Some time ago I received a request for a drag calculation for a very unusual vessel. The Swalinge was built as a coaster in 1977 and converted to a dredger in 1996.
The vessel had the following main dimensions:
Length over all - 82.20 m
Length between perpendiculars - 75.23 m
Width - 14.60 m
Draft - 6.30 m
In the process of modernizing the ship's hull, a rather voluminous bow bulb was added. As I understand it, the main function of this bulbous bow was to provide additional displacement in the fore part of the ship. Its volume added to the displacement of about a hundred tons. As is often the case with the modernization of the existing ship hull, the bulbous bow remained a foreign formation. The shipowner suspected that he was not reducing, but increasing the resistance of the ship. This was especially noticeable in the huge bow wave that the ship formed. Therefore, when the shipowner decided on the next modernization of the hull, the task arose to study the resistance to the movement of the ship's hull. Choren Design & Consulting approached me with this question, which was engaged in the modernization project for this ship.
The shipowner also wanted to understand whether such a bulbous bow is needed on a modernized vessel. In agreement with the customer, we approved the following test program:
- CFD calculation of vessel resistance at fully loaded draft and at ballast draft,
- CFD calculation resistance at fully loaded draft and at ballast draft, without bow bulb (original hull version).
Lines drawings of the vessel before and after the modernization were preserved only in paper form. Scans of paper drawings were used to restore the 3D model.
The result of the calculations showed that the resistance of the vessel without the bulbous bow is lower by 40% for the vessel at fully loaded draft and by 2.5!!! times lower for the vessel in ballast. According to the results of the calculations, it became clear that the bulb practically does not work for the ship at fully loaded draft, since it is completely submerged in water. In the case of an empty ship, when the bulb is above the waterline, a huge wave arises, which increases the resistance at times. That is, instead of reducing drag, the bulbous bow works like a brake. This was rather unusual for me, since for ships with such dimensions, wave resistance should no longer play such a significant role. It seems that the bulb was simply cut into the existing hull without any analysis of hydrodynamics and served as an additional volume in the ship's displacement.
Given the purpose of the ship, the shipowner decided to keep the bulbous bow, but at the same time change its shape. One of the conditions was to keep the volume of the bulb the same as in the previous version. I proposed a new bulb shape and after several iterations, the decision was made to settle on the shape with the lowest resistance. It should be noted that even with such a limited possibility of changing the shape of the hull during optimization, it was possible to reduce the resistance of the hull by
36% in ballast and 26% at fully loaded draft. At the same time, the new bulb looks much more elegant.
In the process of working on this project, we also performed an extension of the cylindrical part of the ship. The CFD calculation showed that the elongation had practically no effect on the increase in the ship's drag.
The use of CFD methods in the modernization of ships allows you to fairly accurately determine the resistance of the ship at the design stage. Considering that when upgrading an existing ship, the customer is usually not ready to order model tests in the test tank, CFD methods are the only source for a qualitative assessment of the resistance of the ship being modernized. The reduction in resistance, in turn, leads to a reduction in fuel consumption and emissions into the environment.
All photos in the article are taken from the Internet and are used as technical illustrations.