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November 21, 2010

Review of W17 Hull Forms

Builder: Is there anything really very different in the Design Approach behind the W17 hull forms compared to other boats out there?

Designer Yes, I think it's fair to say there is … several things in fact.

In the first instance, I am trying to optimize the basically simple construction of a box with a flat bottom, into forms that will be efficient in their passage through the water—not at first glance, such as easy task. They can look so bad in sectional view, but remember, it's the elevation and plan view that will indicate far better how they will travel through the water! After all, a typically Stealth vehicle or plane, does not look so streamlined in section either. So let's go through what has been done for the W17 and why.

Now speedwise, there's nothing so wrong with a boat that has a flat bottom. It's been shown in tests to actually be THE most efficient planing surface and all round-bilge hulls are a compromise away from that ideal, in order to lower resistance at their lower speed or to improve sea-kindliness. So how can we use these flat surfaces to advantage?

Ama bowFirst of all, the design of the amas are quite unique. Whilst we need a flat surface aft to potentially slide or 'ski' on, this can be a source of pounding up forward, so we need a finer entry there. Also, when a multihulls heels, the windward ama lifts out above the waves. No problem when it's a large boat but for a small boat of 16 ft (average hull length of the three W17 hulls), that ama is still not far up. So wave tops will repeatedly impact the bottom, and for that reason, the bottom of the W17 amas are warped as they go forward, to present a Vee shape to the waves coming up under them and eventually running out forward to a fine bow that does not pound but slices. Contrary to several other amas out there, I have chosen to incline the outer bottom face, rather than the inner. This shows two advantages. It not only gives a better Vee shape for the waves but it eases the outer edge from being sharp, so that in a strong squall, the boat can slide sideways more easily and not trip over the more vertical edge that occurs when you incline the bottom on the inner side.

Another unique feature is that these amas are built with a small 'toe-in'. This helps both the climb to windward and also for turning. It's not much, but it's better than the toe-out at the waterline that many other trimarans carry—perhaps unwittingly! Just follow me here.

Ama twistMost amas are set with some inward inclination to the vertical—10–12° is quite common. Their bows are also almost always higher than their sterns above the water. So if an ama is built to be parallel to the main hull (as most are) and that is measured at the deck level, then if you project down the inclined line of the stem and stern, the intersection with the waterline will be further from the main hull at the bow, than at the stern, due to the greater height at the bow. This means that at the waterline of the ama, there would effectively be some toe-out! So on all my tri designs, I build with some toe-in to compensate for this and even to add a small amount of genuine toe-in that I find helps as noted earlier. Not something typically found on other tris.

So now to the main hull. Even if it's basically of three flat panels, there were still options to choose from. They could be set to be quite Vee'd as for a dory and some cats are built like this. This gives more width (and hence stability) as it's heeled and also supports more load as it sinks—both ideal for a dory. But for the hull of a multihull, we don't need that type of stability gain from heeling, as the amas are FAR more effective for that. What we need is low resistance to waves at the relatively higher speeds this boat is capable of. If we incline the sides, what happens to the passing waves? They are progressively pushed out as they rise up the sloped side and attempt to lift the hull. This is a lot of 'added work' on the boat and takes energy that adds resistance to the shape running through the water. Now if the sides were vertical, the waves would simply rise and fall, with minimal added work and resistance. So on the W17, the sides are kept fairly vertical, with just enough slope to make sitting in the cockpit comfortable.

Painted hullAnd what about that sharp lower corner? Should we not round it off? I don't think so! While you'd have a fraction less surface friction, you'd lose an important element of this design. That square corner gives an important resistance to the transverse flow of water under the hull, that would allow side slip. The fact that a tri hull is so long and slim, allows that corner to work positively to help progress to windward and keep flow going more fore and aft than would occur on a wider monohull and early trials with the first boat have indeed shown windward performance to be excellent for a small multihull.

As far as the bottom is concerned, we ideally need this to work as a ski and be as flat as practical so that as much of the surface can run with a very low angle to the flow. Having lots of rocker in the keel line would not help top speed at all. But this creates a conflict, as we need the keel below the waterline to provide buoyancy to support the boat weight and if it were indeed flat like a barge, then the transom would also end up deep under the water. At the bow, we can keep it fairly low, as it will be fine in width and as long as the boat still turns ok, this is a good way to assure the bow stays in the water and keeps the waterline as long as possible. But clearly, designing with the transom underwater is not going to work. But for minimal rocker, we also want to raise it the least possible! So what is that amount? Well, not accepting to compromise top end speed more than necessary, has been my approach for the W17. It's clear that as boat speed picks up, this wonderful flat bottom surface can certainly provide some dynamic lift, so we can use this lift to help keep the rocker to a minimum. For the W17, I have tried to design the boat so that the transom is just afloat with no crew in the boat. "Yikes!" some might say, "what happens when you add the crew?" Well, a boat trims about its centre of flotation and with this hull, that is well aft of amidships, as the stern is much wider than the bow. If the crew are initially added forward of that 'pivot point', then most if not all the sinkage will be by the bow and not by the stern—so keeping the transom at roughly the same level.

Now, as the boat picks up speed, the flat stern sections start to become more supportive, just as a plank (with virtually no depth) that we call 'a ski', can support more and more weight as its speed is increased. We can now use this added dynamic support to carry some weight and slowly bring the crew more aft as speed picks up. This I believe will give the best overall performance for what is basically, a flat-bottom boat. If I had provided the depth under the aft end of the cockpit to initially support the crew with the transom still riding high, then I'd have automatically designed in an underwater 'bustle' that would have added much rocker to the keel line and inevitably limited 'the skiing' that this boat will otherwise do. When a boat is going to be cruising at low speeds and sitting at anchor with a party on board, then that 'bustle' might just be necessary (see most other designs) … but if you then push that hull form to try and reach higher speeds, that adverse rocker will suck the bottom down, add to the trim and create too much resistance to get the best speed.

As in ALL designs, one has to choose ones compromise and I hope this helps to understand how I chose mine. The fact that the first W17 can run at 6–8 kt with the transom lifted to the waterline while carrying a significant excess of weight and then press on to reach over 13 kt in rough water, shows the promising potential of what I am describing here.

For a boat built to the design weight, it should always be possible to run a clean transom.

But if you really WANT to ride a higher stern all the time AND can accept the slower top speed that will go with it, then the simple basic shape of the W17 could certainly be deepened to achieve that. Although this might be a required compromise for those of great weight, it's just not the compromise I have personally chosen for the base design.

Meanwhile, happy water skiing…

Mike, November 2010

I recently had an email from someone who'd been sailing the W17 and he reported this:

"I just had a really good sailor show me how to crank up speed and then turn on the back of a wave to let this boat really go. And GO it did. In fact I had the impression I was sliding on oil, it felt so frictionless by comparison to what I was used to."

So I'm going to coin a new word for the W17 and call this 'Oiling' in future — or 'to oil'. So keep the after bottom surface both flat and sharp-cornered…and have fun guys!

Feel free to send in specific questions or comments via my Questions Form, that I may select to answer through this webpage if considered of broad interest.



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