so many different opinions out there.
not many guys ride single fin shortboards.
cnj beachbreak decent-quality waves.size,base,rake,flex.
6'9"x 21"x 2 7/8" double-wing swallow 6-channel bottom.
wilderness 7.6".rsc rusty 7.5". greenough 4a or 4c 7",8".
i heard kidman,machado,etc. use 9" fins on 5'10" 's.
I love single fin shortboards. I use a 7.5" fins unlimited dobson on my 7-8 ocean atlantic roundtail and a 7" rainbow fins rusty on my 7-6 home made pintail. Both board/fin combos work better on better waves (duh) as both provide good hold and drive. I only notice 'em getting hung up when the wave is really slow. I tend to scoot the fins up pretty far forward in the finbox.
I do have a very old (early 70's) 7-0 stinger (WRV) with a 9" fiin in it, but it's been so long since i've ridden the board (it's a wall hanger) that I have forgotten how it turns. Basically I tend to go board length in feet = fin length in inches, give or take.
There's so many opinions on single-fin surfboards fins because the size, type & placement changes the characteristics of the board waaaaay more than doing the same on a thruster.
Any idea what it originally came with? The shaper usually intends a certain amount of drive & maneuverability to come from the fin when designing the rest of the board.
If you have no idea of the original fin type, how wide is the tail? A rule of thumb that Geoff McCoy used for the Nugget was to place the trailing edge of the fin 7 1/4" from the rear of the board. At that placement, lay the fin down, tip towards the side. The tip should extend past the rail about 1/4" to 1/2". I've used that as a starting point to select fins and figure initial placement on other single fin short boards & it always puts me within 1/2 of where the fin needs to be. If you can't figure out what fin was used in the original design, I wouldn't suggest using more rake than the 4C on such a short board (my 2 cents). Don't adjust the fin more than 1/4" per session, either!
As far as those cats using 9" fins on 5'10"s, I'm sure it has to do with where they use single fins. Single fins rule the barrel, so I'm assuming it's for steeper/faster faces. A fatter, mushier wave on such a board certainly wouldn't require that much fin. On steeper/faster waves, however, there's no fin on the rail to catch the water as you lay the board on the rail. When performing a rail turn under these conditions, a shorter fin could come out of the water all togther before the rail bites into the face. Using a longer fin helps reach into the water when the board isn't flat.
it's not shaped yet,but,at 6'9"x20.75"-21" with a double-wing swallow and 6 channels+i'll ride it on good,easy days,i think the 7.5" 4a or 4c,the 7.5" rsc rusty, the 7.6" wilderness is a place to start.the double-wing will help narrow the tail.
I sometimes ride a natural progression 6'6 single fin and the thing is a blast! I ride it with a 9 inch 2 point skag and sometimes a shorter classic hook style fin. I surf hard and have been for years, and honestly, I can't tell the difference. That board vs one of my hp short boards- sure it's night vs day. That board with a 2" shorter fin... you got me there...
I suppose if you want a tighter turn radius without slipping her out go as long as possible. If you want a single fin and still throw the tail try some short ones. If you think that a fin is holding you back, take a cold shower because you are probably tripping balls... IMHO. Either way, it's always fun to experiment!
A rule of thumb that Geoff McCoy used for the Nugget was to place the trailing edge of the fin 7 1/4" from the rear of the board. At that placement, lay the fin down, tip towards the side. The tip should extend past the rail about 1/4" to 1/2". I've used that as a starting point to select fins and figure initial placement on other single fin short boards & it always puts me within 1/2 of where the fin needs to be. If you can't figure out what fin was used in the original design, I wouldn't suggest using more rake than the 4C on such a short board (my 2 cents). Don't adjust the fin more than 1/4" per session, either!
Ray's description is pretty close to how I learned it. Turn the board over bottom up. Place the fin on its side on the bottom of the board with the base of the fin along the stringer/fin box approximately where you plan to put it. Approximately 25% of the fin area (tip) should extend beyond the rail of the board. This is how I was taught by several old school shapers who were around in the days of single fins.
Beyond the size, fin type is completely personal. Try a few and see what you like.
I have a 5'9" singlefin. I have a 7.5" cutaway on it. The smaller of a board you have the more play the fin really has. Changing in positions, size of the fin, amount of rake, etc will change the feel of the board completley. Try borrowing fins and find out what works best for you.
the rusty fin is a great fin for a more high performance single fin shortie & the greenough 4a is prob. my absolute favorite single fin ever. i don't think you'll go wrong w/ either of them in 7.5" or 8".
good luck! i'd love to see pics of this when you get it!