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

Engine Vents Based on VCR

Requirements

The engine vents can also be sized based on

Table 14-1, “Volume Change Rates (VCR) of

Various Compartments.” It’s important to

keep in mind that the volume of air in the engine

compartment is usually considerably less

than the total volume of the engine compart-

ment. This is because the volume of perma-

nent fixtures—the engines, tanks, batteries,

generator, and so on—have to be deducted to

get the net volumeof the compartment.

Tank volume can be found from

Tank volume, cu. ft.= 0.134× tank capacity

in U.S. gallons

or

Tank volume, m^3 = 0.001× tank capacity

in liters

Average-size gasoline engines and small

diesels, as well as batteries, can be approxi-

mated from Table 16-1.

For larger engines and mid-size to large

diesels, you should consult the manufacturer.

Let’s say that our example boat After

Burnerhas an engine compartment that is

8 feet long with 10.5 feet of beam (2.44 m long,

3.2 m beam). Height, from the keel to the un-

derside of the engine compartment overhead,

on this V-bottom boat is 4.9 feet (1.5 m), and

height from the chine to the overhead is

3.8 feet (1.16 m). We can estimate the average

height as

4.9 ft.+ 3.8 ft. ÷ 2= 4.35 ft. average height

or

1.5 m+ 1.15 m ÷ 2= 1.32 m average height

Gross engine compartment volume is then

8 ft. long× 10.5 ft. beam× 4.35 ft. average

height= 365 cu. ft.

or

2.44 m long× 3.2 m beam× 1.32 m average

height= 10.3 m^3

There are, however, five batteries and the

two large V-8 engines in the compartment

plus a pair of wing tanks, 180 gallons (680 L)

each, 360 gallons (1,360 L) total. Also, there’s

a 15-gallon (57 L) calorifier (water-heater

tank), and a 30-gallon (115 L) black-water

tank. Finally, there are assorted other small

machines mounted, including the forward

portion of the exhaust runs.

5 batteries× 0.5 cu. ft./

battery ≈ 2.5 cu. ft.

2 large V-8 engines× 5.5 cu. ft./

engine ≈ 11 cu. ft.

360 gal. fuel× 0.134 ≈ 48 cu. ft.

15 gal. calorifier× 0.134 ≈ 2 cu. ft.

30 gal. black water× 0.134 ≈ 4 cu. ft.

Other machinery ≈ 8 cu. ft.

Total 75.5 cu. ft.

Net engine compartment volume is

365 cu. ft. −75.5 cu. ft.= 290 net cu. ft.

VCR should be between 150 and 250 per hour.

Use, say, 200, so

290 cu. ft.× 200 changes per hour =

58,000 cu. ft./hr.

58,000 cu. ft./hr. ÷ 60 min./hr.= 967 cfm

or

5 batteries× 0.07 m^3 /battery ≈ 0.35 m^3

2 large V-8 engines× 0.16 m^3 /

engine ≈ 0.32 m^3

1,360 L fuel× 0.001 ≈ 1.36 m^3

57 L calorifier× 0.001 ≈ 0.05 m^3

115 L black water× 0.001 ≈ 0.11 m^3

Other machinery ≈ 0.22 m^3

Total 2.41 m^3

PART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING

TABLE 16-1.APPROXIMATE MACHINERY

VOLUMES

Volume

Item cu. ft. m^3

4-cylinder, in-line engine 2. 50. 07

6-cylinder, in-line engine 3. 50. 10

6-cylinder, V-6 4. 00. 11

Small V-8 4. 50. 13

Large V-8 5. 50. 16

Batteries 0. 50. 01