The pros at PWA (Professional Windsurfers Association) have collected together a bunch of tips on this very topic.
Without quoting the whole thing, it's worth noting some of the highlights:
Anne-Marie: “Pack more bags, but lighter ones... don't put everything
in one bag and make it mega-heavy...they don't like that these days.
You can pack it in 1 or 2 bags, but be prepared to divide it up in 3
If they are really strict about the weight, for example, you can
always put some sails, a boom and a mast in a single board bag you
have around one of your boards.
Which stems from the fact that some airlines are going to be extra strict about weight, individual bag sizes (sometimes due to union issues about the weight that staff have to lift) and bulk.
In all honesty, once you've chosen an airline it's worth calling them up and asking them for suggestions. They'll have dealt with bags of all shapes and sizes, and may ask that you split it up, or pack with protection, or tell you to divide into 2 or 3 bags and pay for extra baggage.