Using my Hokua 9'10 in thruster set up have snapped off three FCS M7 Glass Flex fins at about one every 3 weeks.
Guess I need gruntier quality fins.....thinking about moving to quad set up perhaps.
Been stabbing away at google search but thought would "ask the experts" (that's you folks!) 
I love the Lopez quad fins in my 8'10 Lopez board but they're expensive fins if you keep on snapping them. 
I haven't snapped one either but Gizzie's got somthing unusual going on to be snapping fins so regular. It may be better to find out why they're snapping before shelling out big dollars for a set of GL's....as good as they are.
i went through a phase where i kept snapping the fins out on one side. Spoke to a local Dis. and he said its likely my leggy was getting snagged when wiping out thus snapping em. they were plastics. since gone to a 6foot leggy and have no probs.
Apart from towball saying "run it as a quad" one of the main proponents of running a quad setup (he's a kneeboarder what would he know? lol) says if you doing the quad thing you have two fins taking the turning stresses so there is less stress on each individual fin.
The 9'10 side fins sre placed quite aggressively forward & near the side of the board
If it's a case of them snapping under load, you would think a bigger fin would load up more (more surface area for the water to push against) and have more potential to snap. So the quad theory might work but I wouldn't use a bigger fin than your already using in the front. Maybe same size front and back may work or the other option is just use smaller side fins with same centre your using now.
I've seen a few cases of the flex fins snapping on SUPs. I ride the same board & I'm guessing that they aren't designed for use on a bigger, heavier board like an SUP & therefore snap while under that heavier load. That's why using a carbon fin specifically designed for SUP makes sense to me. My 2 cent.
hey -- gizzie, perhaps you could clear this up
when I started surfing, if you dropped down a wave and were standing straight up and it was over your head -- it was 6 foot ( I AM 5'10")
I usuallly surf waves that are waist to head high -- 3 to 6 foot - my size rating,
I HAVE BEEN RUBBISHED -- 1 to maybe 3 foot is what others rate this. How high ( up to your body length) do you rate 1 to 2 foot waves?
Steve ps what about Garrets wave 100 ft?
your not the fish I'm after
your not the fish I'm after
Regardless of the "mine's longer/mine's shorter" discussion you are definitely not fitting your board out with good quality fins towball
........"I put $45 fins on my $2500 sup".....what's that all about ?
Be careful: if you use too big and solid things, if something is gonna break, I am not sure I want the box to be ripped out the board :-)
Seriously, I would advise 2 kind of fins.
- carbon base fins (the scarfini are really nice, fast fins. Their bamboo sets are awesome). You get the tab resistance of carbon, but the tip of the fin still flexes a bit to absorb the load http://www.scarfini.com/product3933.html?id=B6923A98-0017-16F8-92F5D31E0B0768FE
- C-Drive fins (my favorites). Tough as nails, but the main area is close to the board, so you do not have too much leverage on the box or on your feet (the fins have tremendous drive and grip without adding latency to rail-to rail like fins with area in the tip) www.cdrivefins.com/
Avoid the Gerry Lopez fins, they feel really slow, I don't know why (too much tip area?)
