Motor Compartment Cooling System

The information provided is for a 2002 Duffy 18' Classic. Many systems and components are common across models and years.
It is up to you to determine if this information applies to your boat or not.

Design

Air is mechanically circulated from inside the cabin of the boat, into the motor compartment and out through the external vent. It is controlled by the 12V system master key switch, not a temperature switch.

The external vent is on the bow on our boat.

On newer 18' Duffy boats, the external vent is located on the stern. I assume that it operates in the same way.

A blower and a fan circulate air while the key switch is turned on.

The blower is attached to the vent on the bow. It pulls air from the bottom of the motor compartment, through a 3" duct in the bilge, and exhausts it out of the exterior vent.
The fan on the front of the console above the motor compartment pulls air from the cabin into the motor compartment.

A passive vent on the back of the console provides an additional source of cabin air for the above system.

Notes:

The vent on the bow of the boat is an exhaust, not an intake. It should be facing toward the stern to be most effective, but this exhausts motor compartment fumes into the boat if the front window is open. Placement of the vent on the stearn of newer boats makes more sense.
When the key switch is turned on, both the blower and fan should be running in addition to the fan on the motor controller under the driver's seat. It will not be obvious if one or more of these fail, so regular inspection is required if you live in a hot climate and want to maximize motor ventilation.
Leaving the key switch turned on for a few minutes after docking will help the motor to cool down.

Bow Vent Blower

The original part is Jabsco 35115-0020, 3 inch Blower, 105 CFM, 12 Volt (spec. sheet). About $80. Jabsco states that this is intended for intermitent operation and, if used for longer periods, the maximum motor life is 1,000 hrs (about 2 hrs/week for 10 years).

Attach a cable clamp to one of the existing screws on the fan housing and loop the wires through it to prevent strain on the electrical connectors.

Jabsco offers the blower motor as a replacement part, but it costs as much or more than the entire assembly.

Critical Specifications

Shaft Diameter: 1/4"
Shaft Length: 1"
Shaft Type: D Shaft
Voltage: 12VDC
Mounting Screw Distance: 2 5/16"
RPM: Unknown

90mm Fan

Duffy used a standard 12V 90mm computer case fan (~$7).

Orient the fan so that it pulls air from the cabin into the motor compartment.

The original fan has two wires (12V positive and negative), which was the standard for old computer fans. Modern computer fans have 3 or 4 wires, which provide fan speed feedback and allow for speed control. 3/4 wire fans can be used in this application with the third and fourth wires unused as shown in this pin-out table.

Typical Three/Four Pin Fan Wiring

Pin/Wire	Function	Wire Color
1	-12V	Black
2	+12V	Yellow
3	Sense	Green
4	Control	Blue

External Vent

The vent is a standard 3" PVC Cowl Vent which is sold by several manufacturers. "Three inch" refers to the inside diameter of the vent base. The inside diameter of the deck plate it seats into is 3.25". The vent is pressure-fit to the deck plate.

The blower box below the vent has a flapper valve inside which is designed to cap the vent hose when the blower is off. Our flapper valve shown in the photo was broken near the hinge point.

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