General Concept of the Wavelength 780

General Concept

The current project is the result of a long sailing and work association with the trailerable trimaran and study of its design and engineering aspects.
This boat is intended as a very comfortable, roomy, stable, and easily driven trailerable tri, with a modern, stylish, eye-catching profile on the water and a clear cruising emphasis in construction and fit-out.

Fitted with a fairly conservative rig, compared with other more performance oriented tris in the general size range, the boat, although designed principally as a cruiser, is capable of exhilarating performance for club racing, with speeds of 15 knots plus being readily attained in moderate winds on the prototype. The high beam clearance, a particular feature of this tri relative to others in this size range, coupled with ample, though not extreme, float buoyancy, provides a dry boat in all but the roughest conditions, enhancing its cruising credentials.

The concept of this trimaran evolved as a potential replacement for my existing trimaran. As such, I viewed it as a test bed for the practical and theoretical ideas accumulated over an extended period of professional and personal involvement in trailerable multihulls. I was hopeful that the design also would prove attractive to potential ‘home builders,’ for a variety of reasons.

Some of the factors that I considered important were:

Size – this was critical. I knew that the trend to maxi trailerable designs for extra room and comfort had its limitations, a major one being the stress of shifting around a heavy mast section and having to raise the mast on a regular basis. This was a major limiting factor, if full advantage was still to be taken of the vessel’s trailerability. Maxi trailerables also require a maxi towing rig with the associated problems of extra braking requirements, higher fuel consumption etc. My view was that a boat in the 7.5 to 8.5 metre range provided the best compromise.

As a cruise oriented boat, high beam clearance above the water was considered a must to avoid most of the cold showers commonplace on many of the trailerable trimarans. Higher beam clearance also provides the option for more volume in the main hull and a longer term (rather than weekend make do) cruise layout with dedicated bunks, storage lockers, galley, sinks etc.

As a person with a background in design as a craftsman potter in an earlier stage of my life, I viewed the design shapes of the boat as a three dimensional sculpture in which form echoed function. It was important that the vessel had stylish modern lines that ‘clothed’ the functional requirements of the boat.

The Building Concept

An innovative cedar, ply, epoxy glass composite structure has been engineered to create a strong lightweight monocoque hull.

The main hull is built in four ‘quarter’ sections, based on the vertical divide of the hull along the centreline, and horizontal divide along the sheer line, using a half hull mould and a half deck mould. The mould frames are simply reversed after the first half deck and half hull have been made to produce mirror image pairs.

The major advantage of this approach is that the moulds can be set up in a more restricted building space and secondly, that climbing in and out of moulds is largely eliminated as most of the work can be done comfortably from outside of the mould at a comfortable working height.

The float, half hull and half deck moulds are all assembled in turn on the same common building stock as required.

Final assembly of the main hull halves is accomplished in a simple building trolley, fabricated from hardwood and particle board and fitted with heavy duty castors, allowing the finished hull to be easily moved around and away from the construction area.

Why Ply and Timber

Despite the trend  to foam sandwich “plastic” boats, and extensive use of exotics such as carbon fibre, timber still remains an attractive and less costly option for home builders who may feel more comfortable working with cedar and ply, rather than foam sandwich and the range of exotic (and expensive) reinforcing materials available. Preliminary research on the building concept indicated that the boat, in this size range, could be built lighter than a composite cedar strip plank/glass vessel, and probably equal, or close to the weight of foam sandwich construction, depending on the weights of ply chosen. The bare sailing weight of the prototype, including fore and aft rig, running rigging, motor and anchor was 1130Kg approx. This is certainly comparable with foam sandwich hulls of similar size and volume. Additional full cruising fit-out, depending on requirements, can be expected to add 100 to 200 Kg.

Despite an earlier involvement with commercial foam sandwich production and ‘one off’ cedar strip plank construction for building the often complex curves of modern trimaran hulls, I was still attracted to ply as a simple and economical building material, without the need for higher tech processes (vacuum bagging) or materials (foam, carbon fibre).

The problem for me was that I wanted to build a complex curvature, no compromise shape, normally outside the province of sheet plywood, and not fully achievable, even when ‘Constant Camber’ or tortured ply techniques are used. The normal way of obtaining these complex shapes in foam as a ‘one off’ building project is to lay up foam panels or strips over a male batten mould, or alternatively press them into a female batten mould with the aid of a heat gun to bend the foam. The foam panels are easily trimmed to shape to follow the changing curve of the mould.

Having worked alongside a talented pattern maker in earlier boat building days, where flexible ply was used extensively to build complex mould shapes, I was well aware of the potential of light thin strips of flexible ply for creating complex forms. The problem remains however, that in cutting strips of ply transversely across a sheet of ply, the axis of greatest flexibility, its longitudinal strength is compromised. In traditional cold moulding, this strength is restored when thin strips of ply, usually cut on the longitudinal axis, are laid up in double diagonal (or multiple) layer form and then covered in a light glass cloth. The strength of the layup resides in overlapping all of the joining edges. While successful multihulls have been built this way in the past, it is a time consuming process and results in a heavier hull than need be.

By contrast, in foam sandwich, the core material of foam has little intrinsic strength, and relies for its strength on the depth of separation of two relatively heavy glass skins to provide that strength.

In considering how “Wavelength” might be constructed, I arrived at one of those “Eureka” moments where I could see a way of combining traditional timber cold moulding with the essential elements of foam sandwich construction.
In building the vessel, I have sought to merge these two approaches.

Thin 3.6 to 4mm flexible ply strips, cut across the grain of a flexible ply sheet, are laid up in a female mould as for vertical foam strip construction. The narrow thin ply strips (170 -180mm wide) are easily trimmed with a jack plane to follow the mould curvature and are laid at approximately right angles to the longitudinal axis of the mould. Tech screws at the top and bottom of each strip hold them in place temporarily on the batten mould. After all the ply strips are fitted dry in the mould, light weight, bull nosed cedar stringers are then glued side on, rather than the traditional edge on, along the laid up strips at predetermined intervals. This is the first element in tying the structure back together lengthwise. A lightweight unidirectional glass is then laid up over this structure in one operation, effectively butt strapping the exposed joins between the ply strips, to create the inside skin of the hull. Any necessary bulkheads or other frames are then fitted before the half hull is removed from the mould and turned over. Minimal fairing of the outside faces of the ply strips is required before fitting the outside unidirectional glass layer and any additional glass reinforcement that is required.

 Empirical testing showed that the method, in its basic form, was significantly lighter than an equivalent cedar strip plank lay up and not too different from foam sandwich.

The system is flexible, in that areas requiring more strength, for example under the waterline and on the deck, the ply layers can be built double, where the second layer butt straps the first and where extra light weight unidirectional glass can be added selectively to stress areas. The result of the process is a very fair hull before external glass is added. As this glass layer is thin (plus any reinforcement), there is minimal fairing compound required to fair the whole structure. This compares favourably with foam glass boats where the necessarily heavier glass layups and reinforcement can often require an extensive build up of fairing compound to get a satisfactory fair finish.