Hey I have seen a bunch of videos of the skippers holding the mainsheet downwind while sailing with a spinnaker especially in f18s.In know you want to support the mast and all with the chute up but they are easing the mainsheet what looks to me as if they were pumpping the main downwind. I usually center the traveler and sheet in tight as if I was going up wind and dont touch the mainsheet at all just steer. Can someone explain the how this works and does it work about the same as upwind. Sheet in for power , ease in puffs??? thanks

Works great (trim) in waves to get hooked up. You don't want to ease too much and remove the "backstay" from your mast ,like you said. I've never eased the main in a puff with the kite up. Drive off and if you can't do that dump some traveler.

Alternatively you can cleat the main sheet and play the traveler. With it you can have a much smoother ride, keeping the hull flying by playing the traveler rather then zick-zacking with the rudders.

Cheers,

Klaus

At the F18 Worlds in Long Beach we had several races where the wave action was bigger than the wind. In those conditions, it was really easy to end up stuck on the backside of a wave going really, really slow. It's almost like the feeling of a powerboat trying to get up on a plane, but instead just sitting with its bow up pushing water. Horrible!

A few things really helped the situation:
1. Coordinated main and spin trim with the wave action. When you get caught on the back of a wave and slow down, your apparent swings aft. You need to ease both main and spin to maintain power and get over the wave. When you crest the wave and accelerate again, a well timed trim will promote surfing and adjust for the apparent which has swung back forward. Both main and kite trim are important.

2. Crew fore and aft position. In waves it is really tempting to just stay in the footstrap and keep the bow up, but that can be really slow. When the boat gets stuck on the back of a wave, moving weight forward to unload the stern is helpful. Taking a step forward on the crest can also promote surfing, but you need to be ready to move back as the bows hit the trough.

Jeff has it pretty spot on.
I'd also like to add a few comments.
When sailing in flat water and steady wind the mainsheet stays where it is pretty much.
If it's super puffy you can play it a few inches or leave it out a few inches.
Whatever you do, don't constantly play your traveler, it's slow in all conditions. The only time you should be dumping your traveler is to de-power (again in puffs or if you're trying to sail really high).
As you open up your leech you drive your bows down. This is okay in flat water. However, in wavy conditions, where you are really pumping the main downwind, sheet harder when coming up to a wave or you will push your bows into the wave and slow you down a lot more. At the top of the wave you can ease a bit to open up your leech and push your bows down. But this is not as important as keeping it tight to get up the wave in the beggining so if you can't stay on top of it just leave it tight.
You mentioned you sheet in tight as if you were going upwind, it should never be as tight as upwind, just a bit off of that. Your always going to ease off just a bit. As it gets windier you are going to ease off less and less from your upwind trim. In 20+ you're really only easing a few inches.