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Design Thoughts

by Ian Proctor (abridged)

It is extremely difficult to assess very accurately the difference in performance between clinker and smooth hulls. The International Fourteens have of course, had exactly the same hull designs skinned up in each way and the National Eighteen Footers are about to do the same thing.

Speaking purely from the design point of view I think the following points can be made:

1. A smooth skinned hull of equal displacement to a clinker-built hull, less wetted surface area. This is an inescapable fact and in my opinion should lead to slightly improved performance in light airs.
2. The flow lines round a hull alter their direction at different speeds. Clinker planking attempts to follow these lines at average speeds. although designers can only guess at them. At some speeds, however, the natural flow line must be opposed to the run of clinker planking and, therefore, resistance is increased with some loss of performance. This again is probably most marked in light airs, but in these conditions the flow round the hull is weak and the resistance is probably increased only marginally and performance not affected greatly.

3. Clinker planking does increase lateral resistance. This can make a clinker hull slower to spin from tack to tack, but of course adds to the hull’s grip on the water and is helpful in increasing directional stability at high speeds.

4. clinker planking has a quite marked effect on the dryness of hulls, breaking down the bow wave film. This effect could, of course, be achieved by moulding a spray chine into the hull. Spray chines (and plank edges for that matter) must, of course, increase resistance slightly. This is particularly noticeable when going to windward in only light to moderate airs through a short chop when the hull is pitching down on to the waves.

5. Though no positive proof has been offered, there is evidence that at high speeds clinker planking may be an advantage. The flow lines are then likely to be along each plank as an individual surface and, if properly designed, this can be a benefit to performance. Furthermore, there is an entrapment of air bubbles which probably decreases the effect of negative pressure in the after portion of the hull and assists the planing.

6. There is no doubt that production of hulls would be greatly facilitated in G.R.P. if the clinker shape were eliminated. Not only would this lead to lower production costs, but it would probably also overcome production faults mainly through voids between the gelcoat and glassfibre laminations at the plank edges.

7. I have not done any weight calculations for National Twelves in G.R.P. and do not know how easy it would be to obtain a smooth skinned hull down to weight and with sufficient rigidity. An important point to consider is that the clinker hull is likely to be stiffer than the smooth skinned one, weight for weight.

To sum up: I would accept that the smooth skinned hull would show slight advantages in light weather and might be at a slight advantage in heavy weather. These advantages are likely to be less than those normally shown and quite acceptable in the National Twelve class between boats of different design but, of course, an ultra light weather boat (for instance) would become slightly more of an ultra light weather boat if smooth skinned.

My last point which follows, is only an opinion — I might add that I sailed longer in the National Twelves than in any other class and have great and lasting affection for them Assuming that the object of allowing smooth skin production is to promote up-to-date production methods and to inject new life into the class, I wonder if this is not too late and may possibly do more harm, by disrupting the class, than it will do good.

The National Twelves appear to be in need of resuscitation of extreme types of hull. Smooth skin construction to assist G.R.P. manufacture may be the answer. On the other hand, it might hasten the decline of the class, which is obviously suffering from competition from boats, such as the Lark. As you know, suggestions for changes in the construction rules were made ten years or more ago and I am sure at that time would have kept the National Twelves in the forefront.

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