I've surfed thruster shortboards all my life. I also now kitesurf and ride a quad fin kite surfboard. I'm recently into SUP and have a 9' surf model SUP with 2+1 set up (F-one Anakao). I am familiar with the feel and pros and cons of thrusters vs quads while actually on a wave. What I am less familiar with and, to be honest, struggling with, is the inherent yaw instability of a surf SUP while paddling (i.e. the side to side nose movement with each paddle stroke; especially the first couple of strokes until some forward momentum is gained).
Yes i'm new to SUP and my technique/experience is lacking but I would gladly sacrifice a little something/anything while surfing for the ease of paddling at the moment.
And yes I know a bigger more stable board would do the trick on the paddling front (say a 10' flatter rockered board) but I am a competent enough surfer not to want to sacrifice the wave riding that much. (I'm only 78kg and like the actual wave riding feel of the 9' surf model SUP I have).
So, is a big-arsed centre fin better at keeping yaw stability while paddling or a well designed quad setup?
A bigger centre fin would help slightly, especially if you move it to the back of the fin box, but it will also slow your surfing turns. I'd recommend experimenting with body weight positioning and stroke angles instead. Put the blade in further forward, and away from the board so you start each stroke by pulling towards the board before straightening up (requires twisting the angle with the top hand so you're always pulling perpendicular to the blade).
Hi edhead
When I read your post I thought I had written it, but back a couple of months ago. I am similar to you I am about your weight kite surf first but due to typical poor wind in Autum I started SUPping in March 2011. Tested a few boards and went with a 9' 4 X 30 Oxbow. Good board and straight paddler for its size. But to start had some yaw issues.
I decided to move on so bought a 9' Coreban Pure Fusion and set up with E Quad fins - back to excesive yaw issues - more than ever and stability problems. But now a couple of months on no yaw problems.
How did I improve - well while this was not planned it seems to have been the key. I was lent a 9' 8 x 31.5 wide Angulo Surfa 3 fin set up for 3 weeks - supa stable and straight paddler. I just went out as normal but all my problems went and my skill level improved dramatically and quickly. I wanted to keep this board but no go. So back to the Coreban Fusion. I set it up with the side E quad fins (bigger than standard) and 6.5 Cut away centre fin went OK after a few initial problems but I adjusted very quickly. Now a couple of weeks on I only ride with the E Quads and all good with the exception of stability in some onshore wind days. Yaw problems have gone.
If you can borrow a bigger wide stable board for a few weeks your skills will develop quickly.
I ride a quad surf board for kite surfing and like the quad feel. Good luck.
edhead ,I had the same problem when I started, couldn't keep the bloody thing going in a straight line while chasing waves, sucks doesn't it 
I personally wouldn't change fins you'll just rip it out ,in time , the trick is , if you call it that, is very simple ,it just takes a little patience
when paddling for a wave position your self into a angle where your pretty much pointing long the face of the wave.So at the very worst you'll heading down the face straight an the second trick is learn to lean on the side your paddling as you roll on to the wave, this will engage your rail and your outside fin!!!!
soon you a happy camper
