Subject: The Folkboat-rudder.
To the board of NFIA. Dear Folkboatfriends. 19. oct 2005
We worry about the trend with thicker rudder. Therefor we would be happy if you can take this proposal on your next meeting in NFIA ( november?).
The Swedish Folkboatunion propose that there shall be a maximum thicknes on the rudder under the waterline written in the class rules. For example 45 or 50 mm.
Motivation/diskussion: Last year there has started a trend with thicker rudder. We have been told that ca 6 boats in Denmark has those rudders, among them the winner of the Golden Trophy in Malmö this year. We think that it can cause the class damage in the future if this trend spreads out. It is against the basic principles of an one design class. As an example: The winning boat in the Golden Trophy had a rudder thicknes of 57 mm in the central part of the rudder blade (which probably was built on minimumweight 15 kg). This gives an extra floating power of 10 kg ( 100 N), wich lifts the stern of the boat, thus making the boat faster. It is also possible that it gives some hydrodynamik favour - se below. (Obs: The Golden Trophy winner was very skill, and we really think they would have won, also with a normal rudder).
The class rules has no limitation of the rudder thicknes - what will we see next year?
Technical investigation: We have made some tecnical investigations:
1. We have consulted Lars Larsson, professor in hydrodynamics on Chalmers (Gothenburgs tecnical university), folkboatsailor, and earlier three times Swedish Champion in Folkboat. He thinks that the lifting power 10 kg has a moderate effekt - the same as if the whole crew (250 kg) moves 0,1 m forwards in the boat. Hydrodynamicly it kan be a favor to make the rudder a little thicker, so that the water follows a harmonic bend along the keel and rudder on the boats windward side when it tacs with a rudder angel of about 5 degrees ( to prevent the boat from turning up against the wind). The shape of the leeward side is of less importance. Ta make an even thicker rudder is a disandvantage hydrodynamicly. On the whole Lars Larsson thinks there should bee a maximum thicknes in the class rules.
2. We have measured 19 normal folkboatrudders ( on a club in Stockholm): Result: 11 wooden boats ( among them 1 Lindh and 2 Williams built): thicknes 36 - 38 mm. 5 boats from Folkebådcentralen: 38 - 42 mm. 3 Swedish built plastic boats ( has often IF.boat-rudders): 45 -48 mm. Comment: The newest Folkebådcentralen-boat measured was built 2001 - we have not measured any brand new boat, wich ought to be done before final decision.
It is easy to measure the thicknes of a rudder: For example: Two parallell wooden sticks with a peace of wood inbetween in one end, about 50 mm thick. The rudderblade puts in between the sticks and is easily measured.
3. The price of a standard rudder from Folkebådcentralen is about 1100 euro today including taxes and freight home. A specialy built thicker light weight rudder of good quality would perhaps cost 1500 euro, and is not easy to build by yourself.
For The Swedish Folkboatunion
På mødet i NFIA lørdag den 25 var stemningen, at der fra 01.01.2009 vil gælde en max rortykkelse på 50 mm. fra CWL og ned.