Need suggestions - for tri-fin long board

I will be starting on a new board next week, and would like some suggestions on fin set up.

The board is a the 9’1" longboard blank from Marko (EPS), epoxy will be silver tip from system three, and I have plenty of high density foam for inserts (if needed/recommended for the fin areas)

What I would like opinions on it the type (size and shape) as well as the location of the fins. I would like to build a tr-fin.

Context: I am a newbie, but I can already tell I am going to want something more sporty than the 10’4" monster I built to learn on (It has a single 10" fin and weighs a ton). I also built a EPS/epoxy fun shape, 7’6" from marko which turned out very nice (im just too big and too new to ride it).

any ideas? thanks

Joel Shine

If you don’t already have the blank, specify the heavy density they have and glass it light. Put on a rail to rail tail patch on the bottom to support glassed on fins.

On fins… try a wide-base “Bonzerish” center fin 6.5" high about 4" up from tail with smallish double-foiled side bites set almost parallel with a little cant about 13 1/2" up.

With tail width of 14 1/2" or less, this should work OK. Think longboard with a widowmaker fin layout.

If you want variability, install a longbox in the center with mini-stringer supports.

That’s a stringered blank, correct? If so, I’d advise a Bahne style center box and some removable sides, in about

the same positions John supplied. You get more versatility because you can ride it as a single, single with bites,

widowmaker, or thruster.

If you want to make your own ‘‘biscuits’’ for FCS, I can get you the factory dims. That’s the best way to install in eps.


Thanks guys, especially for the locations and sizes of fine. I will definitely make a fin box for the center fin.

Its a 1.9 pound foam, and it has a standard basswood stringer. I will start a new thread and post some pictures when I get the blank next week.

I am searching the web for the type of fins you have recommended, any vendor recommendations would also be appreciated.

As for fin system vendors, I only know the wholesale/industry side, I don’t know who sells to DIY’s in our area.

If no one here can help, PM me after the weekend.


Here’s a great setup for me…rear box 7" up from tail…side bites 12.77" up from tail, set 1.75" off the rail, toed in to the nose with 6 degrees cant.

I ride a variety of center fins with single-foiled sidebites, but like cutaways best… As a single fin I ride a Harbor fin, which noserides brilliantly.

got the blank yesterday, took a couple quick picts. I will not have any time to mess with it until maybe next week, but I was hoping to get the fins and box(s) ordered. Thank you guys for the recommendations on the side bite locations, how about a size for these things? Any specific product ID would be great so i can find it and order. I have found vendors for side bite, and plenty of singles/boxes - but I have had no luck finding a matching (2+1) set.

My plans for the lamination are a triple 4 oz. deck and double 4 oz bottom. I have 16 lb foam inserts for the fin box(s), i will route out the blank and epoxy these in before glassing. I am also thinking a small insert on the nose and tail, why not? It will not increase weight hardly at all and will basically eliminate the possibility of a ding

“…will basically eliminate the possibility of a ding”

i’m not sure about that.

That blank look very nice though…have fun and post progress pleeeaaasseee.

12+ oz of cloth in epoxy over 16 lb density foam - you cannot ding it even with a hammer, you can rip off the glass with pliers, but the epoxy has enough give (elongation) that its almost impossible to delaminate it with a strike :wink:

In this picture its the green foam, we are forming the coming around the gunnels of a boat we a re building…

We will see, i may just use 5 lb. desity PVC foam (Divinycell) for the inserts, its quite strong also.

last night I ordered the fins/boxes/leash plug. I ordered the side bites from EZ foam, they seem to be the right size from what I have been able to gather here…

And I got the center fin and box and leash plug from Surf Source, 8" cutout fin (white to match bites) …

Any other feedback on my lamination schedule (triple 4 oz on the deck and double on the bottom)? Maybe an extra layer of 4 around the fins?


If you’re doing futures for the sides, you really don’t have to do inserts. The in-the-lam install

is plenty strong if done correctly. But since you’re a boatbuilder and have the divinycell laying about,

go ahead, it will be stronger and the gray d-cell looks cool. I’ve got d-cell inserts on one of my

personals with FCS, and it’s worked out well.


Thanks for the tip. I will just save the insert for the big center fin then.

I will probably fair and paint the board with sterling/awlgrip (its what I know), so the inserts will not be visible.

Sounds good. Awlgrip is awesome stuff. Go for that extra layer through the fins and that lam schedule

will be good for a pretty light end product. It would be good to see how much weight the awlgrip finish

adds, just weigh the board before and after. If you want this thing to be really strong, you might want

want to up the lam schedule a little, but that’s all personal preference.


Hi Mike,

Normally, we can get a much lighter part using epoxy fairing and paint vs. sanded resin (clear or bright) finish. On the last board we did (a 7’6” EPS fun shape), after lamination, we filled the weave with an epoxy fairing compound (very light and sands easy), then paint. There might only be 6 ox of paint on the board as it goes on extremely thin. We will see, I will record all the weights througout the build.

I wonder why the use of stitched fiberglass has not caught on for boards? By that I mean (Biaxial, Uni-directional) They are much stronger vs. plain woven at the same weight (at least 20% stronger)… I have to assume its probably because it is difficult to find in anything less the 6 oz. and it does not go clear (stitching still shows).

Also, it allows for better design of fiber orientation. With woven, all the fibers on the board are running at either 0 or 90 degrees, so half the weight of the glass is not doing anything for longitudinal stiffness.

For the deck of this board, I may use 6 ox Biaxial fiberglass (45/45 fiber orientation) + 4 oz. woven (0/90).

Surf industry convention precludes the use of fairing compound and exterior paint (except in the case

of molded boards). Everything is about getting a transparent lam and finish so that the foam core shows,

or in an opaque lam you want to see the weave and cutlaps. (pinlines are for sissies who can’t do a decent cutlap)

Building boats has a different set of conventions, the lam’s appearance doesn’t matter because you’re going to cover

it up with gelcoat or paint.

There’s no right or wrong here, just different ways of doing things. Some of my best friends are boatbuilders and we

talk about this stuff all the time.

Very true that there is a world of fibers out there not used in the hand-lammed surfboard industry. At Coil, we use some

of those too-stiff or too-dense fibers in our vacuum lam schedule. I hope you saw the knitted carbon/spectra hybrid we

used on our ‘‘concept’’ board. We break lots of other conventions as well.

New days are coming in our industry, and techniques such as what you’ve outlined may gain acceptance.

PS, just kidding about the pinlines (before anybody sends me a nastygram)


The lightest biaxial stitch cloth I’ve seen is 12 oz.

Where do you get 6 oz?


Yes I did see that board, very nice. The spectra is interesting stuff, what advantages do you find over other fibers?

I agree with what you say about boat building being very different. We focus on making something strong, then making it pretty. With boards your trying to do both at the same time - which is definitely difficult in its own way.

The cutting edge of light weight composite (marine or Aerospace) parts right now are usually carbon/kevlar and epoxy infused. But your stuck with the same part. Like one-off boats, boards are shaped for the rider and the conditions - making 2 pieces infusion molds for every shape would never pencil out. What is exciting to me is how CNC milling could make one-off boards (and boats) much less expensive - its still very expensive for a small boat or board.

The 6 oz 45/45 biax we get is woven for us, then we cut it into 6" wide tape.

Hopefully I can get to some laminating this week.


“we filled the weave with an epoxy fairing compound (very light and sands easy), then paint. There might only be 6 ox of paint on the board as it goes on extremely thin. We will see, I will record all the weights througout the build.”

What type of paint and clearcoat do you use? The Awlgrip mentioned above?

I will bookmark this thread to follow the progress, this is very interesting.


for p[personal projects, i usually just use whatever we may have laying around (returned damages) it could be System Three, Sterling, or awlgrip. I have a quart of “federal yellow” that can be used

For fairing we use system three quick fair - it feathers to a fine edge and sands like a charm, on really big fairing (large overlaps on boats) we mix our own fairing with microballoons/silica/epoxy.

right now it look like I will use:

Silvertip laminating epoxy for the glass work

quick fair (epoxy fairing putty) to shape and fair

epoxy primer (either system three or the sterling)

topcoat; either system 3 or sterling

I will take lot of pictures, and probably start a new thread. I will need more help from all of you on the shaping. The marko blank comes 90% shaped, but i wanted to change a couple things (at least I think I do ;))

made a little progress. I refined the shape of the board a bit. gave the nose of the board a little flair on the underside. Also removed about 1/2" of width and reshaped the top side of the rails, but kept the general profile of “50/50” in the middle…

I set the big center box (10.5" box) up 5.5" inches. The bites are 13" up and 1.75" from the rail, and barely canted in. I have some scrap uni carbon 5.2oz, what we used to cap the stringers of a 22’ boat. I thought I would try it in the fin holes - why not. i will use another layer over the top of the boxes to “cap” them - just leaving the slots for the fins to fit into. next I have to grind down the top of the center box

The plan now is to “fair” the foam a bit, then glass. I still plan on using triple 3 oz. on the deck and double 4 oz on the bottom. I would love to get a recommendation on alternative lam schedules ? thanks


Glassing went pretty well, with one major exception (you will see at the end)

Used a epoxy slurry to fill the open cells

Inserted leash plug, will dremel out the opening after glass cures

Deck gets 3 layer of 4 oz. cloth, then another 4 oz in the middle - a “stomp pad”

with the glare you can see there is no resin pooling

I used our large 30" fairing board to sand down the cured glass that overlapped the rail, then glassed the bottom with 2 layers of 4 oz.

before laminating then entire bottom, I reinforced the fin boxes. Over the center box, i used a strip of 6oz of 45/45 biaxial tape. Over the side bit boxes I made very small fillets around the box and put down 2 strips of 4 oz over the box (the opening to the boxes are tapped off). then the wide cloth went over the entire bottom. I was going to use the Uni-carbon tape, but it it way overkill, plus the biax tape is easy to move and work with with


The gray you see it from a little bit of gray pigment I put in to help me see where I had too much (or not enough epoxy).

So then I come back the next morning to be greeted with 2 big blisters!!!

you can see the big one here…

Somewhere, there was some outgassing , it moved through the EPS to the weakest spot and popped out - just like a tire or inner tube with a bubble.

My best guess is some reaction between the glue of the stringer and the epoxy. Sucks. I really spent a lot of time getting the lamination as tight and clean as possible, now I have to cut it out. My plan is to patch with some 4 oz. then go over the middle 5’ of the bottom with 2 more layer of 4 oz., at 50/50 resin/glass ratio that only going to add 1.2 pounds and it will give a lot of strength to the center of the board where my large self will stand.