A crochet pinwheel is a circular pattern where the "arms" of the design radiate out from a center point and curve, giving the illusion of motion. Many variations of this design exist, but the general technique is to cluster stitches together to form the arms (usually tall stitches such as double crochet), shift each cluster over one stitch in each new round to create the curve, and increase the number of stitches evenly in each round as the circle grows larger.
The Starting Round
Using worsted weight yarn and a G or H crochet hook, chain (ch) 6 and join with a slip stitch (sl st) in the first ch to form a loop. This is your starting center for the pinwheel.
Ch 1, then (single crochet (sc) in the ring and ch 2) five times. Sc in the ring once more, then make a half double crochet stitch (hdc) in the first sc to join the round.
Ch 3 to start the next round. A ch 3 acts as a starting double crochet (dc) stitch. Work 3 more dc in that same stitch as the starting chain to create a cluster. Then (ch 4 and work 4 dc in the next ch-2 space) five times. Finish by making a last ch 4 and joining with a sl st in the top of the starting ch-3 stitch.
Sl st into the next dc of round 2. This moves your cluster of dc stitches for this round over by one stitch. You will do this in every round from now on to create "steps" in the stitches that give the pinwheel arms their curve.
Ch 3 to start the dc stitches on this round. Dc in the next 2 dc and in the next ch stitch from the previous row. This gives you the 4 dc stitches for the arm like you had in the previous round. This time, however, you need to increase the circumference of the circle so it lays flat. So you will ch 5 between the dc stitch groups rather than only 4.
Work around the circle in this manner: ch 5, skip next three chains and 1 dc stitch on the next group, dc in next 3 dc and the first ch stitch of the next chain group. Ch 5 and join with sl st to finish the round.
Sl st into first dc, then ch 3 to begin the round. Dc in next 4 dc stitches and 1 ch stitch for 5 dc stitches total--one more than the previous round. You are now curving the arm of the pinwheel a little more. Ch 6 (one more than the previous round), skip the first dc of the next group, dc in next 5 stitches. Repeat this around the circle and end with a ch 6 and sl st to join and end the round.
You can continue in the pattern outlined in round four for as many times as you need to make the pinwheel as big as you want it. The basic principle is to sl st into the first stitch of the round to move over one stitch. Then ch 3 to begin the round, dc in the next number of stitches that is one more than the previous round. Then ch a number of stitches that is one more than the previous round. Work this pattern around the circle, ending with another chain and sl st to join with the first stitch.