Hi Wouter,
Didn't mean to upset anybody I certainly wouldn't suggest making any existing boats ineligible, just thought you were canvasing ideas and they were my thoughts. I have read the F16 rules carefully and if you made the lower section of the mast heavy enough you could make a ultra light top section and still have a tip weight of 6.5kg. as the full length of the mast is being weighed. If the reason for this rule is seaworthiness it may be worth adding a balance point measurment, when I raced in Contender dinghies they had a balance test on the mast to avoid ultra light mast tips. However I do like the simplicity of the F16 rules and can see you have tried to keep them few in number which is why I made the suggestion on materials as it is much harder to play with mass produced Aluminium products and generaly cheaper.
Regards Gary.