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PAWS AND THE NOSEDIVE

John Sears demonstrating the Nosedive. [photo-Dave Eberlin]


This year, I have had a new experience in National 'Twelves. I have also discovered Terminal Velocity. All this has been achieved in Paws which I designed in 1976. The trend throughout the development in Twelve design has been towards wider decklines and finer bows. Paws is 6ft. 3in. wide and has very fine bows. The problem with the latter, however, is that they have less buoyancy. Therefore when the boat is planing it relies on "dynamic lift" to keep the front of the boat above, rather than below, the water. (Dynamic Lift was discovered by Uffa Fox in 1928). This is fine in steady conditions when the occupants move aft until the boat is sitting comfortably-and then the fun begins. As we all know, the wind is not steady. When a gust strikes the boat downwind, there is a momentary imbalance of forces which tends to depress the bow. The only thing that stops it going down is its buoyancy, which is something of which we have very little. Instead, unless the crew does a bionic leap into the transom, the bow continues to submerge which makes everything worse. The rig acts even more downwards and the dynamic lift becomes negative, rather than positive. At this point, lesser mortals may lose their presence of mind. A variety of bailing-out techniques can be practised, e.g. backwards flip,standing dive from the transom (extra points for a double somersault) and the President of the Trent Valley Subaqua Club can provide expert advice on the best method to suit your particular problem. If you do decide to hang on, you may experience the ultimate nirvana of the true nosedive. This will always be one of my most vivid memories. The sensation of flying, after being forcibly ejected from the toestraps in the transom, landing in the sail, while the rest of the boat does a quick Lusitania job sinking bow first, rudder up in the air-fantastic !


However, while this gains maximum spectator points, it does not win races. Various remedies have been proposed such as transom trapezes, bow flaps and helium in the bow tank but none have been practical. Nevertheless, the catastrophes described above can usually be avoided by anticipation and sailing technique, of which more later. In Paws we have a more fundamental problem. Because the boat is wide, it can support a lot of power from the rig, sailing downwind. As the wind strength increases, we naturally sit further and further back in the boat until the crew sits on the helm who sits on the transom. Apart from having to cope with the sensations this brings (loss of vision, etc.) and being unable to steer, this is fine. The bows come up and the boat travels along in style. Then an extra gust comes. The books say bear away and we, do, gently. Slowly, the bows go down again until the boat rides level, travelling like a hydrofoil, with most of the hull clear of the water. The sensation is like ski-ing through a perpetual carwash. This is terminal velocity.


If we try to go beyond terminal velocity, we encounter the nosedive. Once the boat is riding level fore- and aft, any further pressure on the rig will depress the bow with the results described above, The only ways to increase terminal velocity are to move the correctors aft and take ori a heavier crew. There are ways and means of escaping from the nosedive. It is important not to let the water reach the foredeck as this acts as a very effective dive plate. Until that point however, most modern boats remain surprisingly controllable, provided they are kept level athwartships. To lift the bow, the crew must pump the jib hard with frequent hard tugs an the sheet. At the same time, the helm must try to induce a mini-broach by heeling the boat slightly to leeward so that it begins to luff up. Heeling the boat increases the bow wave dramatically and provides much needed bow lift. However, it must be applied sparingly, otherwise a classic full blooded broach will develop. Once the bow comes up, helm and crew must sit out hard to level the boat and move aft to avoid a recurrence.


On the design side, the solution is fine bows with buoyancy. This means veed sections and a deep stem (see Pipedream). A further, development could be the Ogre type transom, which allows the stern to "sink" and moves the centre of buoyancy forward. A larger jib and smaller main will provide more lift up front but performance to windward may suffer. An idea of what the terminal velocity of Paws is can be gained from the fact that we were being photographed from a 25hp dory which was having trouble keeping up on smooth water.



Design Notes

 


The China Doll and the Cheshire Cat had the greatest influence in the design of Paws. The Doll is a very good all-round boat with sections that have a moderately low wetted area, yet give a very firm planing platform. The Cat has a striking rocker profile with a deeply immersed stem which gives the bow a knife-like appearance. The strong, points of the two designs appear to be off-wind ability and windward performance respectively.The theory behind Paws was to combine the best points of these two designs with a few additional features to produce a boat with fine, water- lines, low wetted area and sufficient power, in the sections, to give fast planing. The boat also had to be as wide as possible yet be able to rolltack properly.


The rocker is based on the Cat but with a deeper stem and a semicircular run from the front of the plate-case to the transom. The deep stem enables fine waterlines to be used while still retaining sufficient buoyancy forward. The mid-ship section is based on the arc of a circle up to the waterline, but with the third plank then extended to pass through the rise of floor measurement point. This was intended to give the minimum waterline beam for a reasonable wetted area/displacement ratio. The, chine is above the rise of floor measurement point (unlike the Doll) to provide, some buoyancy in the topsides for roll-tacking. The waterline is simply as fair as possible in its run from the bow through the mid-ship section to its maximum width. In the first foot it is slightly hollow and then runs straight before gradually curving from the bow bulkhead onwards. 'The deckline follows a similar pattern as this is closely allied to the waterline shape. The maximum beam is 6ft. 3 in., including the gunwales.


The transom is loosely based on a 4-plank Doll but with the chine raised and the third plank extended. The other critical section, the bow bulkhead, is based more on the Cat. The garboard is slightly angled and carried as wide as a fair plan of the plank joint will allow. The remaining planks are fitted to suit the overall run of the planks. The garboard twists through 90° from the stem to the front end of the plate case. Thereon it is flat, enabling one sheet to form both planks either side of the keel. The remaining planks twist progressively along the length of the hull with no disproportionate changes in angle between one section and another. The chine angles at each section are as shallow as possible. The line of the chines is intended to match the waterflow, especially at the bow. The land on the top plank was left to give a break-off edge and to strengthen the joint.


Paws has performed well, particularly in moderate conditions. Windward performance is excellent and her displacement speed on all points of sailing is high. Planing shows the odd quirk in the design, although it is basically sound, once the correct technique has been mastered. Paws is stable apart from the occasional nosedive, and roll-tacking is good in spite of the beam and a deep stem. In all weathers she is a pleasure to sail.

John Sears  
 

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