Just after some suggestions for thruster placement on 8 x 21 wide gun, tail is 13"
Gun...Gun or CA Gun. 21 sounds wide for a standard gun.
Any how, I'd start here for that board, thinking it's for overhead waves, but not Pipeline overhead type waves.....also use a probox set up so you can adjust this set up.
from the tail= 4,
Side fins= 13,
Side fins set at 1 3/8 off rail,
Side fin toe= 1/4
I would not spread the fins more than 8 inches fore-aft or it will turn like a Brinks truck. I would recommend 4.75 and 12.75, and toe in the front fins at 3/16" (not the standard 1/4")
I would think that a "Gun" shape should not be too loose
therefore a wide fin spred would be appropriate
I think with the wide spred and Probox adjustability to bring the fin cluster closer if need be would be a good choice
Just My thoughts
I like 7 inch spread on shortboards.
7.5 inch for a minigun up to 7’6".
8 inch on guns, even for Mavericks. It is hard enough to turn a much longer board, even with the increased tail rocker of a gun. The decreased toe-in, also, is a complement to the increased tail rocker.
Of course there is no “right” way…
For that size/type board I typically do rear fin 3" to 3.5" off the tail, and 8" to 8.5" spread (so rail fins up between 11 and 12") …1.25" off the rail, 4 degree cant, and toe-in either 3/16 or 1/4" (smaller fins more toe in, bigger fins less) – which usually comes out pointing somewhere around 1-2" off the nose if you use that method). Seems to work for me. YMMV.
You really can't go wrong with a center box to go with those side bites - whichever side bite brand you choose.
If after switching stuff around you don't like it as a thruster, try it as a 2+1 "Widowmaker" kind of thing? The most dramatic changes will occur by changing the size of your center fin and moving it fore-aft. The center box will give you fore and aft adjustment no plug system will.
Specific fin measurements and placement advice doesn't necessarily work all that well from board-to-board unless all other variables (tail rocker, outline curve and rail foil for example) are equal.
The center box will allow you to compensate for that board-to-board variability and fine tune it to your liking.
Why not make it a quad.........ask for that fin set up and see what happens.
Long story short, fin placement is like a Butthole everybody's got one..............you know the saying.
Some butts work good, some work great, some don't work....Good luck on your rectal journey,
Anyhow this d.d. guy hasn't answered any of the basic questions about his board. So just shove a fin in it and call it a day.
Thanks for the mostly valuable contribution’s.