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

70% Maximum rpm =About 40% Maximum-

Rated Engine Power

(This assumes the propeller is properly

matched to allow the engine to just reach full-

rated rpms at maximum throttle.)

Generator Power and Fuel

Consumption

Generator fuel consumption is best read from

the manufacturer’s data sheet. If you don’t

have the data sheets or specs for a generator

or you are estimating for preliminary design,

you can use the following:

Full-Load Engine bhp =1.7× Maximum-

Rated Electric kW Output

Half-Load Engine bhp =1.0× Maximum-

Rated Electric kW Output

or

Full-Load Engine kW =1.27× Maximum-

Rated Electric kW Output

Half-Load Engine kW =0.74× Maximum-

Rated Electric kW Output

Find fuel consumption using the same

multiplier used for main-engine horsepower

or kW output.

Calculating Tank Capacity:

The Prismoidal Formula

Once you know how much tankage is

required, you have to be able to determine

the volumes or capacities of tanks. The

same methods can be used to find the

volume of other spaces such as locker vol-

umes or compartment volumes. If the tank

or other volume is a regular rectangle,

cylinder, or some other simple shape, find-

ing the volume is straightforward. Often,

however, tanks will be rather odd shapes

fitted under bunks or cabin soles and

against the hull shell. One approach is to

find the area of each of the tank’s ends

and the area of a section through the

tank’s middle. Add the three areas to-

gether, and divide by 3 to get the average

area; then multiply by the overall tank

length. This gives a pretty good calcula-

tion for volume.

A formula that gives more accurate re-

sults is the prismoidal formula.The pris-

moidal formula, in fact, automatically

simplifies to the formula for a sphere,

cone, cylinder, pyramid, and any common

regular geometric shapes. It is the uni-

versal formula for finding volumes, as

follows.

Formula 6-2. Prismoidal Formula

Where (for vertical tanks)

V =volume

B =area of bottom

M =area of middle

T =area of top

H =height

or

Where (for horizontal tanks)

V =volume

B =area of one side

M =area of middle

T =area of opposite side

H =length

Sometimes a single tank has a particu-

larly strange shape with an appendage built

onto it. For instance, a standard wing tank fit-

ted against the side of a hull may have a built-

in protuberance from one end to add volume,

to extend the lower portion of the tank under,

say, a berth. When a tank has such odd

shapes, you divide it into separate shapes

(regions) that are simple to deal with, then

add the volume of each region to get the

total volume.

Knowing volume, you can find tank

capacity in gallons and weight, when full,

from the following tables.

Finding the Capacity of a Tank

Say you have found an area in the boat that

will fit a tank. You have mocked up the tank

roughly with light plywood, doorskin, or

V

B4MT

6

= H

⎛ ++

⎝⎜

⎞

⎠⎟

PART TWO:FUEL SYSTEMS

Formula 6-2.