Hi.
I’ve got a longboard that has been shaped too thin at the tail to install standard box so I’m considering using probox or FCS plugs (don’t want to glass fin on).
I’ll be making a centre fin for this board and using 2 regular fins on the sides.
How large can this centre fin be before the forces exerted on it by the wave end up riping the plugs out.
Brian
I used to put FCS center fins in my longboards, but after several disasters, abandoned the idea completely. I believe FCS did, too. The large center fins on longboards (I’ve used 8.5" and bigger) simply apply too much force to the plugs. These fins had, if I recall correctly, 4 or 5 plugs. Maybe three would be ideal, but I’ve given up on it.
You still need about an inch for the FCS plugs, anyway, so I’m not sure it will solve your problem.
Maybe consider moving the fin box forward and putting in a fin with a lot of rake… like a Harbor fin.
You only need around 3/4" for FCS plugs, plus the rear plug is a couple of inches up from the trailing edge of the fin. We have a couple of boards out there with 6.5 bonzer center fins and so far they are holding up. Double capped the boxes though…
The box is the way to go, instead of individual plugs… too small an area concentrates all that force.
How many tabs do the bonzer fins have? I’ve only had the full box bonzer fins… and none 8" or bigger.
Quad…
If you’re gonna be making the center fin then I would suggest making it with an overhang off the back, with the screw tab on the front. Just cut a couple inches off the tab on the back of the fin so it can slide all the way to the back of the box and a couple inches over. Then install your regular finbox up a couple inches. Hopefully by moving it up you’ll have the inch you need to install it.
Sorry couldn’t resist:
I’ve used 7.5" fins with FCS but for larger I would use the Unlimited boxes either 10.5"
never seen it done but I’d bet you could put two proboxes inline and they’d handle a big fin. Their keel fins are big and one box handles that load fine, but a longboard center fin is taller so would exert more force (if you’re riding it as a single…as a 2&1 a single probox would likely be enough for maybe a 6 inch cutaway).
Of course you’d have to custom-shape the LB center fin to fit the double-probox setup, but it’s doable, and you said you’re making the center fin anyway.
(I’m sure if this is a dumb idea Larry will say so!!)
Thanks for all the good advice. . .
Am going to see if I can move a box 1" further up the board and failing that I like the double probox idea coz it’s very doable
Brian
Hey Brian,
Try using 2 ProBox center boxes inline. You’ll get 8" [203 mm] worth of lateral strength in the box (you’d need 8 FCS plus to match that, and they’re round so it’s really not even a close comparison in strength). Two ProBoxes will give you a little more than 75% of the installed surface area compared to a standard 10 1/2" fin box.
To make up for some of that 25% missing, install the boxes with fiberglass reinforcement in the routed holes and cap with a large tailpatch or 2 to distribute the forces.
You can make a longboard fin with 2 3/4" [70mm] long tabs as shown in the pic. This will give you 5.5" [140 mm] of lateral tab strength, much less stress on the fin base, AND still get the benefits of ProBox adjustability.
~Brian
never seen it done but I’d bet you could put two proboxes inline and they’d handle a big fin. Their keel fins are big and one box handles that load fine, but a longboard center fin is taller so would exert more force (if you’re riding it as a single…as a 2&1 a single probox would likely be enough for maybe a 6 inch cutaway).
Of course you’d have to custom-shape the LB center fin to fit the double-probox setup, but it’s doable, and you said you’re making the center fin anyway.
(I’m sure if this is a dumb idea Larry will say so!!)
Keith,
It’s not a dumb idea 
~Brian
why not stick a standard box in and have it come out the top. then fair it in with suitable material. It’s not like your back foot is going to be on top of it all the time. I think I saw that solution at a swaylocks gathering
Why dont you just glass the center fin on. They look way better than boxes and you can glass it all the way back on the tail if you want.
Glass it on. End of story.
Is that all you got. Round 'ere…
never seen it done but I’d bet you could put two proboxes inline and they’d handle a big fin. Their keel fins are big and one box handles that load fine, but a longboard center fin is taller so would exert more force (if you’re riding it as a single…as a 2&1 a single probox would likely be enough for maybe a 6 inch cutaway).
Of course you’d have to custom-shape the LB center fin to fit the double-probox setup, but it’s doable, and you said you’re making the center fin anyway.
(I’m sure if this is a dumb idea Larry will say so!!)
Hi Keith, No such thing as a dumb idea my friend. I call it thinking outside the box, keep up the creative energy (-;. Mahalo,Larry
Brian install the box as normal a bit further up and cutaway the fin base. I have had to modify a few fins like this where the boxes were incorrectly installed by people.
Hey Brian!
Could you could take your idea, and modify the boxes prior to install by cutting off the little arms on the inside and gluing them together, body-to-body? You’d still have the outside two arms on the ends.
Thanks for all the good advice. . .
Am going to see if I can move a box 1" further up the board and failing that I like the double probox idea coz it’s very doable
Brian
You can do that, but keep in mind, a fins unlimited box doesn’t have to be a “flush” install. I’ve put 'em in thin tailed longboards plenty of times. You rout it in full depth, that is, as deep as you can before going through the deckside at the thinnest point. You can rout out a little deeper forward to get the nose end of the box all the way down (or close). After glassing it in, just grind off all the excess. On these types of install, I usually put a thin (1/4" thick x 1" tall) piece of red oak on either side of the box (a little longer than the box either end) to beef up the install.