Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

ventilation duct is complex, but a simple

approximation can be used for small, ducted

systems up to about 14 feet (4.3 m) long, as

shown in Table 14-4.

Example:In the master stateroom from

earlier, we found we needed between 46.8 and

62.4 cfm, say 55 cfm. This stateroom is entirely

below the aft saloon. The windows on the hull

side are too close to the waterline to open

safely. Vent ducts can run down from the cabin

roof. They enter from a mushroom vent on the

roof and go straight down the cabin side for

9 feet to exit through louver grilles via a 90-

degree bend. Percent loss of blower capacity is

9 feet×2% per foot =18%

90-degree bend =10%

Louver grille =20%

Total approximate loss =58%

Effective vent output =100% – 58% =42% of

blower’s rated cfm output

Required blower capacity is then

55 cfm ÷ 0.42 =131 cfm

NOTE: If the approximate loss is over

80 percent, this simple approach is not ade-

quate, and a detailed duct analysis is required.

The 131 cfm could be handled by a stan-

dard 3-inch squirrel-cage blower (Figure 14-10),

rated at 150 cfm.

Or

Example:In the master stateroom from

earlier, we found we needed between 79

m^3 /hr. and 106 m^3 /hr., say 93 m^3 /hr. This state-

room is entirely below the aft saloon. The

windows on the hull side are too close to the

waterline to open safely. Vent ducts can run

PART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING

TABLE 14-4.ABYC ESTIMATED EFFECT OF

BLOWER SYSTEM COMPONENTS

Percent Loss of Blower

Item Rated Capacity

Ducting 2% per ft.of length,

or 6.56% per m of

length

Ducting Bends—90° 10% each bend

Clamshell Vent Intake 20%

Louver 20%

Screen—^1 / 4 -in.

(6.35 mm) mesh 10%

Figure 14-10. Squirrel-cage blower

(Courtesy Vetus Den Ouden)

down from the cabin roof. They enter from a

mushroom vent on the roof and go straight

down the cabin side for 2.74 meters to exit

through louver grilles via a 90-degree bend.

Percent loss of blower capacity is

2.74 m×6.56% per m =18%

90-degree bend =10%

Louver grille =20%

Total approximate loss =58%

Effective vent output =100% – 58% =42%

of blower’s rated cfm output

Required blower capacity is then

93 m^3 /hr. ÷ 0.42 =221 m^3 /hr.

221 m^3 /hr. ÷ 60 sec./hr. =3.7 m^3 /min.

NOTE: If the approximate loss is over

80 percent, this simple approach is not ade-

quate, and a detailed duct analysis is required.

The 3.7 m^3 /min. could be handled by a

standard 7.6 cm squirrel-cage blower, rated at

255 m^3 /hr. or 3.7 m^3 /min.

As a rule, avoid long duct runs in simple

ventilation systems, particularly horizontal

ones, unless a complete, detailed duct analy-

sis is performed. One 110-foot (33.5 m) mo-

toryacht I surveyed had a vent duct running

from the crew utility room, with washer and

dryer, about 70 feet (21 m) aft to vent into the

engine room. No calculations were done on

the duct blower, which was so underpowered

that airflow was too slow. Not only was this