rive ship are the prime movers that convert fuel into
mechanical work. Candidate prime movers include gas turbines, diesel engines, and
steam turbines. The prime mover (PM) drives the propeller through a main reduction
gear, shaft and clutch. The reduction gear, the mechanical equivalent of a transformer,
steps down the PM output shaft speed, allowing the PM to operate at higher speeds where
it is most efficient while the propeller operates at low speeds where it is most efficient.
For larger ships with top speeds in excess of thirty knots (34.5mph), two propeller shafts
are used with two prime movers per shaft. For a given propeller shaft, the second engine
may be clutched in to provide extra power. A representative mechanical-drive layout is
shown in Figure 1. Note how the propulsion machinery for the two shafts are spatially
separated for survivability.

In order for the aforementioned ship to go in reverse, three options are possible: reversing
turbines, reversing gears, or a controllable pitch propeller (CPP). The latter is used in
most surface combatants. Specifically, a CPP is a complex hydraulic system that runs
through the length of the shaft and uses an oil-piston rod to change the pitch of the
propeller blades. In addition to reversing the ship, the CPP is used in forward speed
control as discussed next.

Lets focus our discussion on a specific ship platform and choose a DDG Arleigh Burke
class guided-missile destroyer. A DDG uses four General Electric LM2500 gas turbines
for propulsion, each rated for approximately 20MW of power. Remember, each of these
prime movers also has auxiliary requirements including fuel service pumps, lube oil
pumps, cooling fans, and air supply and exhaust ducting. In order to discuss speed
control, consider the ship speed versus power requirement curve shown in Figure 2. Note
that the characteristic is approximately a cubic; that is,


(1.1)
(
3
REQUIRED
P
K Spe )
ed
)

Figure 2 also shows that a single gas turbine (GT) can support ship speeds up to 19kts,
the second GT provides speeds up to around 24kts, the third provides speeds up to around
27kts, and the fourth GT enables ship speeds in excess of 30kts. Low-speed operation
is achieved by adjusting the pitch of the CPP while the gas turbine runs at a
low idle speed. Higher-speed operation is then obtained by a combination of pitch
control and governed manipulation of the gas turbine throttle input (ranging from
approximately 1200rpm up to 3600rpm).
(
0 12kts



2

AUX MACHINERY ROOM #1
AUX MACHINERY ROOM #2
MAIN ENGINE ROOM #1
MAIN ENGINE ROOM #2
SHAFT ALLEY
MAIN REDUCTION
GEAR #1
MAIN REDUCTION
GEAR #2
GTM
1B
GTM
1A
GTM
2A
GTM
2B
THRUST
BEARING
THRUST
BEARING
TUBE
SEAL
TUBE
SEAL


Figure 1. Representative Mechanical-Drive Ship Layout (GTM: Gas Turbine Module)


In order to fully appreciate the move to electric drive, we must lastly consider how
electric power is produced on board a mechanical-propulsion ship. The functional layout
is shown in Figure 3. Here we have included three Rolls Royce Allison 501-K34 gas
turbines with associated reduction gears and electric generators, each rated for
approximately 2.5MW (3MW on the newest DDG versions) of power. The total
electrical capacity is chosen so that all vital loads can be supplied if one generator is
unavailable and the remaining two are operated at 90% capacity. As hopefully Figure 3
makes explicit, the propulsion and electric distribution systems are separate, ~80MW of
power is available for propulsion while ~9MW of power is available for electrical loads
and bleed air (which includes sonar masking). For example during normal cruising at
20kts with 3MW of electrical load, four gas turbines must operate (2 for propulsion, 2 for
ship service). This inefficient use of capacity is at the heart of the move to electric
propulsion.
3

10MW
20MW
30MW
40MW
50MW
60MW
70MW
80MW
5kts
10kts
15kts
20kts
25kts
30kts
1GT
2GT
3GT
4GT
REQUIRED
SHAFT POWER
SHIP SPEED


Figure 2. Typical Power versus Ship Speed Requirement


20MW
20MW
20MW
20MW
3MW
3MW
3MW
LOAD
LOAD
LOAD
LOAD
LOAD
LOAD
SM
SM
SM
SHIP SERVICE
SWITCHGEAR
DOUBLE
REDUCTION GEAR
DOUBLE
REDUCTION GEAR


Figure 3. Propulsion and Ship Service Layout on a Typical Surface Combatant
(SM:Synchronous Machine, 3MW: Allison Gas Turbine, 20MW: GE Gas Turbine)
4
Finally, what is electric drive? In a sense, electric drive is really a component of a larger
architecture called an Integrated Power System (IPS). In an IPS, all prime mover power
is first converted into electric power, then it is distributed and allocated between
propulsion, weapons, and other electrical loads as required. This now follows in the
spirit of Star Trek, where Captain Kirk orders Scottie to redirect all power to shields. A
notional IPS system, tractor beam omitted, is depicted in Figure 4. The numerical values
for various pieces of equipment have no direct correlation to a DDX design and are more
intended to aid discussion. The key elements of this system are that there are only four
prime movers (versus the seven in our DDG example ship), 100MW of electrical capacity
is available, each propeller is powered by a combination of electric motor and power
converter, the double-input reduction gear is eliminated, and the CPP is replaced by a
fixed-pitch propeller (FPP). The elimination of prime movers, reduction gears, and CPP
hydraulics is offset by the introduction of larger electric generators, two propulsion
power converters, and two propulsion motors. Through electrical switchgear, any prime
mover can power either propulsion motor or any shipboard electric load.

Before we consider the advantages and technical hurdles of an IPS ship, lets track the
evolution of the concept (briefly). Electric drive by itself is not a new concept. In fact for
the U.S., it dates back to 1913 when the collier Jupiter was fitted with the first electric
drive. This ship was then converted into the first aircraft carrier and renamed the
Langley. Between World War I and II, five battleships, two carriers, and over 50 smaller
vessels used electric drive. The main differences between these ships and the IPS ship
depicted in Figure 4 are that the early ships did not employ power converters and there
was typically no coupling between the propulsion and power distribution systems. For
instance in the early battleships, the propulsion motor was controlled by controlling the
output voltages of the electric generators. During WWII, a shortage of reduction gears
necessitated over 500 vessels using electric drive.


35MW
35MW
15MW
15MW
SM
SM
SM
SM
PC
PC
40MW
40MW
SHIP SERVICE
PULSE-POWER
LOADS
SWITCHGEAR


Figure 4. Notional Integrated Power System
5
Following WWII, double-reduction gears dominated the marine propulsion industry due
to lower weight, lower volume, and increased efficiency (as compared to electric drive).
Currently however, mechanical transmissions are at the limit of the technology and here
is where electric drive re-enters the picture, especially with regards to improving ship
acoustic performance. During the late 70s and 80s, the commercial sector began
employing electric drive in icebreakers, oil tankers, cruise liners, and ferries for many of
the reasons that will be listed below. As a result, some of this drive technology can be
leveraged and militarized (shock, vibration, acoustic performance), helping to reduce the
R&D cost and risk.

An electric drive consists of a propulsion converter connected to a propulsion motor.
What does the propulsion converter actually do? It converts a fixed-amplitude, fixed-
frequency set of AC voltages from an AC generator into a variable-amplitude, variable-
frequency set of output voltages required to control the speed of the AC motor. Well,
lets consider this operation in pieces starting with the end connected to the fixed-
amplitude, fixed-frequency AC generator output, denoted in Figure 5 by Phase A through
Phase C. The three-phase 60Hz voltages are converted into a DC voltage with a 360Hz
ripple by a 6-switch device called a rectifier. The semiconductor devices that implement
switches
may be diodes or thyristors. The devices are numbered in the order in
which they conduct; ideally, two devices are on at any time instance. As a consequence,
&
conduct for
1
T T 6
1
T
2
T
1
6 of a cycle with the line voltage
at the output. When turns
off,
&
conduct and the line voltage
appears at the output. Six identical
intervals result, each with the output governed by a piece of a given line voltage. A
thyristor-rectifier enables the average DC output voltage to be controlled, but requires
additional auxiliary circuitry.
AC
V
1
T
2
T
3
T
BC
V


T1
T2
T6
T4
T5
T3
PHASE A
PHASE C
PHASE B

V
PK
0.86V
PK
time
V
RECT,OUT


Figure 5. Propulsion Converter Front-End Rectifier





6
S1
S2
S4
S3
MOTOR PHASE
V
PH
+
-
V
DC
+
-

V
DC
- V
DC
V
PH
time


Figure 6. Propulsion Converter Output Inverter Stage (One Phase)


The rectifier output is then low-pass filtered to reduce the ripple and passed to the
inverter output stage. The inverter may assume a variety of topologies and the Navy is
investigating several candidates. Possibly the most basic topology is illustrated in Figure
6. This circuit is referred to as an H-bridge inverter. Th