Why trap with kite downwind?

This I don't get. But I'm sure the anwer is obvious because everyone is doing it.

On a windward leeward course you want the best VMG downwind. You get this by flying a hull which reduces drag, move apparent wind forward enabling you to sail deeper at high speed, blah blah blah.

But if you are on the wire, doesn't that make it harder to fly a hull? Shouldn't you be able to sail deeper with a better VMG with both of you inboard? With two sail cats (Tornado classic and Hurricane non SX) you send the crew to lee to aid the wild thing. Why does this logic not hold true for spinnaker boats?

This has been bugging me for a while now.

You are right to a certain extent.  If one crew member is on the wire then it is harder to fly a hull.  However, this extra leverage helps in two ways:

1 - Your speed picks up enough so that the apparent wind comes further forward and you end up on the same line as before but faster!

2 - The crew can get their weight further back for obvious reasons.

The top guys have done a lot of testing and the optimum way downwind is with one on the wire.

(Obviously there has to be enough wind there in the first place)

Originally posted by Scooby_simon   Also, currently, I think, when twin stringing up wind, one on the wire down, when single stringing up wind, don't wire, crew on the tramp, when neither stringing, crew down low.

Give or take a few knots either side that is a pretty good rule of thumb.

When to put one on the wire downwind is all about feel.

Cheers all.

Still not convinced by Worthy's first reason as I would have thought that with both crew "in" you would get the same speed with more depth.

Hard to argue with the experience of pros though. If Bundock/Ashby do it it has to be good enough for the rest of us. I just haven't found it beneficial in practice.

Having the crew on the wire equates to greater rotational inertia (engineerspik) so the balance is easier to control in gusts. (like a spinning ice skater with her arms out - it slows the spin down)(study engineering - if you are already ugly your sex life won't suffer).

But the main reason we do it is to get that weight aft in a blow. There's another foot of boat behind the rear beam that you cannot use when inboard.

Happy to get chapter and verse form other experts.

Dave, it is ALL about apparent wind and getting the lard back when windy. 

Lard back you agree is obvious.  Lets take the Apparent wind(AW) issue.

We know dead running is slow, so we head up to get the AW forward to get more drive.  Nowt new here, and as you heat up, you need more righting moment (RM) to keep the hull in the right place, or you bear off to get the hull back down.  Bearing off means the speed does not rise much, extra RM means the speed goes up (a lot).  It's all about the maths to get the best VMG down wind.  Consider:

Not wiring you are sailing DW at 40degress of dead down wind (DDW), wiring you are sailing at (maybe slightly higher, say 50deg DDW) But 20% faster (or more.  Get the claculator out and you'll find that wiring is quicker !

In practice, wiring is actually even better as (sailing my old Inter 17) as soon as there was enough wind (F3+) I could sail LOWER and FASTER getting on the wire as the AW goes way forward and you can still bear off, speed is higher too.

Wiring works