Thursday, February 17, 2011

Rocker profile comparisons

I'm working through the changes that I need to make to the rocker table design to avoid the problems that occurred this time.

After coming across the beam bending equations in the last blog that describe the rocker profile you would get by fixing a flat surface at the middle edges and bending the ends up I wondered how close the profile would be to the continuous rocker that I had aimed at.

The chart below shows the differences:

As you can see the bend beam is slightly flatter but the difference in the height of the rocker is about 3 mm throughout the section from about 38 cm to 51cm which is around where, at the front of the board, the board will sit above the water.

This represents about 7.5% of the total rocker and when you draw a line horizontally through , say, the 2 cm height line it will result in about 4cm of board in the water when trying to go upwind.

The question is whether this will make a material difference to the board (in particular the upwind performance) that can't be overcome with some additional concave, channels and most importantly riding technique. I don't know the answer to this. My guess is no but would be great to hear from anyone who has a better understanding.

Wednesday, February 16, 2011


So what worked and what didn't

This is the part of the process I've really been looking forward to. To have a chance to put down everything that I’ve learned from the process so far.

Rocker table design
This proved to be a really big one because the mistakes made here caused the rocker and concave to be much less than designed for and so what was going to be a high wind, chop eating board turned out much flatter than expected and so more suited to flat water.

What went right?
The overall dimensions although on the large size were good for having plenty of room around the board to stick the plastic, get the vac connector in place and stretch film to get wrinkles out. However, this was mostly a benefit because it was my first time and I needed the room to fix mistakes. I would aim at next time leaving only about 10-15cm extra beyond the board width for reasons I’ll outline below.

What went wrong?

The biggest issue was that the concave and rocker actually realized was much less that desired and this was due to 3 things:

i) As the rocker table was about twice as wide as the board I had to make and estimate on how much lower the edges of the jigs should be than the middle line to get the desired 10mm of rocker at a point that was about half way to the edge of the rocker table. I estimated that this was going to be about ½ height difference and so to get 10mm of rocker of made the jigs 20mm lower that the centerline at the midpoint of the board. In hindsight this was a really poor approximation because if this were correct it would mean that the table surface (the mold) would have to form a straight line from the centre of the mold to the jigs at the edge of the mold. Instead (after a bit of googling) it turns out that this estimate was 50% off and in fact when you put the actual numbers in for my board you end up with the amount of concave at the edges of the board being just 25% of the height difference between the center and the jigs. So what was to be 10mm can out to 5mm which is actually pretty bloody close to what happened? Also it meant that at the center edge of the board it was 5mm higher relative to the ends and so 5mm of rocker disappeared as well.

The equations governing this is really useful and comes directly from beam bending theory. Assuming that your mold surface is made of material that has a uniform modulus of elasticity, the half width of the board is L and the height difference between the center of the board and the point at the edge where the jigs clamp down is H, then the equation describing the shape the mold surface takes is

Y(x) = -3H/L^3 (L.x^2/2 – x^3/6) - (profile)

So it’s a bit like a flattened out parabola.

If you are aiming for a concave of say c and for my board the edge of board was at x=L/2 (roughly) then you would determine the height difference H that you need to factor into the design by setting Y(L/2)=c and calculating H.

From my calc this number turned out to be about H=4cm instead of the H=2 that I had assumed. One important implication for the design process is that you would first design the centerline of the rocker profile and then use this H to determine the required radius of the rocker line at the edge of the mold. This is necessary as if you do in the other order then you would end up with a hump in the middle of you board.

ii) Second, it turned out to be hard to get the 4mm plywood mold that I used to conform to the jigs exactly as the it ended up causing the plywood to crack if I have put enough pressure on it to get it to conform exactly. This meant that the jig rocker line was flatter that the actual curve of the jig. Again this contributed to the reduction in rocker from the desired 3cm down to 2 cm.

iii) Thirdly, I cut the jig with square edges where the mold surface was going to sit. However, the surface comes in at an angle and so it rested on the inside edge which was higher than mid point where the actual rocker line was designed around. The lesson here is to use a router of the like to shave off the inside edge of the jig.

Together these issues account for most of the loss of curvature. It had been suggested to me that it may have been that the fibreglassed laminate will ‘relax’ after it comes of the jig and I understand that this can be up to 20% of the curve that flattens out. However, I checked the finished board against the rocker table there wasn’t any perceivable relaxation at that stage. Maybe this comes later as the lamination ages.

The rocker table surface

All in all the use of plywood for the surface of the mold was a poor choice. Despite about 6 layers of acrylic paint on the surface to seal it and 4 layers of release agent, when the board came off after laminating the bottom it took acrylic paint and in some cases a bit of the wood with it. Either the release agent was insufficient or the epoxy reacted with the paint. Also, the uneven surface of the wood means the board surface was uneven and didn’t recover that well with sanding.

While I still think that there is something to the issue of using stiffer material, especially if you’re going to put stringer in the board or use wood cores, I think that using wood alone is not the right move. For the next try I think that I’ll lay either acrylic, laminex or maybe a sheet tin over the surface of the wood.

Board design

Rails. Trying to cut the rails to the exact curve of the board edge was a real pain in the arse and turned out crooked. The best tip I’ve come across for this is to just rectangular strips which you can use a straight edge to guide you and then heat the plastic and bend it to the shape you need.
The other alternative which I’m keen to try is to pour the rails using resin with chopped fibres in it for strength. For the current board this would have used around 190gms material (160gm resin +30gm hardener + couple of clumps of chopped fibres and bit of qcell to harden it).

The Inserts

Fill the t-nuts with wax before you put them in the resin. This will stop the resin filling up from the bottom and causing the available depth to be short. This can be a real problem as when you tighten the bolts in to your foot straps, excess resin in the inserts will cause the fiberglass to be pushed off the underside of the t-nut and possibly split the underside laminate. 5 minute epoxy is good stuff to use here to speed the process up.

Vacuum set up

The compress worked like an absolute champion. Because of a leak in the setup when I laminated the top (which I was never able to find) the compress ran continuously for about 7 hours and didn’t blow up.

The vacuum connector made life very easy, the ripstop nylon (with extra holes pushed into it) worked perfectly in place of peel ply and the polyester wadding worker well as breather material. However, the surface of the board came out a bit bumpy and I am trying to work through whether the uneven density of the wadding might have caused this. However, I stuffed up the top layer laminating ( forgot to just the glass to size before I mixed the resin and so the resin had started to go off before I finally clamped it down. This is more likely the reason)
The tacky tape was a really pain in the arse to work with. It was way too tacky and so trying to reuse it (and not waste it) ended up being responsible for damage to the vac film and the subsequent unfindable hole. If I end up using a more uniform mold surface next time I think that I will try just using packing tape around the outside or maybe use plasticine in place of the tape. It might help to form a first line of defense to assist the tape.

PVC tube greater than 8mm doesn’t seem to be up to the job of holding pressures above 15inHg without deforming and running the risk of pinching off. Putting wire strands inside the tube stopped any problems.


QCell is great stuff. I used to little initially because I added an amount equal to the volume increase I was after (e.g. 10% of volume of the resin) but overlooked the fact that a lot of that is air and so when mixed in the volume increase was much less. It seems like the best approach is to mark the volume increase on the side and check it when it’s mixed through. Also, as qcell is often filler, you can just do it by feel. ‘Peanut butter’ seems to be the going description of the right consistency for using it as filler.

When you are putting the resin on the glass, really work it through to get all the air bubbles that will be created in the holes between the weave out. I didn’t work it hard enough and so ended up with a lot of bubbles which will most likely be responsible for the board ultimately cracking.
Get as much excess resin off before clamp it. This resin will flow everywhere on the board you don’t want it to even if you have plenty of breather material.

Don’t let wrinkles appear in the vac film as this will create weaknesses.

Let it cure under pressure for longer than you need to and let it finish the process with the vac film removed before you take it off the mold. Even after 8 hours under pressure the resin still had a slightly waxy feel to it which took another couple of hours to disappear.

Friday, February 11, 2011

Vacuum Bagging

With the core prepared its now time to do the mid-tech part of the process and glass and vacuum bag it. A nervous moment for me as I wasn't sure if all the parts will work together and there we're a couple of big unknowns. The most important of which was what clamping pressure for the vacuum bagging?

I was assured that the PVC foam won't collapse under 1 atmosphere pressure ( the maximum this set up can pull) so the issue is what pressure will squeeze out too much resin and level the board weak. I couldn't find anything definitive on Google and I just guessed that 20 in HG (around 10 psi) seemed like a reasonable - time will tell if it was too much.

Before trying it with glass etc I wanted to do a test run to make sure all the equipment worked that there was no leaks in the system.

The Test Run

For the test run I put the board on top of the polyester wadding ( from Lincraft) that will be used as breather cloth over the board when doing it for real. Otherwise, the set up was identical to how it would be when going live.

So I set up the rocker table and applied a few coats of the release agent (basically carnuba wax like used for surfboard wax). The jar recommends 5-6 coats with 1/2 hr in between to let it set. I just wanted to keep the process moving so I started before the dry run.

I cut the vacuum film to size and cut a hole through in the corner closest to the pump so that I could connect the vacuum fitting for connecting the vacuum line. Install the connector.

I then ran a line of tacky tape all the way around the perimeter of mold surface and made sure the board was going to fit. Tacky tape is basically a strip of stretchy tar and it sticks to itself and whatever else so leave the backing paper on until the last minute. I also found that the tape could damage the vacuum film when I pulled it off after the trial. The trick to getting it off with out damage is to rip the plastic off hard and very quickly. Otherwise the tape will stretch and stay stuck to the plastic.

Tack the vacuum film to the ends of one end of the mold then pull the vacuum film over the mold and tack it to the other end, again by the extremities. Do not stretch the film as but to it so that the tension just takes the large wrinkles. then I exposed all the tacky tape and

Now the moment of truth. Will the setup hold the required level of vacuum???

Great sigh of relief as the vacuum pressure dropped to 20 inHg and with a bit of tuning of the wing nut on the regulator got it to cut out right on pressure.

I let it sit for an hour and it held the pressure for about 20mins before the pump reengaged for all of a few seconds and maintained the pressure spot on.

A couple of things that the test run showed up:

i) the larger diameter PVC tubing that I had for connecting the vacuum connect (10mm compared to the 8 mm that is used everywhere else in the setup) could not stand up to the pressure drop and over the space of about 20 mins it collapsed down. In a live situation where it needs to hold this pressure for hours, it is possible the the tube could pinch off and isolate the pressure under the film from the pressure in the pump and so cause the regulator to no sense the pressure increases it needs to respond to. the solution that ended up working fine was to get about 5-6 lengths of wire and put it inside the tube to ensure the collapsing tube can't pinch off. By contrast the 8mm had no problems at all.

ii) Not an issue but just an observation, the vacuum film was able to stretch around most of the sharp corner created by join between the top and bottom layer. However, it was not complete. Right in the corner it bridged the 2 perpendicular faces with a corner of radius 2-3 mm.

Final Amendment to the Core

I started to get nervous about how much was the right amount of resin and coupled with some confusion about how the excess resin that I was inevitably going to use was going to escape I decided to drill some 1mm holes through the core to allow it to escape if need be. I put the holes about each 5-6cm across and long the board. As it turned out some of the excess resin did push up and into the breather material so it seems like it was a good idea.

Also, as I mentioned earlier I mixed up about a 50% by volume mixture of resin (and hardener) with qcell until it was thick like Vegemite and used this to fill out the sharp corner where the top an bottom layer join so that the glass would not bridge this and weaken the board.

Glass the Bottom

Before you start mixing the resin make sure that your vacuum film is cut to size and the connector is in place.

So here goes. My first attempt at fibre glassing.

I decided to do just the bottom in case I stuffed things up I didn't want to waste all the resin and glass. As it turned out this was a good thing as it made it very clear that I had used too much resin, confirmed that the ripstop nylon did its job brilliantly as a peel ply replacement and confirmed that the vacuum pump was up to the job of holding the clamping pressure for 8 hours.

I finished off adding an extra couple of coats of the release wax and let it dry.

I cut out the 3 layers of 6oz cloth I was going to use and cut it to size with about 4-5cm extra all around. 2 of the layers were cut with the weave running off axis as much as the 90cm width of the cloth would allow. Its not the recommended 45 degrees as the cloth was too narrow. I think it came in at around 30 degrees off the long axis. Apparently this helps with overall strength....

I mixed 750gms of resin as your mate at FGI said about 230gms of resin per layer (about 1/2 sq. m each layer). However, I forgot to take into account the 5:1 ratio of resin to hardener and so ended up with 900gm of resin+ hardener and so ended up with about 250gm of valuable resin sitting in the bottom of the container after I'd drenched everything with the mix. Even with this much left over, so about 650gms of resin, in the cloth, I still had too much. So my feeling it that I should have used about 200gms per layer (1/2 sq m layer).

Lay out each layer of glass, pour about 200gms of resin by running it from one end to the other and work it around with a flat edge of a something like a credit card. Make sure there is enough resin on every part to let the cloth just turn translucent. Any more that this is not necessary and will just be squeezed out anyhow.

Lay down the core and put the ripstop nylon ( used instead of peel ply) and polyester wadding over the top of the core. The idea of the nylon is that it is a woven material and so, under pressure, will allow the excess resin to be wicked away up through the weave and into the breather material. The beauty of the ripstop nylon which was salvaged from an old kite is that it won't absorb the resin and so wont remove any resin in its own right.

The breather material (also called bleeder material) keeps a channel open to allow the air to be sucked out evenly over the surface of the core.

Below is a picture of the layered setup prior to going under the vacuum film.

So now to cover it with the vacuum film and seal it. Run line of tacky tape across both short ends of the table. Tack the vacuum film on one end first and then the other making sure to apply a small amount of even tension to the film to avoid getting wrinkles in it.

Then stick a wade of breather material under the connector so that it remains open and make sure that this material drapes of the breather material covering the board as you need to maintain an open channel for evacuating the air.

Then run the lines of tacky tape down the long edges.

Then apply the vacuum.

Below is a before and after of the vacuum pump in action.

If the pump gets the air out and the seal is good then it just a waiting game. If the pump runs continuously or never gets over a certain level then there is a leak somewhere and that really sucks because it can be a bastard to find them. Firstly try finding an wrinkles in the plastic that have been pushed into the tacky tape. Fine wrinkles can let enough air in to undo your pumps good work. Try stretching them out and pressing then harder into the tape. Sometimes the air rushing in is audible - a great help. The worst case is that there is a pin prick hole in there somewhere.

I found one hole that have been caused by the film being pulled down over a small piece of gravel that ended up on the table somehow. A wade of tacky tape did the trick.

Below is a shot of some of the excess resin I was referring to above being squeezed out the end and wicked up into the breather material. Amazingly, the separating action of the nylon meant that all this excess resin went into the breather material an almost none was left on the top surface of the board.

The photo below shows you the resin that rose up through the holes I drilled in the board.
Its a good idea to sand these with some 50 grit sandpaper before the top layers go on so that you get a good mechanical bond between the dry and wet resin.

So after 10 hours on the table, time for the great unveiling. .......

When I took the board of the table I was amazed at how little stiffness there was in the board when it had the bottom on it. I was a bit stiffer than the foam but still bent under its own weight. As Andres at Balmoral Boards pointed out its not until the top layer goes on that yo turn this single layer of fibre glass into the 'I' beam that has all the strength that composite structure delivers. And want a difference it made when the top when on. It went from bending under its own weight, to being able to support me standing in the middle of it while the tips spanned between 2 chairs.

Top Layer

Before you do the top layer make sure you fill the footstrap inserts ( the t-nuts) with candle wax to avoid resin going inside. Wax is a good idea as you can just hit it with a match and get rid once you've finished glassing the top and you drill out the holes.

I repeated the process to lay down the top layers of glass and this time mixed up 600gms total ( 500gms resin 100gms hardener) and this was just enough ( probably another 50 gms would have been a good idea).

Cloth down, resin down, work it around and in and get rid of all air bubbles in each layer before the next one goes on.ripstop nylon, breather cloth, vacuum film. Suck the air out.

A word of caution
I reused the vacuum film which turned out to a stupid idea. The tacky tape was so sticky that I wasn't able to get the film off without stretching it in places and as it turns out I must have put a hole in it somewhere ( which I never found). The net results ( after trying for about an hour to find and plug the hole) was that it continued to leak for the whole 8 hours it was under pressure and so the vacuum pump ran for u 8 hours straight. Fortunately it survived but the contrast with the bottom glassing where the pump engaged every 15-20 mins or so was staggering.

The finished product

Finally, after a nervous night of not knowing if I'd wake up to a blown vac pump and rockerless, concaveless board, it was all done.
I peeled off the film and material on top and the board remained stuck to the rocker table ( as you would expect. As I'd let it cure overnight the resins still slightly waxy which I took to mean it still had some time to go before it was completely cured. So I just left the board stuck to the table for the day and let it completely harden.

So then after, cleaning off the excess glass with the Dremel and using a surform to plane the excess resin back to near the rail (not back to expose the rails yet as I want to experiment with this a bit).... the near final product was ready!!!!!!

All that remains is to drill the fin holes, clear the wax out of the footstrap inserts and paint it....
Next blog will be a run down on what worked, what went wrong and what I'm going to try next time.......

Thursday, February 3, 2011

Preparing the core

So now its time to get into making some progress on the core.....

I traced the outline of the board (CrazyFly Raptor) onto butchers paper and cut it out. This made it an easy job to rescale the baord size by placing folds along the centre lines of the paper. I took 2cm off the length and 3cm off the width.

Remember to account for the width of the rails (10mm) and reduce the board core size accordingly.

I transferred this across to a plywood template of 1/2 the board. I made a half template to help make sure that I will get the board exactly symmetrical. A plywood template will also be more robust and reusable. In the BroKite video they make their templates out of 3/4" marine ply and cut out the entire board. This lets them run a router around the outside of the template and cut out the board and the inserts very precisely.

To make the upper deck of the board I used exactly the same template but shifted it in from the ends about 15cm and traced the top deck out in quarters using the same profile.

The long holes marked near the tips are where the reinforcement for the fins will go. These will be cut out and filled either with epoxy thickened with q-cell. The fins I measured up have a hole spacing of 47mm.

While fin insert locations look close to the tips, remember that the rail will add another 10mm.
Its useful to draw the center lines on the board to help with the later alignment on the rocker table.

You can then just cut the PVC foam out with a Stanley knife.
Working surface

I used waxed baking paper over the surface that I was working on to make sure any runaway resin didn't stick and it worked like a charm. I hammered nails into the surface when I was holding the rails in place while the glue cured so the surface needs to be pretty solid and expendable.
The Rails
As I mentioned earlier I made the rails out of 1/4" ABS plastic sheet. I traced the outline of the board onto the sheet and cut it out with a hacksaw blade.

This was a real pain in the arse to do. It was slow and ultimately the outline was shaky and so the rails' thickness waxes and wains along the length of the rail which makes it look like crap. On googling around it seems that what might have been a better idea would be to cut straight strips 10-12mm wide as you can do this with a jigsaw and a guide so that the width is perfect. Then flame the surfaces of the strips over a gas burner (stove or other exposed flame is fine).
This does 2 things. It oxidises the surface of the plastic which makes it a better surface for the epoxy to stick to and it softens it so that it can then be formed to the required curvature without the with thickness varying along the length. There is a brief snippet in the Brokite video of them doing this.
Again, I used the slow curing epoxy and thickened it with Qcell. This turned out to be trickier than I thought. In the first batch I added too much qcell and waited too long to start using it and so it went off in the pot. It went from workable liquid to a solid mass that couldn't be used in the space of about 2 mins (after having been mixed for 20mins or so). I'm guessing the the QCell might shorten the pot life as well?

Then, in the second batch I put too little in and a fair amount of it flowed away when it was put on the rail edge and board. In hind sight I think the best approach would be to get it to honey consistency and put it in a squeeze bottle (like a tomato sauce bottle) and squeeze it onto the rail. Using shorter cure time resin would be far better but I was no where near quick enough to use 5 minute epoxy which goes off in 2-3mins. Alternatively using polyester resin might work well as its typical curing time would be short enough but not too short.
When the glue was in place and the rails positioned I held everything together by using old kelgecell offcuts that I nailed in place to squeeze the rails in all the way around the board. and then placed weights around to stop it from bulging out. The idea of standing the nails off from the board is that if the nails were butted up against the rail then any resin that flowed away would then flow through the hole in the wax paper underneath and stick the board to the working surface.

Fin positions

Cut out the fin slots to get them ready for filling with epoxy. Firstly cover the top and bottom of the opening for the fins with masking tape. Then cut the tape away from the top side. This will keep the resin inside the fin support area and make removing the excess resin that will inevitably get on the board as easy as peeling the tape away.

I used the epoxy resin with about 20% (by volume) of QCell. It seems that one of the keys to getting this to work is to mix it really thoroughly and air on the side of adding a little more hardener than required. If it takes too long to start curing (10 min seems too much) the qcell separates and floats to the top of the resin which is not the desired outcome at all. Qcell apparently makes the resin a bit stronger but I did it mostly here to save using expensive resin.
!!!!!! After you pour the resin in make sure you give it a good stir to get any air bubbles out as these will weaken the cured resin.

Footstrap Inserts

I used 1/4" stainless steel T-nuts for the footstrap inserts and filled around them with 5 minute Araldite cause I didn't want to have to wait for too long to get things moving along.

The holes for these are made all the way through the board and then filled with resin. I just used a softdrink bottle top to trace out the hole size. I've made the distance between the instep bolts 18" and the spacing between the bolts on each foot are 6.25". Because I stand with my feet splayed out like a duck I've put more angle on each footpad. I put the instep bolt 3/8" above the centerline and the outside one 1" below. The other boards I've looked at have the instep bolt on the centreline.

You want the top of the t-nuts to be just below the surface of the board and you want the bolt when screwed all the way(through the footstaps and pads to not damage the setting by cracking the resin below the nut. To get the t-nut sitting in exactly the right spot I used the following approach - I thinned out a Klegecell offcut so that when the screw was passed through it into the t-nut and flush to the surface of the klegecell, the threaded end of the screw poked out a millimeter or so. The kelegcell piece cahn then be sat across the hole to make sure the t-nut is in exactly the right spot. The hole around the t-nut can then be filled with 5 minute epoxy and the screw/nut connected to the klegecell can be inserted into the epoxy. Leaving the screw in prevents epoxy getting inside the thread and making it difficult if not impossible to screw the bolt in.

Although I am probably going to use 12mm bolts in the final board, I worked with 16mm bolts so that I could make sure that there is enough of a gap between the end of the screw and the epoxy in the insert to avoid damage from over tightening or using long screws later.

Make sure to work the t-nut up and down to avoid any air bubbles being opened in the epoxy when you insert it. If air bubbles do open up it would be wise to grind the resin out from around the bubbles and re-fill them.

In the photo below I placed some baking paper under the klegecell but that was a bad move as it creased and then caused a crease to be set into the top surface of the insert. Its also not necessary as the klegecell is easy just to sand back once everything is set. Afterwards I topped it up with 5 more 5 min epoxy and sanded it flush with 50 grit sandpaper.

Filling around the edges of the top deck

As I understand it, fibre glass doesn't deal well with sharp corners and so its necessary to fill the sharp corener that exists between the edge of the top deck and the lower deck.

Apparently this is right up QCells alley. It needs to be thick like 'peanut butter' according to Trev so you can spread it on and it will fill out the sharp corner made by the join. I made mine too thin and so I didn't get the filling effect I was after (see the photo). So my plan is to redo this before glassing. I sanded off the edges to help.
Final core
So after a bit of sanding with 50 then 150 grit sandpaper and the dremel tool to clean the rails up here is the final core ready to be glassed. the rail thickness is still all over teh shop and so I will try to fix that after its been glasses as I may be able to use the liquid abs to fill out the rails a bit once. I haven't quite worked out how to do this yet but I've got Trev working on it.
At this stage the core wieghts 750gms. So with c. 1.3kg of resin plus the fiber glass the board is looking like it will come in at around 2.4kg before accessories. If I can hit this weight for the first attempt then I'll be pretty happy. I haven't tried my hand at vacuum bagging and fibreglassing before so its likely to come out a bit heavier. Time will tell.


Tuesday, February 1, 2011

The materials

So, time to commit some real $$$ and buy the material for the board. Everything I needed I was able to source at FGI and Dion Chemicals in Brookvale in Sydney. Here's the list:

-1/2 sheet of 6mm Klegecell (2.4 x 1.2 m) $100. Enough for 5 board sized pieces which when laminated (2 x 6mm layers) is enough for 2 boards + another board using a combination of foam and some other core material.
-2 litres epoxy resin + slow curing hardener (40 minute pot life) $70
-7m, 6 oz e-glass 900mm wide. This is enough for 3 layers each side including layers with weave running diagonally across the board $6/m
-750ml of Q-cell (from Dion Chemicals as they sold small quantities) for thickening the resin when using it for a filler.
- ABS plastic for rails. 10mm strip 2.5x the length of the board. This can be heated and bent into the shape of the rails.
- stainless steel T-nuts and 12mm screws . these are the nuts that will be glued into the board and the footstraps will bolt onto. 4 cost $8 from Bomond Trading, also in Brookvale. I saw on the Brokite video that they use flange nuts with ABS around them. The stainless T-nuts took a couple of weeks to get hold of because they weren't so common in stainless (mild steel are readily available). The flange nuts are also a dime a dozen and you can get them straight of the shelf as they are commonly available and look like they would do the job just as well.

Other accessories
-Standard Mold Release $26
-Tacky tap (for creating an air tight seal between the vac bag and the mold surface) $15. -Apparently can use window mastic from bunnings.
-Vac bag connector $13
-Vac bag film 1mx 3m $5/m
-A piece of 1.5mx0.9m rip stop nylon from an old kite. This is the peel ply that goes over the top layer of glass and allows excess resin to be wicked away.
-Polyester wading (using in making quilts) from Lincraft. This is used as bleeder/ breather cloth over the top of the peel ply and keeps the space above the board open enough to allow the air to be draw off. $15 3x2m piece.
- old credit card for wetting the glass out with resin
- coffee cups for mixing the resin in

Couple of things worth noting
ABS plastic - I found this a challenge as it is quite expensive $80 for piece about 1.5mx 0.8m ( an offcut from Allplastics here in sydney.) Expensive because you only use a small amount of it 10mm width around the perimeter of the board.

There are alternatives, namely, reinforcing the rails with carbon fibre matting or Kevlar (thin stripes would do) or using resin thickened with Q-cell and pouring it into a channel around the board that would act as a mold (and throw in some chopped fibre to reinforce the resin further). These other methods may end up being cheaper if your only going to build one board as you won't have all the excess plastic if you can't get a small enough off cut. Also, according to some designers, they allow you to do more with rails which some claim to be a very underestimated feature of a board.

Liquid ABS
I used ABS because I had some on hand but I made the mistake of cutting the rail shape directly out of the sheet rather than heating a straight strip and forming it around the edge of the board. This meant I had lots of offcuts left over which I thought meant were just going to be waisted. However, a friend mentioned to me that a friend of his who makes boards in Melbourne swears by using Liquid ABS to pour the rails.

A little googling turned up what Liquid ABS is. Its simply ABS plastic dissolved in acetone. I took the offcuts and cut them up into pill sized pellets and poured in the acetone and within about 20 mins it was like peanut butter and when left out for the acetone to evaporate off it reconstituted in solid ABS plastic what was indistinguishable for the original. Turns out this is a brilliant glue for ABS plastic parts and also also for casting to make ABS parts.

It seems like you could use it as a replacement for epoxy in the poured rails discussed above and I am going to have a go at using it to fill in around the t-nuts that footstrap bolts will screw into as well as the reinforced sections that the fins screws will pass through.

Cost per board

As you can see from above there will be a fair bit of excess material given the minimum quantities you can purchase. In particular the Klegecell, ABS plastic. If you pure calculate it on a per baord basis and ignore the minimum quantities its going to cost around $150 in material for the board. Footstraps and pads are extra.

You can also see that there is real economies of scale with the foam and the resin/hardner also gets much cheaper as you buy larger quantities. Also, starting with 12mm foam and carving it rather than laminating would also be cheaper as 12mm foam is less than 2x the cost of 6mm.

My plan is make at least 3 baords. Two will have 2 layers of 6mm foam and the 3rd will be made from 6mm marine plywood with large panels cut out and replaced with the foam. This one will be a light wind board like the Slingshot Glide.