An Historic Error?

C

Chainsaw

Brmmm Brmmm
Am doing some rudder maintenance and replacing sloppy plastic gudgeons. Had a search round the old threads on rudders/set up and found this anomaly:

http://www.laserinternational.org/rules/measdiag.htm

In my book, the 78 degrees refers to the bisected angle between the top leading edge of the rudder assembly and the leading edge of the foil of the rudder. (the bit that’s actually in the water.) Sure my book is old, but no other measurement has changed (only tolerances on mast distances has reduced from +/- 12mm to +/- 5mm.) Perhaps a pic will explain better:

rudderangle.jpg


You can see that 0 degrees is not the bottom edge of the rudder cheeks.

As it is, the top trailing edge of my rudder is not parallel with the trailing edge of the alloy cheeks. I read in an old thread that this is a giveaway that the rudder is swept too far back, contributing - perhaps not greatly - to weather helm. But there was no general agreement on this forum that a guy could file a bit out of the rudder and swing it forward. Posters have muttered warnings about measurers etc...

Was this rule purposely changed or is the new drawing inaccurate and the error carried through – causing the measuring debate (if any)?

My rudder is ten years old and made in Australia. It's some kind of molded reinforced plastic. The boat and equipment was used in an international regatta around 2005, and to my knowledge has remained unadjusted.
 
My understanding of the rule is the angle between the bottom of the rudder bracket (alloy cheeks as you call it) and the leading edge of the rudder blade cannot exceed 78 degrees.

The rear of the bracket can be used for a reference, but I do not believe there is a spec for that measurement.
 
My understanding of the rule is the angle between the bottom of the rudder bracket (alloy cheeks as you call it) and the leading edge of the rudder blade cannot exceed 78 degrees.

The rear of the bracket can be used for a reference, but I do not believe there is a spec for that measurement.
 
Your drawing is only part of a bigger drawing.

I think that the line just below the rudder head does NOT have anything to do with the 78 degree meassurement

The line shown just above the 78 has to do with the length of the foiled blade. There is another corrsponding line at the bottom of the blade but your drawing is cut off and does not show that other line.

For the purposes of the 78 degree measurement, the bottom of the metal and the leading edge of the blade have always been the reference lines.
 
I made the pic close up to highlight that the line is not parallel to the cheeks of the rudder head. Here's the bigger picture
biggerpicture.jpg


...showing the construction lines for the length of the foil (527mm) which run parallel to the trailing edge, as they should, and also a width measurement of 203mm which has a construction line at the same angle as the length construction lines. But what I think is the 78 degree line is nowhere near parallel to the other construction lines...no way is it a printing error. It was either drawn wrong, or it's meant to be that way. The book is: Laser Sailing for beginners and experts, by Dick Tillman 1975 - Appendix 4 Measurement diagrams.
 
http://www.laserinternational.org/rules/rules07.pdf
ILCA By-Law 3: Measurement 2. (b) Equipment:
If any mast, boom, fitting, centreboard or rudder is the subject of a protest as to size, shape or location, measurement thereof shall be governed by the drawings and tolerances set forth in the Measurement Diagrams (Ref: By-Law 1 - Rules)​


So the official measurement diagram is what governs for the rudder. That seems pretty clear to me that the measurement is taken between the bottom edge of the rudder head (ie the metal plates) and the leading edge of the rudder blade. I don't think there is any error. I tried my hand at drawing in Paint, maybe this is clearer:
rudder.JPG
​


Also, you CAN adjust your rudder forward or backwards to get the required 78 degree angle:​

PART THREE
OPTIONS AND EXCEPTIONS TO PARTS ONE AND TWO
15. RUDDER​


(d)
To achieve the maximum 78 degree rudder angle relative to the bottom edge of the rudder head, the leading edge of the blade may be cut away where it touches the spacing pin.

(e)



To restrict the rudder angle to maximum 78 degrees relative to the bottom edge of the rudder head, the lower forward spacing pin shall be wound with flexible adhesive tape.​


 
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