One of the boards I’m working on now is a 1 lb EPS foam quad. I was concerned about keeping the fin boxes in with the light foam. (Yeah, glass on’s are lighter, but I want to play with different fins). I think I have an idea.
So here are a few pictures of my test. I think it will work nicely. Any suggested improvements are welcomed.
The method involves using 2, possibly 3 strips of balsa, coated with epoxy resin and sunk in the foam before you route out the fin box holes.
I stumbled around for a while figuring out how to cut the slots for the balsa strips. Two methods came to mind. This one:
and this one:
Ha ha. DO NOT USE MEHTHOD NO. 1. YOU WILL DIE. Or at least melt your hair to your scalp. I used a flat knife blade heated with a candle. Cut like butter. I didn’t, but you will want to use a straight edge to guide the hot knife and some means of controling the depth of cut. Punching a hole in through to the deck will make your language deteriorate and any pets run for cover. Maybe a piece of tape that you don’t mind smelling as it burns under the candle. Anyway, cut the slot, coat with resin and drop the strips in and let them dry.
Route the box holes:
And you just pretty much rock.
The torsion from the fin box is now translated into shear across a wide area of foam and the whole load spread out over a larger area. On the first board I will probably use 3 strips at 1/16 inch, say 4 inches long.
I think this will work on CompSand and any application where you would consider installing a block of heavier foam on wood for fin stability. And as I think about it, instead of layering up 2 or 3 layers of cloth to stablilize the box as is normally recommended, you might think about just one layer and three stips of balsa for a lighter install and less sanding. And as I think further about it, I guess you could use this method for glass-ons too.
sounds like it may work. You may want to have those balsa strips go to the rails to give you some more stability. It should not change the flex. To help out with exotherm a thin cured precoat of epoxy/chopped fibers should protect the 1lb eps. Then an extra slow curring epoxy/chopped fiber mix should be fine.
I’m liking it! the less balsa I have to stick in the foam the better. Just to remind you that you shouldn’t make the strips of balsa touch the deck. It would be like gluing the bottom and deck together, lessening the shear (flex) properties of the EPS. I like that you can use scraps of balsa for this instead of making/routing a fancy block into the foam. Let us know if it works.
As for the exotherm issue, I’m just not having that problem. I just set a big FU box in a longboard using a 50/50 mix of resing and microballoons and no trouble. That mix is easily stronger than the foam.
On the balsa strips, just a little resin will do. That should keep any exotherm issues minimal.
It’s truely a great idea. I think there’s room for improvement in your approach. Consider how much stronger the whole system would be if you let three pieces of wood into the picture instead of two and let them in a 45 degrees to the bottom of the board. I’m not an engineer but I suspect the strength gain would be exponential. Think of how much more purchase the sticks would have on the foam surface. Along with the fact that the torsion on them is redirected so well as thus there little or now chance off foam compression under heavy fin loading.
The panels for your fins are done. I’ll be foiling during the next days.
Future came up with a way to tie all three thruster boxes together a couple of years ago. The Vector System I think they called it. A spin off of this would be to take your idea a little further and run those balsa insterts on a diagonal and tie each set of rail boxes together with them. Tge end result would be creating the strongest part of the board.
It’s the “Vector Suspension System”. It uses cabon fiber rods and molded connectors to lock all three boxes together. The center rod runs forward from the center fin for about 2/3 the length of the board. There’s a junction on the center rod that connects a cross rod to the outside fins. They used to have pics on their website, and it was their “big thing”. I just looked at their site, and there’s no mention of the system anymore.
I’ve used this system in the past, but I haven’t talked to those guys for awhile.
The strength/stiffness of the install is principally governed by how strongly it contacts one, or both, decks. The balsa strips will spread the load laterally along the principal stress axis to the glass. Should help a fair amount.
It will work better and better the closer the inserts are to the major stresses on the fin, the greater their surface area contact with the glass, and the wider the contact with the glass. The width acts as a torque multiplier. This is one reason why my surftrucks boxes are so strong - they are 1.3 inches wide, and this added width more than makes up for the lack of length.
That’s the long way of saying I would put the two strips about 1 inch apart fore-aft, just in front of and behind the max camber on the fins.
And stop thinking about distributing forces to the foam…it acts as a sandwich middle in shear, but it is not going to help a whole lot in stabilizing the finbox. You can get far more stability with far less weight if you focus on connecting to the glass appropriately and put those inserts near max camber.
Rich, unless I am missunderstanding (that never happens) I don’t think the 45 degree angle will do much for you. Blakstah is right, though, more contact with the glass on the bottom is going to add significantly to the strength of the set up.
Anyway, thanks for validating my minor brainstorm. Carry on.
i reckon if fcs plug was 2 mm bigger in diam it would be perffect
they are so pee easy to reapir cracked plugs
and with new adhesive there are new possiliites!
futures box are heavy and create a hinge effect between the fins
and stiffen the tail horribly.
i can see why you would use a box though
1 pound foam is so weak wrt finbox support
imo 1 pound foam dont belong without sandwhich unless you use a vent
FCS derives a lot from being round and thus requiring less tools for installation (a drill instead of a router). If they were larger in diameter they would be more stable in the bottom deck. Instead of going that route, FCS patented making a hole to the top deck and filling it with filler/resin. Two decks is stronger than one, it works incredibly well. The biggest issue is the flex of the top deck changes around the plug and you need to patch the top and bottom of the board to make it work. Most glassers skip patching the top and end up with little rings in the deck that can eventually fracture.
Red-X uses both decks, too.
For bottom deck only installs, the box width at max camber is the main determinant of stiffness. When Futures started getting installed in boards with less glass and softer foam they invented a suspension system which is really a finbox stabilizer which is a MUCH more difficult way to solve the problem than making the boxes a half inch wider at max camber.
I like the Lokbox well for bottom deck only installs. The points on the ends of the box are a little too small radius’d though, and I’d also make them wider at fin max camber and less wide especially near the rear of the fin where lateral forces are quite weak.
For installations only into the hull, spread the load laterally into the glass. It will not interfere with longitudinal flex, and will stabilize and strengthen the box. Lots of people also use slightly higher density foam as an insert around the box. This will definitely not hurt but the main virtue is still more efficient load transfer to the glass.
The glass is just so much stiffer than the foam than any forces on the finbox will eventually need to go to the glass, you might as well start the design problem by figuring out how to make it work better.
(Not to hijack the thread, but if that is true about FCS plugs eventually causing death rings at the deck, and I definitely believe you, blakestah, in the long term isn’t there a [simple] way to reinforce the FCS mount at the bottom, like say a glass patch insert that extends out beyond the plug? I’m thinking a sort of daisy-shaped patch [4 relief cuts] inserted with the plug…?? Which would make it less of a multi-step process to put the things in, too, BTW)
Very clever. Seems like a great way to distribute load over a larger area in what is otherwise a pretty weak material. Why not extend the balsa all the way through the deck? That way you’ll engage both sides of the glass sandwich, further dispersing the stresses. If you used some sheet pvc you’d leave less of a mark because it’s white.
(Not to hijack the thread, but if that is true about FCS plugs eventually causing death rings at the deck, and I definitely believe you, blakestah, in the long term isn't there a [simple] way to reinforce the FCS mount at the bottom, like say a glass patch insert that extends out beyond the plug? I'm thinking a sort of daisy-shaped patch [4 relief cuts] inserted with the plug...?? Which would make it less of a multi-step process to put the things in, too, BTW)
The FCS install manual covers all the necessary steps, and it works. Patches should be applied on the hull around the plugs, and on the deck around the ring. 1 inch in all directions around the plug/ring. This is not a problem that occurs if installation is done “by the book”. The problem with the ring if unpatched is that the load becomes highly concentrated on that circle, eventually causing failure. If the glass is double thick there, the load gets distributed over more glass, and its all good.
For those who haven’t figured it out yet, the vast majority of finbox failures were not installed “by the book”, irrespective of which finbox it is.
((((Thanks for that, blakestah. I’ll look again, but is the patch going on top of the deck lam? That seems strange, putting it on top of the lam–seems like the patch should be in direct contact with the plug for the best strength))))
((((Thanks for that, blakestah. I'll look again, but is the patch going on top of the deck lam? That seems strange, putting it on top of the lam--seems like the patch should be in direct contact with the plug for the best strength))))
Yup.
From the FCS install manual:
“When glassing the underside of a surfboard add two extra patches of glas approx. 150x75 mm, oval shape, around fin area to increase the strength. The deck of the surfboard should also have a minimum of two layers of glass over the fin area.”
The 75 mm recommended width is about the diameter of the plug plus one inch on either side. I know at least one glasser that doesn’t drill all the way through to the deck to avoid having the rings, even though this fails to tie the plug to the deck and leaves it weak…
