Hollow wooden board strong enough?

I’m doing a hollow wooden board 5’5’'. But need advise if its gonna be strong enough or I should add some more wood for strength? 

Skeleton is lasercut in 4mm plywood for ribs and 2x3mm plywood for stringer (mirror cut), thinnest parts are 8mm. 

150mm between ribs. Extra 4x4mm wood strips 80mm appart for stiffness and more glue surface. Rails, tip and tail are to be solid in balsa. Deck in 4mm balsa (100x1000mm sheets).

The idea is to epoxy glass it 4+4 and 4. 

Skeleton as on picture weighs 0,5kg 

Any thoughts about this, am I way of or on the right track? 


Additional are the Goretex automatic vents to be trusted for ventilation? 



Kinda depends on the decking you use. But if you have doubts, add some reinforcement in the critical areas. The goretex vents are commonly used for this. You can get more insights from https://www.woodboardforum.com 

The frame looks good, is that a Paul Jensen design?

I’ve always thougth that most of the strength of the board came from the glass and a stringer. It might’ve been Bill Thrailkill that described the glass and stringer as a big letter ‘I’, with the top ends extending outward to the rails. In my mind, I figure as long as I get the desired shape using whatever medium I choose (wood, foam, cardboard), once glassed heavily enough, the board should be strong enough to hold my weight. If you go by this theory, it looks like you would be fine. However, if this was my project, I’d be on the cautious side and end up doing something like 6x4 or 6x6 deck, 6 bottom. 

Either way, pls update this post with your progress. 

I do not exspect it to work, sorry, but the 4mm deck will not last. For the bottom it will behave like an eardrum (hope it is the right word, acc. to my dictionary this is not only the anatomical eardrum, but also works for audio)

meaning it will work, as long as you do not hit anything.

I cannot comment on your plywood frame, I have never worked with a plywood frame, i use balsa only, but you can test it easily by yourself, put a plank to distribute the weight and then add weights, or better stand on it. If this lasts, it will last…

The deck, especially the later feet, knee and hand area, will bent down, it may spring back, but it will be a weak spot. The area betwenn stringer and rail, between to ribs is not strong enough.

But good news, you can always add additional layers of glass.

But I would suggest another way: Laminated at least the deck (bottom optional) from the inside too, a 3oz cloth should be enough, laminating in and outside results in an H-Beam; the higher the better. So if you will use 6mm balsa for the deck, it will be even better. Laminating the inside can eliminate the need for the second glass layer on top, so it still will be light.

How it works: Prepare your deck, apply a cheating layer of resin, very thin (on the later inside!) This will close the pores of the balsa and prehibits that to much resin will be sucked away from the following glassing. After fully cured, glass the inside, using a fair amount of resin, better a little more than usuall, the glass shout be really wet!

Let it sit until tacky, but still not cured. In the meantime prepare thickened epoxy as a glue and apply it onto your frame, then glue the deck onto your frame (still tacky inside). You definitely need to support the rocker or better use a rocker table. Do if wanted, the same with the bottom.

You may check some of my builds in detail (and my mistakes and faults) on www.woodboardforum.com, username: olddude.

A first impression of my last build, you can find here on swaylocks, searching single fin 6’6

Had to create a new account to reply. Ill look into your builds. 

Thanks for the advise. After the comments I think Ill glass it at least 64+6. 

Didn’t consider the fiberglas on the inside of the deck, that might be a good idea. Othervise adding a veneer like you did on top of the deck could maybe be an option for me. I have some 1mm 3d Veneer (see picture) laying around that could add some more strength and a harder surface to avoid it denting. Furthermore I decided to add a bit more “hard” wood (not balsa) to the frame. 

 

Veneer, depending of the Wood, distributes a point load very good, but finally I usw it more for optical reasons than technical. A Layer of veneer, depending on wood and glue, results in an additional weight of about 500g. With veneer, you may just epoxy the Outsider, without any glass at all. 

If you warnt to go as light as possible, I would not use veneer. Balsa with a cheater coat and a 160gr/m2, should do the job, at least for no point loads. It should result in no dent, of a knee bumps onto the deck, or a foot is standing upon. The inside laminate should prevent that the entire area bents downward.

Fortunately you can always test it. Use your thumb and apply pressure; if it feels to soft and bends put another Layer of glass. My sons board, a 3 piece, has a 4oz,6mm balsa, 2oz inside lamination and it is definitly stabile.

https://www.woodboardforum.com/forum/design/workshop-tool-ideas/3156-shortboard-5-11-x20-1-8x2-9-16-collapsible-the-transformer

 

 

I would listen to surfdude/olddude. Also check out everyone’s builds on woodenboardforum. There is a lot of knowledge and talent there.

I’ll add that most of my boards are/were 4mm plywood frame with the same plywood for the decks. My frames were more whole than yours the only lightening I did was with a forstner bit. Depending on how you decide to glue your decks and rails should also add strength to the board. I’ve always had my rails “sandwiched” between the decks instead of the rails being wrapped around the outside. Whether or not it adds appreciable strength is a good question that someone smarter than I am could probably answer.

If you are still worried about deck strength, want it to look nice, and don’t mind a little extra weight you can use plywood for the decks and cover it with veneers.

Also don’t forget to add some sort of backing for your finboxes and leash cup before you glue both decks on. It can be done after, but it is an unnecessary pain.

Good luck with the build and keep the pictures coming!

My thoughts exaxtly with the point load. Ill add the veneer. Dont Think Im in trouble with the weight. Cant see it go above 2kg in any possibly way (before veneer and glassing) 

Thanks for the reply. I’ll definitely look more into that forum. 

I have decided to ad more strength in the rail, so they can contribute more to the overalll stength of the frame. 

Maybe a good idea to sandwich the rails, I understand the idea and it makes sense in theory at least. 

I’m want to avoid plywood deck. I used it for my last build (4mm) and it was a huge overkill. That board is indestructible but heavy. 

Jep, I’ll remember the blocks. 

Nice… Looks plenty strong given your demensions and glassing schedule…!!!.. 

Thanks for all feedback and advice.

Update with photos. Super slow process, because of to much good surf ;) 

Board is shaped and next up is the glassing. Ended up weighing 1950g preglass. Decided to go with sandwiched rails, goretex airvent, futureboxes and probably a 6+4 / 6 glassing. 

Super exited to get it glassed and wet. 

 







Great looking project!