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

Formula 8-1. Circumference of a Circle

Circumference=π×Dia.

Where

π= 3. 14

Dia.=tube outside diameter, in. or mm

Example:For instance, if we had a gen-

erator powered by a 12 bhp (8.95 kW) engine,

and we wanted to use half-inch aluminum

schedule 40 pipe for a keel cooler, how much

would we need?

The aluminum has to be painted with bot-

tom paint to minimize fouling. Generators are

run not only underway but also when a boat

is at the dock or at anchor, so the cooling

area required has to be based on the worst

case—zero external water flow—or station-

ary operation. Nominal^1 / 2 - inch schedule 40

pipe has an OD of 0. 840 inch. Wall thickness

is 0. 109 inch. This is less than the thickness in

Table 8-1, meaning the area could be slightly

less than indicated, but more cooling area is

OK, so

We need 0.60 sq. ft./bhp from the table.

12 bhp × 0 .60 sq. ft./bhp = 7 .2 sq. ft.

Circumference=π× 0. 840 in. = 2. 638 in.

2. 638 in. × 12 in. length = 31 .656 sq. in. per

lineal ft.

31 .656 sq. in. ÷144 sq. in./sq. ft. = 0 .219 sq. ft.

per lineal ft. of pipe

7 .2 sq. ft. required area ÷ 0 .219 sq. ft. per

lineal ft. = 32 .9 ft. of pipe

Or

We could use DIN 2448 21. 3 ×2 aluminum

pipe (21.3 mm OD, 2 mm wall). This is less

than the wall thickness in Table 8-1, meaning

the area could be slightly less than indicated,

but more cooling area never hurts, so

PART THREE: EXHAUST SYSTEMS

TABLE 8-1. KEEL-COOLER EXTERNAL SURFACE AREA PER INSTALLED POWER

UP TO 85°F (29.4°C) SEAWATER TEMPERATURE

STEEL, Painted—^1 / 4 to^5 / 16 in.(6.3 to 8 mm) Thick

Surface Area

sq. ft./bhp m^2 /bhp m^2 /kW

Generators and stationary vessels (dredges, etc.)1. 00 0. 093 0. 125

Slow-moving vessels (towboats, etc.)0. 50 0. 046 0. 062

Free-running vessels, under 8 knots 0. 40 0. 037 0. 050

Fast free-running vessels, over 8 knots 0. 34 0. 032 0. 042

ALUMINUM, Painted—^1 / 4 to^5 / 16 in.(6.3 to 8 mm) Thick

Surface Area

sq. ft./bhp m^2 /bhp m^2 /kW

Generators and stationary vessels (dredges, etc.)0. 60 0. 056 0. 075

Slow-moving vessels (towboats, etc.)0. 30 0. 028 0. 037

Free-running vessels, under 8 knots 0. 24 0. 022 0. 030

Fast free-running vessels, over 8 knots 0. 20 0. 019 0. 025

COPPER or COPPER NICKEL TUBE—Unpainted

Surface Area

sq. ft./bhp m^2 /bhp m^2 /kW

Generators and stationary vessels (dredges, etc.)0. 21 0. 020 0. 0262

Slow-moving vessels (towboats, etc.)0. 15 0. 014 0. 0183

Free-running vessels, under 8 knots 0. 12 0. 011 0. 0147

Fast free-running vessels, over 8 knots 0. 10 0. 009 0. 0125

Formula 8-1.