John Owen constructs a versatile and economical boat tent from a polytunnel
After acquiring a Moody 33 MK1 in need of a total restoration, my friend and I needed somewhere to carry out the work. The boat was moved to a boatyard that could provide the space we needed, so we turned our attention to constructing a cover to keep the weather out. Initially we considered building a timber frame, but then we had the idea of using a polytunnel.
We searched online and found that the latter option would be substantially cheaper, and duly purchased a 12m x 5m frame for £100 from an internet auction site.
We collected the frame and set about adjusting it to our needs, starting with extending the legs of the frame. We did this using 2m-long scaffolding poles: to connect the polytunnel hoops to the scaffolding we used a short piece of tube that would fit inside both the polytunnel tubes and the scaffolding tubes. It was necessary to bend the connecting tubing by 10° as the polytunnel hoops were not a complete semi-circle. The polytunnel came with ground tubes, which we didn’t need, although they did have stop collars which we repurposed for use on the connecting tubes.
We were now ready to erect the frame, but where to start? Normally when erecting a polytunnel it’s easiest to work from one end to the other, but this one would be 2m above the ground with the four middle hoops over the boat and the two ends extending past it. For this reason we started at the centre, hoops three and four, and worked outwards. We fixed the two scaffolding legs into position on each side and connected them together horizontally with more scaffolding tube. We also used more scaffolding tubes as temporary supports while we prepared the polytunnel hoops.
The hoops were assembled on the ground before being passed up onto the boat. Then they were positioned over the legs: with one of us on the boat, holding the hoop in place, the other could fit the connecting tubes in each end and fix it to the top of the legs. The first hoop we erected had to be held up temporarily with a couple of guy ropes while we put the next one up and connected them together with the ridge pole. We also tied the hoops to the toerail of the boat to further stabilise the structure. With the two middle frames in place we then worked our way to the ends, putting up the remaining hoops. With the end hoops not being over the boat we used a ratchet strap to hold them to the correct width.
It’s curtains for you
Finally, with the frame assembled and in place it needed to be fully covered. We tackled the sides first: a friend who worked for a used commercial vehicle dealer gave us the idea of using the curtains from the sides of lorries. When ownership of a lorry changes, the curtains bearing the previous owner’s logo are replaced, and this dealer was willing for us to take some away rather than disposing of them.
The curtains needed to be on a track so we could open and close the sides. The curtains were 450mm longer than the legs and needed to hang vertically, so we had to find a way to attach the track to the hoops. We cut holes the size of the tubes in blocks of wood then cut the block in two, through the hole, to leave a semi-circle in each piece. We then attached a plate to the longer piece of wood and bolted them back together to clamp around the pole, as shown above. The plate was used to attach both the top cover and the track. We bought the tracking from a local supplier of body parts for commercial vehicles.
The corners presented more of a challenge as they needed to allow fixings on the side and the ends. These corner brackets were made from 75mm box section steel, 150mm long. From halfway along two adjacent sides were cut away, and what was removed was welded to the remaining two sides to create fixing tabs. Two holes were then drilled in the corner of the remaining box section to allow it to be clamped to the tube as shown. A block of wood and a plate were bolted to the side to match up with the other side brackets, and an extension plate bolted onto the other side for the end of the tunnel.
The top cover
When the frame was erected we shortened the polytunnel a little and it was now only 11.4m long. The distance over the hoop from fixing point to fixing point was 6.6m. We had decided that the best way to fix the top cover would be to roll it around a beam the same length as the frame, and this would use up 600mm of cover on each side. We also wanted to leave a small pelmet of 100mm to cover the curtain track, so we therefore needed a cover for the roof of 12m x 8m. After conducting an internet search we found a 12m x 8m reinforced clear PE tarpaulin of 200gm per square metre at a reasonable price: the supplier also sold an 8m x 5m sheet which we needed for the ends of the tunnel.
To fix the sides we used a 75mm x 50mm timber beam made by joining three 4m lengths together with glued and screwed scarf joints to create a beam 11.4m long. These beams were temporarily fixed to the side brackets with G-clamps. We draped the cover over the top of the frame and held it in place with speed clamps. The cover was then marked at 425mm from the edge, as we had calculated that was where the bottom edge of the beam needed to be when the cover was fixed. To fix the cover we first held it in place with staples and then screwed clamping strips of 75mm x 25mm x 1.5m timber over the top. The beam was rotated 360° so that the cover was completely wrapped around it, then it was screwed to the bracket to fix it in place. The diagram below shows the cover as the red and blue lines: the red is the cover above the beam over the top, and the blue is the cover below the beam that leaves the 100mm pelmet.
The process was then repeated on the other side. To tension the cover over the top of the polytunnel the clamping brackets were slackened off and pulled down on one side and then the other.
Filling in the ends
We needed the ends of the tunnel to be removable in the summer and in place for the winter. We used the 8m x 5m sheet and cut 3m off each end: this gave us two 5m x 3m sheets each with a cut edge and three edges with eyelets for fixing. We used the same method as with the top cover, ie fixing of the sheet was the same as the sides, with a clamping strip and a turn around the beam. Once this was done, three eye bolts were fixed through the beam to provide fixings for lines that would hold the ends of the top cover down. As it was late spring, and we would not need the ends filled in, the cover was rolled up and lashed to the top of the beam.
Fitting the curtain sides
We were given the details of a local commercial vehicle body parts factor, who sold replacement track for curtain sides at £5 per metre + VAT. We needed 11m for each side and 5m for each end, a total of 32m. The track came in 3m and 4m lengths, so we bought four of the 3m lengths and five 4m lengths. We used one 3m and two 4m lengths on each side, and one 3m and half of a 4m length on each end. The track was fitted by simply screwing it to the underside of the beams.
The curtains we decided to use were a little tall for our needs, so the rollers were removed from the top and the curtains shortened by 200mm, then the rollers reattached. The curtains were fed onto the tracks with the buckles on the inside, the first one going across the front and partway down the side. The second one was cut to fit the remaining side. The third one went across the back and partway down the other side, with a final one finishing the side to the front corner. Once all of the curtains were in place a hole was drilled in the side of each track, at the ends, and a small bolt put in place to stop the curtains from coming off.
We needed to hold the bottom of the curtains down, and fortunately had just replaced the guardwires on the boat so we were able to recycle the old wires for this purpose. We attached the wires to the base of the scaffolding poles all around the frame, using an additional two eyes across the ends cemented into the ground where there weren’t any poles. Once all the wires were in place they were tensioned with bottlescrews in two of the corners. The curtains still had the buckles attached, and they hooked underneath the wires and were fastened. At the ends, the eyes had rings attached, and we squashed these slightly so that they would fit into the buckles.
To finish securing the curtains where they overlapped, we used short bungee cords. We attached eyelets to the bottom of the curtains and hooked the bungees to them. At the top, we hooked the cords to the roller frame on the adjacent strap.
The polytunnel in use
All in all, the polytunnel boat tent has worked well, and the curtains are great. We only need to open up a corner for access in the winter, but in the summer we can open both ends and the sides to help keep it cool. The covers for the ends went up easily when they were needed and, with a bit of sun, it gets warm enough inside even in the middle of winter to allow us to carry on working.
There have been a couple of issues with the top cover that, with a little more knowledge of polytunnels, could have been avoided. Firstly, polytunnel frames get hot in the sun, and this can cause a deterioration of the cover. A foam tape, called anti-hotspot tape, is applied to the tubes to insulate them from the cover. Our frame had most of its anti-hotspot tape still in place, but unbeknownst to us the covering layer had come off, and it is this thin film which prevents a reaction taking place between the foam and the covering sheet. We discovered this on a wet autumn day when we noticed some drops of water on a couple of the frames: closer inspection revealed some small holes in the sheet where it was in contact with the tube. An internet search revealed both the cause of the problem and the solution – an anti-hotspot repair tape that could be placed over the existing anti-hotspot tape. The same supplier also sold polytunnel repair tape in
To make the repair, we loosened the brackets on one side, allowing us to push up the side beam and thereby slacken the entire cover. This enabled us to first fit the anti-hotspot repair tape and then a strip of 50mm polytunnel repair tape to the inside of the cover. We also cut some 150mm strips from the remaining bit of cover we had left and fitted these between the hoops and cover. On the outside, we applied a strip of 75mm repair tape over the area it had been leaking from. The repairs have been fine throughout the winter, and the supply of repair tape has been useful for a few minor repairs.
The second issue we have had is condensation on the inside of the top cover. We didn’t have any problems until we put the end covers up, in the middle of October. A few weeks later we opened up as usual and got dripped on: the inside of the top was covered in water droplets. We have alleviated the problem by fixing a second lightweight polythene sheet on the inside with a small gap between this and the outer sheet: this catches the drips and directs them down the edge of the tent away from the boat and us.
The only thing I might consider changing is the top sheet. The one we’ve used is fine, and with a little TLC and some repair tape should see us through to the end of the restoration of the boat: but, as stated earlier, it is 200g/sq m. We have since found out that the sheeting normally used on commercial polytunnels is a lot heavier at 700 to 800g/sq m and has a life expectancy of five years.
What it cost:
Polytunnel frame £100
Scaffolding tube £100
Tarpaulin sheets £170
Timber for the side/end beams £50
Track for the curtain sides £200
Other bits and pieces £50
We already had some items, for example the scaffolding joints, but even with everything added in the whole structure has cost us less than £700. To my mind it has been well worth it for the comfort it has provided when it has been pouring with rain outside and throughout the cold weather. We were able to work throughout the winter, so our estimate of two years to complete the restoration remains achievable.