Ways of determining weight

The weights of the complete engine plant can be determined using the following methods:

1. Using known individual component weights.

2. Using unit weights from similar complete plants.

3. As a function of the known main engine weight.

4. Using weight groups which are easy to determine plus residual weight group.

Using individual weights

Here the weight of water and oil in pipes, boilers and collecting tanks is part of the engine plant weight. The weights of all engine room installations and small components should also be determined.

Engine plant weight using unit weights

If the weight of the plant is established using unit weights of similar complete plants, these will contain specifications for each detail of the engine plant—even the electrical unit, although there is no direct connection between the weight of the propulsion unit and the electrical unit. Ideally the weights of propulsion and electric unit should be treated separately. If the unit weights of existing ships are used as a basis, these should always be related to the nominal power (100%). The designs of the systems should be similar in the following respects:

1. Type of propulsion unit (diesel engine, steam turbine, gas turbine).

2. Type of construction (series engine, V-type engine, steam pressure).

3. RPM of propulsion unit and propeller.

4. Size of ship and engine room.

5. Propulsion power.

6. Auxiliary power.

Given these conditions, unit weights, often ranging from below 0.1 to above 0.2t/kW, give reliable estimates (Krause and Danckwardt, 1965; Ehmsen, 1974a,b).

Determination of engine plant weights from main engine weights (for diesel units)

Although the determination of the weight of the engine plant as a function of a known main engine weight is in itself a rather imprecise method, it will nevertheless produce good results if basis ship data are available. In the absence of manufacturers' specifications, the following values relating to a 'dry' engine (without cooling water and lubricant) can be used as approximate unit weights for diesel engines:

slow-speed engines (110-140rpm) 0.016-0.045t/kW

medium-speed engines in series (400-500 rpm) 0.012-0.020 t/kW

medium-speed V-type engines (400-500 rpm) 0.008-0.015 t/kW

C is also around the upper limit where ships have substantial additional machinery weight (classed as part of the engine plant), e.g. tankers, reefers.

Gearbox weights

Gearbox weights are based on catalogue specifications. Factors influencing the weight include power, thrust, speed input and output, the basic design, i.e. integral gearbox, planetary gearbox, and whether the gearbox is cast or welded. For welded single-reduction and integral gearboxes giving a propeller speed of 100 rpm, a power-related weight of 0.003-0.005 t/kW can be assumed. Where propeller speeds n are not fixed, values can be chosen within the normal limits:

Cast gearboxes are approximately three times as heavy.

The use of weight groups to determine engine plant weight

Using easily determined weight groups to calculate engine plant weight is primarily suitable for diesel units. The weight of the unit can be divided up as follows:

1. Propulsion unit

Engine— using catalogue or unit weight Gearbox— using catalogue or unit weight

Shafting— (without bearing) using classification length and diameter For material with tensile strength 700 N/mm2, the diameter of the shaft end piece is:

d = 11.5(PD/n)1/3 d in cm, PD in kW, n in rpm The associated weight is: M/l [t/m] = 0.081(PD/n)2/3 Propeller— A spare propeller may have to be taken into account.

The following formula can be used for normal manganese bronze propellers: Wprop = D3 • K(t/m3)

for fixed-pitch propellers: K ^ 0.18AE/A0 - (Z - 2)/100 or

-(Z - 2)/100) with ds the shaft diameter Controllable-pitch propellers for cargo ships K ^ 0.12-0.14 Controllable-pitch propellers for warships K ^ 0.21-0.25 Ehmsen (1970) gives weights of controllable-pitch propellers. Fixed-pitch propellers on inland vessels are usually heavier than the formulae indi-cate—the same applies to ice-strengthened propellers and cast-iron spare propellers. Electrical units

Generators powered by diesel engines operate via direct-drive at the same speed as the engines. For turbo-generators, the turbine speed is reduced by a gearbox to a speed matching the generator characteristics. The shaft generator arrangement (i.e. coupling of generator to the main propulsion system) has the following advantages over an electricity-producing system which incorporates special propulsion units for the generators:

1. The electricity is produced by the more efficient main engine.

2. The normally cheaper fuel oil of the main engine is used to produce the electricity.

3. There is no need for any special servicing or repair work to maintain the generator drive.

The use of a shaft generator often requires constant engine speed. This is only compatible with the rest of the on-board operation if controllable-pitch propellers are used and the steaming distance is not too short. Separate electricity producing units must be installed for port activity and reserve requirements.

In the weight calculation, the electrical unit weight includes the generators and drive engines, usually mounted on the same base. Switchboards and electric cables inside the engine room are determined as part of 'other weights' belonging to the engine plant weight. The weight of diesel units is:

The output of the individual unit, not the overall generator output, should be entered in this formula (Wangerin, 1954).

There are two ways to determine the amount of electricity which generators need to produce:

1. Take the sum of the electrical requirements and multiply this with an empirical 'simultaneity factor'. Check whether there is enough power for the most important consuming units, which in certain operational conditions have to function simultaneously.

2. Determine directly using statistical data (Schreiber, 1977).

Other weights

Pumps, pipes, sound absorbers, cables, distributors, replacement parts, stairs, platforms, gratings, daily service tanks, air containers, compressors, degreasers, oil cooler, cooling water system, control equipment, control room, heat and sound insulation in the engine room, water and fuel in pipes, engines and boilers. This weight group is a function of the propulsion power, size of ship and engine room and standard insulation. As a rough estimate:

The lower values are for large units of over 10 MW. 4. Special weights—on special ships

1. Cargo pumps and bulk oil pipes.

2. Cargo refrigerating system (including air-cooling system without air ducts). Weights of around 0.0003 t/(kJ/h) or 0.014 t/m3 net net volume.

The refrigeration system on refrigeration containerships weighs ~1t/40ft container with brine cooling system, ~0.71/40 ft container with direct vaporization. The air ducts on refrigeration containerships weigh ~0.8t/40ft container with brine cooling system, ~1.37t/40ft container with direct vaporization. The insulation is part of weight group 'E&O'. Its weight, including gratings and bins, is ~0.05-0.06 t/m3 net net hold volume, or 1.9t/40ft container when transporting bananas, 1.81/20 ft container when transporting meat.

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