Festo's Fin-Ray structure is a beautiful and surprising thing. You could just build the fin if you don't fancy going the whole hog. If you're up for a longer, more involved build, follow the full instructions and hey Festo, you’ll have a motorboat fit for your bath or garden pond.
This kit requires the use of a sharp object, always be careful when using sharp objects!
Cut out all the printed parts below (they're all numbered so you won't get mixed up!). Use watered-down PVA glue to stick the paper parts so you can easily cut the pieces to shape. Take care to get the right material for each part…
1Expanded polystyrene. You need a piece about 2cm thick, or build up the thickness by hot-gluing sheets together. Don't use too much hot glue too quickly, you'll melt the material!
22 12mm balsa wood. If your sheet is a bit thicker or thinner it doesn't matter.
66 Create a block of balsa by joining pieces together. Block needs to be 27mm x 26mm x 26mm.
77 Balsa wood. If you've got a 14mm square section, use that. Otherwise create something similar. The grain should run along the long edge of the block.
88 Stiff plastic from a CD.
99 Stiff plastic from a CD.
1010-15a Stiff plastic from a CD.
1110-15b Flexible plastic like acetate.
1216 Plastic from a milk container.
1317 Stiff plastic from a CD.
1418 Thin acetate.
2When all the parts are cut…
Wallace's Safety Tip!
"Go careful when you're cutting CDs chuck as they can splinter."
When all the parts are cut, remove the paper prints (except parts 2, 3 and 16). As you remove the paper from the fin parts (numbers 10-15, a and b), write their numbers on with a permanent pen so they don't get mixed up.
1Take part 10a and fold part 10b around it, folding on the dotted lines.
2Hot-glue them together – don't use too much glue as these parts have to be accurate or the fin won't work. Go careful not to burn yourself too!
3Repeat for parts 11-15.
4Take part 16 and place it paper-side-down on a table. You should be able to see the dotted lines through the paper. If not, transfer the positions of the dots to the plastic side with a permanent pen.
5Now take the constructed part 10 and put a little hot-glue on the side of the hinge. Glue it where the first dotted line indicates. Remember if you use too much hot glue on the acetate, it'll melt and you'll have to remake the part.
6Add the other parts in order, just like the first. There should be one dotted line left at the end (where it says "glue" on the paper).
7Now join the other side, part 16, to the first. Start at the wide end and work towards the narrow. This is a little fiddly so take your time! When you get to the end, glue the tip together. Trust where the marks are, even if they look wrong!
The fin is complete!
If all you're making is the Fin-Ray, you're finished! Even if you're going on to make the full boat, have a quick play with the fin now because it'll be attached to the boat soon. Try gently squeezing it and twisting it – it should curve nicely.
4The fin drive mechanism…
1Take part 4 and glue part 6 to it. It goes at the back, right in the middle. The wood grain should run front-to-back, not side-to-side.
2Take a skewer and punch a hole it needs to be dead-centre of the end of part 7. Go all the way through.
3Push the skewer through the hole in part 4 so they're in line. Hot-glue together. Before it sets, check the skewer is vertical. When it sets, strengthen the base with more hot glue.
4Glue both parts 5 to the front to form a wedge shape.
5Glue part 18 into the gap between parts 6 and 7. This forms a smooth low friction surface for the fin to run on later.
7Glue a skewer onto part 8 as indicated by the dotted line. These need to be secure. Do the same for the other part 8, but on the other side as shown.
8Glue parts 8 to the sides of the fin-ray structure. It should overlap the sides between the first and second cross-spurs.
9Place the fin in position. Add the triangular part 17 on top of it. If you've cut the holes carefully, they should all line up. The triangle points towards the fin, not the front.
10Wiggle the triangular piece left/right. The fin should bend and flex neatly.
11Check the slot in the triangle piece. It should be smooth edged, so a skewer will slide smoothly along both walls. If it doesn't, work it with a craft knife until it's smooth. Keep all your fingers above the blade!
12Trim the skewers so they only stick up about 1.5cm.
13Add the plastic washers (part 9) to each of the skewers. Hot-glue each to the skewer, but not the part that rotates. And check everything still moves freely.
5Make the boat deck…
1Test-fit the boat's deck (part 2) onto the drive mechanism. The skewers should all fit through holes in the deck. If you look down through the large hole, you should be able to see the slot of the triangular piece.
2Hot-glue the deck into place once you're happy!
Build the motor gearbox. If yours is the same as ours, build it up as shown. If not, you'll need to come up with something close.
Make the cam by inserting a skewer through the holes in the cogs. We've used an extra cog and hot glue to help hold it in place. It'll take quite a lot of force, so make sure it's right!
5Test-fit the motor and cam in place with part 3. Try turning the motor. It should make a complete turn without snagging or feeling stiff. Adjust if you need to.
6When you're happy, glue part 3 in place, and then the motor. Make sure everything is still turning freely while the glue sets.
6Attach the motor and electrics
1Solder a wire from the battery holder to the motor terminal as shown.
2Solder a wire from the other motor terminal to the middle connector on the switch as shown.
3Solder a wire from one of the edge connectors on the switch, to the free wire on the battery holder as shown. Add batteries and test. If the motor doesn't run, it's better to fix it now than later!
4Glue the switch and battery pack in place.
5Turn the motor on and see if everything works.
7The fin-al bit!
Finally! Test-fit the float. When you're happy the motor will run with it in place, glue around the edge with hot-glue. Don't use too much at once, or you'll melt the polystyrene!
Fill a bath with cold water – 10cm should be deep enough.
Carefully place your Fin-Ray Motorboat into the water. The fin should be under the water; the wooden deck should be above it. If the fin hasn't sunk far enough, add a little weight to the boat by blu-tacking a couple of coins onto it.
Now for its maiden voyage! Turn the boat on and watch it race! Alright… crawl.
Does it turn left/right too much? Does the nose wobble too much? If so, try adding a keel at the front. This is just a piece of wood sticking down from the nose. Dig a channel in the polystyrene and affix it with hot glue. Now try another voyage!
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