Longboard for competition

Here is one project:

I can get easily EPS with 4 meters by 70 cm by 30 cm, with 25 Kg/m3 (dense).

It´s to put a glued stringer with 3 mm thin.

1st question: does this blank can be CNC cutted by the KMS machine and stay perfect?

2st question: Would it be preferebly made by a normal blank of poly?

This board is going to be covered 4 oz glass with 3 mm PVC and then 4 oz glass, all epoxy.

Does it need to be vacuum glued?

Wich would be your preference?

(PS it´s to be very strong, flexible and light)

Construction help needed…

I wouldn’t use a stringer if you’re going compsand (PVC-foam sandwich)

Ofcourse you’ll need to use vacuum bagging.

25kg/m³ core + sandwich will make your board bulletproof. Ok, thats not true but 15 kg/m³ will be enough to make a strong compsand (windsurfers are using 10-15kg/m³).

From what you’re saying I can deduct that you’ve never build a board and you do not really know what you are talking about. Building a competition longboard as a first board and doing it in sandwich will lead to a bunch of frustrations!

I would use the 25kg/m³ foam + (double)stringer and lam it with 2x6oz bottom and 3x6oz top (or 2x4oz and 2x6oz if you want it lighter and don’t want to waste too much money on your first board). No vacuum bagging needed.

95% chance your first board will float and will be able to catch some waves. 1% chace that it will be a competition board (depends on what competition you’re in).

search the forums for compsand!!

With 9´2", by 22" and in EPS it must have a stringer… I had one covered with PVC and become to stay, after one year, very flexible and then with a little wave, she broke in half…

I´m not going to shape it, my shaper is !!

I deliver the “pieces” (EPS adn PVC) and he is going to shape and glass…

but I have also a vacuum machine and when the board is beeing covered, I will use it.

I just don´t know if the CNC machine cut´s well the EPS, quite clean…

then, after covering, the rails will be made in glued balsa.

It will stay one stringer and the rails balsa as a structure and the coverage with glass and PVC…

With pigmented glass, I believe she can stay in the 6,5 - 8,5 Kg…

what do you think???

Luis,

you are really going to want to look in the archives about how to do this. It is not a short answer. You may need to have the eps core somewhat shaped before you vacuum the sandwich skin on it. Stringer is truly optional, especially if you intend on having the balsa rail build method, the solid wood will take over the load bearing from the stringer anyway (and introduce a whole different set of variables into the game but that’s a whole different story)

Have a good read throught the stuff here and on compsand.com where there is more specific information about the vacuum and balsa stuff going on…

The more thought goes in before you touch foam, the better the result is…

CNC is no problem on EPS - Just slow down the feed rate if you have tear out. With the EPS from MARKO or US Blanks you can keep the feed rate the same as PU.

See this picture…

it has not a stringer… that´s why she broked…

(EPS - 25 Kg/m3 + 4 oz + PVC + 4 oz + paint)

When I bought it, his weight was 5,5 Kg (+ or - 10 pounds)

for me it´s to much lighter, to ride a bigger wave for example…

this board was very technical to ride, supercontrolable, but as I said very light…

5.5. kg = 12.1 lbs.

Is that cored-skin both sides? Putting a center stringer in a cored-skin board will increase load-bearing by yielding a much

stiffer structure. Thicker skins or different skin materials might be better in terms of increasing load-bearing while keeping

board ‘‘feel’’. I’d bet if you go over to compsand.com Paul and those guys could advise you.

And definitely talk to your CNC operator about the EPS, or they’ll talk to you. Some machines don’t cut EPS very well, and

there’s almost always a change in cut rate or something to deal with. But they’ll like it better with a stringer, stringerless

gives 'em fits.

I hope I didn’t offended you in your first post, it was to warn you about the difficulties of sandwich.

It’s because I screwed one up, altough I shaped 3 normal boards before it.

Like said above, a stringer will give not enough strength, it will more likely damage your sandwich skin than giving strength.

Look to it this way: you know I beams (http://en.wikipedia.org/wiki/I-beam). the strenght is in the outer plates, the middle plate needs just to be capable to keep the two outer plates apart and to carry the shear load.

To make your board stronger you’ll have to add more glass! I don’t think the EPS core is dented much in on your picture, so it is not the foam that failed, it’s the GLASS!

On conventional boards a stringer is good because it prevents the foam on the centerline from denting in (that’s what the sandwich skin does, it spreads the load on the EPS), ofcourse a stringer gives some stiffness. But when your stringer is stiffer than your board it will load your skin a lot.

More glass is the answer!

(Take a look at windsurfboards, they are ALL stringerless)

Edit: I don’t believe thicker skins will do a lot, thicker skins are good to prevent denting (that’s why windsurfboards have thicker skins), but not the breakage resistance (not so much as the glass).

Hans, fijne dagen he!

Love it, you always come with a engineering point of view!!

You will find a few compsanders talking thicker skins, I find another advantage to thicker skins, it gives me more meat to shape and that means better margin for error, note I use balsa sheets 2 to 3 mm instead of 1 to 1.5mm so thick is really quite relative… but the extra wood gives some room so you don’t end up shaping through the skin… a humble garage-comp-sand-yarder perspective!

Yes, I wouldn’t go thinner than 3mm for the same practical reasons. With my screwed up sandwich board I even sanded trough the 3mm trying to correct the mistakes I made ;). But I assume that the board shown in the pics has at least 3mm. I’m not sure but I do not think that there are many foam-skin builders who use less than 3mm.

@dave

danke, prettige feesten voor u ook.

het is toch gek dat je mij hier vooral in de examens vind. :smiley:

In het jaar heb ik zoveel andere zaken aan men hoofd.

Ik hoop ooit nog eens aan een boardje te beginnen en liefst zo snel mogelijk.

Och ja we zien nogwel. Voorlopig deel ik alleen mijn “engineering point of view” ;).

Well, the PVC is 3 mm, and for me is ideal for this construction…

the use of 1 layer of glass, PVC and another layer with 4 oz, I, really don´t know if it is a good structure with a stringer, in a 9´2"…

without a stringer I have the experience…

nevertheless the windsurf boards are one of a kind construction, because the shaper almost doesn´t deals with the shape himself, but with the outside construction. That is the most important part of the construction and not the blank, wich almost works as an “schock absorber” of the outside…

some of them simply… don´t have anything in the middle…It´s like a boat hull… but this is another kind of “history”

For instance, I question myself, sometimes if I get a very light poly blank and cover him with 170 g/m2 carbon layer (top and bottom) and 4 oz glass fiber with pigmented resin (all in polyester resin…)

It will stay bullit proof, but I believe it wiil stay light (I saw one board with 320 g/m2 with no glassfiber, and it was light…)

a light core with heavy glassing won’t be strong. Because the foam will dent in or the tears apart and your glass layer folds! that’s where the sandwichlayer comes in, it prevents the glass from “folding”.

without a sandwichlayer your board will also dent like hell!

the glass is only strong in tension (and compression if it can’t fold).

That’s the theory, the foam won’t get any load, it doesn’t add strength. But it has to keep the glass layers in place, otherwise the glass fails!

(BTW it is a misconception that carbon is strong, because it is not! Carbon is stiff and brittle! It breaks easily with shocks.)

The carbon brakes “easily”, but breaks over a tension point above of the glass fiber…wich is good, but terrible to repair…

a lot of windsurf boards are made of carbon, plywood and glass… in vacuum of course and the board needs to have a little hole to breed…

That’s true, but in shocks it’s the ability to absorb energy that counts (without plastic deformation) and therfore glass is better than carbon (kevlar(Aramid) and dyneema(UHMWPE) even better). E=F*x, where F is the yield strength and x strain at yield strength.

For windsurfboards the stiffness of the carbon is needed, a stiffer board is a faster board, it deforms less and absorbs less energy but will break more easily.

If you look to waveboards, where speed is’t that important but strength much more, they mostly use kevlar and less carbon.

BTW: There is much more use of HDfoam than plywood. I assume you are thinking about the starboard wood construction wich is a thin wood veneer over HD foam. Wood is commonly used under the footstraps but almost never the whole board (to heavy).

The only full wood sandwich board I know is the “Fanatic RIP wood” (1998) but my local surfshop couldn’t sell it because it was to heavy (8kg) while the foam version weighted 6.8kg.

Edit: to be scientifically correct

The formula given higher is not really true actually it is the integral from 0 to yield strength from F*dx.

Which is ~ (F*x)/2

One thing not mentioned was shape.

All modern epoxy styro boards have lots of belly (thru panels or rounded) and lots of dome in the deck.

All modern Cobra made sailboards use very little carbon (maybe 3 strands at the most), so mostly get their stiffness and breakresistance thru shape, rather than materials.

Hard to tell the cross sectional shape by the pic.

I suspect cross sectional shape is the most important strength resistance against breakages, and for controlling flex also, of course.

Flex is a very bad thing for durability, as it destroys the integrity of the rails first and formost, and it seems the rails are the tru anti breakage element.

???

use lighter foam…

12 to 15 kg density at about 55mm for the finished core thickness

put on perimeter stringer with 15 to 20mm wide pvc

overlap the pvc skins over the perimeter stringer

the pvc should be around 60 to 80kg density and the lowver density skins are better if you go to 5mm on the deck

use 2 to 3oz glass and epoxy resin to adhere the skins to the core

you must use a vacuum bag to put the skins on for about 8 hours perferably longer if you have cold environment

if the temperature is under 25 degree celcious maybe 24 hours for a reliable cure

after the skins are on, finish shape the rails and glass the board with 6 oz

lap the bottom glass around to the deck and let the deck glass hang

use pvc inserts around the fins boxes

the finished board weight will be around 12 pounds

use and epoxy fill coat and a polyester finish coat

if you want the board really strong use a lightweight wood like paulownia, balsa or cedar

cedar and paulownia will be more durable and a bit heavier

balsa will make the lightest strongest board for its weight but if balsa gets wet it gets soggy

the board will require a vent

if you make the board to stiff and limit it range of movement it will snap easily

stiff does not mean strong in a surfboard

its not a load bearing wall, it is a dynamic flexible panel

carbon is no good

this is a correct schedual for a performance longboard and will yeild a very durable and light surfboard

center stringer do not have any meaning in this construction and will ruin the board

cedar skins at about 3mm thick will make the stongest lightweight board possible

incidentaly mike sabin said to me (he builds big yachts) that without heat curing it is best to leave epoxy clamped for 3 days

3 days

this is the man that built an open 40 and sailed it around the world with 2 little kids, has built many sandwich boards and is a factory floor manager of a large yacht manufacturer… when replacing mast spars whilst sailing around the world . he told me they would break again if they were not clamped and cured for 3 days… when your life depends on it

Was that board stagger lapped? I’ve haven’t done stringerless yet but I’m curious to know.

laps do f all all in properly made sandwich boards

they are strong before you even glass them

like stupid strong

stringer/poly is a joke compared to it

Quote:

Was that board stagger lapped? I’ve haven’t done stringerless yet but I’m curious to know.

My board just broked, when I was passing a wave ( a very little one), but the wave beat her right in the middle.

As you can see in the photo, she´s broked in half almost perfectly… and there wasn´t ANY mark of tireness material

As I said ths construction for me is almost perfect, but to complete it, it would be with a stringer covered wit a 6" layer of carbon all over the stringer, glass, pvc, glass and balsa rails with the inside rail reenforced with glass or carbon.

This strucuture will give some elasticity (but not much…) and it will not stay very heavy…

I´ve been in the Compsand forum and I stay delighted wit on board wich the owner is over the board (inverted) and testing her flexibility with is weight… imagine the perfect weight, with the perfect shape, with the perfect flexibility of the board, and the only factor that makes the difference is the surfer…

(At least I knew that my only problem in surfing was only me… and so I couldn´t give any excuses, as I give now… eheheh…)