Hi Pete,
Welcome to Affinity Forums :)
Not entirely sure if this is what you are looking for but here's a sample file showing two possible solutions (each one inside its own layer - only the first is visible when you open the document):
shape_sample.afdesign
1. Erase Blend Mode:
- go to Document Setup, Colour section and check Transparent background
- draw a white rectangle with the same size as the document (to fill the background)
- place your turbine outline (closed shape) layer on top of all the others in the Layers panel and set the Blend Mode (the dropdown on the top of the Layers panel) to Erase
- export the file as PNG
2. Compound object
- go to Document Setup, Colour section and check Transparent background
- draw a white rectangle with the same size as the document (to fill the background)
- place your turbine outline (closed shape) layer - i've used a cog shape as an example - on top of the white rectangle layer in the Layers panel
- select both the rectangle and the turbine shape, press and hold ⌥ (option/alt) and click on the Subtract icon on the main toolbar to "cut" the turbine shape out of the rectangle.
- export the file as PNG
