Please somebody help me. Simple question.
› DELFTship forum › Hull modeling › Please somebody help me. Simple question.
 This topic has 7 replies, 4 voices, and was last updated 11 years ago by John R. Coil.

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February 24, 2012 at 18:36 #35615aseathParticipant
Hey Everyone! Ive recently started to use Delftship. I have a question.
I have downloaded one of the boats from your design database. The ship I downloaded is 152m long and 23m wide(beam). I need the hydrostatics and resistance results. But those results has to be for 90m long and 20m wide ship. So I want the hydrostatics and resistance results for exactly the same hull design/shape but not for 152x23m hull, but for 90x20m.
I went to Project Settings and changed the values from 152×23 to 90×20, and then went for hydrostatics. And it still shows me the results for a 152×23 ship. So how can I make the software to display for me hydrostatics for 90x20m and not for 152x23m.
(Changing the points in the control net didnt work for me because I wasnt able to make it correct).
Please somebody help me.
Thank you very much! 
February 24, 2012 at 20:06 #35616John R. CoilParticipant
As you’ve noted (Transform > Scale) doesn’t always result in exactly what you want with only 4 places after the decimal available and in DelftShip there appears to be no way around that limitation. In the past I’ve tried multiple iterations with very limited success.
I know this may sound weird but since a foot is smaller than a meter it follows that 0.0001′ is less of a variation than 0.0001m would be … so if you are willing to trust the units conversion you may be able to get just a scooch closer than otherwise possible.
Considering that getting a substantial boat built to the nearest centimeter (never mind a 1/10 of a millimeter) over its whole length/width may be good practice may I suggest you need not be so concerned?

February 24, 2012 at 20:11 #35617robin szemetiParticipant
load the 152m x 23m ship, then use Transform>scale
enter 0.592 for longidutanal scale
enter 0.869 for transverse scale
enter some number you like for vertical scale … you will have to experiment here, becasue you are not using the same scale for all axes,now you have the correct sized model, maybe you get the correct dynamics

February 24, 2012 at 21:52 #35618aseathParticipant
Thank you rszemeti! It worked. I used 0.88 for vertical scale. Even though I dont know which factor would give the best values to the ship.

February 24, 2012 at 23:18 #35619MarvenKeymaster
To make sure you end up with exactly the same underwater shape the vertical cale should be:
desired draught/model draught

February 25, 2012 at 05:23 #35621John R. CoilParticipant
Thank you rszemeti! It worked. I used 0.88 for vertical scale. Even though I dont know which factor would give the best values to the ship.
Heh! Looks like I win today’s Misinterpretation Award!
Sorry, I thought you were asking about problems getting an exact final value. ðŸ™‚

February 25, 2012 at 22:08 #35625aseathParticipant
Its okey Rurudyne. Its just my bad english.
Okay I got another question. I wanna know total resistance on my ship at different speed. In Delftship I go to Resistance and it gives me a graph and says that f.example at 20kn the resistance is 2500kN. But if I go to FreeShip and make the resistance calculation for the same dimensions it gives me Rtotal= 540kN. So is it 2 different values? I understand that 2500kN sounds more logical, but then what does that Rtotal in FreeShip shows?

February 26, 2012 at 11:24 #35626John R. CoilParticipant
Nothing wrong with your English. I just misunderstood what you meant when you mentioned control points.
As for the other I may be able to help you a tiny bit.
There are a number of different ways to calculate resistance and some seem to rely on on the assumption that your hull isn’t too different than the series of examples on which they are based. Somehow, and I’m not saying I know how, these schemes come up with a type of hull that’s “typical” and if your hull is indeed similar then they do a pretty good job.
But if your hull is not similar, as indeed my relatively high prismatic coefficient but still balanced sailboats (no “power stern” or anything like that) I’ve been puttering around with seem to be dissimilar, then the results can be squirrelly. The Delft series included with the free version of Delft Ship just doesn’t behave rationally with my models: looking more like a roller coaster than a resistance curve.
It seems likely to me that FreeShip just uses an entirely different type of resistance calculation. Either one based on a different “typical” hull or one that actually tries to calculate resistance using some of the available equations and, likely as not, a healthy dose of assuming stuff.


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