Backdrafting (Station Wagon Effect) - Four winns FREEDOM Owner's Manual

Figure B7: The effect of boats moored along side.
K. Backdrafting (Station Wagon Effect)
Backdrafting or the "station wagon effect" is caused by
air movement over or around a boat creating a low pres-
sure area or suction area around the stern that can in-
crease CO level on the boat. Backdrafting can be af-
fected by relative wind direction , boat speed, and boat
trim angle. See Figure B8 Backdrafting - Airflows Over
Boat and Behind Transom".
Under certain speed and operating conditions the low
pressure area may form in other regions and permit car-
bon monoxide to enter the hull through openings that
are not on the back of the boat. Boat factors which may
affect CO concentration:
Figure B8: Backdrafting - Air flows over boat and
behind transom.
1. Intentional or unintentional excessive trim angle
causing high bow angle. See Figure B9.
2. Excessive or unequally distributed weight.
Freedom™/Horizon™ Owner's Manual
Figure B9: Inefficient trim angles.
3. Canvas Configurations - Under various conditions, add-
ing or removing canvas may raise or lower CO levels.
See Figures B8, B9 & B11.
Exhaust Fumes!
Hull exhaust from your boat can cause exces-
sive accumulation of poisonous carbon monox-
ide gas within cockpit areas when using protec-
tive weather coverings (while underway or while
stationary). Provide adequate ventilation when
the canvas top, side curtains and/or back (aft)
curtains are in their closed protective positions.
4. Opening and closing ports, hatches, doors, and win-
dows may raise or lower CO levels on board a boat.
When airflow is moving forward inside the boat, CO
may be entering the boat. See Figures B10 and B11.
Figure B10: Desired air flow through the boat.
Figure B11: The effect of canvas configurations.
5. Operating a boat at slow speeds with a following wind
should be avoided. Consider changing direction,
adjusting speed, or both. See Figures B12.
Section B
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