Sewing a Beach Catamaran Trampoline (from Scratch)

A group of my friends all have a particular sort of beach catamaran - a Paper Tiger. It's a 1970's design and was quite popular here in New Zealand (and also Australia). So I kept my eye out for one on Facebook Marketplace and one day at lunch time one popped up for free! That weekend I borrowed one of my friends trailers and picked it up. I painted it up and sailed it for a season and had great fun! That winter I noticed that the main beam had corroded (stainless steel on aluminium) and it was a miracle it had made it through the summer. So I fixed that up and had another great summer out of it. But this summer the boats trampoline started to die. The edging started to pull away in a few places, and when sailing at Lake Rotoiti, a hole the size of someones ankle opened up. I tied it all together with paracord, but by the end of the season it was clear that something had to be done before I ended up in the water.

I wend to the local boat shop and asked where I could get the fabric for a boat trampoline and the guy looked at me like an alien. "There's a sailmaker in the port who can make you one" was his response. Online wasn't much help either. The most useful thing I found was this video by the manufacturer of industrial sewing machines. And that was it. Even here on instructables there is only one instructable about making a boat trampoline, and it's mostly how to stitch around an existing one.

So I set out to build myself a trampoline, from scratch, for minimal cost

Materials

1. Trampoline Fabric (from a local recycling center $40NZD - it looked like it had never been used!)
2. PVC cloth (from a billboard)
3. Bonded Polyester Thread (from Aliexpress $30NZD)
4. Stainless Steel Spur Grommets and tools (from Aliexpress $30NZD)

Tools

1. Scissors
2. Ruler, tape measure, marking pen
3. Strong sewing machine (doesn't need to be walking foot, though that helps)
4. Clothes pegs

I have a nice sewing machine. It's a lovely Bernina 830 record from my grandmother. It's a lovely machine, and I was hesitant to sew these super-thick plastic fabrics with it. I gave it a few test runs and it did OK, but it really wasn't that happy. It would have done the job, but I wasn't that enthusiastic. So I kept my eyes out on all the second hand places. A few weeks later an a friend rings me in the evening "There's an industrial sewing machine for free a 30 minute drive away."

So we hook up a trailer and zip on out. It's a Juki TR-7 that's been sitting in a barn for the past few decades. The Juki TR-7 is a real mystery machine. It's old enough and rare enough(?) that there's no manual online and only a few people even mention it. However, I think it's an amazing piece of engineering. It's so simple. There's like three gears and a bunch of bushings. And it uses a fascinating tension takeup wheel rather than the normal secondary oscillating arm.

So after getting it home and applying some oil to all the places I could think of, I try sewing with it. It stitches OK, but it's a clutch motor. This means that the motor always spins, and the foot pedal simply engages/disengages the machine with a clutch. The motor spins at 2800RPM. Each revolution makes the needle go up and down. With a 5mm stitch length, it works out to OMG Waaaayyy tooooo faaasstt. And the speed controllability is zero. It's either on or off.

Fortunately, sewing machine motors are fairly standard, so after sending a $235NZD to a local sewing supply importer, I have a lovely 550W brushless motor that can stitch from one stitch per second to incredulous speeds with perfect controllability, and enough low speed torque that it would probably make it through your finger. As a bonus, it lost at least 10kg. The old motor was probably close on 15kg, and the new one less than 3kg.

So for me, that was the first part done. I'm putting a bunch of photos here because it really is a beautiful machine.

If you don't have an industrial sewing machine, you can probably do it by hand-turning any machine (find the cheapest at your local junk store?), or just asking a friend who has one really nicely.

Picking the Thread

You want strong thread, and you want UV resistant thread. There are two things that meet this criteria "Bonded Polyester" is polyester with some sort of coating. "PTFE" thread is apparently even better, but it's also more expensive.

The thread thickness is also something to consider, There are a dozen ways for thread thickness to be measured. Tex, Deniers, mm. And then there's the number of strands.

Anyway, I ended up buying some Sanbest bonded polyester in 200D/3 size from Aliexpress. That means that it's made of three strands of 200D thread. Which means it's 600D all up (Denier is a unit of weight per meter). In theory this works out to Tex 60 or so, but I see on the spool it's labelled as #40. Who knows! But ideally it's UV resistant (time will tell), and it sure is strong. It'd cut into your skin before snapping.

I know I show white thread here, I bought a spool of white and black, and decided to set up the machine with white (easier to see against the black fabric), but sew the actual trampoline with black (hopefully more UV resistant)

Setting up the machine tension

This is a strange thread, and it took a while to adjust the tension of the machine for it. You see, the top and bottom of my machine have different mechanisms of adjusting the tension. The bobbin routes the thread around a sharp corner. The top squishes it between two places. The bonded polyester is quite low friction but is quite thick. So compared to normal thread the bottom tension was too tight (has to bend the thread) and the top tension too loose (slips through the plates easily). After a bunch of fiddling I managed to get it to put the stitch where it should be - in the middle of the fabric.

Do the tension adjustment on the actual trampoline fabric. I found it made a big difference too - trampoline fabric is a plastic with quite large holes, and it needed a different tension from cotton fabric, even with same thread.

Design

The paper tiger has a somewhat silly design by default. In the original design, the trampoline is laced to holes drilled through the deck. The deck overhangs the hulls by 30mm or so, and the original plans call for lots of holes down each side that you lace through. The problem is that the deck/hulls are wood, so they become a place that rots out. For that reason, it was soon replaced by.... a dozen eyelets each side screwed into the wood. Now it's at least harder for the water to get into the wood. But you're now expecting self-tapping screws in tension to keep you out of the sea. A few months after I got my boat I had some self-tappers pull out, so I went to the modern design, which is to have zero holes in the wood, and lace it only to the two beams - using a piece of dyneema or wire rope to hold the edges. I just laced my existing square trampoline to the edge ropes, but the modern design has a fitted trampoline with pockets that the ropes go through.

I decided to make my new trampoline the modern design.

So I looked at some modern boats, saw how they did all there lacing/pockets etc, and then decided to do my own thing. Your solution here will be boat specific, but some key points are that you need to reinforce any eyelets, think about where things will wear etc. etc. In my case, because the trampoline sides are held up by rope, I got to use my engineering knowledge and pull a catenary curve out of the toolbox. It's the shape a piece of rope will form when hung from two points. It's a bit like involute gears - all gears end up involute, even if they don't start that way. Similarly, the trampoline will end up with a catenary curve over time, but it would be nice if it started that way to reduce material stretch.

I did my design in FreeCAD so I could check clearance of hatches - and deal with the fact that the hulls taper slightly front-to-back and the beams being different lengths.

If you have an existing trampoline, probably just copy it. Don't forget to add seam allowance for pockets/hemming. You'll probably want every edge hemmed.

Also, think about the order you'll do the seams in if you have any pockets.

Transfer to some test fabric

I wanted to make a test to check that my design was good, so to transfer the design to the test fabric I used a projector. I hung my test fabric (a bed sheet) from the wall, and projected an image onto it. I zoomed the image until it's dimensions matched my CAD files, then traced around it with a sharpie. I cut it out, stitched it up, test fit it ... and, well, the fabric was stretchy enough it didn't really tell me much. But it at least looked like it would fit and I knew the order to sew everything in would work!

Transfer the design

Tracing the design onto the fabric was fiddly. The trampoline mat I had was not particularly flat, so I held it down with some (not quite heavy enough) boxes. I then meticulously marked it out. To keep things very square, I drew lines down the threads in the fabric, so I knew that it was exactly square relative to the original thread.

Sew

I have surprisingly few photos here, but it really wasn't hard. It's about the simplest sewing project you can do - it's mostly long straight (or gently curved) lines, just lots of them. I did at least 5 rows of stiching on all the load-bearing seams, spaced (mostly) 4-5mm apart. Managing this much fabric can be frustrating, but I found rolling it into a tube made it way easier to handle.

I found the fabric held a crease fairly well, so for doing the hems/edge pockets, I folded it strongly, creased it with my fingers, and then added clothes pegs to hold it in place (pins wont work on fabric this thick).

One thing I don't have a picture of is doing the edging with PVC cloth. But again, it really wasn't hard. I just cut a 10cm wide strip from some billboard cloth, pre-creasing it, holding it in place with more clothes pegs, and then stitching it.

My machine is not a walking foot sewing machine, and in a few places the PVC cloth did pull/pinch. But with a bit of management it wasn't too bad. I used a teflon foot to reduce the surface friction and I think this helped, but am really not sure.

I did a quick test fit on the boat, and decided on a grommet spacing. I then spent half an hour banging them in. No drama. A couple whacks on a punch to make a hole (and a sacrificial bamboo chopping board). A couple whacks on the anvil/die to set the gromet.

I did a couple tests with the gromets back at the design phase. I tried a few varieties (plain and spur) and tried loading them up. I tried different amounts of fabric to see if they would pull out of the fabric, and then eventually went "They'll probably be good enough". Getting stainless spur grommets is hard, and yes you do want spur grommets. My tests showed they had a much higher pull-out force.

Maybe if you have a dedicated supply store for something nearby you can find them, but I resorted to Aliexpress and just chancing it on them being any decent grade of stainless steel.

So yep, bang them in.

I did have to sharpen the punch for making holes. I think it was meant for leather (or something else softer than trampoline fabric). A few minutes with a cordless drill and some sandpaper put a fairly decent edge on it.

My boat has some blocks (pulleys for you landlubbers) that need to go through the trampoline. So after I had all the edging done, I mounted it back onto the boat, marked where they were, took it off, and edged those with PVC so the fabric wont tear at those points. This is the part that put the most load on the sewing machine. In places I was going through 6 layers of the PVC cloth (where the edging for the holes overlapped with the edging on the edge), but the sewing machine handled it like a champ. No drama, no struggling noises.

The trick here is shown in the video I linked at the start at about 6 minutes in - you cut a strip of PVC 10cm longer than the hole, fold it as you would edge it, and then cut slits. Then you can insert it into the hole so that it overlaps on all sides Now when you sew it down there aren't any weak spots.

If you are doing this with a weaker machine, this is the part you are most likely to have to do by hand, as it is where you will be going through the most layers. But compared to all the edging, it's not that much.

And there we have it - all done and ready for the next sailing season. I haven't taken it out on the water as there hasn't been any wind (most of the wind we get where I live is thermally generated, and so past autumn there isn't enough heat in the day to drive the wind) and it's cold even in the middle of the days now, but maybe I'll take it out for a gentle bob just to test it out.

Once I started sewing, this project took two weekends, and I reckon you could do it in a single Saturday if you didn't waste as much time overthinking it as I did.....

If I include the cost of the sewing machine motor, this project did cost a few hundred (New Zealand) dollars, so it wasn't particularly "cheap". But now I know that, even if this trampoline isn't perfect, I can make another one if something goes wrong.

There isn't any magic in a boat trampoline. The thread is just a heavy duty thread. The fabric can be found or sourced, the machine doesn't need to be a $4000 long-arm-walking-foot thing. It may not be the best "first sewing project" but I would say its simpler than making a T-Shirt as there are no fancy seams.