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Ever wondered why the bottom of boats often have steps?
The image above shows the wave profile around a rib. If you take a closer look at the white hull you will see two creases or features which run the length of the hull; these are known as ‘spray rails’. Different boats have differing numbers and complexity of spray rails, but primarily these features have two main functions:
- To generate additional lift and promote planing.
- To divert spray away from the hull which hopefully acts to reduce resistance and keep the passengers dry.
TotalSim have been putting their latest methodology for hull resistance calculations to the test to determine whether CFD can pick up the subtleties of spray rails and at the same time provide some interesting insight.
The hull used in the demonstration was drawn by us and please accept our apologies as we are no Naval architects! We do know a lot about CFD and fluid flow and hopefully you will find this post interesting at the same time of demonstrating some of TotalSim’s capabilities.
A closer look at spray rails.
The image to the right shows a close up of the spray rail. You can see from the diagram that they separate flow away from the hull and generate lift at the same time.
In our example two large spray rails were added. Both hulls were then tested at a constant speed and a fixed sink and trim. By fixing the attitude we get a clearer picture of the spray rails’ effect and allows for a simplified comparison of the performance.
How did they perform?
Two simulations were carried out using TotalSim’s current best practice for this type of analysis, which was recently validated against the NPL Hull series. The mesh and solve process is highly automated.
The hull with spray rails generated 16% more lift and 3% less resistance/drag. A significant increase in efficiency.
So we now know that the spray rails are generating extra lift, as expected, but how about the spray issue?
The images below shows a side by side comparison of the hull with and without spray rails. We can see that the rails are acting to divert the flow away from the hull, reducing the potential for wet passengers! So far so good and they are behaving as we expect.
A little more insight.
If we dig into the solution a little further we can apply the same methods and analysis techniques as we use throughout our consultancy business. The image below shows a comparison of the hull, with and without spray rails, and shows the vertical pressure component. This is the component of the pressure force (not including hydrostatic) that acts vertically e.g. a vertical surface could have no contribution. This helps us to show which parts of the hull are helping generate the additional lift.
We can see that regions formed on and upstream of the spray rails make the most significant contribution. These are marked 1. on the image.
One feature that stands out is the peak just downstream of the inboard spray rail, marked 2 on the image above. There is also the discontinuity between the peak from the internal spray rail and the peak at 2. Further analysis using the volume fraction combined with an isosurface of the interface gives further insight. The inboard spray rail clearly separates flow from the hull surface, as shown by the top half of the image below, marked 1. This separation appears to entrain air outside of the spray rail, shown by the streak along the hull in the bottom half of the image, marked 2.
We hope you have found this article interesting, If you would like to know more about how TotalSim can help with your marine and hull resistance problems then please get in contact,
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