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
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 hulls 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