# EFFORT Streszczenie raportu

Project ID:
G3RD-CT-2002-00810

Źródło dofinansowania:
FP5-GROWTH

Kraj:
Netherlands

## Transom immersion of a hopper dredger - case study report

Title: Transom immersion of a dredger

In work package 5 of EFFORT several case studies have been carried out for application & demonstration purposes. This is the first case study.

Transom immersion is a most common phenomenon for hopper dredgers due to their characteristic fullness together with the deep dredging draught (the operational freeboard of a dredger is only one third of the statutory freeboard). Hopper dredgers have large transom immersion and it is assumed that the effects of it on the total resistance are considerable (transom immersion is important for other ship types too, but to a lesser extend).

To avoid the immersion of the transom the buttocks of the stern must be stretched so, that the transom is fully above the waterline, or they must be curved upward. The disadvantage of the first solution is that the ship length will increase with an amount of approximately 10 m! That increase will have a lot of consequences and must be stipulated as unrealistic.

The disadvantage of the second solution is the fact that the flow will not follow that curvature and will separate from the hull. Then, the resistance will increase too and when the curvature starts in front of the propeller also vibration problems can be expected when the separated flow will enter the propeller plane.

The use of potential flow CFD is normal business nowadays to improve the hull of a dredger, but the stern waves are not predicted very well because of the transom immersion (there is definitely no potential flow anymore). A viscous flow CFD code must be used for this matter.

For this reasons a study has been carried out to investigate the use of viscous CFD for this purpose.

Unfortunately, the code in this case was not able to deal with a free surface. So, the applicability with respect to the sternwaves was not investigated.

Nevertheless, this study remained interesting because a comparison was made between a transom stern and an extended stern. For the flow and resistance matters, the following results can be noted:

- The extension of the stern to avoid any transom immersion is significant and can be judged as unrealistic in practice.

- The computed wake field is not affected by the simplified approach of an extended stern instead of an immersed transom.

- From the propeller plane to the transom further downstream gradually a difference in pressure is noticeable, but rather little.

- In general, model scale and full scale show the same trend.

- Full-scale computations for the shallow water condition show more influence on the axial flow, but only slightly.

- The computed resistance coefficients learned that the right order of magnitude of power increase due to the immersed transom was predicted.

This result will lead to further investigations:

- Improvement of the stern without extension with the help of viscous CFD.

- CFD codes must be developed to a full employable tool to enable prediction of the stern waves.

In work package 5 of EFFORT several case studies have been carried out for application & demonstration purposes. This is the first case study.

Transom immersion is a most common phenomenon for hopper dredgers due to their characteristic fullness together with the deep dredging draught (the operational freeboard of a dredger is only one third of the statutory freeboard). Hopper dredgers have large transom immersion and it is assumed that the effects of it on the total resistance are considerable (transom immersion is important for other ship types too, but to a lesser extend).

To avoid the immersion of the transom the buttocks of the stern must be stretched so, that the transom is fully above the waterline, or they must be curved upward. The disadvantage of the first solution is that the ship length will increase with an amount of approximately 10 m! That increase will have a lot of consequences and must be stipulated as unrealistic.

The disadvantage of the second solution is the fact that the flow will not follow that curvature and will separate from the hull. Then, the resistance will increase too and when the curvature starts in front of the propeller also vibration problems can be expected when the separated flow will enter the propeller plane.

The use of potential flow CFD is normal business nowadays to improve the hull of a dredger, but the stern waves are not predicted very well because of the transom immersion (there is definitely no potential flow anymore). A viscous flow CFD code must be used for this matter.

For this reasons a study has been carried out to investigate the use of viscous CFD for this purpose.

Unfortunately, the code in this case was not able to deal with a free surface. So, the applicability with respect to the sternwaves was not investigated.

Nevertheless, this study remained interesting because a comparison was made between a transom stern and an extended stern. For the flow and resistance matters, the following results can be noted:

- The extension of the stern to avoid any transom immersion is significant and can be judged as unrealistic in practice.

- The computed wake field is not affected by the simplified approach of an extended stern instead of an immersed transom.

- From the propeller plane to the transom further downstream gradually a difference in pressure is noticeable, but rather little.

- In general, model scale and full scale show the same trend.

- Full-scale computations for the shallow water condition show more influence on the axial flow, but only slightly.

- The computed resistance coefficients learned that the right order of magnitude of power increase due to the immersed transom was predicted.

This result will lead to further investigations:

- Improvement of the stern without extension with the help of viscous CFD.

- CFD codes must be developed to a full employable tool to enable prediction of the stern waves.