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January 05, 2010

W17 Trimaran — Preliminary Design Concept and Development

Once having decided that I wanted a small, inexpensive day-sailing trimaran to get out on the water as often as possible, I figured I had better use my own fairly extensive design and building experience to come up with a unique design that I could share with others. The first thing I do with ANY design is to identify the target user and the attributes that I want the boat to excel in. In this case, the user was to be me and the top attributes were to be speed, comfort, relative dryness and ease of handling, both sailing and ashore. And all this from a rather small budget along with a relatively short build time using readily available materials. Hmmm… quite a challenge!

So where to start? The budget and build time would lean me towards plywood hulls, and having already written about the various options on my website, it was pretty clear which hull shape I should consider. Something with a narrow flat bottom that was kept well below the waterline tied to flat sides that would simply divide the water without much disturbance. This would be easy to build and sit neatly on a flat surface or trailer.
But would this simple shape give me enough performance to satisfy my hunger for speed and efficiency? I would have to find out.

So when I had the opportunity to test sail a small tri of this configuration, I jumped at the opportunity. Mark Gumprecht, a fine furniture-making contact from the North East, had made a number of boats for his personal use and friends using this narrow-flat-bottom configuration and as he was looking to sail on picturesque Lake Champlain on the Vermont-New York border, it was a great fit. So last September we got together and had a great time test-sailing his little 12' folding trimaran and talking boats. So what was the result?

Well I have to say that although this configuration was 'technically acceptable for a day-sailer', I was pleasantly surprised as to just how well the hull slipped through the water with minimum fuss. Whenever I sail any boat, I crave that feeling of efficiency and ease that goes with a balanced, well-conceived design—and this little boat had it in spades. That's not to say there were not some things that could be better, but the promise of this hull shape convinced me that this offered a viable and justifiable choice for my new boat. While the little boat as designed was great for flat water, Lake Champlain where I sail can really kick up some wave and I felt that two areas particularly needed attention—the low cross beams (akas) and flats on the bottom of the amas.

Just making the boat bigger would help its general seaworthiness, so I set out to decide on a length. Being as I wanted something not too heavy to handle ashore while alone, I felt that 16' would work fine for me. Any shorter would kill the performance I wanted, much longer could make it cumbersome for one—and I'm hardly 'a spring chicken' any more. ;-) But 'the length exercise' only started there.
Using plywood, meant that scarfs would shorten the available length by 2"-3" and then the side curvature would likely double that. So now I was down to 15'‑6" .

Hmm… I thought. I have 3 hulls to build and I'll be cutting a foot or so off the ply to build amas with a suitable length for this. What a waste! So I changed my plans for a W16 and decided I would design the amas for the one scarf and 15'‑6" but make the single central hull a bit longer with an extra scarf, and so the W17 was born.

I've already mentioned that the flat bottom amas caused some concern. It was really only a minor thing, but as windward amas frequently come out of the water yet are still close to the surface, there's a lot of sporadic contact with wave tops. The 4" flat on these ama would slap down and create annoying noise, if not some added resistance. Making a vee or rounded bottom would be getting away from the simple main hull shape and result in the two hulls not looking even remotely the same, so being as esthetics are important to me, I scrapped that thought. I finally came up with an idea to incline the flat bottom a bit.

To see what this would do to the lines, we made a ¼ scale model in cardboard and the result gave an attractive straight line to the raised knuckle. Originally, I had thought to place this raised knuckle on the inside of the ama, as it would allow the slightly straighter and deeper flat side to be on the outside—theoretically at least, providing a very slight lift to windward from the asymmetrical shape. On a second look however, not only was the asymmetry extremely small, but the new inclined surface would become more horizontal as the windward ama lifted! So here I was making changes without really solving the original issue. Finally, I decided to flip the hull completely around and move the sloped ama bottom to the outside. This now created a nice easy form to contact waves and the straight chine line looks good also.

Next issue to address was the low cross beams or akas. These had been recessed down below the deck line on this small tri and while this looked neat, it lowered the beams to an unacceptable level for any rough water. So the first thing was to raise the beam above the main deck. I could have also raised the main deck and freeboard to keep the same recess but more freeboard is more windage and I saw no advantage to it. With the akas raised, they now needed to curve down to the amas that were below the main deck level and a way of creating them was devised without making it too complicated for a first-time builder.

On the small tri, there was a lot of impact with waves into the front face of the forward ama and this broken water generally found itself in the cockpit. Just raising the beam will reduce this impact but at the same time, this longer boat will be sailed in rougher conditions than is possible for a 12‑footer. (I know this as fact as when I tried to push the envelope with this small boat, I was promptly reminded that you cannot sail in short waves much higher than ⅙ of the boat length without a high risk of pitch-poling. I tried but finally succumbed. The experience was not totally negative though. Nothing was broken and it made me think of the best procedure to right such a boat should this ever happen again.)

So back to the cross beams. The W17 will have a ply fairing on the nose of the forward beam and even if spray and waves still succeed to come aboard on occasion, it will not be there long as I also decided to build in a raised cockpit floor to make it quite self-draining through scuppers to the outside. I built a model of the fairing to experiment with this, as I not only wanted it to look good, I also wanted to be able to build it fairly easily from thin flat plywood. This pic. of one model gives an idea of how the forward W17 beam will look at the outboard end.

I was initially quite skeptical about the one-side leeboard that was built on to the small tri, but to be honest, the builder had done a good job with the mechanics of it and it really worked quite well. In this case, it was probably the best solution as his sailing area really demanded something pivoting and a centerboard case would have half filled the small cockpit.

But once I chose to raise the cockpit floor, I now had some space for at least a narrow board. But long keel slots are not my thing in a boat that's already short by many performance standards, so I needed another solution. I also needed a pivoting solution as the lake has some shallow areas and hidden reefs but a fully built-in, pivoting board would need at least a gasket and these are notorious for jamming if they don't just tear and fall off.

My mind was then taken back to a solution I had found for a Div II sailboard I still often enjoy. Although built for a large dagger board, I had replaced the original dagger for a narrower board that would pivot back at least 25 degrees before it hit the back of the case. So I drew up a design that would work in a similar manner, except that I also inclined the dagger board case more to get this W17 board to incline to at least 30 degrees. That would lift the tip a useful 300 mm (12") off the bottom and have such an inclination that it would more likely slip over an obstacle or at least, not stop too quickly. The case will also have place to house a small crash area of optional replaceable foam and the dagger board can be pulled up forward to totally remove it if need be. I think it's a good compromise and the resulting slot is now only 450 mm long and mostly filled with the foil, regardless of its depth. The top of the case is very well supported by the cockpit floor and I chose not to raise its sides above the floor, so that the cockpit will not only drain into it but also allow a reasonably nimble person to comfortably sit on the now flush cockpit floor while sailing. An additional advantage of this 'floating system' is that I can fine tune the location of the board to get the boat perfectly balanced.

As I decided to bring the aka and rig back further than was possible on the 12‑footer, this meant that there would be a higher load on the forward beam compared to the aft one; perhaps 75% of the total and I finally chose to design the forward beam for 100% of the full ama buoyancy. So when planning the assembly of the cross beams into the amas, I initially planned for the forward beam to go down into the ama and be bonded to a bulkhead. Although theoretically a strong solution, this would mean building the deck around the webs and complicate the alignment and assembly. Though I'll still keep it as an option, I finally decided that the loads at the ama/aka intersection were really not that high so I've gone for a mating of the parts at the deck interface to allow an easier assembly, free of precise alignment with any bulkheads. After surfacing the beam ends to suit a defined angle, the two beams can now be laid on the ama deck and simply drilled and bonded in place with one bolt at each beam end.

Sailing comfort starts to get more important to me with age and I've made sure that this boat will not aggravate me in any way. The 2 meter long seating area each side of the cockpit will be at the right height for those without super long legs and I've built the gunwale strip on to the outside of the hull, so that it's not digging in my back if I do decide to sit down on the floor out of the wind—something I enjoy to do in some conditions. The cockpit seats also have sloping backs as well as handy storage compartments and when you want to move out a bit more, I've made sure there's enough raised flat surface to sit on without any discomfort.

Whether you ever want or will need to go farther outboard than this, will decide if you fit outboard wing nets, but there will be good places to mount a relaxing 78" × 36" (2 m × 0.9 m) trampoline on each side if preferred. As without nets, it's a 1 metre stride to the ama, the forward cross beam has been made wide and flat on top to provide a non-slip walk area to more easily reach there for the rare time it may be required.

The rudder will be a modified version of an idea used on one of my kayaks, whereby the rudder assembly pivots from a transversal hinge at deck level (see pic). The first sailboat application I saw of this was on a very small one-off tri that I recently test sailed, and I really liked the way it felt.

It's a spade rudder that, unusual for a small daysailer, drops down under the stern so that it's not only less accessible for air to suck down but can also be a little smaller as it works more efficiently with the flat bottom acting as an upper 'end plate'. It lifts up with a portion of the hull bottom attached and fits like a glove when it's down. Its construction is quite simple with sides of thin ply over a simple S/S (stainless steel) tubular shaft that operates inside a fibreglass tube. For the W17, it will have additional side brackets that will permit it to operate at slow speeds in a half raised position for coming ashore or avoiding some obstacle. The hold-down line will also pass through an auto-release cleat, so that the rudder will not be ripped off by something not seen.

A boat of this length can use a fair amount of sail when the winds are light, but one needs to be able to get rid of that surface when the wind picks up. So I've designed this boat to have a roller-furled jib and also take an optional, asymmetrical reacher from a bowsprit.

As I did not want a long sprit on such a short boat, I chose to mount the forestay about 300 mm back from the bow to give a good slot between the foresails. This meant that the sprit could be only 600 mm long and still work admirably. Sprit needs good lateral strength as well as vertical and I solved that without a plethora of wires.

I've made the sprit from a flat board that is glassed over, and it will sleeve into a flat pocket just over the deck in front of the forestay. Such a flat board about 100 mm wide will be very stiff laterally and not require any side stays. One then clips on a simple bobstay to a stem eye to take the higher vertical load and all is set.

Although I have designed the boat to optionally use the rig of a day cat—the Prindle 16 is a good fit—I have personally always felt that a rotating wing mast would give a wonderful boost in performance if some neat way to build one could be worked out. As I have been studying that very issue over the last 2 years, I was most interested to see that this small tri already came with one designed and built by Mark Gumprecht. Mark has actually built more different sizes of wing masts than anyone I know—five, I believe he told me—so his concepts are certainly worth looking at. This particular mast was 17' long and built very light and slim [and of quite different structure from other Gumprecht masts and the Waters-Wing]. Although it had one diamond, it still flexed more than I felt comfortable with, yet in some pretty rough conditions it still stayed in one piece. Although of simple design, it was beautifully built and finished and the whole mast barely weighed 1 lb per foot. For my part, I had already worked up a stiffer design that I planned to use on my W22 and with a few tweaks proposed by Mark (a more rounded nose), I feel confident it will perform well if not grossly abused. So I've now downsized that design to suit requirements for the W17 and the building plans will include details on this development that, to be honest, is a rare thing to find these days.

With wing masts on all the big racers, one may ask why that is? I can only imagine it's because no one wants to take the responsibility of some possible failure in the future and as the designer has ZERO control of either how the mast is built or how it's loaded in use, this is quite understandable. However there's a great shortage of good info out there about wing masts and with good design, they are not so impossibly difficult to make either and, in fact, well worth the effort and discounting labor, cheaper than an alloy one.

Regardless of material or form, there is always a risk of mast failure and even someone getting hurt, but that can happen with anything pushed to its limit. So if you're ready to accept that risk then I'd certainly go for a good wing mast on a fast trimaran like this. The only failures I've personally heard of came from neglect and poor storage etc, though I am sure they've happened while sailing hard from time-to-time. Even one of Mark's creations was accidentally 'tossed over the side' one day when the owner forgot to wire down the turnbuckles on his large trimaran while at a mooring—but despite some inevitable contact with the gunwale rail, the mast survived unscathed and was reportedly sailing again.

The Gougeons used to sell a design built from sheathed wood strips but no longer do so. It was actually a very large section compared to those of Mark and myself and although I feel the design somewhat outdated now, a good number of masts were built to the design and still useable. Carbon fibre masts are at the other end of the scale and I'll cover that subject more when I find time to write about wing masts in more detail. Let's just say that a wing mast for a small boat can still be built without expensive carbon fibre if well designed and constructed.

I've only covered a few of the details that a designer is faced with while detailing a new boat, but I hope it gives an idea of the process, at least for the W17. One thing that I'd like to add is my development of using sheathed ply for much of the structure.

Under the section on Plywood Construction there is reference and a photo to plywood boats that have been literally soaked in epoxy and yet they have failed far faster than other boats with no epoxy at all!. This is because of both the poor preparation and also, that far too often a cloth sheathing is laid over internal structure that will not permit the glass to actually lay close to the plywood it is trying to protect. The result is that water gets in behind the glass and is then trapped there, guaranteeing that the wood will rot out in record time. To avoid this issue, many designers specify that there be NO glass sheathing to the interior—only resin. But even then I have seen rotting behind many coats of plain epoxy—generally due to poor preparation, excessive humidity or poor workmanship.

But consider this. In those areas subjected to high external pressure from either water or impact, it's the interior surface that will tend to crack and that needs the extra support from a good glass surface acting in tension. So, what to do?

I now consider I have mostly solved that issue for my designs as I specify that where needed at all, the interior surface of the ply used, is to be pre-sheathed with glass on a totally flat surface, before being mounted on any internal structure. This allows one to also better limit the amount of resin used and therefore keep the panel reasonable light. Amas in particular are very subject to external pressure from being submerged as well as impact from being so exposed. In addition, their interior is often neglected or inaccessible and so I certainly recommend to pre-sheath all plywood used for the W17 amas—including under the decks, as I would rather add 15 lbs now than have 20+ lbs of moisture in there later causing havoc and a quick end to all the work that was put in.

For many small boats, this pre-sheathing can also allow the optional use of far less expensive plywood if carefully selected by sheet and used for low stressed areas. Typically, sheets of half-decent 6mm lauan ply can be found for as little as 16 the cost of marine ply and although not as resistant or strong, it can be 'strong enough' for many uses including many large above water areas of the shell on a small boat. W17 build instructions will give tips on this and make suggestions on how to deal with voids etc.

(For much larger boats with a greater investment of time and other costs, marine ply is still the better choice however.)

The W17 will have an easy folding system, based on rugged stainless steel hinges that are readily available. Due to buoyancy and wave action on the amas, the greatest stress will come on the lower attachment under the beam—particularly the forward one. So this one will have special attention and its attachment/bolting surface doubled. Regular butt hinges (heavy duty) will serve for the operational upper hinges, backed up with spacers to take the compression, while the highly stressed underside will use double strap hinges with far longer tongues and more bolts. (Ed: The welded underside strap-hinges have since been replaced with a custom designed latch that eliminates all welding.)

W17 SailplanTo fold away the boat, one will just lower the mast (a one person task) and then simply pull 4 pins and fold in the amas over the top of the main hull. Any trampolines fitted will stay in place and the overall beam will then be about 7'‑3" or 2.2 m.

As for the W22 trimaran, the aim of all my current designs is to offer great owner satisfaction through a boat that cannot only be built at fairly low cost, in reasonable time and with just average skills, but also sails and handles really efficiently with a wonderful thoroughbred feel to it, while also being both drier and more comfortable than others of its size.

If these design goals match those of your own, you owe it to yourself to take a good look at the new W17 design. It will be a really fun boat to own, sail and take out your friends, skeptics or otherwise—and it's why I intend to build and own one for myself.

Handling ashore will be almost effortless too. The flat bottom shape is very easy to mount on wheels and a flat-bed trailer will take it anywhere.

ON to the next article on the W17



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