Published by the Open Canoe Sailing Group

Meanwhile Rod and Jan & Dave were having their own tussle, being close together at the buoy. With sails dropped the paddle race began. Rod's light boat and double paddle kept him up with the twin paddles of Jan & Dave and neck and neck they thrashed towards the line, but Rod slowly gained and crossed ahead of them. Peter, making sure his twins were put properly to sleep, made a creditable paddle to the finish.

Walt and John hope you all enjoyed the start to the season, and would like to thank all those who helped clean up the site and thanks to Keith for laying out the buoys and Bernard for the paddling instruction. Oh! Don't forget the band.

(Was civilisation created by winners? Answers on a postage stamp please. And for the 2nd year running I saw my first swallows of the year at Rudyard. - Ed.)

Those Dangly Bits (JohnB)

We have been having some serious thinking about centreboards, leeboards and rudders - those bits that dangle beneath the boat and, being out of sight, are so often out of mind. The drag created by these items is far greater than the drag of the hull alone and this should encourage us to pay more attention to them.

Firstly, the section. The most successful we have used so far is a NACA 0012, or something close. I say something close because if you read the preamble to the NACA series you will see that their test sections are made on profiling machines to a tolerance of two thousandths of an inch! Don't let's kid ourselves that we can get nearer than somewhere close! I suppose, to be realistic, the best we can hope for is a decent streamlined shape.

The most important part of the board by a long way is the leading edge and the first third of the chord. It needs to be as near perfect as you can make it; so take your time, cut a cardboard template, this bit really matters. The total area of the board below the boat or waterline needs to be about 3%

Optimum Centreboard Profile.

Optimum profile for a centreboard or leeboard for a 44 square foot sail (190 square inches).

of the sail area. In the UK, with their sail area of 44 square feet, this means a board of 190 square inches.

The profile of the board is also important. The upper two thirds is a parallel, the tip is 0.5 of the chord. This will give a near perfect lift distribution as well as being slightly easier to manufacture. Remember that all these proportions apply to the part of the board that is in the water; you will have to add on extra to allow for the case etc. The thickness of the board should be close to 10% of the chord, in this instance 1 inch. From the taper down the thickness should decrease consistent with maintaining the 10% of the chord ratio. To produce a board that is even close to this specification is demanding work but without a degree of accuracy you will be wasting your time. Ideally it is work that should be done on a tape-controlled contour mill; doing it by hand you will need to progress slowly and cautiously.

The board described here used a NACA 0012 section, and like all foil sections it has laminar flow at the leading edge and turbulent flow at the trailing edge. The transition from laminar to turbulent flow occurs either where the designer wants it or at the first bump or irregularity. NACA 0012 has this transition near the leading edge, which makes them less susceptible to building errors. It also allows them to run at higher loadings before they ventilate or stall.

Rudders need a thicker section than the board. The thickness of the blade should be at least 13/14% and possibly 15/16% of the rudder chord. This is because the rudder, when turned out of the flow line, is operating at a higher angle of attack. A centreboard normally runs up to about five degrees angle of attack but rudders are often 10 or 15 degrees out of the flow line. The thicker section will support a higher angle of attack before it stalls.

I know that much of this will go against the grain; it seems counter-intuitive. One would imagine that a thin blade had less drag but it is not the case; it's a case of the truth being stranger than fiction.

Auf Wiedersehen, Pets! (TimF)

I would like to take this opportunity of saying thank you to all members of the OCSG for the enjoyment Ben (14) and I (41-ish) have shared with you through the mid-nineties. Ben first started sailing with me in a Little Pete, making contact with the group on Windermere in '93. More recently we sported a Chris Hare boat and Ben kept the old Grumman of Jenny & KevinL in the family.

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