Flange mount blowers are generally mounted with the flanged discharge port fastened directly to a flat external surface to minimize required ducting. The inlet port can then be connected with duct hose to the area requiring ventilation. Length is the dimension along the fan motor axis (C in the diagram) Ignition protected - DC voltage models comply with USCG regulation 183.410 and ISO 8846 MARINE, as well as ISO 9097 MARINE DC motors also comply with BSEN 50081-2 for suppression of electro-magnetic interference, and are CE marked AC models are not ignition protected