# Powerboat trim angle study

#22

So here it comes - the at-all mention ;): @radenpm9, @ggiraldo, @Zoran, @zgaryulo, @bkocijan any progress on your end? I didn’t find the time to work on it in the meantime but would be happy to give quick hints / help if somebody is currently working on it! Would love to see some first results.

Best,

David

#23

Hi @dheiny and everyone,

I have my mesh the same as last time, with the rare refinement artifacts, expecting someone to come up with a solution. I think it is related with the domain being defined as a solid from CAD.

Anyway, I don’t think this is a show stopper that will prevent the simulation from running, I just think it could induce some errors. But for first approaches this could be acceptable.

I will run the simulation and report back.

Greetings,

#24

Hi @zgaryulo,

Here is needing the moment of inertia values, in [kg-m^2] units and in a system with origin at the center of mass.

Also please clarify, the given center of mass and weight of the hull are for the full geometry or the halved one shown?

#25

Hello all,

I’m interested in being a part of this! @zgaryulo if it should be possible, could you please mail me the paper at srajendran@simscale.com. Thank you!

I tried to work on the mesh - please have a look into this project https://www.simscale.com/projects/sjesu_rajendra/powerboat_trim_angle_study/
It was kind of on a trial and error basis for the bounding box and other geometry primitive sizes, that I was able to achieve some uniform refinements. Thanks @ggiraldo for the geometry. I had this as the starting point to create the mesh.

Looking forward to interesting learning experiences out of this project.

Regards,
Sam (@sjesu_rajendra)

#26

I will just ask for help here, because I literally copied all the parameters in your mesh computation, and I am still getting the weird refinements. I can see that most of them are the same as mine, only difference refinement levels.

I am just scratching my head here, any detail in your findings is welcome.

Greetings,

Edit: I found something I had missed, it is running, will report back.

Edit 2: Fixed! Just for the record, the problem was the size of the refinement box for the meridian interface. Changing dimensions for it to be just smaller than the domain was the fix, thanks Sam!. I also now have two meshes, one coarse with ~500k cells and a finer one with ~3.8M cells.

#27

Hi everyone, for the record, the refinement pattern issue is fixed. You should not see the problem for newly created meshes. Best, Babak.

#28

Hi @zgaryulo,

I am using the Savitsky paper to determine the trim angle of a catamaran. I see you already have a validated Excel spreadsheet to calculate it so, would it be possible for you to share your Excel file with me?

Thank you very much.

Regards.

#29

Dear all,

Let me assist here. This topic is of great interest to me and I will try to explain the the differences in expectation between the naval architecture and the CFD simulations.

Firstly, Daniel Savitsky’s first paper on the subject was in 1964 which is a very long time ago and before CFD existed. His method of calculation does not predict the trim angle at a certain velocity moreover the lift force and resistance based on a certain angle and speed. So, for CFD we are after a steady state simulation at a certain angle at a certain velocity. There is a very big opportunity for CFD engineers here as the limitation of Savitky’s method is that it only works for simple prismatic shapes which are seldom used now. So, as you can see from the bottom of the hull, there is no variable deadrise (for CFD engineers - the cross-sectional shape does not change from front to back). We know that a variable deadrise hull works much better now and this is where CFD can accurately predict the the lift and drag of the hull very easily. However, what is required is to prove that CFD gives the same results as the tried and tested Savitsky method on simple prismatic shapes.

I have viewed some of the mesh set ups and there are a few anomalies. The model must be inclined upwards in order to produce dynamic lift in a steady state. The shape of the front of the boat although under simulation will produce dynamic lift based on the shape, in real life it is a little different - the boat will pitch up and for a time will be pushing water until it gets fast enough to reach the steady state simulation where it is supported by both bouyancy forces as well as the vertical force component based on the inclined surface (hydrodynamic lift). You will notice from the results of the Savitsky formula calculations the drag forces are lower with with the trim angles are lower even though velocity increased, there is less boat in the water at this time. Savitsky method predicts this with formula i.e. calc wetted area and trim angles based on centre of gravity first then do the resistance calculation.

Hope this helps. On the SimScale CFD training, there was one particularly interesting topic which is the quasi-steady state or local time-stepping which I am not sure how it works to predict the hydrodynamic lift and drag forces of a hull. Perhaps someone can help me with this. There was not a lot of information about how to perform this type of simulation but I believe this may be the simulation which may be the best to get a powerboat trim angle and resistance calculation.

Regards,

YL

#30

Dear all,

I have re-read the earlier posts on this topic. It seems there was a problem with the mesh. And most of those really interested to do this simulation were staff of SimScale. Also, I noticed that there were requests for technical information e.g. Savitsky paper, excel spreadsheet. And since many months ago interests seems to have waned.

Just so that you are aware, I have Savitsky’s 1964 paper, his 1976 paper for rough and smooth conditions, his 1979 paper (re-evaluation) and others as well. And I also have the excel spreadsheet. I will forward the links via our SimScale technical account manager Akrem. If you are interested, please get what you need from him.

Besides this, I am the holder of the patent for a watercraft which uses a novel means of planing. So, if your interest is the old planing regime, all you need to get you up to speed…you will be able to get from Akrem. Also, as we have signed a mutual non-disclosure with SimScale, if you are interested in the novel means of obtaining hydrodynamic lift, please let Akrem know also ;).

Best wishes,

YL