Longboard Fin Question: is poplar plywood strong enough?

General Discussion
1

Hi,

I want to buid a 9.5’’ longboard fin.

I’m thinking about to build the core of 8mm plywood, outside 4 layers of 4 oz glass on every side.

So far I have my fins from birch-plywood and now I’m asking myself will that be strong enough?

Or do I need a glass layer in the middle of the fin?

 

Best regards,

Andi

 

2

Andi,

My vote is that the is no need to glass a layer in the middle. What you are planning is strong enough.

Glass-on fin? All that 4oz and whatever you use to attach it to the board will be good.

3

Hi Andi-

4 x 4oz both sides should make it very strong, maybe too rigid. Do you want it to flex? I made a smaller one 8" for a FCS box with plywood outsides (no glass outside) and 2 or 3 x 6oz in the middle. It flexes maybe 1"  total at the tip but I don’t think the plywood likes being bent, I hear little creaks when I bend it hard. I finished the outside of that one with paint and water-based polyurethane instead of fiberglass.

One nice thing about glass in the middle is that it is easier to work on, less chipping of the plywood as it is foiled.

I am a beginner, hopefully the experienced will join in with their comments.

-J

4

 

 

.The way you turn it should be fine…

5

thanks for your suggestions.

will it change anything if I do not build a glass-on-fin?

think that the hole for the skrew could be not tight enough…

by the way: what glassing would you do with birch plywood?

6

I have never used poplar plywood.  Ar you sure all the layers are Poplar or is it just the exterior veneer which will be mostly foiled off?

 

The glass in the middle can help to form the Halo, and reduce the amount of  clarity destroying glass required on the exterior.  Getting the glass in the middle is the challenge. and I imagine more so on plywood than a cut of solid hardwood.

 I did an 7.5"  Ipe fin with ~ 6 layers of 4 oz in the middle,IIRC, and 4 on each side which were foiled, then one more 4 oz to cover the foiled fiberglass.

 

It has a nice flex pattern but ultimately I went back to a thruster set up on the board I designed it for

7

The heading says polar, but your post says birch. Most types of plywood are spec’d by the exterior veneer and the core plies can be who-knows-what.

If I was going to do a box fin and wanted the ‘wood look’ I’d build it like a fin I had years ago. The core was glass cloth, built to equal the thickness needed to fit the box. A layer of plywood of indeterminate species was added on both sides to gain thickness for foiling. After the foil was done, they added another layer or two of glass. Made for a nice looking fin witha  wood surface and a generous glass bead.

 

Also, birch is a hell of a lot stronger than poplar, or most of the wood used for fins (redwood, balsa, etc). In fact, 19 ply Baltic birch plywood is used to build many of the large scale speaker boxes used for arena systems by companies like JBL, EAW, EV, etc. Those have to be strong, and very rigid.

8

excuse the confusion.

I started the thread with the idea to use poplar, but now i tend to do it with birch.

Simply because I don’t want to try something new.

The new board, for that is the fin, was enough new ground…

 

9

You could lam 1/8" clear Lexan sheeting between the ply layers…

10

If you are going to use birch plywood, be sure it is Baltic Birch and not the Birch “veneer” common at lumber yards and Lowes/HD.  Baltic Birch is often used by DIY long-skateboard builders – laminating 3 or 4 layers of 1/8" BB plywood.  You will know it is BB because it comes as 5’ x 5’ sheets of multiple thin birch layers.

I believe the glue used in Baltic Birch plywood is not waterproof though.  However, I believe the glue in Finnish Birch plywood is waterproof (been a while since I bought some Finnish Birch).