Originally Posted by Delta-T
A squirrel cage fan is much more efficient than a prop fan for moving air. With small hoses a high static pressure is needed to move air. Hard to achieve with a prop fan. The fan you remove should have specs on it or you should be able to look them up online. Specs with have a rating of CFM (cubic feet per minute) at like 0.25" of static pressure. If you are going to use a different fan make sure the specs match.
Each 90 deg bend in the hose is like adding 6' of straight hose in back pressure (static pressure). Try not to add any additional bends to the new setup. And the wire reinforced plastic hose is the worst hose for air flow. I know there are little to no alternatives to use and it is used in almost every boat. And with out equalization there can not be air movement. Meaning you need a way for the air to enter into the compartment to be able to remove air. So the fans being used for moving said air needs to have a good static pressure rating. I just looked up a fan for this use and there was no SP rating, only CFM. Be careful a 250 CFM rated fan means with no load on it (no static pressure), unless otherwise stated.
As I "study" this installation it seems a little shaky in terms of careful engineering specs. No reference to SP in the blower specs seems like an important oversight? It was built in 1976. "Its good enough" doesn't meet my requirements. It has lasted almost 40 years this way?
There are two shell vents on the stern. One connected to the blower discharge and the other one to a piece of duct hose that opens into the engine compartment near the stuffing box. I understand that one is the inlet and should reduce SP. It includes that 90 degree elbow at the vent (that is two) and the wire reinforced plastic hose. It would be interesting to figure out the actual cfm being moved by this system. I am guessing that the blower's capacity is used to establish the "actual" volume of air moved. It must be a fraction of the blower's capacity. Wire reinforced plastic ducting, restrictive shell vents, general configuration of the engine space and connecting lazarette, ??? It looks like a crap shoot to start with?