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The GOSSIP

Number 131 / March 2002

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Published by the Open Canoe Sailing Group

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So Which Way Is Up? (JohnB)
(Adapted from a pamphlet by JohnB, which appeared at the winter meet.)

Sometimes, when I am watching our fleet plug its way to windward, I am struck by how far we have progressed over the years. There has been a continual slope of improvement, both in terms of gear and of us, the sailors. In the early days the technology and rules were largely based on previous ACA experience. We had little understanding of our own and we needed years to acquire it. We had fun learning. Within a very short time, however, the desire to point higher and go faster was in evidence. First Gunter and then Bermudan rigs began to appear with purpose-built hulls and asymmetric leeboards, culminating in DaveS's fully battened sail on the Selway Fisher 'Waterman' in 1995. At the time this was regarded as quite a brave move and we all watched with interest, but of course like most of Dave's ideas it worked beautifully. Most of us regard these developments with pleasure and interest. It manifests a certain vitality within the group; we are alive to new ideas and that can only bode well for the coming years.

It has also been suggested that, as a result of global warming, the weather patterns are changing, as we are either sailing in F1/2 or it's a F5/6 and there doesn't seem to be much in between. Whether there is statistical evidence to support this remains to be seen (we are currently in correspondence with the Met Office regarding the matter). If it were so, then perhaps we might have to revise our rules concerning sail area.

There is always a tendency to develop along more or less established lines, based on what has gone before ("Onwards and upwards", we cry), but if only we knew which way was up! We develop our rigs in accordance with the sail aerodynamics of bigger boats (where the research is done) and forget that our mastheads barely reach the level of their booms. Recent reading has led me to the conclusion that one of the fundamental problems we face is that our canoes are operating well below the clean air flow, where flow is turbulent in direction and speed. The angle of attack and velocity for our little rigs is constantly changing, even within the height of the sail. At this level the velocity can vary between zero at ground level and 25 knots at the head of the sail.

In a recent paper by Mike Brettle in the AYRS journal, Catalyst No. 5, 'Wind Profiles and Yacht Sails', he gives the following figures for the first 10 metres (see table on the right):

However, there is no good reason to think that more suitable rigs are not there waiting to be found. If we have the courage to step outside the conventional perhaps we could be pleasantly surprised - a rig with more power from its 44 sq. ft. and a lower overturning moment?

Two rigs described in the pamphlet appear below. (Ed.)

Height
(M)

Wind speed
(Kts)

10

20

5

19

2

17

1

16

Sail Aspect Ratios (JohnB)

Low Aspect Ratio Fenced Sails
Some years ago they tried putting very wide booms on the America Cup boats, in an attempt to minimise the losses around the foot of the sail. The original aerodynamic work had been done in the Junkers wind tunnel in Germany, where they found that fences would decrease the induced drag,

resulting in an increase in drive of 15/20%. One often sees such devices on aircraft wings. Could such an arrangement be used on canoe sails to advantage and what might it look like? The following is an attempt to understand the concept and some of its construc-... (original text was truncated at this point - Ed.)

Low aspect ratio fenced sail.

Such a rig has a very low aspect ratio (0.6), which makes it tolerant of changing angles of attack and is most favourable to the use of fences. The top and bottom of the sail are fenced, which will reduce the induced drag considerably, with much higher lift/drag ratios. The mast is fully aerodynamic and may be over-rotated, thus reducing the turbulence behind the mast and vastly improving the airflow over the leeside of the sail. The final advantage this rig may offer is its very low centre of effort, only some 4.5 feet above the waterline. Its main drawback is the difficulty to reef, although the soft sail part could presumably be contained, leaving only the wing mast (some 12 square feet).

High Aspect Ratio Sails
It may seem perverse to switch from low to high aspect ratio but a study was made a little time ago that suggested that this could be a way of increasing the drive from our rigs. To some extent this has already been born out by the new sails being run by Solway Dory. They have extended the luff

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