QUESTION: Can you please describe some basic ways of using plywood for the construction of a cost-effective multihull and explain how each shape might effect boat performance?
ANSWER: Although we will primarily consider a number of design options for using plywood, most of these will also lend themselves to any flat panel—and that could even include foam slabs with fiberglass skins, although this might not be recommended for the under-keel portion of the main hull where a denser core would generally be needed. (Sections C & E)
So let's look at six symmetrical options. Here's a sketch of each… with an indication of a ½‑round section offering the same volume for comparison, being as that's the shape with the least surface area and lowest skin friction. While the first 2 (A,B) are primarily potential designs for a narrow trimaran amas, the last 4 (C,D,E,F) could work within limits, for either the main hull or ama, though I do not personally recommend shape D for the reasons given below. There's also no reason that the designs cannot be combined for a trimaran, with one section used for amas and another for the vaka (main hull).
Section A is what Lock Crowther used for the amas of his successful Buccaneer series.
With a fairly deep central Vee, the hulls slice fairly easily through the water and also come down into wave tops pretty gently too. The relatively deep Vee will provide good directional stability and also help to reduce leeway or side-drift. However, the single chine will add some unavoidable form resistance, particular if used for a main hull and combined with much rocker. [It's important to note that with all these plywood shapes, I personally believe that it's the rocker that adds resistance. If the knuckle line can start low and stay straight, then the form resistance from a knuckle on a very narrow hull should stay very low in most reasonable conditions]. As far as construction is concerned, the shape involves three underwater joints that each require careful preparation for a good fit, so that makes this a middle-of-the-road option as far as labor is concerned. Also, the lower sharp corner is vulnerable to damage and will need extra glass or other sheathing for protection.
Section B is one of the easiest to build, as there is only ONE underwater joint eg: two sheets of ply could be wired together, the interior simply filled with an epoxy slurry and the joint then glassed. However its narrow section means that it needs to be very deep to find the equivalent volume. Such a section has been used on some Wharram-like catamarans and because of the depth, it was considered not necessary to add additional foil surfaces to control leeway, so overall it was a cheap and easy solution if the draft was not going to be an obstacle. The section does carry a fairly high penalty as far as its wetted surface is concerned though and the deep sections, while providing enormous directional stability (tendency to go straight which is fine for distance cruising), would also be very resistant to turning. This would make this section a less attractive option for a boat to be used in narrow, restricted waters. Without additional foils, such a shape generally has poor leeway resistance too and such a design would need a high keel rocker to improve its turning ability and this puts significant limits to its speed. But when combined with an open deck (a la Wharram) this shape has at least proven to be seaworthy and seakindly.
Section C is another very easy section to build with only two very simple joints to make. Chine logs (wood battens fitted internally at the knuckle) could simply be glued to the side ply and then both planed off together to fit the closing bottom ply. This is the system used primarily by Bill Kristofferson for his early quick-to-build K-designs. See:smalltrimarans.com/blog/?p=723#more-723. It is also used by Richard Woods for several of his catamaran designs, as well as for the main hull of his new Strike day-tri's. [My own W17 trimaran also toots this shape to good effect and in this case, the bottom joint is made with a simple epoxy fillet, glassed over both sides - see the ABC System under Construction Methods].
While most of these simple shapes cannot give the ultimate lowest resistance for all speeds and often provide less floor space, the SECTION C design is rather the exception and has proven to work well compared to both simpler options as well as more sophisticated shapes. If the bottom is widened, this design can provide enough space for a main hull and with a little vee up forward, will not slam if kept deep and underwater at the bow and certainly works well for a simple-to-build trimaran main hull, if the following additional design pointers are taken into account.
Care needs to be taken to be sure the bottom surface is not too wide—particular in the forward half of each hull—in order to prevent slamming, but the section has less depth penalty than Section B. The flat of bottom will not only permit the boat to turn more easily but will also provide a good surface to sit on (either trailer or sandy beach), provided that either the bottom ply is thick enough or provided with sufficient internal stiffening. An extra layer of glass would be sensible for the flat bottom or even the luxury of some abrasive-resisting Kevlar. This design provides more effective buoyancy low down than most of the other simple sheet options and if the bow is kept low in the water, the chine is kept fairly straight and its resistance minimized as the water is now parted mostly sideways—a relatively easy effort for a long, narrow hull. The low-in-the-water bow will slow its turning, but for a trimaran, this should not be of major concern. For a catamaran however, the two keels will likely need a more 'compromising rocker' to maintain adequate manoeuvrability.
Section D is a wide version of Section B and this is the way that Wharram catamaran hulls developed in order to give more internal space. See: wharram.com
As mentioned under Section B these hulls generally have an easy motion, though the wider ones clearly have extra form or wave resistance, and that is easy to understand. As the wave moves up and down on the hull, wide Vee hulls also force the water horizontally aside and this effort creates more wave resistance than a hull with more vertical sides. When combined with a shape that has more wetted surface than that of a semicircle or even a U or box shape, it is clear that such hulls will inevitably be slower. Additionally, hull sections with excessive Vee up forward, cause a rapid increase in buoyancy that throws the bow high in the air, only to fall again when the wave has passed. This starts a repeated pitching action that is noticeably less with the Section C having more parallel sides. However, the vee'd hulls are easy to build and no doubt this is one factor behind the successful sales of Wharram designs worldwide. The other factor is that Wharram catamaran designs have an appealing link to their Polynesian past and still ingeniously incorporate numerous rope lashings to attach the rig, rudders to the hull and hulls to cross beams etc, so that all provide a flexibility to resist ocean wave forces. Wharram designs also set very low rigs and therefore have significant stability reserve compared to many 'more modern' faster designs that carry a higher risk of capsize unless their beam is increased.
There are limits to that also, as when a multihull is hit with an exceptionally vicious wind gust, if the sail is not immediately released to weather-cock, the boat will capsize in the direction of least stability. When the design beam is increased excessively, this direction then becomes diagonal or even fore & aft, so resulting in a pitchpole, so a balance of Beam to Length is always necessary.
Section E is the first of two multi-chine options shown here. The use of two narrower strips of plywood for each bilge, brings the shape far closer to that of a round bilge hull and this lowers its frictional resistance through the water. It will not be the same as a round bilge though as it still has 2 knuckles per side that will cause some turbulence as they interact with the ever-changing water pressure due to passing waves. This can be aggravated if the chines take much of a banana shape in profile, as they so often do when trying to simulate a rounded hull. Construction is also made more complex and it probably takes nearly twice as long to build either of these last two hull forms, than it does say Section C or D. In fact, one may argue that building a multi-chine hull like this, takes about the same time as say a strip-cedar hull that can be totally round-bilge, with the only extra time being that to cut out, set-up and fair the male building mold. Total material cost will likely be higher for the latter though unless one can share the mold cost with other builders. Section E has a flat bottom, so there are 'just' a total of 4 knuckles to fit and fair. The flat-of-bottom can help to get the volume of buoyancy into the hull design and also be low down, and this shape will allow a boat to turn more easily than that of Section F that has a sharper keel on the centerline. The shape also still sits nicely on a trailer. However, the flat bottom will require to be mated with a good sized foil (center or dagger board), as leeway can be an issue otherwise. The note about bottom protection for Section C, equally applies here.
With five knuckles, section F has the most construction work and precision fitting, but the slight Vee at the bottom can help with directional stability and also provide slightly more resistance against leeway than Section E. But both these designs have some merit and one might be justified in selecting Section E for a boat doing a lot of short tacking in a restricted area, whereas the extra directional stability provided by Section F would make steering more relaxed for longer tacks. A wide version of the quasi 'U'-shaped Section C will sit better on its bottom and likely offer the least leeway and lowest wave-making resistance, despite it's boxy-look in section.
One thing about designs like Sections E & F with added chine complexity, is that other building methods now start to become competitive re time and cost. In particular, apart from the strip-cedar method already mentioned, one might consider hybrid designs that use the combination of a fiberglass solepiece for both a more ideal underwater shape and durability, combined with plywood topsides where straight panels do not significantly affect the resistance. In fact, these designs can be designed to look both fast and attractive. Three such designs that come to mind are the Cobra Catamarans (>30'), the L7 trimaran, and my more recent W22; see references below. (Even some car designs have used fairly straight knuckle lines in the past, with the Citröen DS19 and BMC Mariner being two interesting examples that come to mind.)
There are also full lapstrake designs that use narrow strips of plywood for each of their strakes from gunwale to keel. This does permit one to arrive at fairly round-bilge shape, thought the laps will add some resistance, and there's no denying that the multiply strakes with gentle sheer, make for a very pretty hull. (See photo).
Regardless of the hull shape, one needs to keep in mind that the construction of the hull or hulls for any sailboat, are only a fraction of the total work—perhaps 1⁄3–and in some cases (as for sections B,C,D), even less. This fraction will also depend on whether one is using an existing rig and mast and how much internal work is planned. But the main point is to consider any decision about compromising on hull design to save build time, in relation to the time for the overall project.
See also specific articles under Construction Methods - # 1, 2, 3, 4, 5, 10, 11 & 12 and Compound Curves.
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