LogoSmall Trimaran Design

Designer's Comments cont…

Not wishing to sail on wet paint, a decision was made to launch with just a primer coat of epoxy, and pale green is what the builder had on hand. Then, despite valiant efforts by builder and supplier, the curved mainsheet track failed to clear customs in time, so a jury-rigged rope horse had to suffice that gives almost zero downhaul when off the wind. Cockpit drainage holes were initially left undersized and their venturi covers were initially not fitted. Fore deck spray deflectors were also not yet installed and for the weather to be experienced, were greatly missed. There was also no time to find 'the sweet spot' for rig and daggerboard placement, so the initial helm was reported as 'somewhat light or neutral'. But all these things proved easy to fix and here is how this was achieved.

Based on this initial experience, the main issue that justified a deeper look, was one of too much water in the cockpit when in rough water. It was concluded that several reasons had come together to cause this and they have now all been addressed and even for the first boat, this is no longer an issue. When sailing fast in rough water without the spray deflectors in place, significant water can come over the bow that could normally be avoided (see videos). The boat is only 17 ft long after all. Initially, this added quite some weight to the boat as the drain holes were made too small. But with larger drains and the venturi's now in place, it's already a greatly improved situation and once the spray deflectors are installed, it will become 'a non-issue', at least to the extent that any 17‑footer can be expected to stay dry.

At speed, and with no seal over the DB slot at the exterior, water also tended to surge up the daggerboard case and flood the footwell. But since then, the case construction plans have been upgraded to add what I've named a 'surge suppressor' within the case and this will certainly reduce the upward flow for all future boats. Another step could be to add a horizontal seal that fits flat over the rear of the daggerboard trunk and tight against the board when it is in the up position but so far, this has not been shown to be necessary.

[One factor that could affect the design, is the challenge of building to a design weight. This weight control challenge and variation, applies to ALL boats built of wood—but much less to boats mass-produced in glass or some plastic. 400 lbs is not a lot for a 17' tri and will not just happen automatically. Although part of this control will be in technique and self-control over the use of glass and resin, a large part will come from the availability of light woods and ply in the specific part of the world you're building in. This should not be a major issue in North America, as solid timber of light, straight-grain cedar or spruce is fairly plentiful as well as light plywood of lauan and okoume. But for building in the tropics where sometimes only hardwood is available and even plywood is made using similar woods, it could be an issue. This can add significant extra weight as while 4 lbs per sheet may not seem much, that alone would account for about 60 lbs on such a boat! From a design perspective, the ideal solution would be to recognize early-on if there's likely to be a need for a built-in margin and if so, 'tweak' the main hull form below the waterline to add a little compensating buoyancy. Small tris are particularly vulnerable to this, as all the weight is initially carried by one slim hull.]

To add a further margin against up-surging water I finally agreed to slightly raise the cockpit floor, but felt that the change should be quite moderate as the headroom to duck under the boom would be directly reduced. So the change was limited to 25 mm and this is now incorporated into the updated plans. To assure that it is possible for future builders to achieve the weight target, some of the optional lighter plywood thicknesses were switched to become 'the prime choice' in low stress areas, and the interior design of the cross beams also further optimized along their length, to contribute further weight savings without compromising strength.

As far as sailing performance in the water, the boat shows good posture (trim), a stable, steady ride and despite the choppy conditions during her first open-water trip, moved well through the water, pointing up and gaining on monohulls twice her size. She also reportedly tacks well and steers with precision. The fact that the owners would even take such a new, small boat for a 130-mile shake-down cruise/race in open water, showed the confidence they had in the general design and construction and despite some rough water, everything was reported to still feel very solid when they returned home some days later.

It has been noted that with the long cockpit, while great for lounging with friends, there is a tendency for the sailing crew to sit too far aft. This causes the centre of lateral resistance to move aft and leaves the helm rather too neutral for most of us. While the solution is to keep the DB down and forward and also move more forward, I decided when adding the surge suppressor to the plans, to move the DB case slightly more forward also, to be sure the boat will indeed be 'sweet handling with a comfortable, eager feel'.

If after the steps already taken, someone still wants even less water in the cockpit on those rough days, there are still two quite feasible options left. One would be to fit recessed seals over the slot under the boat and the other is to fit a short case over the slot inside the cockpit, of about 125mm (5") height. However, as both solutions carry some inconvenience of their own, neither of these is presently planned or deemed necessary.

So after her first 130 miles afloat [Nov 8th 2010], the W17 has clearly shown in these conditions, to have a fine smooth ride, great stability, rugged strength and excellent performance for her size - particularly upwind; and according to all accounts, is a joy to steer and sail. And now that the initial issue over cockpit water has been solved, future boats built to design weight, will all be able to demonstrate that the original design goals have been met.

Mike/

Nov 16th 2010
Weight and drainage issues are now resolved. A few tweaks to the scantlings and main hull have now added a small margin for the builder and a full build go-ahead is now given. Care will still need to be taken on weight control, something that will always be true for ANY small trimaran that initially supports all its weight on one slim central hull. Adding more buoyancy could detract from performance as the lines could be too compromised, but I'm now confident we've got the best performance profile for a typical average crew weight within this simple hull form.
If you know you will be sailing regularly with very heavy people on board, then I suggest you contact me for a customized recommendation, as all small multihulls are very weight sensitive.

mike


W17 compared to the WETA 4.4 … can you?
Although this has no connection to the early reports on this boat, I'd like to make a brief comment about some misleading remarks that have been made on various Forums, where some individuals have attempted to compare the WETA 4.4 with the W17.
I have a lot of respect for the WETA 4.4 and already gave it a good and fair rating in my Review of nine Small Tris. But, other than both being good, desirable small sailing tris with excellent performance, they are of totally different species. The WETA 4.4 is a very sporty, spray flying, thrill-giving little boat that gets a lot of its performance through the use of hi-tech, lightweight materials and therefore, the boat is costly for its size. But for a sporty trimaran equivalent to an equally-wet beach cat, this is an interesting and exciting option. As for beach cats, it can and will capsize at times, but being small, one ama can readily be flooded and the boat righted, with claims it's even easier than for a beach cat.
The W17 by comparison, is a larger more stable sailing platform that will capsize far less often and that offers its performance in a quieter, smoother manner, coupled with more comfort and space. As far as the sometimes distorted claim that a WETA has a higher payload than a W17, this is a total play on numbers and should be meaningless to those who understand a little more about volume and buoyancy. Of course, the WETA has the buoyancy to support 4-5 persons, but it's normally hot performance would then be severely hampered with that weight and drag. By comparison, the W17 could probably take 8 if you load it in an equivalent fashion. But as I have no interest in promoting this boat for anything other than an enjoyable performance experience, I would limit that to 4 with 2 persons being the ideal crew for the boat. In such a comparison, the best performance for the smaller WETA would generally be with only 1 aboard. Made professionally, these boats would probably cost closely the same, but they would offer two entirely different experiences. One, the thrilling on-the-edge performance plus the occasional dunking, while the larger W17, would offer smooth fast sailing and the ability to make longer trips with space for picnic and camping gear and a far lower risk of capsize. The lighter, rounded boat would likely be faster in light weather but the W17 should hold its own in the higher winds and rougher water when its very moderate sailplan would not be a penalty. Your choice … they both perform their very different design goals admirably, though the larger boat is clearly more versatile.