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

get. There are some sources for custom

6-bladed, 140 percent DAR props, but that’s

truly the max. If you need still more blade

area, you have no choice but to increase

diameter.

Minimum Diameter and

Blade Area

What’s needed is an easy way to check blade

area and relate it to diameter using the DAR.

I’ve worked out a formula that will allow

a quick estimate of the minimum diameter

required for either displacement or planing

hulls.

Formula 1-1. Minimum Prop Diameter

as a Function of Blade Area

or

Where

hp =total brake horsepower for each

engine

kW =total flywheel power for each

engine

DAR =disc area ratio (or blade area

ratio); expressed as a decimal

kts =maximum speed in knots

DARs FOR REPRESENTATIVE PROPS:

2-Bladed Sailer = 0. 24

3-Bladed Standard = 0. 50

3-Bladed Wide Blade = 0. 70

4-Bladed Standard = 0. 70

4-Bladed Wide Blade = 0. 95

5-Bladed Standard = 0. 95

5-Bladed Extra-Wide Blade = 1. 20

These formulas make a few simplifying

assumptions, which apply quite well to most

average boats:

e =efficiency = 0 .55 for displacement

boats and 0.70 for planing boats

wf =wake factor = 0. 9

propeller depth =1 to 3 feet (30 to 90 cm)

below the waterline

Full engine power is used in these formu-

las, not shaft power. The reduction for

shaft losses has been incorporated into the

formulas.

Using this formula, it’s a quick job with a

pocket calculator to see if a given boat is in

the right ballpark for blade area.

Example: Say you have a 40-foot (12.2 m)

sportfisherman powered with twin 400 hp

(298 kW) diesels, and it has standard

3-bladed props, 24 inches (61 cm) in diame-

ter. The boat gets a top speed of 30 knots. You

would find.

or

In other words, the 24-inch (61 cm), 3-bladed

prop is too small in diameter to have ade-

quate blade area and is likely to cavitate.

Given enough clearance and the right

reduction gear, you could go to a 28-inch

(71 cm), 3-bladed wheel. A more practical

solution—in most instances—is to go to

more blades, a wide-blade-pattern propeller,

or both.

Trying a standard 4-blader gives

or

which is spot on.

PART ONE: DRIVETRAIN INSTALLATIONS

Min.Prop.Dia.,in.(Displacement Hulls)

1

=

225 hp

DAR kts kts

×

××

NOTE:kts× ktsis the same as kts^1.^5

Min.Prop.Dia.,in.(Planing Hulls)

160

=

× hhp

DAR ××kts kts

Min.Prop.Dia.,cm (Displacement Hulls)

1

=

0081 .5kW

DAR kts kts

×

××

Min.Prop.Dia.,cm (Planing Hulls)

1384. 2

=

kW

DAR kts kts

×

××

160 400 hp

0 .50 DAR 30 kts

27. 9 in.

30 kts

×

××

= Min.Prop.Dia.

1384. 2 298 kW

0 .50 DAR 30 kts

70. 8

30 kts

×

××

= ccm Min.Prop.Dia.

160 400 hp

0 .70 DAR 30 kts

23. 5 in.

30 kts

×

××

= Min.Prop.Dia.

1384. 2 298 kW

0 .70 DAR 30 kts

59 .9c

30 kts

×

××

= mmMin.Prop.Dia.

Formula 1-1.