R.J. Boonstra

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Master thesis (1)

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Keel-Rudder Interaction

A look into the wake of a sailing yacht

Master thesis (2019) - Rep Boonstra , Riaan van 't Veer , Lex Keuning , Ido Akkerman , Austin Kana

In the early 00 of this century, Keuning et al. (2007) looked into the hydrodynamic forces on the rudder and the influence of the hull and keel on these forces. Among others, they have assessed the lift and drag forces on the rudder. One of the outcomes of this study was an asymmetric lift curve; for negative rudder angles the rudder seems to stall at angles more than five degrees smaller than for the positive rudder angles.
The goal for this research is to find and clarify the physical phenomenon which induces the rudder to stall at smaller rudder angles when subjected to a negative rudder angle (during bearing away). The main question to be answered in this report is: What physical phenomenon is at the basis of the asymmetric stalling behaviour on the rudder of a sailing yacht?
Towing tank tests are used to validate the data from Keuning et al. (2007). After which RANS CFD simulations are conducted in NUMECA to compare the towing tank tests to and to visualise the wake of the yacht in attempt to clarify the phenomena found.

A number of conclusions were found in this study. Firstly, the results of the towing tank experiments showed similarities to the previous experiments by Keuning et al. (2007). Differences are found in stall angles for positive rudder angles. These differences raise questions on the correctness of either of the experiments.
Secondly, no reasonable explanation is found for the negative drag forces found in the towing tank experiments. It is expected that these originate from the set-up of the rudder.
Thirdly, the lift curve found in the experiments is confirmed by the CFD data for the test cases. The physical effect behind the early stall behaviour of the rudder is still unknown. It is indicated that a part of the decrease in stall angle, a couple of degrees, is caused by the influence of the keel, when the disturbance passes on the low pressure side of the rudder. The hull is responsible for the remaining decrease. The CFD data indicates an influence of the vorticity of the keel and the boundary of the hull to cause a disturbance on the rudder.
Further research in necessary to clarify the results found in this study.