about 20 multihulls between 30’ and 60’ using this vacuum-laminated system.

Owner Mike Allen has designed a series of ‘Visayan Catamarans’ using CC – ranging from 32’ to 60’. Here is their VS 50 footer.

Source