Semi-Displacement?

Metacentric height vs lenght and displacement

I appreciate that this thread is lifted to a higher level.
Much nonsense has been written on this theme.

I wonder if the definition of the relative velocity is relevant. The Froude number in relation to displacement and length are not enough. Is there a better dimension that relates more closely to the hull shape?

What I think about is the longitudinal meta-centric height. This should be combined with transverse meta-centric height in some way. These two dimensions relates to the hull form, displacement, length and width, also above the waterline. These dimensions and the relationship between them should show some of the wave making character. It is not only the question of wavelength, but also its amplitude.

Added to this, of course, comes the planing surface boundary to identify the hydrodynamic lift, with or without spray strips. Without longitudinal spray strips it would be difficult to identify the planing surface.

Just a thought

js

 

Ad Hoc & Leo,
You are essentially talking about the same thing, but are seeing it from different points of view.

Ad Hoc uses the wave profile and hydrostatics to explain the concept of local buoyancy and squat change at different speeds.
Leo uses the hydrostatic buoyancy plus local dynamic lift/suction forces (and relative moments) to arrive to the same result.

But both explanations are based on the single concept - pressure distribution around the hull. It is just a matter of convenience whether you prefer use the pressure through one calculation method of another.

As Ad Hoc has explained, pressure distribution around the hull generates a wave profile, which in turn can be used to calculate the local hydrostatic lift.
But the initial hydrostatic pressure field is modified by and additional pressure distribution, created by water flowing around the curved hull surface, as Leo is stating.
Which is again exactly what Ad Hoc is saying.

Hence they are both legitimate ways of seeing things, because they are physically strongly interconnected (or, better - indivisible).

 

It's even more complicated than that. The water also squats around the hull.

Cheers,
Leo.

 

Yes, as I said in an earlier post, it depends on which point of view you prefer. My preference is to calculate the sinkage force and trimming moment directly.

Cheers,
Leo.

P.S. Were the Italian football team greeted with a barrage of rotten tomatoes on their return from South Africa? I miss the old methods of football criticism ;P

 

I don't know of anything simple in deep water.

In water of finite depth, some simpler calculations of squat are available for slender hulls. These approximate formulas use the section area curve rather than the actual hull offsets.

Leo.

 

Semi-displacement Hull Drawings

Hi,

Have you found any semi-displacement ship drawings? I would also like to see exactly how the hull looks like. It should be nice if someone could post a Freeship file with a semi-displacement hull.
Thanks
APP

 

Dear APP,

See the attached file.

 

I couldn't open the '.fbm' type of file.

how about posting an image?

 

APP,

Why you can't open it?
What version of Freeship do you have?

 

thx, I forgot I did a "restore" on my computer

is that some default hull?

 

Yes, NPL Series / 100

 

Perfect! Thanks a lot.

I take the opportunity to ask you:
------------------------
Please refer to http://www.westlawn.edu/news/WestlawnMasthead06_June08.pdf
"Practical Speed and Power calculations June 2008 page 12" Table:
Buttock Angle vs Speed
Semi-Displacement Boats:
Speed- Lenght Ratio 1.5 to 2.5
Buttock Angle 3 to 6 deg.
Angles to the at-rest waterline
------------------------
Have you by chance calculated the SL Ratio and the buttock angle of your above posted semi-displ. Freeship Hull? and how you do it for the buttock angle? (sorry I am not an expert).
Thanks
APP

 

When you get the file to load and it's all working well, you might have to get some corrections to the hull offsets. Last time I looked at the file it was produced using a table of offsets published in:

Bailey, D.,
"The NPL High Speed Round Bilge Displacement Series"
1976, RINA.

There are about 5 typos in the original table.

Good luck!
Leo.

 

Dear APP,

Trying to help you please see the attached files.
These files could be opened through any kind of CAD software.
Following the process DIMension command from Menu Bar - before that please enable the Snap mode - you can measure any distance and / or the areas and / or some volumes you may need to find.
If the CAD software send you back an error, please check if the line-points that you locked are section points or you have to make small correction to the Lines' continuity on sections' points to the (Hull) polylines.
I am sure that will give you an elementary experience to measure a Hull and to find alone the way to achieve a solution on what you asked.

 

Sorry, I am not an expert. I just needed to know the buttock angle degrees. However, I noted in above mentioned page [http://www.westlawn.edu/news/Westlaw...d06_June08.pdf ] that there is a drawing with an explanation of the buttock angle, so the matter is solved. OK.

PS: To note it is not just a buttock angle but the "one quarter (1/4) beam buttock angle".

Thanks again
APP