i am trying to understand why the Mandurah swell chart is so much less than Rotto. I can understand that the Cott buoy is inside the reef/Gage Roads harbour, so the swell is accurately less, but why is Mandurah typically less (like, measuring as mid-way between Rotto and Cott). Perhaps more specifically, is the Mandurah buoy an accurate indicator of the swell at Gearies?

Mandurah swell is definitely affected by the 5 fathom bank
As for the readings:
Rottnest Buoy seems to be located west of the West End meaning nothing is between it and the southern ocean
www.transport.wa.gov.au/imarine/rottnest-island-tide-and-wave.asp

Mandurah Buoy is right up against the Fathom Bank reefs but still fairly offshore so at a guess, the more south the swell direction the more influence Cape Bouvarde and the outer reefs will have on the reading.
What ever the case, i think the swell at Avalon or Gearies will be less than the reading at the buoy.

www.transport.wa.gov.au/imarine/mandurah-tide-and-wave.asp

www.transport.wa.gov.au/imarine/coastaldata/nauticalcharts/pdfs/WA913_cape_peron_to_dawesville.pdf

Great info - thanks