Hello all. I'm new to the forum but have found the topics very informative, so wanted to introduce myself and join your community.
I'll get right to it. I have a 2019 Naish 120 liter crossover SUP ( 7'6" x 30") that I've been learning to SUP and wing foil with. I'm six feet tall and weigh 200 lbs (~90 kgs ). The board is really stable with the foil, but can be slightly tippy when set up as a quad SUPing. I'm thinking of adding a center fin to help with stability when SUPing. What is the largest fin you can recommend? As background, I'm also trying to get the wife into SUPing on lakes and bays. So extra stability would help her on this smaller board.

Just use the foil mast without then fuselage and wings. Naish actually make a nub for the bottom of your mast to protect it.
JB
Gonna try this. Personally the tippiness with quad is annoying, but by paddling harder it can be overcome. For the wife, that wont end so well. The mast might do the trick.
I've used two 9" dolphin fins in mine and it works for windsurfing without the foil and that might also work surfing and flat water paddling.
Quad fins plus two 9 inch twin dolphins in the center? It certainly can't hurt to try . The wife has a ginormous 10 foot SUP. I call it the barge and almost throw my back out getting on top of the car. I'd love for her to downsize and use the 7'6" just for tooling around the bays.
Pasquales,
Try getting her a lightweight 10'. Plenty out there even secondhand. My girlfriend had a tank of a Molakai plastic board which was so heavy she hardly ever went in the water because of the weight. Bought her a Starboard whopper ( 10x34") . Even in litetech construction it only weighs 10.6kg ( carbon is 9.2 but bloody wxpensive). There are a few out there second hand at the moment. Despite the width it surfs pretty well as well.
Here is the Naish NUB (Mast Bottom Cup) that you can simply put on the bottom of your mast and use it without the fuselage and foils.





Apologies again for the photos coming out sideways. Seabreeze photos are very frustrating. Lozza, please fix if you can ;)
regards,
JB
Pasquales,
Try getting her a lightweight 10'. Plenty out there even secondhand. My girlfriend had a tank of a Molakai plastic board which was so heavy she hardly ever went in the water because of the weight. Bought her a Starboard whopper ( 10x34") . Even in litetech construction it only weighs 10.6kg ( carbon is 9.2 but bloody wxpensive). There are a few out there second hand at the moment. Despite the width it surfs pretty well as well.
My preference is for the wife to use the 7'6" Naish, albeit if it is stable enough. Then I can make a case to get a smaller foil SUP . Currently eyeing a 6'0" F-One Rocket. But if it is unstable to her, I'll sell the Naish. I'd say my chances of making this work are pretty slim. She likes the board because our dogs can climb aboard and go for a ride.