Stringer in small foilbord?

[kriskris]

I am thinking about building a small strapless foilboard. EPS or XPS Foam with carbon skins 12 oz on bottom and 2 x 12oz on top with extra reinforcement for foil attachment and wrapped rails. Probably cedar wooden block built in to strenghten this are too. Dimensions approx. 32 x 17 x 2 inch. Basically slightly scaled down Slingshot Dwarfcraft. My understanding is that loads on board are carried mainly by the carbon fibre skins. Is addition of wooden stringer necessary? Would it provide significant strenght or stifness? Is this lay up sufficient? Any words of advice would be appreciated. Thanks, Kris

[BWD]

If using eps or xps I would definitely use a stringer for a foilboard. For a short surfboard probably not. But the foil adds compression loads to mount the foil, cantilever loads to ride the foil, and shock loads when you run aground. These light foams cannot take that.
If you do rails mounted foil, do a double stringer setup where each box is in a stringer. If simple 4 holes/inserts, put fat stringers in with the holes or inserts in the stringers. Stringers do not have to run nose to tail.
You can shape stringerless and route or cut slots for short stringers in just in the foil/stance area.

[kriskris]

Thanks BWD. So if 2 stringers are incorporated (the way you suggest) do you think my planned lay up schedule would be sufficient?

[RalfsB]

Yes, I also found 2 stringers to be the most practical. Stringer can be few mm wider than the hole, like if you are using 6mm screws then stringers can be 10-12mm wide, full board depth, length like 5-10 cm back from the aft hole and 10-15 cm forward from the front hole. To make stringers more stress resistant you can laminate some glass or carbon around them. It is convenient to put some bronze barrel nuts (or something similar) into the stringers so you can use shorter screws and do not have screws going all the way through the board.
Regarding the layup schedule, you can put an extra sheet of glass/carbon cloth on the top and the bottom so that it overlaps the stringer area.

Here is a picture of a carbon-laminated stringer I used in one of my projects

[downunder]

Did u make the carbon plate as well?

[RalfsB]

Did u make the carbon plate as well?

No, that is a commercial KFbox-to-plate adapter, made by Alpine

[TomW]

If you laminate 400g of uni D carbon 150mm wide on each side you don't need a stringer. I find it easier to do this. I tacoed a foil board with a 3mm central stringer, it didn't have enough uni d carbon on each side.

[windmaker]

Yes, I also found 2 stringers to be the most practical. Stringer can be few mm wider than the hole, like if you are using 6mm screws then stringers can be 10-12mm wide, full board depth, length like 5-10 cm back from the aft hole and 10-15 cm forward from the front hole. To make stringers more stress resistant you can laminate some glass or carbon around them. It is convenient to put some bronze barrel nuts (or something similar) into the stringers so you can use shorter screws and do not have screws going all the way through the board.
Regarding the layup schedule, you can put an extra sheet of glass/carbon cloth on the top and the bottom so that it overlaps the stringer area.

Here is a picture of a carbon-laminated stringer I used in one of my projects

06.jpg

Doesn't water get into the wood from the screw holes?

[TomW]

I'll say it again, you don't need a stringer of wood. If you are using 20 -30 kg m3 foam, put a 80kg m3 foam block that's about 25cm long and 16cm wide under the mast. From bottom to deck, the whole thickness. This is primarily to withstand the compression of screwing the mast plate . You have to have solid epoxy or plastic plugs where the mast bolts/ inserts will be. Use 400 g of uni d CF about 15-16 cm wide on each side. That will give you a strong beam. Then add lamination with rail wraps, the schedule is depending on your foam density, but if using 30 kg m3 foam, then 2 x 6 oz. Glass top and bottom.
Because you need to make what is basically a cantilevered beam, your laminate should be symmetrical top and bottom.
If you are going to add footstraps and do jumping you'll need to beef it up.

[JakeFarley]

Been thinking about building a low tech foilboard using 2" EPS with a cross hatch wood (probably poplar) stringer as below. Simple 6 oz. fiberglass epoxy layup with a wood veneer bottom and a top pad. Any thoughts on this?