Making simple paddlewheels...

Cut some styrene sheet into strips 3mm wide and some 5mm wide, the size of wheel (and of the styrene sheet) will determine how many of each you will need. The wheels I'm making will be just over 100mm in diameter. You could buy styrene strips in suitable widths but it would cost more...

The 5mm strips are then cut to suitable lengths for 'spokes' for the wheel. 16 are required for each wheel if they are to have 8 floats...

You also need four centre hubs for each wheel. They can be circular or, in this case octagonal. I cut a 20mm wide strip, cut eight 20mm squares (two wheels) then cut/filed the corners to make 8 sided shapes... four of them are shown here;

Draw a pattern onto a sheet of paper, I used graph paper to get the 8 spokes but it could be done using plain paper and a protractor or 45 degree set square...

Use masking tape to attach the spokes for one half of the wheel to the pattern, glue one of the centre hubs in place with polystyrene cement and weight down with a heavy object until the adhesive sets...

Once it has set, remove the tape CAREFULLY and attach the other hub to the back, fix in place with modelling clamps or clothes pegs, use the pattern to assemble the other side of the wheel. Then make another two items for the second wheel...

Use the 3mm strip to construct the outer rim. It needs to be cut so two strips come together along the centre line of each spoke and do not extend past it like this...

I did this 'by eye' though you could mark it all out (taping the spokes to the pattern again would help too)

When you have all the pieces cut, they can be glued in place and clamped until set. Assemble all four wheel-halves.

The inner rim is made in exactly the same way but half-way between the hub and outer rim...

When it's all set, file or sand the ends of the spokes level with the outer rim and remove any excess cement/styrene from the joints...

ALTERNATIVELY - if you have a 'circle-cutter' (or a compass, a steady hand and lots of patience) you could make complete circular rims - in which case the 3mm strips are not required.

Find the centre of each wheel-half (I use a small watchmakers-type screwdriver) and drill to fit the driveshaft you intend to use...

Fixing them to the driveshaft! The easiest way is probably to just glue them on with araldite but it could be difficult to get them straight and equally spaced apart - and if the glue holds them firmly enough to the shaft, they'll most likely never be able to be removed. You could get them equally spaced by glueing a short length of styrene tube between the two halves using the same length for each wheel (a piece of biro ball-point pen casing would do) but they'll still be impossible to remove from the shaft.

A meccano wheel could be bolted to the inner half of the wheel (like the one shown next) which would make the wheel removable, in which case, make the centre hub the same size as the brass wheel (a plastic one with brass boss would be lighter and, if made from styrene, could be glued in place instead of using bolts - I don't have any of those so don't know if they are styrene or vinyl type plastic)

I'm going to use two of the brass collets you can buy from model shops - though these are old meccano ones. A length of styrene tube which fits closely over the collet is cut to the required length (in this case 35mm) and a hole drilled in the centre to take the screw that fits the collet (don't use the grub screw supplied) The collet is pushed inside and the screw inserted and screwed into the collet. Then a length of styrene tube that fits closely into the first one is glued inside up to the collet and sawn off (do this in both ends) Another still smaller tube is glued inside that and also sawn off, this is to give a larger surface to attach to the wheel halves. See the diagram on the right, and pictures of the actual components shown below. File/sand both ends flat and assemble on a length of shaft using polystyrene cement. I used a large pulley fixed on the shaft then the wheel parts (ensure the rims are on the outside of the spokes) then another large pulley 'free' on the shaft, a spring and a third pulley fixed on the shaft with the spring pushing the parts together.

Before the cement sets, check the spokes are aligned correctly and rotate one of the wheel halves as necessary. A length of rod (such as a cocktail stick or wooden skewer) can be placed into the wheel where a paddle float will go. Rotating the wheel, the rod should line up with the shaft. Try this in all positions on the wheel, if you assembled it accurately, they should all line up!

Once the glue is set, remove the whole wheel from the shaft :)

Cut strips to fit on the spokes between the inner and outer rims and cut into lengths to form the floats (need eight for each wheel) in this case they are 15mm wide and 40mm long. Glue four around each wheel and clamp in place to set, then fit the other four. The result should be two 'usable' wheels each with eight floats...

A little extra detail can be added by glueing 'slices' of hexagonal or square rod onto the hub (and if you want, the frames and/or the floats too) When painted they'll look like bolts...

I've just painted these in red oxide primer but they can of course be any colour you like, I may paint the floats grey to resemble wood, but for all-steel wheels they can be left as they are...

Making Round Rims...

Using a 'compass cutter' (available from most stationery or art supply shops) cut four discs from a sheet of styrene. These should be of a radius equal to the distance from the centre of the wheel to the outer 'points' of the octagonal rim ....

It's best to score it very lightly and go over the line several times (the tool is quite flexible and will drift off its line if the blade grips the styrene too well) then turn it over and score the other side. Pushing down on the centre spike of the tool will mark where the centre is on the other side... ...When both sides are done, gently bend the styrene and it will split to produce the disc. Lightly sand the outer edge making it smooth.

Then reduce the radius by around 3mm to 5mm and cut again to produce a ring. Make four of them.

To bend the styrene now when it's as little as 3mm wide, it will be neccessary to grip it in some type of pliers, use ones without any 'teeth' or grooves that would mark the surface of the styrene (I'm using pliers designed for adjusting relay contacts). Bend a little at a time until the ring can be removed from the disc.

Set the compass cutter to a radius equal to the distance from the centre to the outer 'points' of the inner frame and, as before, make four more rings...

Then use some of the scrap styrene to make the centre hubs for the wheels, you need eight!

Use these components instead of building the rims from straight pieces of styrene (and the 8 sided centre hub). I found it easier to align the parts by adding onto the graph paper, complete circles drawn with a compass and taping a drawing pin to the back with the point sticking through the paper and onto which the centre hubs can be placed (open out the holes in them with a 1/16th drill or very small 'watchmakers-type' screwdriver)