Glassing on fins (keel) and using roving???

Hi again everyone,

Many, many thanks for everyone who replied to my last post.  I hope this is my last post/question!  I can’t see this one specifically in the archive, although i’m sure it’s there somewhere, but apologies if this is a double question.

Just a couple of questions:

1)I’m glassing my fins when they are attached to the board, rather than before.  I take it that I glass them on after I’ve laminated/glassed the bottom of the board as opposed to the foam, sorry for stupid question!

2)Last one, I’ve seen on previous swaylock discussions, that there can be issues with fin strength when it comes to glassing on your own fins? When using the roving stands, is there anything particular I need to do?  I’m intended on pre-cutting them and then setting them either size of the fins.  How many ropes do I need to use for each side of each fin, I think the rope has around 10 strands…

Many thanks for your anticipated and appreciated help,

Cheers, Neil


I was a fin and hot-coater for some years.

Here’s a quick rundown.

Pre-cutting the roving is a mess.

Cut length enough to go down the length of the fin base x no. of fins x 2.

With a little more to extend past the fins.

Cut to length with a pair of sewing scissors kept in acetone. Sharp ones.

Off a roll of roving it’s about 8 strands.

When placing roving, dip your hands in acetone to wet your gloves.

Lower roving in resin bucket with a twisting motion.

Keep dipping until saturated.

Wring roving through your hands.

Lay in place and cut to length.

Don’t forget to extend roving past fin base at both front and back.

Use your thumb and forefinger to push roving into the corner.

Pinch ends and apply cloth.

Sqeegee flats and sides of fins. Remove excess resin.

Pull out air bubbles or pull them up at ends of roving.

Hopefully you can accomplish all this before resin kicks.

Solar-ez makes this job fool-proof.

Mount your glass-ons with bias cut cloth ONLY !    Forget the roving.     Glass cloth only, is far stronger.    Bias cut cloth is the secret to the increased strength.   I abandoned roving in the  D  fin era,  (1960) and went with bias cut cloth.     Never had a broken or cracked fin from that point on.   In the pre leash period, lost surfboards made regular trips to the beach, cracking or breaking fins off.    Using roving is a time saving production method.    If you want strong, use the method I’ve advised.     To this day, it is the only method I will use to mount a glass-on fin.


Where do u get bias cloth? What is it exactly?


Barry that was a damn good description on mounting fins. how do you initially tack the fins on? I use 5 minute epoxy and sometimes just brush resin around the sides. Lots of ways to do it.


You can cut any cloth on the bias. Instead of making your cuts parallel to the weave the cut is made on a 45 degree angle to the pattern. So, when you glass your fin onto the board the strands in the cloth do not run vertical and horizontal in relation to the fin’s orientation, but rather at an angle. This allows the cloth to flex without putting a lot of tension on the fabric.

I did a quick p-shop rendering of a fin outline with glass cloth image overlayed. Obviously, the weave of the cloth is not scaled to the outline of the fin, but this should clearly illustrate bias VS the way most folks do fins.

Pic #1 shows the method typically used. while #2 shows the cloth laid and cut on the bias

Sammy’s description is perfectly stated.     The only comment I can add, is that I would always trim the bottom of the fin to match the bottom contour of the placement location, for maximum surface contact.     I then tack the fin in place with a small batch of lam resin.      After the resin sets well, you can proceed to lay on the glass layers, again with lam resin.    All this, prior to hotcoat.


I’m gonna give Bill’s method a try next board and just wanted to add I tack mine on with a bit of hot glue. Mike



How do you build up the good looking, tight fitting, fillet around the base of the fin if you don’t use roving and glass patches?




The 45/45 fiber alignment, of the bias cut cloth, allows the cloth to be easily tucked into the 90 degree fin/board junction.      The tighter you can tuck the cloth, the stronger it is, and the reduced fillet area produces less drag.      I start with the largest pieces of cloth, and proceed to smaller and smaller pieces.    I like a stagger of 1 to 2 inches, depending on the size of the fin.      That way when you sand, you only cut into the ends of the cloth layers, leaving the longer fibers intact.    The tight fin/board junction,and a well foiled fin, dramatically reduce drag, adding up to greater speed across a wave.   Helpful?


The industry standard is pretty much Barry’s description, and myself having glassed thousands of fins over the years if done correctly is fool-proof.  

What IS the optimal size for a fillet if you want to go fast?  …  here’s a guy who did a good job on what he calls “interference drag reduction”.  Not on a surfboard, but pretty damn cool anyway, if you scroll thru the construction pics.  Lots of hot wiring, foam, and fiberglass work:

result = a 65 hp engine flying a plane at 230 mph!

a few words from the designer: "Interference drag reduction, as I’ve applied it, is more than just reducing the interference between boundary layers in the corners. What I’ve tried to do is arrange the wing and fuselage and canopy so the combination produces a sort of “Poor Man’s Area Rule”, as the late John Thorp is said to have called it. I spent a lot of time shoving things around on the general layout drawings, paying attention to the wing root fairings, and carefully positioning those things that have to be there anyway, like the canopy (and even the wheelpants), and reshaping the fuselage itself. If you get it right, you can reduce the drag of the wing/body combination, and that’s what I think happened on the AR-5. I think I got it approximately right. The exhaust streak that forms on the fuselage side above the wing root is remarkably straight. I love that exhaust streak.

I noticed, after all this manipulation at the drawing board, that the shape that resulted happened to lend itself to a unique kind of construction that allowed me to build the fuselage by hot wiring it the same way Burt hot wired the wings on the VariEze. No fancy compound curves to carve. In fact, very little foam carving at all. The shape of the fuselage comes out almost automatically. The .4" thick sandwiches that form the majority of the primary structure are all cut to shape and thickness with the good old hot wire. Pretty darned neat!"

Sorry for the sidetrack, just thought a few of you might be interested…


I think the difference between a fillet and a square angle would be insignificant.  A board with a fin box vs a board with a glass on fin, don’t seem too much different to me (fin and placement being equal).

   Well, your thinking is flawed.   In 1959, a surfing buddy of mine  kept getting a cracked fin, when his board made solo trips to the beach.     So he did a heavy fillet, lots of glass, lots of rope.    You know, if some is good, more is better, eh?      Not so, his board was now much slower.     So, he started sanding, and refining the fillet.     The more he took off, the faster the board would go.     The lesson was not lost on me, or on my buddy Mike, an engineering student.    After that lesson, I abandoned roving and large radius fillets, in favor of the sharpest possible fin/board junction.    The use of bias cut cloth is common today, in the glassing schedules used in composit aircraft construction.


“After that lesson, I abandoned roving and large radius fillets, in favor of the sharpest possible fin/board junction.”

Hi Bill -

Would a standard box/plug attachment not yield the sharpest fin/board junction?  I.E. NO fillet?


Yep.    One of the many advantages of the FU box, and the one most overlooked and misunderstood.


I have a similar question, so I’m going to piggyback on this thread.  I finished some plywood speed dialers for an XPS/Epoxy fish I’m working on.  This is my understanding of how to seal and glass on after reading a bunch of threads/youtubing:

  1. Seal the fins with one layer of cloth and resin per side

  2. Tack the fins onto a laminated and sanded (not hot coated) board

  3. Use roving and/or bias cut cloth fiberglass patches roughly the size of each fin to glass them to the board

My question is, how many layers of 4oz or 6oz per side do you laminate the fins to the board with?  NOT including the one layer per side already used to seal the fins.


Now you’ve really opened up a can of worms!  I’ll stick my neck out and tell you what I do.  It may be overkill but I’ve repaired MANY fins with cracks on the sides, water absorption along the leading edge, cracks along the fillet, and of course (my favorite), fins that are entirely missing in action - requiring custom fabrication of a suitable replacement…

First, make the fins themselves strong enough that they can survive a hit.  They will receive a hit and it’ll likely be the leading edge which is an especially vulnerable spot with plywood fins with little skinny ‘halos’ around the edges.  I prefer laminating at least 10 layers of fiberglass as a spine and add the wood to that.  Once foiled, you’ll have a somewhat reasonable reinforced edge that’ll protect the wood and give you something to which you can attach your capping layers after the wood is foiled.  I’d recommend at least 4 layers on the wood side to protect the wood and to give the fin some strength against side torque during hard turns.  When I read “NOT including the one layer per side already used to seal the fins”  I figured I’d chime in.

When glassing them to the board, cut a BUNCH of fiberglass patches.  IMO they don’t all have to go clear up the sides… maybe a few per side but mostly at the base with 1/2 going up the fin and half against the bottom of the board.  The length of the patchs should be slightly longer than the base of the fins but again, they don’t all have to be.  Some can be a bit shorter.  I cut as many as 20 patches per side for each fin and sand them to suit after the hotcoat/fillcoat is cured.

The 45 degree ‘bias’ thing that Bill T. recommends is a good idea but with so many layers, they don’t all have to be exactly 45 degrees…

Until you’ve lost a fin on a bottom turn, or had one flex and crack, or leak at the leading edge after you barely tapped a rock, etc. it is easy to dismiss all of this as overkill.  I don’t have a problem with that. Ask any repair guy about fins in general - glass-ons, boxes, plugs, plywood, etc…  You’ll likely get an ear full.

PS - it doesn’t hurt to add a laminated patch over the entire fin area before attaching the fins.  It can be done while the bottom is being laminated.  I wouldn’t sweat for a second if your board ends up heavier than a ‘standard’  shortboard off the rack.  Those are notoriously disposable and especially prone to fin issues.

Love the thread, I’ll post photos of my current build and how I (what I call gusset, I’m just a carpenter) a fin on/in as i’m doing it, curious to get feedback. As a leashed old man that does not enjoy swimming after my board I dont often pick up the pieces from a runaway but I do like to think I ride hard and have only lost one fin in almost a decade of riding my planks. For now I will say I use multiple layers of 3ml glass cut at a 45 to thread weave to rienforce the fin/bottom joint extending 2" up the fin (hardwood fin) to allow some flex. Seems to work for me.     

I busted my fin off and epoxied a sixteen ounce budweiser can on as a joke. The joke was one me becuase it worked really good. it fell off after a couple of days. Makes you wonder about fin design. Every friggin fin on the planet looks like the dorsal fin of fish or dolphin.

   Nobody in the fin business has the balls to step up and try something new.