Introduction
A single propeller with a rudder is the most common arrangement of vessel propulsion and steerage with design guidance available [1, 6]
The use of twin propellers is common to ferries and cruise vessels etc. yet appropriate design guidance is needed. A programme of scale model testing for single propellers [6] has been extended to twin propellers. The results will be presented and a suggested basis of design proposed for rock and concrete mattress protection.
Thinner mattress protection types can produce significant savings in vertical quay walls which will be outlined in the paper.
Testing
Scale model testing was undertaken using two 150 mm Ø open propellers. To replicate actions in berths, the following effects were tested: -
- with and without rudders
- propeller clearance
- propeller separation
- relative propeller rotation
The testing was carried out with a range of model rock sizes and also with a sealed thickness of insitu concrete mattress protection. This testing was an extension of a previous testing programme for single propellers. [6] It has allowed the effect of twin propellers to be demonstrated and appropriate design guidance suggested.
Berth Scour Protection
Guidance for twin propeller actions is presently not well developed and often significantly overestimates the scour protection that is needed.
The paper will present an established design method for rock protection under single propeller action. [3, 6] This guidance will be similarly extended to twin propellers based upon the testing undertaken.
The performance of mattress type protections as ‘Sealed’ or ‘Open’ types will be described. ‘Open’ protection types allow flow entry and high trapped flow pressures underneath. This aspect significantly effects performance and the protection thickness needed. A design method of ‘Open’ protections by Raes et al will be referred to.
For insitu concrete protection formed as a ‘Sealed’ protection, design methods for both propeller suction and flow will be presented in a simplified format for both single and twin propellers. A beneficial combination of insitu concrete mattress protection to structures with rock falling edge aprons will be described. The performance of rock falling edge aprons is well recognised and can be designed to cater for edge scour.
REFERENCES
[1] PIANC Report 180, (2015) Guidelines for Protecting Berthing Structures from Scour Caused by Ships.
[2] PIANC Bulletin 109, Römisch, K., & Hering, W., (2002), Input Data of Propeller Inducted Velocities for Dimensioning of Bed Protection Near Quay Walls.
[3] PIANC Report of Working Group 22, Bulletin no 96 (1997), Guidelines for design of armored slopes under open piled quay walls.
[4] BAW (2005), Principals for the Design of Bank and Bottom Protection for Inland waterways, Bulletin 85, Karlsruhe.
[5] Hawkswood, M.G. Lafeber, F.H., Hawkswood, G.M., (2014) Berth Scour Protection for Modern Vessels, PIANC World Congress, San Francisco, USA.
[6] Raes, L., Elskens, F., Römisch K.W., & Sas, M. (1996) The Effect of Ship Propellers on Bottom Velocities and on Scour Near Berths and Protection Methods Using Thin Flexible Revetments, Proceedings 11th International Harbour Congress, Antwerp – Belgium.
[7] Hawkswood, M.G., Flierman, M., De Haan, R., King, M.G., & Groom, J.A., (2016) Propeller Action and Berth Scour Protection, PIANC-COPEDEC IX, Rio de Janeiro, Brasil.