Hi Robi,
Wouter`s comments on finding the groove downwind are great, nothing much to add. You need to set the spin, sheet in, sheet in main & jib, as if going upwind, then head up until the boat accelerates so much that it forces you to bear off, in 15knots this shoud definitely happen. Then need to steer up or down to just keep the weather hull dry. It could be that you are not heading up ENOUGH to start with, apparent wind should push you down so you are reaching deep, but it feels like you`re beating. If you don`t head up at the beginning of the leg you will not build enough apparent wind to accelerate, and will just sail along slowly relative to someone who has got it right.
On upwind, I find that I can`t point much like some of my opposition, but I foot for speed and often get to the mark ahead, some of us are just 'footers'. Suggest you call up Matt and ask for some advice on settings for mast rake etc, that can make a big difference. Also ask Phill, I`m sure he`ll give you some basic numbers that will work. Then you need to just go out and try different stuff at different times, noting how it changes the power.
Mast rotator slightly out of the right setting can kill your pointing upwind, suggest you lead all controls out to the beam ends, remember my previous post when I was asking the questions ?
If things are not working well, need to change ONE setting at a time and see if it helps.
Sounds like you lacked power, need to use a bit less prebend with that much weight on board, a bit more mast rotation.
You could also be hooking the leech, this kills upwind pointing and speed. Try ease off the mainsheet a foot or so if you are struggling with this again.
Also need to get your crew to lose 40lbs .