As far as I can see the F16 class do not rule that you HAVE to use a spinnaker or even have one fitted. Basically it only limits what you can maximally use.

If your boat still measures in (overall weight) without the spi-gear then you can race without it. If not then just add a little weight (also allowed by the class rules). Of course you will be racing first in wins or of the fully fitting F16 rating despite the fact whether you are fully fitted or not.

I think the only minimum requirement stated in the F16 class rules is that your craft is at least 104 kg in 1-up mode and that you have a pair of rudders fitted. Also if you have daggerboards then they should also be a pair.

Other then that you can sail upside down if you think that that will make you faster.

Wouter