ShipWorks

Hydrodynamic Ship Design Forewords

In the following pages hydrodynamics hull design guidelines are presented. Since the wide theory behind the matter and due to the not simple laws leading the phenomena and the interaction between the involved parties, the main purpose of the present session of the manual is to give a beginning support to those are approaching the matter. Therefore, should be taken in mind during the application that, experience of the designer and his perspicacity is the first tool to be adjusted.
The present session of the manual has the purpose to give some information on how to approach the hydrodynamic design of a vessel.
Should be clear, from the very beginning, that hydrodynamic problems can not be presented in the following few pages, therefore herewith some guidelines are going to be presented in order to give a support to whom are approaching this field.
Introduction and Back-ground
It is a common experience that, when a ship is sailing at certain speed, an effort is required. As matter of fact, the environment, in which the ship is moving, exert a resistance to the ship.
The resistance exerted from the environment, for a Ship, can be divided into two main components, the first related to the water and the second to the air.
If from theoretical point of view, it is easy to understand that the generation of the resistance is connected with displacement of the fluid around the advancing hull, quantify this resistance it everything but easy.
Even without taking into account the complication due to the presence of two contiguous fluids, air and water, and their interaction, the prediction of the resistance of a Ship and then the power to install on board it's a complex matter.
Because the determination of the resistance of the hull is the main task of the CFD, in the following a brief presentation on how the Resistance can be managed and then propulsion power arranged on a ship is discussed.
Resistance of the Hull
Classically the Resistance of a vessel is divided into some components according to the experimental way followed to determine it.
Everything starts from Froude observation from which was understood that different model scale of the same hull generates on scale wave pattern when the ship speed is scaled according to Froude similarity. In other words a given hull sailing at a certain speed so that

known as Froude Number, have the same value, generates the same wave pattern independently from the model scale.
From this occurrence borne the opportunity to analyse the Hull resistance as made by two different contributions.
The first one, known as Frictional Resistance R F , is just related to the resistance encountered from a submerged plate sailing at a given speed. Being a plate a two – dimensional object its contribution to wave generation is negligible, therefore the total resistance measured on the plate, is assumed as pure frictional resistance. Following a parametric study an analytic formula have been delivered. From that formula a reliable Friction resistance can be predicted once wetted surface and speed, together with physics characteristics of the fluid are known.
The second one is the Wave Resistance R W . Tank test gives the measurements of the total resistance of the hull, once from this value the frictional resistance is subtracted the remaining part is called Wave Resistance. Evidently this procedure is the same to assume the contribution to the hull resistance, a part from the frictional one, mainly related to the wave component. This is very near to be true, but should be taken into account that there are also other contribution due to viscosity of the water, vorticity and others that even if they are generally negligible they have an influence on the total results. This second contribution, could be also known as Residual Resistance, in order to don't forget that the wave contribution is only a part of the wide contribution even if it is main one. In the following we shall refer to Residual Resistance.
Since the minor contribution don't follow the same scaling law of neither frictional or wave resistance same discrepancies can be found in the true scale results.
That's why the final results of the tank tests are usually corrected according to a Trial Allowance based on full scale measurements comparison with tank test results.
Based on what above, the full scale resistance of a vessel can be predicted as follows:
Measurements of total Model scale resistance by means of a Resistance Test to carry out in a towing tank;
Calculation of Model scale Friction Resistance by means dedicated formula.
Calculation of model scale Residual resistance ;
Calculation of water density ratio between Sea and Tank test water: ;
Calculation of model scale ;
Calculation of full scale Friction Resistance by means dedicated formula;
Calculation of full scale Residual resistance ;
Determination of full scale total resistance ;
Possible Correction of total resistance above determined applying Trial – Allowance factor.
Should be noted that the Trial – Allowance is determined according to designer own experience, therefore it is commonly applied to the power more than the resistance. This is because full scale measurements can be easily taken in terms of power, on the contrary it's very hard to get information on full scale about Hull resistance.