Or3.02 m×2.68 m×1.98 m headroom =16 m^3
1 m×1.43 m×1.98 m headroom =2.8 m^3
16 m^3 −2.8 m^3 =13.2 m^3VCR should be between 6 and 8 times per
hour.
6 ×13.2 m^3 =79 m^3 /hr.
to
8 ×13.2 m^3 =106 m^3 /hr.(NOTE: It’s customary in passenger spaces to
use gross compartment volume, as we have
here. As we’ll see, net compartment volume
is employed for machinery spaces.)
We would need to install blowers or pas-
sive ventilators that provide this flow rate to
meet our VCR requirement.
HOWMUCHAIR PERPERSON? For passenger
spaces, you should also check the minimum
amount of ventilation based on the amount of
air required to keep each person comfortable
in fairly warm weather. For reasonable com-
fort, each crewmember needs about 15 cubic
feet of air per minute (cfm) or 25 m^3 /hr. Ob-
viously, a small passive vent delivers more air
when it’s blowing hard than when there’s
barely a breeze. What we need, however, is
ventilation when it’s hot and there is little
wind. Accordingly, the boat should be fitted
with vents that’ll deliver 15 cfm (25 m^3 /hr.)
per person in light winds, say 4 knots. Table
14-2 gives the cross-sectional vent areas that
will give the required flow in just 4 knots of
breeze. It assumes a roughly standard cowl
vent sitting on a water trap, or a good-quality
mushroom vent.
Standard cowl and mushroom vents
come in diameters ranging from 3 to 10 inches
(7.6 to 25.4 cm) and have minimum opening
cross sections per Table 14-3. (NOTE: Proper
cowl vents should unscrew so they can be
closed with screw-in deck plates in extreme
weather conditions, and they are designed to
rotate to face into or away from the wind as
needed.)
Example:If a boat routinely carries six
crew, you will read the required 90 cfm
(153 m^3 /hr.) off the table, which would be
provided by 92 square inches (594 cm^2 ) of
vent area per person in a summertime 4-knot
wind. Table 14-3 shows that this would call
for seven or eight 4-inch (10.2 cm) vents.(Seven 4-in. [10.2 cm] vents at 12.6 sq. in.
[81.1 cm^2 ] each equals 88.2 sq. in. [568 cm^2 ].)
Few boats have this number, so it’s no
wonder that so many are uncomfortable
below on hot days. In fact, although 3-inch
(7.6 cm) vents are quite common, they’re
really too small to give enough ventilation.
Three-inch (7.6 cm) vents should be used only
for passenger-compartment ventilation on the
tiniest craft, where you may have no option
but to go this small. If hatches or windows canPART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING
TABLE 14-2.VENT REQUIREMENTS PER
PERSONNo.of Area, Area,
Crew cfm* sq.in. m^3 /hr.* cm^2
1 15 21 25 135
2 30 37 51 239
3 45 51 76 329
4 60 64 102 413
5 75 77 127 497
6 90 92 153 594
7 105 110 178 710
8 120 131 204 845
9 135 157 229 1,013
10 150 190 255 1,226
11 165 229 280 1,477
12 180 278 306 1,794
13 195 335 331 2,161
14 210 404 357 2,606
15 225 484 382 3,123
16 240 578 408 3,729
17 255 686 433 4,426
18 270 809 459 5,219
19 285 949 484 6,123
*Approximate vent flow for the specified vent
area in a 4-knot breezeTABLE 14-3.ROUND VENT AREAS
Dia.,in. Area, sq.in. Dia., cm Area, cm^2
37. 17. 645. 6
412. 610. 281. 1
519. 612. 7 126. 7
628. 315. 2 182. 4
738. 517. 8 248. 3
850. 220. 3 324. 3
963. 622. 9 410. 4
10 78. 525. 4 506. 7