To prevent cheating using the grandfather rule.

The grandfather rule is there to allow an easy access to F16 sailing / events using very similar but non-compliant boats. In smaller (local) events it was envisioned that this allowance would be applied very leniently = open F16 class. However, at important events like the Global Challenge the rules would be applied more stricktly = (closed) F16 class.

As an onwer of a non compliant boat you take a small risk; the head of the local organisation or the board of a local class association may not renew your dispensatie next year. But apart from that you are pretty save. Once dispensation is given, it will be valid for the remainder of the season except at very important F16 events (2 a year ?)

Personally, I wouldn't worry about 3 cm additional hull length. I don't think any F16 sailor would protest you for it at any event short of the GC. And even then the local class organisation may choose to protect you by issueing a dispensation when you are less performant overall due to other reasons like being overweight, having less sailarea etc.

Wouter