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Hull Ebb

Vessel Delay Analysis

Delay analysis

 

There are numerous historical well as recent and predictive datasets. System that provides real-time information about water levels, currents, and other oceanographic and meteorological data from bays and harbors “Now casts” and predictions of these parameters with the use of numerical calculation models are available. In certain locations this information is very important to track because changes to the bathymetry due to dredging or as a resulted in changes in water currents or other oceanographic effects.

 

A program by ….is capable of predictions of ship motions (i.e. displacements, velocities, and accelerations) for a ship advancing at constant speed, on arbitrary headings in both regular waves and irregular seas. The irregular seas are modeled using either the two parameter like Bretschneider, the three parameter Jonswap, or the six parameter of Ochi-Hubble wave spectral models. Both long-crested and short-crested results are provided; short-crested waves are generated using a cosine squared spreading function. In addition to the 6DOF responses, will predict the absolute motion, velocity, and acceleration, as well as the relative motion and velocity for various locations on the ship. SMP95 will calculate the probabilities and frequencies of submergence, emergence, and/or slamming occurrence for various locations on the ship. It also incorporates recent innovations for calculating added resistance in waves based on the work by Wen-Chin Lin and Arthur Reed, as documented in their paper "The Second Order Steady Force and Moment on a Ship Moving in an Oblique Seaway"

 

 

Standard power

 

Navigation is typically formulated to provide safe and efficient passage for a selected ship under specified transit conditions. The design ship and transit conditions may be selected to represent the “maximum credible adverse situation,” the worst combination of conditions under which the project would be expected to maintain normal operations. A project that successfully accommodates this situation can be expected to perform well with a full range of smaller ships and less difficult transit conditions.

(1) Design vessel - For deep-draft projects, the design ship or ships are selected on the basis of economic studies of the types and sizes of the ship fleet expected to use the proposed channel over the project life. The design vessel or vessels are chosen as the maximum or near maximum size ships in the forecast fleet based on the characteristics (length, beam, draft) of the ships being most representative of the potential economic advantage to be found in the forecast ship fleet.--For small craft projects, the design vessel or vessels are selected from comprehensive studies of the various types and sizes of vessels expected to use the project during its design life. Often, different design vessels are used for various project features. For example, sailboats, with relatively deep draft, may determine channel depth design; and fishing boats, with relatively wide beam, may dictate channel width design.

 

(2) Operational conditions selected for design can strongly affect a navigation project. A deep-draft project should be designed to allow the design ship to pass safely under design transit conditions. Normally, extreme events are not considered in specifying design transit conditions. Ship operators can usually suspend operations during these rare events without undue hardship.

 

 

 

(a) Operational factors to be specified for design transit include:

• Wind, wave, and current conditions.

• Visibility (day, night, fog, and haze).

• Water level, including possible use of tidal advantage for additional water depth.

• Traffic conditions (one- or two-way, pushtows, cross traffic).

• Speed restrictions.

• Tug assistance.

• Underkeel clearance.

• Ice.

 

Normal operational conditions are strongly influenced by individual, local pilot, and pilot association rules and practices. For example, pilots usually guide a transit only when conditions allow adequate tug assistance. There may be operational wind, wave, or current limitations on the ability to safely moor a ship at a terminal or berth. Turning operations and maneuvering into a side finger slip may be limited by tide level and current conditions, including river outflow. Energetic wave conditions at the seaward end of the entrance channel may prohibit pilots from safely transferring between the pilot boat and ship. Such operational limitations may well be controlling factors in determining whether or not a safe transit is possible, and navigation project design should be consistent with these limitations.

 

Ice navigation. Winter conditions along northern sea coasts, estuaries, and large lakes can cause ice to be an occasional, if not chronic, concern for safe and efficient navigation About 42 percent of the earth experiences temperatures below freezing during the coldest month of any year. The presence of ice is accompanied by longer nights and increased fog and precipitation. Shipboard mechanical equipment, instruments, and communications apparatus are less efficient and more prone to failure in cold temperatures. Aids to navigation become less effective, and maneuvering in ice is much more difficult, hence ships experience difficulties. Sea ice nomenclature and map notation symbols are defined by the World Meteorological Organization (WMO 1970). Ice thickness and structure are key concerns. Multi-year ice, sea ice more than 1 year in age, can be over 3 m thick, but it is found only in the Arctic Ocean and its marginal seas and near Antarctica.  Therefore ships that regularly navigate icy waters must have exceptional structural strength and propulsion power for safety of the crew, equipment, cargo, and environment. Special hull, propeller, and rudder designs reduce resistance and help clear lanes.

 

Icebreakers may be needed to escort ice-strengthened cargo vessels or to periodically clear shipping routes.Factors to be considered for ship operation in ice rather than temperate conditions are:

• Ship maneuverability is retarded.

• Ice forces can divert ships from their intended course.

• Darkness is more common.

• Low visibility is more common (fog and precipitation).

• Winds can be very strong.

• Visual aids to navigation are less effective.

• Shipboard instruments are more prone to malfunction.

• Assistance or rescue by tugs is more difficult.

• Crews are more strained and fatigued in the face of these challenges.

 

 

 

 

Standard displacement and trim

 

Design draft is the distance from the design waterline to the bottom of the keel. Ship depth is a vertical dimension of the hull, as shown in the figure, and it should not be confused with ship draft.  Draft may not be uniform along the vessel bottom for both deep- and shallow-draft vessels. Normally, draft near the vessel stern (aft) is often greater than near the bow (fore). Two useful indicators of such variations are: trim - difference in draft fore and aft, list - difference in draft side to side. Maximum navigational draft is the extreme projection of the vessel below waterline when fully loaded) is needed for navigation channel depth; mean draft is preferred for hydrostatic calculations. Waterline beam (width of the vessel at the design or fully loaded condition necessary for navigation channel width. Maximum dimensions of the above-water part of a vessel are also critical to ensure adequate clearance. Maximum beam is the extreme width of the vessel. Lightly loaded draft is the minimum vessel draft for stability purposes, from which vertical clearance requirements, such as clearance under bridges, can be determined.

Trim is generally defined as the longitudinal inclination of a ship, or the difference in draught from the bow to the stern. It is controlled by loading. In general, at low speed, a ship underway will squat by the bow. The practice is to counteract this squat by trimming the ship by the stern when loading. The rule of thumb is to provide an allowance of 0.31 m to account for trim in waterway design. The normal approach for a vessel is to assume a trim rate of 3"/100 ft of length or 0.25 m/100 m. for PTP for maximum vessel size of 440m, 1m is use.

 

 

 

 

 

Monitoring speed

 

Ship performance is measured through different method that range from Non-liner motions functions of equipments, stresses and impacts, Inertial forces and Damage to equipments, and speed reduction.

 

 

Ship motions can be investigated in 4 different ways:

 

  1. Theoretically        -  Analytical and numerical methods
  2. Experimentally     -  Using model tests
  3. Empirically           -  Statistical observations
  4. Directly               -  Ship trials

 

Theoretical investigations –     Derive simple analytical expressions for describing surface of the seaway and determine the ensuing vessel motions.

 

The studies include the following:

 

-         Important characteristics of the vessel and the environment

-         Prediction of motions

-         Insight into acceptable values of motions

-         Knowledge of average performance to be expected including stability and resistance

-         Basic ideas regarding motions stabilization and ways to achieve it

-         Guidelines for model tests and full-scale trials

 

However, since ship motions are highly complex, a combination of ways has to be used.

 

In conjunction with theory, experiments are carried out to predict ship performance and  ship trials are conducted in order to correlate model and ship results. 

 

Computer simulations are also used to study the ship performance.  This is usually in the areas where complex behavior as non-linear cannot be predicted analytically and performed experimentally. 

 

IRREGULARITY OF THE SEAWAY AND THE HISTOGRAM

 

The degree of irregularity of a seaway can be shown by the shape of its Histogram.

 

         i.e. Frequency of occurrence for the individual wave characteristics.

 

Histogram is basically derived from the record of wave elevations as shown:

 

 

 

Also, another way of plotting the Histogram is to plot the cumulative distribution as by diagram below:

 

 

 

 

 

 

It has been found by experience that the theoretical Rayleigh curve fits the histograms for the wave height (double amplitude) very well.  Further discussions on this subject will be mentioned later.

 

 

Vessel Motions in Irregular Seaway

 

Though the principal of linear superposition has to be adhered to, it has been found from experiments that the response of ships in irregular seas indicated that the linear spectral techniques yielded unexpectedly accurate results in seas as high as seastate 7.

 

Thus, practically linear theory can be applied for up to moderate sea state cases.

 

The basic steps in predicting the vessel motions are given as follow:

 

1.                  Choose a suitable wave spectrum S(w)

 

2.                  Calculate the encounter frequency wE, as a function of the wave frequency w, vessel speed V, wave speed e, and heading m: wE = w(1 – V.c-1 cos m)

[m = o0 for following seas, 180o for head seas]

 

3.                  Calculate the encounter spectrum S(wE) = S(w) x dw/dwE

 

4.                  Evaluate R.A.O =

 

Where it is assumed that;

 

(a)    The response to each component wave is independent of the response to the other waves.

 

(b)   The response is a linear function of the component wave amplitudes.

 

[ * a commonly used wave input parameter is the wave amplitude for           translational motions or wave slope for rotational motions ]

 

 

5.                  Plot the response spectrum SR(wE) = S(wE) x (R.A.O)2

 

 

 

The frictional resistance of the large, full bodied ships will very easily be changed in the course of time because of fouling. In practice, the increase of resistance caused by heavy weather depends on the current, the wind, as well as the wave size, where the latter factor may have great influence. Thus, if the wave size is relatively high, the ship speed will be somewhat reduced even when sailing in fair seas.

 

  • Increase of the propulsion power may only result in a minor ship speed increase, as most of the extra power will be converted into wave energy. Erosion will start, and marine plants and barnacles, etc. will grow on the surface of the hull. Bad weather, perhaps in connection with an inappropriate distribution of the cargo, can be a reason for buckled bottom plates.
  • In principle, the increased resistance caused by heavy weather could be related to: Wind and current against, and b) Heavy waves, but in practice it will be difficult to distinguish between these factors.

 

  • The loss of power is itself a function of the hull design. hull fouling with barnacles and tube worms may cause an increase in drag (ship resistance) of up to 40%, with a drastical reduction of the ship speed as the conSequence.In general, for every 25 ?m (25/1000 mm) increase of the average hull roughness, the result will be a power increase of 2_3%, or a ship speed reduction of about 1%.
  • Resistance will also increase because of sea, wind and current, as shown in Table 4 for different main routes of ships. The resistance when navigating in head on sea could, in general, increase by as much as 50_100% of the total ship resistance in calm weather.
  • On the North Atlantic routes, the first percentage corresponds to summer navigation and the second percentage to winter navigation. However, analysis of trading conditions for a typical 140,000 dwt bulk carrier shows that on some routes, especially Japan-Canada when loaded, the in creased resistance (sea margin) can reach extreme values up to 220%, with an average of about 100%.
  • Unfortunately, no data have been published on increased resistance as a function of type and size of vessel. The larger the ship, the less the relative increase of resistance due to the sea. On the other hand, the frictional resistance of the large, full-bodied ships will very easily be changed in the course of time because of fouling.
  • In practice, the increase of resistance caused by heavy weather depends on the current, the wind, as well as the wave size, where the latter factor may have great influence. Thus, if the wave size is relatively high, the ship speed will be somewhat reduced even when sailing in fair seas.

Because waves are highly variable within a specified sea state, it is prudent for final design to perform parametric studies for similar ships and, in many cases, to conduct studies to develop realistic estimates. Such studies could consider:

 

(a) Analytic studies, using strip theory or other theoretical calculation methods as developed by naval architects.

(b) Interactive, real-time ship simulator studies.

(c) Physical model studies, using radio-controlled, free-running scaled ship models with wave response measurements.

(d) Direct, onboard ship measurements while transiting through the entrance channel.

 

Physical model studies to aid in probabilistic navigation channel design are also used for evaluation. Direct field measurements of ship motion are valuable additional studies, but extreme conditions controlling design are not easily captured. Field measurements are dependent on available ships and environmental conditions during the limited duration of the measurement program. (Wang et al. 1980).

 

Exxon International Company published a report based on Empirically Method for Estimating Vertical Ship Excursions in Waves.entitles “Underkeel Clearance in Ports” in 1982. outlined a  procedure to estimate the allowance required for a tanker due to wave-induced motions, Based on the model tests conducted at the Netherlands Ship Model Basin (NSMB) (Koele and Hooft 1969) .the responses of a loaded, untrimmed 200K dwt tanker in shallow-water waves were estimated. The majority of the tests were conducted with ship speeds of 7 knots. Based on SOREAH model tests (1973a-c) and some theoretical predictions for a 21K dwt tanker performed by NSMB (1980), Kimon (1982) extrapolated the 200K dwt tanker responses and other vessel sizes.

 

 Deep-water wave data can be used to determine general trends in wave characteristics for the project area, but complexities in local bathymetry and shore orientation will produce a local wave climate that is different from the offshore data. The effects of the direction of water currents ebb and flood tidal currents, on the waves must also be taken into account in determining the characteristics of the waves being encountered by the ship in an entrance channel. It is important that wave characteristics represent the waves that the ship will encounter since the motions of the ship are the result of the ship’s response to the waves.

 

Predicting ship response in wave is a major problem without easy solution. Physical models can be used to predict ship response from monochromatic waves, if model ships are properly scaled, constructed, balanced, instrumented, and tested, lack of data results in difficulty in verifying proposed models to predict the motion of a ship induced by waves to compensate for this motion.

Loss of speed on turn

 

 

  1. monitoring speed- power performance and correct to standard condition i.e “ no weather”

 Speed deterioration as hull deteriorate

 

  1. log book analysis – slowing down because of fog
  2. notes roughness for fouling for about half delay
  3. bad weather- (1/2 voluntary + ½ involuntary) for about 1/3 delay

 

 

 

Computer program for time-and-motion simulations which applies predicted tides and tidal currents, historical or projected ship cargoes, drafts, and arrival times at the ocean entrance, and cargo transfer rates at port can also be used. The same model can simulate ship transit times with various dredged channel geometries. Waiting times are computed as the difference between transit time simulated across the shoals and transit time without shoal restrictions. Deeper channels will tend to be more efficient sediment traps and can shoal at faster rates (Trawle 1981). However, a deeper channel might tend to localize shoaling and could reduce the length of channel to be dredged and cost of maintenance dredging

 

Randomly occurring combinations of variables affecting transit times, such as strong winds, river discharge- and wind-induced depth changes, high waves, low visibility, and ice conditions can be added using a Monte Carlo approach. This method requires enough repeated simulations of the same input variables with random values of stochastic variables to encounter the full range of combined extremes. Statistics of transit time, waiting time, and associated costs can be computed from these data. A more complete discussion of vessel traffic flow simulation models is given by PIANC (1997a).

 

Ship costs are generally proportional to operating time, either under way or at berth. Reductions in time navigating the port approach, at the dock, and departing translate into cost savings. Other cost factors include impacts of ship arrivals on longshoremen, mechanical equipment, and cargo staging at the port. Transportation costs without channel improvement must be estimated as a baseline against which to measure cost-effective optimization of channel excavation. The savings realized by a range of excavation depths and channel configurations should be compared with corresponding dredging and disposal costs. The ideal optimum will achieve the maximum net savings, but environmental quality effects, financial capabilities, and user preferences can affect the final project design.

About the Author

marine engineer

Hull Ebb
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