The Mink Raft
Mink rafts and cage traps are provided by the Norfolk Mink Project to our volunteers. However if you wish to build your own or live outside the county of Norfolk this guide will help you through a step by step process from start to finish.
Our mink rafts are based on the game conservancy trust mink rafts, but we try to use recycled plastic stokbord when possible instead of plywood. Plywood can be used but we recommend treating it with a garden paint, such as Cuprinol garden shades or at minimum varnish the edges as otherwise the plywood will delaminate. DO NOT treat plywood with oil as this can contaminate the water.
Materials Required To Build Four Mink Rafts
It is more economical to make four rafts at once since you will get four base sheets out of one large polystyrene board. We have therefore provided a list of materials to make four mink rafts. Most of these can be purchased from hardware shops or building supplies merchants. We estimate an approximate cost of £150-200 to build four rafts.
2x 2440mm x 1220mm x 6mm Stokbord black
1x 2440mm x 1220mm x 9mm Stokbord black (will make 16 tunnel tops, alternative such as plywood can be used)
1x 50mm expanded polystyrene insulation boards 2400mm x 1200mm x50mm sheets
8x BZP eye bolts nuts & washers 152mm x 6mm
16x M6 x 70mm BZP roofing bolts & nuts (Screwfix or CPC), other suppliers might only do 80mm which is fine but they will need cutting down once fitted.
2x 3.6m x 22 x 200 sawn imported softwood red/whitewood FSC (with spare for one raft)
Spax 4CUT or Screwfix Turbo Ultra screws 4x40mm pk200 (or equivalent) if you can get a pk100 do that as only 32 needed
Spax 4CUT or Screwfix Turbo Ultra screws 4x25mm pk200 (or equivalent) if you can get a pk100 do that as only 64 needed
16x Corner brackets 1½” x 1½”
3x M6 large flat washers pk10 (24 needed)
4x 1m length of 12mm doweling
*Plastic barrier mesh or equivalent, usually comes in a roll of 100cm width (cut to size)
*Staples for staple gun (hand stapler works just as well)
*optional but recommended
4x 25cm handy basket (can be tricky to source, try school supplies shops. I purchase from www.medwaysplastics.co.uk in bulk).
4x Oasis florist foam, 25cm brick (local florists or Amazon)
School clay, comes in 12.5kg block, 650g per raft (local supplier or www.glsed.co.uk)
Kiln dried sand, comes in 25kg bag, 350g per raft
4x Machine turned round pointed posts 1.50m x 50mm p/treated HC4 green FSC
4x 6” nails
12 meter of 6mm diameter polypropylene rope, 3m per raft
How To Build a Mink Raft
Mink raft base
The base of the mink raft is made out of two 1220 x 610 x 6mm sheets of stokbord which sandwiches a sheet of 1200 x 600 x 50mm expanded polystyrene in the middle. The stokbord comes in a large sheet of 2440 x 1220 x 6mm which is then cut to the dimensions mentioned, and the polystyrene comes in a 2400 x 1200 x 50mm and again cut to the dimensions previously mentioned. Some suppliers may cut this to size before delivery so it is worth asking, this also makes it easier to deliver and handle. The picture below shows a sheet of stokbord cut into four pieces, do the same with the polystyrene but at the required dimensions. I recommend a circular saw for the stokboard as this will give you fairly straight cuts but a jigsaw can also be used. For the polystyrene you can use a sharp knife, hacksaw, fine tooth woodsaw or something similar. Personally I find a hacksaw blade in a handle works well for the polystyrene.
You now need to cut a 237 x 152mm rectangular hole for the tracking pad in the center of one of the stokboard sheets. Take one of the stokbord sheets and find the center, to do this I measure in 305mm on the short side, making some marks and 610mm on the long side again making some marks; now join the marks with a long straight edge to make a cross which will give you the center. From the center of the cross we can now measure out the hole for the tracking pad. The width is 76mm and the length is 118.5mm. Measure this out as in the picture and join it up. I find a chalk stick or a yellow/white colouring pencil good for marking marks on the stokbord.
You can also mark where you need to drill the holes for the bolts at this time; these are simply 2” in from the edge as shown in the pictures (for plywood we recommend 3” – 4” from the edge).
Now that you have marked out one sheet, you can take another sheet and clamp them together using spring or G clamps. I just use a couple of old chairs to rest them on so I have free cutting room below for my jigsaw. You could also use some blocks on a bench to raise it up and achieve the same thing. Now drill small holes in the corners and cut out the rectangle in the middle with a jigsaw. I find cutting two sheets (one raft) at once is best. Once the tracking pad hole has been cut out on one sheet you can use it as a template for the rest. Make sure to check that the basket for the tracking pad fits and does not drop out. If the hole has been cut slightly too big a strip of metal builders band can be used to make the hole smaller again ( e.g. Builders band – Screwfix ).
There is also an option to add a couple of finger holes when cutting out the tracking pad hole; this is to aid in pulling out the tracking pad when checking for footprints. The easiest option is to drill two large holes in two opposite corners and then cut out the tracking pad hole, I use a 19mm spade drill bit to do this but a similar size will be fine.
Now that you have cut the hole for the tracking pad, unclamp the two sheets of stokbord. Place a 1200 x 600 x 50mm sheet of polystyrene between them, making sure that they are roughly lined up and clamp them all together. Drill a 6mm hole through all three sheets at the positions marked earlier – 2” from the edge. Bolt them together using two eye bolts and four roofing bolts to form the base of the raft. The clamps can now be removed. (Note: Plywood rafts drill 3” – 4” away from the edge).
Use a 6mm drill bit and drill all the bolt holes.
You now need to cut a hole into the polystyrene for the tracking pad. I find a hacksaw blade works very well and gives a clean cut but other tools can be used as previously mentioned.
Turn the raft over and trim down the bolt threads to within a few mm of the nut using a grinder with a cutting disc. A hacksaw can also be used but will take a little bit longer. If you used 70mm roofing bolts you will not need to cut them down, you will only need to do the eye bolts.
All that is left for the base of the raft is to fit some mesh. This is optional but it does provide grip for an animal (e.g. mink) to climb up onto the raft. I use plastic barrier fencing mesh (green) 1m wide roll, but any similar plastic mesh will work. The approximate size is 50cm x 25cm but with the barrier mesh it is easier to count 3 holes down and cut across the 1m roll on the 4th hole down, leaving a strip with 3 clear holes. You then count in 12 holes and cut the strip in half on the 12th hole making two pieces of mesh for one raft, which can then be stapled onto the raft base with a staple gun.
The base of the mink raft is now done!
Mink raft tunnel
The tops for the tunnels are made from stokbord. This time though we use 9mm which are cut from a 2440 x 1220 x 9mm sheet. If you are only making a few rafts this might not be the best option as a sheet this size gives 16 tunnel tops. A similar thickness of plywood can be used but the dimensions should be the same, particularly the width as you may need to fit a trap inside.
The tunnel tops are 720 x 242mm, and as before use chalk or colour pencil to mark it out. The best layout to get the 16 tops is shown below. Number 16 might be a touch wider once cut but this isn’t a problem. This sheet can be tricky to handle so I recommend two people lift it into place for cutting. I use a circular saw but a jigsaw can be used although you might get a few wavy lines.
The sides for the tunnel are cut from a 3.6m length of pressure treated 22 x 200mm sawn softwood plank which is cut every 718mm (which allows for saw blade width). From one 3.6m length you will get five sides, so 2x 3.6m will do five rafts, leaving some spares. Once cut you can paint them with garden paint. This is optional as they are pressure treated but helps to darken them up so they are not as obvious. Fix the tunnel top to the sides with 8 x 4mm x 40mm screws, and two ‘L’ brackets on each side with 4mm x 25mm screws.
There are two main reasons to have the option of narrowing the tunnel entrance on a mink raft. One – if you are using a trap which does not have a fixed otter guard and two – some otters can dig out the clay in the tracking pad which means it needs topping up quite often.
This is where you can make use of the leftover squares of stokbord cut out of the centers of the mink raft base. The picture below shows the layout, which will give you 6 x 1in x 20cm strips of plastic, enough for 3 mink rafts. Once cut out measure in 10cm and mark the center point. Measure out 1.75in on both sides of the center mark and make a mark. Cut out two 12mm holes as shown (not the center mark), this will give you a gap of around 3 inches from the edge of one hole to the other.
Now take the strip of plastic with the two 12mm holes you have just cut out and place it on top of the tunnel. Line it up with the front edge and the inside edge of the tunnel. Use a sharp tool e.g. a sharp nail to mark the center of each hole on the tunnel top. Use the plastic strip as a template.
Drill two 12mm holes using the marks you have made on the tunnel top as guides. Repeat this process for both ends of the tunnel. Secure the strip of plastic with the two holes to the base of the raft between the tunnel sides, using two small screws (see picture). This also helps to prevent warping, making sure there is a 20 cm gap in the tunnel large enough to fit a mink trap. Cut some 12mm dowelling to fit into the holes, around 23-25cm in length. This will keep otters out but still allow mink, water voles and other small mammals through.
Take the 6mm polypropylene rope and cut a 3m length, remember to burn the ends to stop any fraying. Tie each end to the eye bolts on the raft using a bowline knot. The pictures below provide a how-to guide if you aren’t sure how to do this. If you cannot tie a bowline then use a knot you are confident with, which won’t come loose. Once complete, tie a simple loop in the center of the rope. I normally tie the loop once on site – in flowing water one side of the loop will need to be shorter to keep the raft parallel to the bank. This will be passed over a post hammered into the ground and used to secure the raft when it’s installed on-site. For each of the 4 x pointed mooring posts (1.50m x 50 mm) knock in a 6” nail approximately a foot from the at the blunt end (top) of the post. Blunt the point of the nail almost flat – this will stop the wood from splitting. Make sure the loop just fits through the post – it must not be too loose. It can also be a good idea to add a second rope to make sure the raft is secure.
There are four sections to cut from the main block to get it to fit in the handy basket. You will need a long sharp knife and possibly a ruler if you want to score the cuts first. The image shows the first two sections to cut, stand the block on its end and cut downwards, 10mm in on one side but pointing to the corner on the other side (so you are cutting a long wedge). The yellow line shows the cut for section 1 and the red line is the cut for section 2, once section 1 is cut turn it around to do section 2. This can be tricky to cut, keep an even pressure when cutting downwards.
Get sections 1 & 2 and trim them up for fitting into the basket. To do this I use the 1” piece as a guide and trim section 1 & 2 to length. Once that is complete just trim the height if needed.
Place section 1 into the basket like so and push it down snug, if it is sticking above the main block trim it down. Repeat the step for section 2 on the other side.
Use the 1” section that was cut off the end to make two small wedges (section 3 & 4) for the ends. Once cut place one in each end and trim if needed. These can also be tricky to cut; don’t push the knife too fast. If there are gaps in the corners, use any spare bits, trim and push in place.
Below is an image of how the GCT suggests to cut the block, I personally find this a bit trickier but it gives you another option. Here is also a link to their website with a guide of how they cut it – (www.gwct.org.uk).
Mixing the clay
The clay mix is made up of school clay & kiln dried sand. Weigh out 350g of sand and 650g of clay and put into a bucket or bowl for mixing. You will also need some water – I recommend using a mug of water and adding small amounts at a time until the consistency is right. Use your hands to make sure it is well mixed with a paste-like consistency – you do not want any hard clay lumps. Once mixed, place it on the foam in the basket and spread out to fill the basket. I use a paint scraper but anything similar can be used to level it out.
All the components are now complete for the mink raft and you are now ready to set it up and monitor for mink.