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Constant Camber

This technique was pioneered by 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 its 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 reasonably fair hull shape can be created without the need for much internal framing other than bulkheads.   I add a note of concern here, as 'butt joining' these otherwise rigid panels will be a highly stressed potential trouble area, where water may later infiltrate. 

(Footnote: My own way around this would be to let any butt strap only serve as a partial connection on the inside, and to then grind out the butted joint on the outside in a broad vee, to lay in fiberglass tapes of various widths, so that the joints are made secure throughout the full thickness of the shell, prior to exterior sheathing).   

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.

In terms of performance, the CC system is what I would call 'a mixed bag'.   While the form offers a gentle ride in rough conditions, its not an ideal one for a main hull in many other aspects.    First of all, the shape is very vee'd and this not only offers far less foot space within a main hull but creates section shapes that have a fight with passing waves.   This will depend on whether the vee'd shape is at the waterline or not.   (On the larger, well loaded boats say 40ft or more, it's possible that the vee'd part is mostly underwater, so the potential problem I will outline here, may apply less to such boats).  

The potential issue is easier to understand if we first imagine a light CC boat at anchor facing head-on waves.   As each wave peaks pass down the hull length, the water molecules are forced horizontally outward by the vee'd sections if they are much wider above the still water line.   If we now put such a hull into motion against such waves, we can visualize the wasted energy in doing this useless, horizontal work. In addition, this action also creates a more disturbed surface that the wind picks up and blows over the boat as spray, making such boats wetter than they need be.   Further, as the vee'd shape is often retained forward giving a more 'old-tradition' bow overhang, this gives a high increase in buoyancy right up forward, resulting in the bow being thrown quite forcefully upwards, and then, once the wave has passed, the bow falls deeply back down into the wave trough. This starts an exaggerated pitching action, aggravated by the highly vee'd shape and not only adds more resistance of its own, but also upsets the air flow over the mast and sails that are now being thrown quite violently back & forth into the oncoming wind.  

But to be clear and fair, all boats will pitch to some degree as the boat interfaces with waves of varying heights.  However, when the sides of the boat are more vertical, the buoyancy pick-up is much less extreme and that wasteful 'horizontal action' from excessively vee'd hulls, is much reduced.    This combination of factors not only limits the speed of such boats using such a vee'd shape, but can also introduce what has been called 'a nervous, squirrely feeling' starting at a Speed/Length0.5 ratio of around 2 (which is really not fast for a good multihull), as the waves create unequal side forces on the hull at any one moment, possibly requiring a larger rudder to be fitted.   (The longer the boat, the less the effect though, so the 52ft Juniper would probably not even notice this).

Coupling all this to the loss of foot space inside, personally leans me well away from all hulls heavily vee'd near the waterline and those created by the present Constant Camber system more often fall into that category.

Construction notes:

Although the hull shape can be somewhat controlled with guide frames and bulkheads, this system (like that of Cylinder Molding) 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.



Photo by kind permission of Chris White and Onne van der Wal. (www.vanderwal.com)

Advantages:
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:
The negative thing is that the system is fairly 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.

Since 2000, I have personally become less enthusiastic for the very Vee'd shape offered by Constant Camber, as despite the  appearance of a smooth elegant shape,  there are performance compromises to consider.    It's all too easy to view a rounded hull (monohulls also) and get 'carried away' by its generally sweet form, but forget that it's the resistance to FORWARD motion that is the most important one to consider.     Consider this.   A boat makes 'a hole' in the water directly related to it's weight.    To move forward, water needs to be moved aside.   If this is done mostly at the surface, waves are too readily created, so ideally, this 'parting of the water', should be done as deep as possible below the surface.   Deep 'U' or box shapes achieve this better.   Vee'd shapes give little buoyancy for their depth and surface friction, and as they move through waves, plus up and down while pitching, they push surface water outwards in a pumping action that does nothing to help forward motion.   CC shapes also tend towards a banana shape in profile and this further encourages pitching which is something we want to negate and not encourage.    Foot space is also compromised compared to a deeper box or U-shaped hull, so unless the hulls are very large, interior space may be unnecessarily penalized.   While CC shapes may permit a gentle up and down ride in waves, my personal view is that hulls shapes with a deeper 'U' or box form and straighter lines in profile, will offer faster hull shapes for the important extended mid-range speed.      You may read more about this in other articles on this website.


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 42ft Constant Camber cruising trimaran, by going to this website. http://svrikki.net/Build/RTT/The_Build.html

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.   (But even here, later reports have described many rot issues with water infiltration into plywood).    Thankfully, a small trimaran for daysailing will be a lot less work than this one.

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.

 

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