I never thought I would say this but I actually agree with Macca here.

Without weighing the boat I don't see how you can be F18 compliant.
The rules require that the measurer put a sticker on the hull stating the all-up weight, which has to be the same number as stated on the certificate.
It is also a lot less work to weigh it once instead of at every mayor event.

Measurement Rules: http://www.f18-international.org/rmeasurem.htm