| AUCKLAND, NZL-(1-12-2002) Chopping and changing bows appears to have been common practice in the Louis Vuitton Cup so far. Among those considering a change, Prada were the first to go public while Team Dennis Conner chose to alter the shape of USA-77 Stars & Stripes’ bow during the rebuilding of the boat after its sinking earlier in the year. And these two teams were not alone when it came to some front end surgery, other teams are also thought to have modified their bows in some form since the beginning of the event. Double knuckle bows appear to be the favoured shape once again, but why has this configuration been such an important development?
The reasons lie with a few basic ACC fundamentals, one of the first and most significant factors is the weight of the bulb in the keel. At 22 tonnes it’s perhaps easiest to think of an ACC boat as simply a means of carting this massive weight around the course. With the rest of the boat weighing around 3 tonnes in total, the shape of the hull and its distribution of buoyancy quickly becomes an important factor with the weight of 15 saloon cars slung beneath.
Knowing where the bulk of the buoyancy needs to be influences the lines of the hull, especially as under the rules no hollows are allowed. The result is that what happens at one end affects the other.
But on top of this, designers are trying to get as much waterline length as possible. The measured length is taken 200mm above the waterline. The basics of naval architecture are that long is quick, (at least for non-planning conditions), so the longer the waterline, the quicker the boat. Extending the length of the boat on the actual waterline increases the steepness of the bow if the line is going to pass through this measurement point. Having a vertical bow might seem to be the best option, but the fact that there is no overall length restriction, or penalty. This means that there is more waterline length to be gained when the boat is heeled, so a bow overhang beyond the measurement point is a good thing. Hence the double knuckle bow.
Once the decision has been made to go for extra length, the boat needs to be heeled to exploit the overhangs. But with 15 cars slung beneath in the keel, heeling the boat is not that simple especially for a beamy buoyant hull. Making the hull narrow and reducing the form stability of the hull helps the boat to heel, but a narrow hull can reduce the total buoyancy required to support the massive bulb in the first place. Sinking the boat slightly when she heels helps her to pick up waterline length quickly too.
A narrow boat means less wetted surface area and therefore less drag, particularly good for downwind sailing.
So narrow is good, but the only way to ensure such a narrow boat still floats is to use most of the available volume within her maximum beam. The result is a sharp turn at the bilge and slab-sided topsides. Going too narrow also means that the required buoyancy has to be squeezed out of the maximum length. This increases the angle at the bow and stern has the effect of reducing the length of overhangs and reducing the amount of extra length that can be picked up when the boat heels. The result is often a shorter boat.
While setting to with a chain saw or some high tech filler has changed the shape of several front end and with it the fortunes of some, the bottom line is that bow profile and the waterline length issues remain just one of the issues. No matter how good the bow job, most designers will confirm that such radical changes are only part of the story. Albeit in a very important and often easy area of the boat to spot.
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