Centre-board and rudder-blades on FJ’s

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Deriva e timone sono determinanti nelle prestazioni e nella sicurezza di uno scafo, richiedono precisione e competenza.

Nota Bene: questo articolo va considerato per l’epoca in cui è stato scritto, relativamente a tecniche di costruzione e conoscenze idrodinamiche che oggi hanno subìto delle evoluzioni. Malgrado questo conserva una sua interessante valenza, specialmente nella descrizione dell’importanza del profilo, per la deriva FJ.

This article was published in the International FJ Bulletin of April 1973, written by J. de Waard In this article J. de Waard writes about a test of the influence of the thickness of a centre-board on the boat speed and the resistance to drift. This article is a shortened version of one I wrote for the Bulletin of Dutch Flying Junior Organization.

When i started in the FJ, my boat had a metal cetre-board. To my disappointment I found out that with this boat I could not point as high in the wind as those boats, wich had centre-boards of 18 mm (about 3/4”) thickness, the difference was approximately 5 degrees. PLAST-O-MATIC boatyard in Beverwijk, Holland ( to wich I am closely related) built a centre-board case of 40mm (1 5/8”) width, the maximum allowed for FJ.

From diagram 1 we can read that given a certain angle of leeward drift (A, B, C, D and E), the pressure of the centreboard against this drift KY is greater when the centre-board has more thickness. The centre-boards I made had a thickness of 20mm (7/8”), 25mm (1”), 32mm (1 5/8”). The centre-boards were compared sailing in a test-bank. Sailing, the results were measured with a finish line fastened to the mainsheet traveller-track, and a calibration on the transom, the big line in the transom indicates when the line is parallel to the keel of the ship (at the end of the line was a trout fishing spinner).

We found out that pivoting centre-boards (fig II) did not give any advantage in speed or drifting angle. When we compared the various centre-boards the 36mm (1 ½”) one gave 30% to 40% less leeward drift than the 20mm (3/4”) centre-board.

Centerboard profile 2

The total resistance of boat plus centre-board was lower with the 36mm one, probably because the boat goes through the water at a smaller angle when leeward drift is less. Comparing several centre-boards of 36mm, we found out that:

  1. The tickest point of the centre-board should be at 1/3 from the foreside.
  2. The foreside of the centre-board should be round (radius 2mm according to tank results, in waves radius 3 to 4mm (approx 1/8”) proves to be better) (fig III).
  3. Aft and underside of the centre-board should be cut if at a right angle, every roundness here hurts the performance. The thickness should be between 2.7mm (1 ½ “) and 3mm (1/8”), thinner aftsides do not improve the performance.
  4. Windward performance improves if the sides of the aft 1/3 of the centre-board are made hollow ( with sand-paper or plance files) to about 2mm

Centerboard plan

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