So I was out today and yesterday with my 7' board. On my last wave today, I snapped the fin in the back of my thruster formation, i just rode a little to far with the wave into shore . Anyways, the two twin fins still remain. My question is, is it worth buying new fins or will the twin setup now hold up well? With a board this size, will it even matter? What is the difference is how either setup performs?
The side fins on a thruster placement are further forward than a twin placement would normally be set. So riding your thruster as a twin would be less stable than a board designed to be a twin. You can compensate a bit with much larger side fins if you're set on riding it that way, but it would be best to get it repaired.
For some reason he's making it sound like a bigger deal..... is it a glassed in fin? If not no problem... just go get another fin. If you're on a tight budget you can probably get one on Craigslist or Ebay for $10.
As far as performance....(with the utmost respect I'm saying this) If you're riding a 7' thruster and are scraping fins in the sand you aren't going to notice any difference in performance whether it be two fins or three.
Now if you're riding the 7' thruster (otherwise known as a minigun) at Pipeline then yes you're going to notice a difference! I'm assuming currently you're surfing waist high waves or so somewhere on the east coast and the response of a board that size in that kind of surf is probably that of a longboard.