RESISTANCE AND POWERING

There can be no absolute terms of optimum form for ships. The designer must make

many compromises. Even in terms of resistance one form may be better than another at

one speed but inferior at another speed. The shape of a ship hull is determined by many

competing influences.

For ease of construction, it should be a rectangular box;

for adequate transverse stability, it must be wide;

for adequate strength as a beam being bent in a longitudinal plane, it must be deep.

All these factors influence the shape of a hull, but often the primary factor is the dynamic

interaction of the hull with the water. The interactions that govern the resistance of the

hull to steady forward motion--a resistance that determines the choice of propulsive

power--usually demand the greatest attention from the naval architect. While the

minimization of resistance is a common design goal for all ship types, other

hydrodynamic performance characteristics such as seakeeping and manoeuvrability

performance may be more important for some ship types such as passenger vessels and

surface warships.

Until somewhat over a century ago, predicting the speed of a proposed ship was all

art and no science. Then, as steam engines replaced sails, half the difficulty was

removed: naval architects no longer had to guess what wind forces and directions

would be available to push the ship along. The other half of the difficulty lay in

predicting the resistance of any given hull form to being moved through the water

at any given speed. Naval architects still have no precise quantitative understanding of

that problem. Since the early 1870s, however, the insights of William Froude have

produced generally satisfactory ways to estimate resistance. These are usually based,

either directly or indirectly, on the use of model basins. Naval architects still have a

lot to learn about predicting the speed and power of boats and ships. Their

estimating methods are nearly all derived from empirical evidence, either from

model tests or from existing ships. Adding to the difficulty are hard-to-predict

influences such as sea conditions, loading conditions, and degree of fouling (for

example, barnacles) on the underwater hull.

Let’s start this discussion of the components of resistance by considering the

case of the deeply submerged submarine. Let us assume that the craft is well

shaped. That being the case, the only important source of resistance to forward

motion will be the friction caused by the viscosity of the water. The energy used

to force the water aside at the bow will be fully regained as it closes together at

the stern.

•Frictional resistance

•Wavemaking resistance or residual resistance

Components of Resistance

Friction: Contrary to popular opinion, a ship's frictional resistance is not so

much between hull and water as it is within the water itself. Let’s try to

explain.

As the hull slides through the water, a thin coat of the water will attach itself to

the the hull and move along at the ship' s speed. Owing to the water' s viscosity

(stickiness), this innermost layer will try to induce the second layer to come

along; but that same viscous property will cause the third layer to resist the

forward motion of the second layer. The cumulative result is that many layers

will be caught up and tend to follow the hull, but each will be less affected than

its interior neighbor. Eventually, as the effect gets smaller and smaller, it

disappears altogether at some finite distance from the hull. The body of water

that is dragged along is known as the boundary layer. Its thickness will be

influenced by the relative roughness of the hull surface and will gradually

increase from bow to stern. The accumulation of trailing water at the stern is

the wake. So, while we can reduce frictional resistance by keeping the hull

clean and smooth, we only fool ourselves by coating it with grease, a trick that

has been reinvented a thousand times but never with success.

Wave-making. If our submarine rises to the surface, a new source of

resistance will appear: wave-making. The force of the bow pushing against the

water will now produce waves. Since the waves must overcome gravity, they

will absorb energy. Of course there are other waves created, too, such as those

at the stern. We can generalize by saying that wave-making resistance arises

from variable pressures between hull and water surface working against gravity.

Another source of resistance closely related to wave-making is that of eddies

forming behind blunt appendages, shaft struts, rudders, and so forth. In a well-

designed vessel this source of resistance is not large and is usually lumped in

with wave-making. The wave- and eddy-making resistances are often referred

to collectively as residual resistance.

Effect of Speed, Size, Main Dimensions and Hull Form Parameters

on Resistance

Major components. Most ships and boats, then, have two major components

of resistance to forward motion: friction (overcoming viscosity) and residual

(largely wave-making, overcoming gravity).

Naval architects try to minimize frictional resistance by providing a smooth

hull and then trusting to the operators to maintain it in good condition. This

requires frequent cleaning and the application of high quality paints to resist

both fouling and corrosion.

Naval architects try to minimize residual resistance by developing good hull

forms.

In most merchant ships, the frictional resistance is perhaps three to four times

as large as the residual resistance. In relatively high-speed vessels (such as

passenger ships, naval combat craft, yachts, or ferryboats), the two resistances

might be about equal.

Except in a small minority of cases, air resistance is so small (perhaps only a

percent or two of total resistance) that naval architects feel safe in ignoring it

altogether.

Reducing wave resistance (Bulbous bow forms)

Central aim: Among a naval architect's most challenging responsibilities is that

of selecting an engine that will drive a proposed ship at the right speed. Once the

vessel is placed in service, failure to meet the owner's specified speed will cause

the designer acute embarrassment and penalty. On the other hand, if the vessel is

found capable of appreciably greater-than-specified speeds, the embarrassment

arises from having wasted the owner's money on an oversize engine. Hitting the

bull' s eye is no easy task.

Procedure. In selecting an engine the naval architect must go through a

multistep procedure culminating in an estimate of the required horsepower.

Brake horsepower (BHP) if a diesel or gasoline engine,

or shaft horsepower (SHP) if a steam turbine.

BHP and SHP are identical in a direct-connected diesel engine.

In geared engines BHP goes into the reduction gears; SHP comes out. The

difference, caused by friction within the gears, is usually less than 3 percent.

Explaining the procedure for estimating power is easier if we start with the

engine output (BHP or SHP) and work our way backward to see what happens

to that power and how it is finally dissipated.

Some of the energy that goes into the propeller shaft never reaches the

propeller. A small fraction (perhaps 1 or 2 percent) goes into overcoming

friction in the shaft bearings and the seals that keep water from seeping in

where the shaft passes through the hull. Propellers convert torsional energy

into thrust. In doing this they are seldom more than 70 percent efficient, so the

useful energy coming out of them is appreciably less than what went in.

Propeller efficiency can be readily estimated from published charts derived

from numerous model propeller tests. What is more difficult, however, is to

assess the complications brought on by interactions between the propeller and

the hull. As we explained how friction produces a body of water, called the

wake, that moves along with the ship. The propeller operates within this

relatively slow moving water. That gives the propeller a firmer base for

producing thrust and so it regains some of the energy lost in overcoming

friction. At the same time, in drawing water into its sweep, the propeller

reduces the water pressure on the stern. That "hull suction" (technically called

thrust deduction) gives the propeller more work to do and so more energy is lost.

Naval architects generally lump the wake gain and thrust deduction to produce

an overall estimate of their combined effect called hull efficiency. Typically, this

hull efficiency may come to 1.10 in a singlescrew ship and 1.00 in a twin-screw

ship.

What happens to the energy

Effective horsepower

The product of propeller efficiency and hull efficiency is called the propulsive

coefficient. This is used to convert the power delivered to the propeller to the

power that would be required to pull the ship through the water if no propeller-

related complications were involved. This is sometimes called towrope horsepower

(THP), but more commonly effective horsepower (EHP). It can be estimated from

the three independent components of resistance already explained: friction,

wave-making and a small increment for appendages.

Numerous experiments have shown that frictional resistance will vary directly

with the ship' s wetted surface (which is the surface area of the underwater

hull), with the ship's speed raised to an exponent somewhat less than two and,

of course, the degree of roughness of the hull. Other factors are the ship' s

length and the viscosity of the water, which will vary slightly with temperature

and degree of salinity. Long ships have relatively less frictional resistance per

unit of area.

Estimating frictional resistance

The task of predicting wave-making resistance is considerably more difficult.

Naval architects simply do not understand in any quantitative way the physics of

how a ship creates waves. Strictly mathematical approaches to the analysis have

been under study for several decades, but most design methods in use today still

rely on empirical evidence from model basin work or derive their conclusions

from measured performances of existing similar ships. In some instances, model

basin researchers have tested large numbers of methodically related hull forms.

These are called standard series, and their results have been analyzed and

published so that naval architects can derive a good approximation to the wave-

making resistance per ton of displacement for any normally shaped hull form.

Once wave-making resistance has been estimated, wave-making horsepower can

be derived. The sum of the two horsepowers, plus a modest increment for

appendages, will produce the total effective horsepower.

Estimating wave-making resistance

Required engine powers are usually estimated by the following step-by-step

procedure:

1. Measure the proposed vessel' s wetted surface from the lines drawing and use

that to estimate frictional horsepower.

2. Estimate wave-making resistance and corresponding horsepower

requirements from published standard series results or from available data on

existing similar ships. If time and budget permit, the services of a commercial

model basin may be engaged not only to check estimates, but perhaps to refine

the hull form by testing variations on the original design.

3. To the sum of the frictional and wave-making horsepowers add an increment

for appendages that were not fitted to the model. This may amount to about 7

percent for a single-screw ship with a rudder as the only appendage. In a well-

designed twin-screw ship it may come to 12 percent.

4. The sum of the powers found in the first three steps will be the estimated

effective horsepower, EHP. Dividing that figure by the propulsive coefficient

leads to the horsepower that must be delivered to the propeller. The naval

architect then adds a few more percentage points for shaft losses to arrive at

SHP. Another small increment will provide BHP.

Estimating required engine power

5.Having carefully developed estimates of SHP or BHP (often to four significant

figures), the naval architect must now add a prudently generous margin in

recognition of real-life conditions the ship will have to face. To this point all

calculations have been based on an idealized set of assumptions: perfectly

smooth hull, fair weather, and smooth seas. The usual margin for service

conditions is a rather arbitrary 15 to 25 percent. Another margin may be

appended if the type of engine is such that its output may be expected to

diminish over the years. I think you may now see why this procedure for

arriving at the necessary horsepower should be called an estimate and not a

calculation .

6.With the above-derived estimate of required power, the naval architect is

ready to select a standard engine or turbine, usually right out of some

manufacturer's catalog. Since off-the-shelf units will seldom provide the exact

power required, prudence dictates going for a standard model of slightly more

power than the estimate.

Engine Reduction

Gear Bearing Seals

Screw Strut

BHP SHP DHP

THP

Shaft

Ship Train Drive

Brake Horse Power (BHP)

• Power output at the shaft coming out of the engine before the reduction gears

Shaft Horse Power (SHP)

• Power output after the reduction gears (at shaft)

• SHP=BHP - losses in reduction gear

Delivered Horse Power (DHP)

• Power delivered to the propeller

• DHP=SHP – losses in shafting, shaft bearings and seals

Thrust Horse Power (THP)

• Power created by the screw/propeller (i.e. Propeller thrust)

• THP=DHP – Propeller losses

E/G R/G BHP SHP

Shaft Bearing

Prop. DHP THP EHP

Hull

Relative Magnitudes?

BHP>SHP>DHP>THP

EHP is approximately equal to THP (usually slightly less)

Screw propeller

Nozzle propeller

Froude's insight. Before Froude's basic work, starting in 1868, various

engineers (including Benjamin Franklin) had tried to predict ship speed based

on model tests. They had all failed, however, because they did not understand

how to extrapolate the model results to the full-size ship. Froude realized that

the difficulty lay in the fact that the two major components of resistance

(friction and wavemaking) followed different physical laws in scaling up from

model to ship. One of his unique contributions, then, was to treat friction and

wave-making separately in going from model results to full-scale prediction.

Froude understood the laws of similitude. These tell us how physical

characteristics are affected by scale.

Model Basin Theory

Froude observed that when ship and model were both moved at the same

Froude number both produced identical (to scale) wave profiles. That is, if the

model showed a wave crest at the bow and another at the stern, the ship' s wave

profile would show exactly the same pattern. From this he reasoned, correctly,

that ship and model wave-making resistances would vary directly as their

displacements when both were moved at the same Froude number. As for

frictional resistance, simple towing tests on a series of flat planks (essentially no

wave-making) showed him that the viscous resistance varied directly with the

wetted area and with the speed raised to a power somewhat less than two.

Froude's number:

Froude's technique. Armed with the concepts spelled out above, Froude

was ready to use his model basin. He had built an exact scale model of some

ship. He then towed it through the water of his tank at the Froude number

corresponding to the ship' s design speed, and measured the pounds of pull

required to maintain that model speed. This gave him the model's total

resistance. He then calculated how much of that total was contributed by

frictional resistance. Subtracting that from the total resistance gave him the

(wave-making plus eddy) resistance. And now you know why the cumulative

wave and eddy component is called residual resistance. Now Froude was ready to

use his model to predict the full-scale ship's resistance (and from that, EHP).

First, the ship's frictional resistance was predicted using much the same

formulation as that used for the model, with a correction for relative roughness.

Second, the ship's residual resistance was taken as the model's residual

resistance multiplied by the ratio of their displacements (with, of course, ship

and model speeds both at the same Froude number). As you will notice, he kept

in mind that the scaling laws differed between frictional and residual

resistances. Finally, he added together the two individually arrived-at resistances

so as to provide a reliable prediction of total full scale resistance, and from it

EHP. The diagram in the following figure illustrates Froude's logic and may

help clarify the thinking outlined above. Today, more than a century later, naval

architects still exploit Froude's method. Numerous improvements have of

course been developed, but the fundamental approach is still the same. Until

hydrodynamicists can completely solve the basic physical laws of wave-making

and the complex flow around the hull, naval architects will continue to rely on

Mr. Froude's model basin techniques.

0

200

400

600

800

1000

Eff

ec

tiv

e H

ors

ep

ow

er,

EH

P (

HP

)

0 2 4 6 8 10 12 14 16

Ship Speed, Vs (Knots)

POWER CURVEYARD PATROL CRAFT

The figure shows the 360-ft-long model basin at The University of Michigan's

Department of Naval Architecture and Marine Engineering.