A sheepshank knot can shorten a rope while you are a kayaking. It is not a strong know but can come in handy if you need to take up slack within your rope.
Step 1: Create a large Z within the middle of your rope.
Step 2: At the end of each side of the Z create a small loop facing inwards.
Step 3: Take the lower portion of the Z and bring it through the left loop.
Step 4: Take the upper portion of the Z and bring it through the right loop.