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

Washdown garden hose on deck usually lies
in serpentine loops; increase effective length
by 50 percent.

30 ft. garden hose × 1. 5 =45 ft.

or

9 .1 m garden hose × 1. 5 = 13 .6 m

Total effective length of all hose (pipe) and
fittings:

14 .87 ft. fittings + 18 .25 ft. internal hose +

45 ft. garden hose = 78 .1 ft.

or

4 .52 m fittings + 5 .56 m internal hose +

13 .6 m garden hose = 23 .7 m

Friction head (hf) is

or

(Note: Used the constant 140 for smooth hose.)
Total apparent head is

35 ft. friction + 46 .2 ft. pressure + 10 .25 ft.

static = 91. 45 ; use 91.5 ft.

or

10 .6 m friction + 14 .3 m pressure + 3 .1 m

static =28 m

Acceptable Flow Speeds

If water flows into a pump too slowly, it will
cause problems such as recirculation,
reduced pump efficiency, and even failure to
pump. Similarly, if water flows too fast, it can
cause water hammer, noise in the piping, and
cavitation in the pump (vibration and reduced
output). Flow speeds (fluid velocities) for
water pumping systems should be in the
ranges shown in Table 19-4.

Nine feet per second (3 m/sec.) should be

the absolute maximum. You want to be a bit

under this in all applications. Continuous do-

mestic operation is for items like pressure-

water systems or bait-well circulation.

NOTE: Restrictions at the nozzle of a

hose (like a fire hose) can be used to in-

crease velocity and pressure at the nozzle.

This does not affect the flow speed through

the rest of the system, though you would

need to include the added head from the

nozzle reduction in calculating the system’s

friction head.

Formula 19-2. Flow Speed (Fluid

Velocity)

or

Where

gpm =gallons per minute

lpm =liters per minute

ID =pipe inside diameter, in. or mm (Take

from the pipe tables in Appendix B.

Hose ID is the nominal ID of the hose.)

You can find the minimum and maximum flow

speeds for varying pipe sizes from Table 19-5.

Returning to our deck washdown, we can

see that a 12 gpm (45.5 lpm) flow is comfort-

ably under the maximum for a^3 / 4 - inch (DN

20 mm) pipe and nowhere near the minimum

value. So the flow speed is acceptable. You

can calculate the precise flow speed using

Formula 19-2 for the exact ID of^3 / 4 - inch (19

mm) hose, but inspection of Table 19-5 shows

it wouldn’t be required here.

Velocity , m/se c.

21 .2lpm

(ID, mm)^2

=

×

Velocity , ft./sec.

gpm

2 .45 (ID,in.)^2

=

×

hf, ft.

10 .44 78.1ft. (12 gpm)

140 (

1. 85

= 1. 85

××

× 00. 75 in.)

4. 86 =35 ft.

Chapter 19:Selecting and Sizing Pumps

hf, m

82 .54 23.7m (45lpm)

14 (1. 9

1. 85

= 01. 85

××

× cm)

4. 86 =^10 .6m

TABLE 19-4.ACCEPTABLE FLOW SPEEDS, WATER

ft./sec. m/sec.

Minimum for proper pump performance 2 0.6

Optimum for continuous domestic operation 3 to 5 0.9 to 1.5

Maximum for high discharge (bilge pump,

fire main, washdown) 9 3

Formula 19-2.