HD-PU inserts into EPS - with PU-glue or epoxy

I´m building a twinfin board with a 2lbs EPS blank.

Finboxes will be FCS II and I´ll use some big self foiled fins similar to Asher Pacey 5.79 twins.

I´ve routed some reinforcement inserts out of HD-PU (apr. 6lbs/cuft sensity) that will leave a frame of apr. 1/2inch around the plugs (+a bottom under the plug).

The inserts will protrude into the channel a bit.

Most people seem to use epoxy to glue the inserts into place.

But that will make sanding/reshaping the channel cumbersome.

I have a high strength PU glue (like titebond/gorilla glue and such), which I use for glueing stringered EPS blanks.

This glue sands quite good. I think this would be a good option for glueing the HD inserts into the blank.

Any concerns about the glue not beeing strong enough ?

(my EPS gluelines are stronger than the EPS itself, testpieces always broke in the eps beads and not in glueline)


I forgot to say:

With the PU glue I would put some lead weights on the inserts to prevent the glue under the plug from foaming.

The inserts fit very snug into the routed pockets. So theres only a very small amount of glue used (and not a foaming sponge sorrounding the finplugs).

I use Gorilla Glue for my inserts, except that I install them into the blank before shaping.  

One thing I 've been doing with my insert installs was to glass the core side of the insert material with a couple layers of 6oz.  Then I template the inserts and install them with the glass sid down - basically providing a fiberglass “floor” for my fin system install.   Now the bottom of the finbox is supported even more strongly than the top plate.  

I tried something similar on my last build…

Thank you for the great advice.

Next time I will insert them before shaping. Seems way better.

I’ll use my pu-glue.

The glassing of the bottom of the insert is a great idea. Must be bombproof. Thank you for sharing. I will try that next time.

Years ago one of the fin system developers told me that the primary cause of failure for any of the fin systems  - including glass-ons - is due to the surrounding foam moving around under load more than the glassing and the box or plug.   Stabilize the foam in which the box or plug is sitting and they’ll hold up fine and maintain their adhesion.  

No matter Glue you use, just one “stronger” than foam. Fin box is stiffer and stronger than foam, only way to strenghen system is to increase surface of softer foam wich take forces to reduce pressure stress. Windsurf builder trick is to lam under+wall+go on board with same fiber renforcement so plug with is hd foam is fully encapsulated with long fiber and connected to skin. I do this with fusion in soft eps for sups, fins break first.

I’ve seen lots of finbox failures in which the surrounding foam gave way in spite of serrated edges around the box, fiberglass liner, caps, etc.  

Aside from a double stringer spaced to support a single box on each side (not applicable here) one of the best ways to support the surrounding foam is with little mini-stringers placed perpendicularly around the plug.

I think the first time I saw this done was on a board made by Ambrose Curry… It makes a lot of sense if they are glued in properly.  Of course on the board in question, the outside channels might pose a challenge.

Thank you all for your comments. I´ll try the glas/shell thing for sure. And the star-shaped pod as well.

In fact, I thought about a X-shaped or star shaped woodstirrer reinforcement.

But I went with the HD insert solution because of the channels on this board.

I´m a civil engineer, mostly designing/calculatin big compound bridges.

Transferring forces from a steel-structure into the concrete-strucucture is very similar problem to anchoring forces with a plug into foam.

It´s about how to distribute / lead concentrated forces over into a “softer” volume.

(anchoring wind energy pods into soft ground is another task with similarities)

I enjoy tinkering with such things on my boardbuilds.


I glued the HD inserts with the PU glue. Won´t have time to reshape it and set the plugs before the weekend.

I´ll hand in a picture later on.

The board and rail gets quite thin towards / behind the trailing edge of the fin. I tried to shift the recess for the insert as close and as far back as I felt comfortable with. The insert will be a bit excentric to the finplug. (moved a tiny bit inwards and forward).

And I reduced the depth a bit, to not go through to the deck or even cut away the whole rail behind the fin.

I hit the maximum possible depth before cutting through to the deck quite well. I could almost look through the bottom of the recess.

Next time I´ll set the inserts before shaping. No problem to shape the rail and hitting parts of the plug when it´s glued in prior shaping.

But this time I had to avoid routing away a section of the already shaped/finished rail.

The Glass trick i try to describe is same principle. I lam Ud fiber strip in box that Come back to bottom each side perpendicular to box axe, so it connect strongly under and side of box to skin prevent box rotation in foam when fin is hurt.

I made my first board with futures boxes. It´s an asymmetrical Mini-Log / Stubby with twin/quad setup. I made it for a buddy who is a strong and big guy. (6´8", very fit, basketballplayer)

I decided to reinforce the twin box and the big quad box with plywood inserts.

(length of the struts adjusted to the fin area / force, bigger struts for the twinfin)

The board is stringerless, the bottom is glasse 5+5oz and over the finplugs are 5oz patches on top of that.

Should be bullet proof. Time will tell.


As lemat indicates,

Looking at what sailboard builders are doing is always a good idea.

Windsurfing finboxes and mast tracks take much higher loads and are installed in much softer foam. (10-15 kg/m3)

HD inserts get you far, and to get truly solid a connection to the top skin.

Footstrap plugs are just installed in HD inserts without connection to bottom skin, I guess these loads are similar to surfboard fin plugs.

I did the HD insert solution on the board before (see previous post) with 80kg/m³ PU foam.

I wanted to try both ways.I think both should be quite solid.

I made HD-insert routing jigs (for cavity and for insert) for the fcs II plugs on the previous boards.

Instead of making two new jigs for the futures, I went for the mini stringers this time.

I mostly build boards for myself and use fcs plugs for my personal boards. But my buddie rides Futures so I installed futures plugs for him. Don´t think I will use futures very often. After building the futures jigs for the finplugs, I was to lazy to make even more jigs for the inserts. To many jigs for using futures on just one board.

Routing the slots was fast and easy and I enjoyed planning down the stringers with the mini handplane.

So it was good fun and a good way to try something new.

I´m not trying to find the fastet work routine or something. Board building is just a hobby for me and a way to spend my free time.

This solution is of course way to time consuming for a industry setup. But it´s a different thing for a hobbyist.For me it´s all about trying new things and having fun building stuff.

I bet your way to instal fin box will be realy effective ! An ultimate improvement could be to connect wood plate to a reinforced deck lam. I will do it like this next time i install foil insert.

When I routed the slots, I thought about that. (routing through to the deck)

But I thought I would be overkill for futures finplugs. And I would have come out through the wing (front quad plug). Didn´t want to touch the wing.


Nice boards you’ve posted!  I think the additional strength provided by the mini-stringers will be shocking.  I also like the idea of bridging through to the deck laminate but if it’s not reinforced with additional reinforcement patches up top, it’s not unusual to see the deck cave in around hard plugs and reinforcements and create a sheer point where the glass breaks down around the plug.  In normal use and then some, you should be OK with either of those fin box reinforcements.  In the old days FCS recommended the connection to deck and patches on deck for their single plug installations.  A lot of guys skipped those steps and some did OK with them.  A lot of guys who skipped those steps had problems though.

Might be over kill, but I put in 1/2" wide balsa strips that run across the board to secure my boxes, and I add a layer of glass over the boxes. I’ve had large keel fins feel like they will break loose when I did shorter pieces, but I use proboxes.