Double bow bulb can be calculated for in DS

[otaku]

Here:

http://www.ibiblio.org/hyperwar/NHC/CRS/propulsion.htm

one can find a double bow dome designed for the DDG-51. It can apply to various hulls, so long as the correct/appropriate modeling is customized for a given hull. For months i’d kept forgetting to model this in DELFTship just to see what would happen to my hull.

With only the flattest wedge piece extending about 3 feet forward of the hull but not farther ahead than the sonar dome on my hull, my model only sees a minor decrease in resistance starting at 34.5 knots. Below that, the resistance for the most part either didn’t increase, or it was on the order of about 10 pounds of resistance added.

A naval engineer in know was (seemingly) pleasingly or curiously impressed that DELFTship calculated for this. But, maybe he’s never played with DSFree. After thinking about it, a 2nd dome is just more surface area that may positively or negatively impact flow and stability calcs DSF & DS pro already account for.

Later i thickened (or widened, rather) the forward wedge piece but it increased the resistance at lower speeds, and did nothing for the higher speeds. If i had the Pro version, this might be minutes of work at a time. But, with Free, of course, i can look forward to eons of tweaking and still not know what negative or deleterious effects i may cause in my model since i don’t have the ability to auto fair curves and cannot see flow lines.

A possible nice feature in a future version of DELFTship might be the ability to set resistance and speed targets and then supply a simple range of bulb or appendage sizes and then DELFTship proposes some number of iterative and saved and report-compared outputs.

It might also be fun to be able to shape these bulbs (2nd or just one primary one) and then move them up and down the Z axis and vary the bulb volume. Since in the case of naval sonar domes a destroyer/cruiser-sized hull is likely to use a variant of the USN SQS-56 or similar sonar’s dome, one doing what i’m doing might model the primary dome on that. Tweaking the 2nd dome could be easier if code is added so that we could use SNAME/DTMB bulb samples and choose our own. A real navy will choose something that’ll support a hull’s main range of operating drafts, speeds, and so on.

Adding in DELFTship some layers to handle one or more bulbs, flaps, wedges, and other appendages and an ability to unconfusingly include and exclude them from hydros reports would be super nice. Maybe this part is doable now, but i’m having a heck of a time moving some rudder nodes and lines from layer 0 to the rudder layer i already have. I’ll have to re-read the manual because my efforts in a 15 minute attempt were fruitless.

Just some thoughts and ramblings. [file name=06_2nd_bulb_wedge_piece_persp_port_bow_viacad_wvfrnt_obj_rvrsd_angle.jpg size=18368]http://delftship.net/delftship/media/kunena/attachments/legacy/files/06_2nd_bulb_wedge_piece_persp_port_bow_viacad_wvfrnt_obj_rvrsd_angle.jpg[/file]

Attachments:

• 01_2nd_bulb_wedge_piece_profile.jpg
• 06_2nd_bulb_wedge_piece_persp_port_bow_viacad_wvfrnt_obj_rvrsd_angle.jpg

[otaku]

Some more views

Attachments:

• 03_2nd_bulb_wedge_piece_persp_zebra.jpg
• 05_2nd_bulb_wedge_piece_persp_port_bow_viacad_wvfrnt_obj.jpg

[otaku]

Another view… [file name=02_2nd_bulb_wedge_piece_persp_gausssian.jpg size=24950]http://delftship.net/delftship/media/kunena/attachments/legacy/files/02_2nd_bulb_wedge_piece_persp_gausssian.jpg[/file]

Attachments:

• 07_2nd_bulb_wedge_piece_persp_port_bow_viacad_dxf_.jpg
• 02_2nd_bulb_wedge_piece_persp_gausssian.jpg

[otaku]

For another perspective, here are some Froude number values

An Australian team used the DTMB 5414 model and arrived at
FN values of 0.2755 for 20 knots and 0.4136 for 30 knots. Their model testing of the DDG-51 hull is described at:

http://www.cyberiad.net/library/pdf/tsl00a.pdf

and this is (IIRC) a model of 466 feet at the waterline and 505 feet LOA. For my DELFTship model these are the major values:

base model (no 2nd bulb/dome)
Design Length: 562′
LOA: 559.5
LWL: 524.16
Block: 0.4388

modified model (2nd bulb/dome, thin)
Design Length: 562′
LOA: 559.5
LWL: 524.17
Block: 0.4388

modified model (2nd bulb/dome, thicker)
Design Length: 562′
LOA: 559.5
LWL: 524.17
Block: 0.4391

The Froude numbers at various speeds:

Speed Base Model Thin 2nd bulb Thicker 2nd bulb
17.5 0.272 same as base same as base
20 0.310
26.5 0.411
27 0.419
30 0.466
38 0.590
38.5 0.598 [file name=11_2nd_bow_bulb_3.jpg size=66956]http://delftship.net/delftship/media/kunena/attachments/legacy/files/11_2nd_bow_bulb_3.jpg[/file]

Attachments:

• 10_NO_2nd_bulb_wedge_piece_resistance.jpg
• 11_2nd_bow_bulb_3.jpg

[otaku]

3rd chart of speed resistance…

Note, in case you can’t recall them, the column headers for the values are:

V v Fn Rf Rr Rtotal Pe

Attachments:

• 12_2nd_bow_bulb_2_wedge_piece_resistance.jpg

[otaku]

I spoke with an architect about my double bow in DELFTship and i suggested that it shouldn’t be surprising that DELFTship can calculate the resistance since the added bow is just added resistance.

Then, just as he started to add, “But…”, we both started talking about the wave interferences a bad second bulb would create rather than resolve. So, he said, “It’s GREAT that DELFTship is making calculations but is it calculating for wave interferences?”

So, my question is, are DELFTShip Free and Pro versions able to account for different waves and the consequential forms/interferences?

Will there be a way to visualize the waves based on speed, bulb relative positions, and draft? (I’m not sure how much trim and other attitudes play into it other than the general hull shape surface wetted and surface exposed changing the stability and temporarily and locally messing up other calcs…)

[Author]

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