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The GOSSIP

Number 138 / October 2002

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Published by the Open Canoe Sailing Group

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Adding A Jib - One Sail Or Two? (JohnB)

Members may be interested in a little sail analysis program called SailPowerCalc, which calculates forward drive, overturning force and leeway angles etc. In spite of its inaccuracies, like only approximating the imperial measurement and adding an extra percentage for roach, it is still probably better at guessing than we are, so it's worth a look. We ran the program a couple of times to compare a single mainsail with a mainsail and jib of similar area.

The change in area from 44 to 52 sq.ft. is the result of the added roach and to reduce the area would mean changing the sail dimensions which, by the way, must be entered in metric.

When we begin to compare the analysis of the two rigs the changes in performance are clearly seen. The apparent wind speed is approx. 14 knots at an apparent angle of 22 degrees. The angle of heel is high, about 26 degrees, but this is calculated with the crew in the 'Tim Fenner' position. In the program this is called the RMC and for an open canoe it is about 1.2 kilograms/metre/degree, although clearly this will increase as the crew sits out.

The rigs tested on SailPowerCalc.

We can now consider those numbers that are perhaps more germane to the question of one or two sails. The aspect ratio is 3 in both cases.

Main

Main & Jib

Sail drive in forwards direction (Kgf)

4

5

Heeling moment (Kgm)

30

28

C of E as percentage of rig height (%)

39

35

Drive/heeling moment ratio

0.146

0.164

TyroneC sailing the training boat he built for the 10 sq. M association. Very similar to our own boats.

The whole trend of the results is towards improvement and it is indeed as Frank Bethwaite posits in his book High Performance Sailing, that jib sail rigged boats are more efficient than single sails. The sail drive, that is the drive that is actually pushing the boat in the direction you want to go, has increased by some 25%, the height of the centre of effort has been lowered by 6 or 7% and the drive/heeling ratio is improved.

These results are encouraging, so who will be first to turn them into practice?

 

Sea Cows (JimB)

The GOSSIP has been very good this year with well-written yarns. The coastal meets in Wales and Scotland have provided most of the interest, I think.

As F&M (Foot & Mouth Disease - Ed.) put a damper on things last year, the coast gives a fresh dimension. Sea cows are immune. Some of the creatures sighted made it sound like a Capt. Cook voyage of discovery - bottle-nosed dolphins, porpoises as well as rock climbing, tidal and toilet considerations all gave an extra dimension to our freshwater forays.

Apart from the gulls and midges I only saw a seal, the first for me. I had been lost in the enjoyment of my new boat (on Sunart, I presume - Ed.) so I did not see it until it was only a few yards in front; then I realized it had a face and was watching my approach. It submerged with a swirl and I sailed over the spreading ripples. I wonder what it thought of my three new bottoms as they passed over? (I hesitate to suggest, Jim; answers on a postcard, folks, and a bottle of beer to the winner.) To return to my opening remarks, the GOSSIP will only be as good as we make it, so keep writing. Roy will sort it out. (!)

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