4. Hot wire cutting the foam blank

So we have superbly crafted templates for our board, and built a top notch hot wire cutter.  Now comes the first step in building the actual board.

4.1 Inspecting and preparing the EPS Foam

The EPS foam comes in a 8’x4’x2’ block that is unmanageable in the workshop.  The first step I take is to cut down this huge block into four 8’x2’x1’ blocks (actually I should point out that Combat Polystyrene did this for me, great service that saves a lot of hassle).  The reasons that I cut to this size are (i) I am not building any boards whose length is greater than 244cm, (ii) the boards aren’t wider that 60cm, and (iii) the rocker line and depth of the board means that it can be cut out of a block of 1’ depth.

If you do want to build boards that are long and wide, you may want to section the 8’x4’x2’ block in a different way.  Or, you can try gluing blocks of foam together, but this can lead to problems later.  Even if you use very light contact adhesive to do this, the hot wire cutter will not like cutting through the glue, and when you are sanding the glueline you tend to get high and low spots which can really test patience.  If you must glue blocks, the best glue I have found is Copydex (after trying all sorts of contact adhesive that melted the EPS!), but be prepared for trouble ahead.  Whatever you do, don’t use epoxy to glue the blocks!  Your hot wire is a knife through EPS butter, but will never melt epoxy.

The next step is to make sure that your block of foam is square on all edges.  If you don’t do this, you find that there are all manner of problems establishing a centreline on the board and mould later in the process, so its worth doing now.

If the block is square all round, cushtie.  If not, at least you know, and can make one side of the block a baseline for all measurements.

The easiest way to establish whether an edge is square is to use a dirty great set square (cheap at www.screwfix.com) as shown in the picture.  If you find an edge that is not square, mark it as such and move on.  If you find that you have no square edges, I’d seriously consider bollocking the supplier and sourcing some new blocks.

4.2 Attaching the profile templates

Now, the profile templates needs to be attached to the foam.  Remember to make sure that the templates have had drafting tape run around them to stop the hot wire sticking on splinters (which I forgot to do in the photo below, doh).

The trick here is to make sure that the templates are in exactly the same position on each side of the block.  The best way to do this is to attach one template to one side of the block using 4” wood screws in left hand hole at each 15cm station.   When you are positioning the first template there are a couple things to watch out for.  First, you want at least 2” of EPS foam between the bottom of the foam and the template. This is because the foam under the template acts as a mould later, and a mould held together with a wafer of foam in the middle is likely to break.  Second, remember that your hot wire cutter can only cut to a certain depth (mine cuts to about 10” I think).  So if the distance between the template and bottom/top of the foam block exceeds this depth, you won’t be able to cut in one go, which is what we want!  Be really careful screwing the screws in, and do it using a non-electric screwdriver – overtighten the screw and it just destroys the foam that’s holding the screw in place.  Now, measure the exact position of the nose and tail of the template and transfer these measurements to the other side of the block, accounting for any non-square edges.  Then, attach the remaining template again using 4” wood screws.

4.3 The first hot wire cuts 

Now we get to the exciting bit, the first real build piece, and the point where you get to see the rocker line of the new board.  Make sure the workshop is well ventilated (the fumes from the hot wire cutter are not good).

If you have never used a hot wire cutter before, it’s probably worth have a few trial runs on scrap foam before launching into the first proper cut.  It takes time to get the tension in the wire set up correctly, and also to play with the current through the wire so that you don’t cut too quickly or slowly.   The set up I use allows me to make a 2’ cut through a block of foam at a rate of about 0.5cm per second.  Any faster, and the heat in the wire tends to stretch the wire to breaking point.  Any slower and you’re getting grey hairs before the cut has finished.  I guess the optimal rate of cut is equipment dependent, but that seems to work for me.

There are two cuts to be made.  The first liberates the deck, and the second the hull.  Why do the deck first?  Because if are going to have any problems, you want to find them in the deck cut – it doesn’t matter so much if it goes wrong.  On the other hand, you want the hull cut to be perfect, because you need the foam that is cut away to be used as a bottom mould later.

Using the hot wire cutter to make 2’ cuts is a two-person job.  My wife tried it a couple of times and found that she got bored very quickly, and consequently lost concentration.  If the wire deviates from the template, you end up with ridges on the foam, not good news, especially if you also end up with grooves in the bottom mould as a result.  So I needed a steady, experienced hand: enter Rex.  Rex claims not to enjoy the hot wire experience, but really you can see that he is digging it.  He gets amply repaid when I have to do shitey jobs on his boat.

So, the block is set up, and the hot wire is cooking.  I haven’t got any photos of a cut in progress, but really, there is not much to tell.  You (and your partner on the other side of the block) slide the wire in at the nose end of the template, and very slowly follow the template to the tail of the board.  The keys to a good cut are to ensure that both ends of the wire are moving in parallel, and that you don’t force the wire.

If the wire does break half way through, it’s not the end of the world.  Cut the washers off the end of the wire, and give it a sharp tug to remove it from the foam.  Cut a new length of wire, attach the washers, and pull it through the foam (not attached to the frame or power source) so you get to the point at which you stopped cutting.  Reattached to the frame and power and off you go.

The feeling of lifting the top off the foam is marvellous – it’s the first time you get to see the curves of your board.  Look at Rex checking out the deck of the speed board.  He just knows that mother is going to fly.

Once you have completed the deck cut, keep the offcut slab on top of the deck and turn the whole block over.  Now for the hull cut.  Again, take it slowly and carefully and run the full length of the hull side of the profile template.  You now get your first look at the rocker line of you board.

This is where you can get to check the accuracy of your templates.  The speed board that I am building here has a completely flat section for the first 70cms or so from the tail.  The look of satisfaction on my face hasn’t come from substance abuse in the workshop, but is because the rule shows the hull to be absolutely perfectly flat.  Top job.

The first thing I do after making these first cuts is to mark a centreline on the hull and deck of the board, and also on the bottom mould.  Then, I mark the nose and tail points on the on the board, and also on the bottom mould.  It is good practice to mark up these reference points now, it becomes more difficult to do so later on.

The next job is to make the ‘V’ cuts. 

 

4.4 Making the ‘V’ cuts

If you were to take a jigsaw across most boards, between front and back foot straps, you would notice that the bottom of the board is not flat.  On most boards, the profile of the bottom of the board is ‘V’ shaped , especially in the section 1m from the tail.  Some boards do not have a ‘V’ cut hull, for example AHD have concave hulls, and the Starboard Hypersoinc is far from flat!.  But the majority do, and this is the hull design that will be considered in this board.

The degree of ‘V’ varies from board to board.  A high degree of ‘V’ gives directional stability, but is slow.  No ‘V’ gives a skittish fast ride.  There is no hard rule as to the optimal degree of ‘V’.  That said, it is unusual to find a board that has any ‘V’ in the front half of the board.  This area tends to be flat.

In considering the amount of ‘V’ for our board, we are restricted by the process that is used to make the ‘V’ cut.  In essence this involves moving the profile templates in such a way as to take a wedge of foam from the bottom of the board.  The foam that is taken off the board has to come off in a single piece, as it needs to be added to the bottom mould. 

We need to reset the profile templates to make a ‘V’ cut.  To do this, take out all the wood screws, with the exception of the screw in the 135cm station, and the one right at the tail of the board.  Then, carefully take out the tail screw, and drop the tail end of the template by 6-10mm.  The screw at the 135cm station acts as a pivot for the template. 

How far to drop the template depends on the width of the foam blank, and how much ‘V’ you want.  I always make a spreadsheet that compares the ‘V’ in the donar board with the ‘V’ that would be cut by dropping the board by ‘x’ mm.  By varying ‘x’, you soon find the optimal drop for the tail of the template. 

Now we see the reason for drilling pairs of holes in the profile template.  When screwing the template back into the foam, use the hole that was not used first time.  This way, you are screwing into fresh foam, and not trying to reuse a hole.

Remember, only drop one template to start with – we need to make a cut before dropping the second template.   Now, break out the cheapest drafting tape that you have, and run several strips of tape from the tail to nose of the board, on the side that is not going to be cut.  One strip must run along the centreline of the board, as shown in the picture. 

Now run the hot wire cutter along the drafting tape and the top of the template, taking a wedge of foam.  Start from the tail of the board.  When you get close to the 135 station, the wedge gets very thin, and the wire starts to skip out of the foam.  If this happens, it’s time to stop the cut.  If you continue, the wire will cut in and out of the foam leaving an unpleasant uneven surface.  Once you have finishing cutting, be sure to keep hold of the wedge of foam and stick it to the bottom mould using cheap drafting tape.  Note how I have labelled the wedge I’ve cut here with a tag of ‘RHS’ to make sure I put this wedge on the left hand side of the bottom mould (RHS of hull = LHS of bottom mould).

Once the first wedge is removed, drop the other profile template by you chosen amount, put drafting tape down the other side of the board with one piece on the centreline, and then make your second ‘V’ cut.

The hull of the board has now been shaped, but the board is still looking somewhat rectangular in shape.  The next stage is to cut out the plan shape of the board.

4.5 Cutting the Plan Shape of the Board

This stage is just great – it’s when you really start to get excited about the new board.

First of all, spend some time marking out the board;

  1. Make sure a centreline is marked on the top and bottom of the board. 
  2. Mark up your 15cm stations across the full width of the board, on both the deck and hull.
  3. Now, use your design sheet and plan shape template to mark out the bottom of the board on the hull of the foam blank.
  4. This only gives you the outline of the bottom rail of the board.  The board is actually slightly wider than this, as we have not included an allowance for the additional width of the rails.  Using the rail profiles on the design sheet, work out the additional width of the board at each 15cm station.  On a freeride board this will be something like 14mm at the centre of the board.  Mark out the rail widths at each station, and then use the plan shape template to draw the true outline of the board.  In the picture you can just make out that I have used a green marker for the hull outline, and then a red marker to add the additional width for the rails.

Now take your small hot wire cutter, and set the wire to cut at 90 degrees.  Then cut around the plan shape of the board, leaving a tolerance of 5mm between your cut and the outline of the board.  Don’t be tempted to cut too close!

The board should now look something like this.  It’s worth weighing the board at this point, as you get an indication of the volume of the board.  The speed board weighed in 700g, giving the board as it stands 77ltrs of volume – but more foam has to come off and a laminate has to go on, so its only indicative.