Making and mounting a plumbing pipe tunnel fin

Tunnel fins are a great way of supercharging a board, and can be added to any suitable board quite easily. The look of a tunnel grows on you once you ride one !All the tunnel fins below were made from plumbing pipe:Ok here's the low down on it.* Six inch plumbing pipe is good, cut it in half or slightly more than half.* Cut a 3 inch to 4 inch length of pipe.* The tunnel should be mounted directly behind the centre fin, with a gap of about an inch between the two at the closest point* The pipe must be foiled very carefully, just as all fins must be foiled very carefully. Foiling can be done on both sides or on one side only, both ways work very well. Because plumbing pipe is thin it is easiest to foil it on one side only (just on the outside ) but the inside should be made smooth and fair, and there should be no sharp edge at the leading edge .* Foiling plumbing pipe is easy enough however the plastic is quite resilient so sharp paper is needed, start with 60 to 80 grit sanding fore and aft and work down through the grits to 400 or 600 grit. if the trailing edge becomes sharp then once foiling is finished sand a flat surface on it about 1mm across, this is faster.*Mounting the tunnel: with a tunnel foiled on the outside a slight angle of apparent lift needs to be added as if you set it up parallel the fin will actually be angling slightly downwards which isn't what we want. We want the actual flow to either run parallel to the bottom or slightly downwards to give some positive lift against the bottom.So, the bottom line is that for the fin foiled on the outside only the middle or apex of the trailing edge should be about 2mm further from the bottom of the board than the apex of the leading edge, this is a reliable place to start. Giving another 1mm to 2mm of lift ( 3 to 4mm in total )will spice things up considerably due to the greater lift angle, this is all good especially if the board has a lot of tail rocker.If you are foiling both sides ( inside and out )then the starting point is parallel to the bottom and dialling it up say 2mm from there.* attaching the fin: because the fin shape is so stable it doesn't need a lot of attachment strength. If you have a good fit then a light sanding with fairly fine sandpaper and a few drops of epoxy ( even araldite in a tube or something like that ) will do nicely. make sure that the join to the board is nice and smooth, no big fillet is required, just a tiny bead around the fin base.Alternatively make a tunnel for fcs tabs:*Best to go for just the centre fin and take the side fins off, re. fin position probably whatever position you are using already. The tunnels work well with side fins also but probably your side boxes will have too much toe in , you could try it though but most likely it will feel best with the main fin.PS It's easiest to do the basic foiling of the pipe and then cut it in half as then you have something to hang on to, fine finishing can be done after cutting the pipe in half. As mentioned before slightly more than half a pipe is good, with a 6 inch diameter pipe use about an extra inch over the halfway mark.