With my 7M Superfreak UL, skipping across the tops the the chop, fully planning for sure and committed to the harness the board starts to feel really weird, not sure exactly but it seems I'm slipping sideways down wind and need a bigger fin?
Also struggling with rig control, the board wants to head upwind and then of course luff up, I'm trying to rake the rig back and at the same time push hard on the front foot to direct the board downwind.
Am I asking too much from this SUP board with tail rocker?
Suggestions
I reckon you are expecting too much.
Wider the board, bigger the fin.
You'd need about a 40cm plus in it (?) if it were a windsurfer
I think also lack of straps meaning you can't drive it away really hard with your front foot?
Caveat: never sailed a SUP 
You don't really need a big fin unless you're standing out on the rail in the straps. I could sail my old 12'6" with a 6.4 and plane upwind with a 9.25" fin. Is the board a thruster? Maybe try taking out the side fins a try a centre fin with more area if you have one. Also, why are you trying to rake the sail back? This is how you steer into the wind when not planing so could be half the problem.
Agree with Mark your probably expecting too much from the board. Ive sailed SUPS and they certainly are'nt performance boards
MY first windsurfing board was WINDSTAR with triangle sail.
Looks like SUP from early stone era.
Anyway that is possibly the best way to discourage anybody from windsurfing forever, but if you passed this stage you are probably committed.
What I only suggest 
- windsurfing is MUCH EASIER on right , modern equipment...
Wrong tool for the job.
A SUP is a Stand Up PADDLE board, not a light wind planing windsurfer.
No straps and high speeds, why would you bother?
Never sailed a SUP but willing to put in my 2 cents worth.
One reason for luffing up could be that your weight is still not fully into the harness. Try moving your back foot closer to the centre of the board and see if it straightens out. If it does, it means that it is your weight railing the board up and this will make it turn into the wind. Only solution I know of is to just put more weight into the harness.
Not sure about raking the sail back. This just happens naturally due to you moving more to the back of the board as the wind hits the sail. If anything the sail should be raked a little forward to force the nose off the wind.
Sorry about the other problem you mentioned about slipping sideways downwind. Not sure but if you haven't got the steering sorted, you won't be going fast enough for the fin to bight. Maybe the steering problem should be sorted first and see what happens.
Along with the rest of us, anyway, anyhow, anymeans to get on the water (sailing that is)
what are you doing gayboarding?
Yes, the board is a thruster, center fin is around 9" or so.
I'm raking the rig back because that's how I have observed people sailing while up to speed and planing.
And I am discovering if the rig isn't raked back the wind wants to jerk the sail forward and catapult me.
When you are planing you steer primarily with foot pressure not tilting the mast forward and back, correct?
I know where you are coming from Beaglebuddy. My take is that not all WindSUP's are created equal - there are some that plane early and have good speed but not many. My recent experience is on a Kona 11.5 TT and I really only got it dialled in the other day. I was using a SuperFreak 7.0 UL rigged in "ultrasoft" model (see ULUS on the Superfreak forum) on the Kona in 7-12 k in ****ty knee high waves. But it was fun - the sail pulled like a tractor in and out of transitions and the board had the glide to keep moving when gybing onto the waves and swell or riding clew first down the line. But you could shut the power off by luffing or over-sheeting. In 12 k, I was planing at good speed in the straps (I dunno, no GPS but easily over 20 knots board speed).
I reckon a longboard for light wind wave sailing has to have the length to transition to a plane cleanly - it should be planing on the wave and have thin enough rails to crank into the turns. I can't see how a short tug-boat SUP with tail rocker and corky rails can do that as well.
Anyway, I reckon your board may not be a planner - but that small fin won't be helping. Have you rigged the Superfreak like Jeff Henderson's video? Might have too much drag and loading the fin up. Took me 6 months to learn to rig mine properly and I don't think I'm finished learning yet. I am a slow leaner and think I know better. Turns out I don't :-) Having footstraps helps balance in a planing stance, but I don't use them much on a wave - feet are moving around all over the place.
Good luck getting your board dialled in, and remember the long boarders motto - Having fun is half the fun!
The SUP is an 11' X 31" NSP, I had a mast track installed. For a surfboard it has a relatively flat rocker but like all surfboards there is some tail rocker.
Using it as a SUP I have no problems turning it because I'm 100KG and very tall, for an average sized person it's a beginner board but for me it's just perfect.
I like to sail it because the wind here is very marginal, gusty with holes.
I haven't learned to waterstart because there really hasn't been enough wind, when I'm sailing if there is anyone else out they can usually waterstart but they are always smaller than me so until I get more proficient and can feel comfortable with even bigger sails to waterstart I need a board I can uphaul.
I have Starboard GO 155L but unless I'm fully powered up and planning this board is no fun to slog on and it's not as fun to ride swell and small waves on.
Where I sail I can wave ride the blast back out around the break.
I don't understand everyone's comments about not raking back the mast, I see everyone sailing that way, go to the picture section on this website and everyone powered up is raking the mast back, "closing the gap".
If I don't rake back I risk a catapult or having the boom jerked from my hands and smashing the nose of the board. Settling into the harness it gets a twitchy pulling forward feel until I rake back a bit.
Dr. Duck I'm rigging to spec not ULUS but I plan to try it.
Isolating the problem now, it seems when powered up I need to start raking the mast back, and to do that I need to start moving towards the back of the board, and when I do that the tail rocker caused the very long nose to rear up and that's when the problems start.
Like the guys say, wrong tool.
It's OK in light winds, I use a SB 12-6 up to 15 knots. Use it to cruise around or freestyle for exercise. Fins don't make a big diff in my experience.
But for planing?? 15-20 knots and no harness with a 6.5 is real good workout, and it's almost fun. But not planing.
