it is a 4" Rule, my engine is a Yanmar 4JHE. I have it wired by way of relay to the engine panel so when the engine key is on, the blower is on. I can stop it with an on/off toggle if I choose. I do it for a couple reasons ... to supply fresh air to the engine, which it needs. To keep the engine compartment a bit cooler. I have the intake hose right next to the HO Alternator to pull heat away from that so my temp sensor doesn't shut down the charging. And, I have the exhaust exit out in the cockpit by the aft lazerette, protected by a hood, near my feet when at the Helm, and this comes in handy when it's chilly, or during overnights when we want a bit of warmth.

cheers, John