19m semi displacement / planing hull "Port service boat"

Hi All,
To finish the work let we shall return there, whence started to make this model - to program Shape Maker with which have made the first steps to receive a good ship with impetuous silhouette.
I have made again model on Shape Maker, identical that already saw in Rhino and Maxsurf Modeler, i.e. with deadrise bottom frames without rounding of bottom in CL.
But unfortunately in the forum of Boat Design it is impossible to publish files with extension shm, for this purpose it is necessary to be content only dxf file of Lines plan, txt file for Offsets Table, Perspective view with Design Hydrostatics,Perspective fore view and Bodyplan view.
You can see the zip file!
_________________________
NA Razmik Baharyan

 

Dear Rabah, you can zip "shm" files and publish it on Boat design. You can find some more examples og shm files on http://msi.as/Downloads/SurfacesExample.aspx

 

Hello All,
You can see the Cross curves of stability of this model making with Orca3D-trim, heel and VCG=0
_____________________
NA Razmik Baharyan

 

@Rabah

Dear Sir,

I have worked out a preliminary hull and find that there is a considerable difference between our resistance values. Kindly find attached the files for power vs speed. My hull requires only 350 kW (effective power) for 25 knots as compared yours which is 600 kW (effective power) for same speed. Also note, how the Savitsky pre-planing and planing curves are almost merging to form a single curve for my hull.

Also the wave contour at the aft of that hull is something worrying. Please have a look at the attachments. Your model was in the opposite direction in Maxsurf so had to be rotated 180deg.

I worked out a different hull from very basic ideas and find that the I get much lesser resistance. I will not post the maxsurf file here at this stage. I can send it to you privately by email if you want.

Best Regards,
Devu

 

It is look's little bit strange for me - too big difference in power. How it was calculated? Same Weight and COG? Same Main dimensions?

 

Of course it is very surprising that there are such great differences. What we do not know is how the two models that have been studied are similar, nor how the designers have done calculations.
Super software, poorly used, can give surprising results. It is very important to know the theory before giving the "Calculate" button of the software being used. To what extent are the two designers applying the theory correctly? That is why it is not possible to say which of the two is right. It is clear that one of the two (or both?) is wrong.

 

Hi Devu,
1. Orientation of the coordinate system is made according to recommendation in the Help of the program Orca3D for calculation of Planing Analysis by the method of Savitsky /see the file/.
2. If you has read closely that document which I published for my model calculation displays two power for speed 25 kts /see post No 39/:
PE total = 472,5 kW
PP total = 945 kW
Whether you make a variance between them?
PE is an effective power necessary for overcoming resistant of the waters at movement of the ship.
PP is the specification power by the catalogue of the producer firm on which it is selectable engines / in our event general necessary power for two engines/.
3. But for my model at 25 kts we have Displacement-Froude number Fnv = 2,4 <3.
It means „Forget about a mode of gliding!”, i.e. will be only semi - displacement mode of movement.
_________________________
NA Razmik Baharyan

 

Dear Sirs,

I have used the Maxsurf model provided in the last post. When I opened it in Maxsurf it was in the opposite direction. Kindly see the Maxsurf coordinate system attached. I tried running it as it is but "Maxsurf Resistance" was dragging it backwards so I rotated it 180° and ran. If you say that power by Rhino is 472.5 kW, you may be right.

I have not tried my hull in Rhino, only in Maxsurf. I do not use Orca yet. But I think, when I start finalizing the Lines plan for the vessel, I will also verify with Orca.

The difference in our results may also be because of the hull form difference. In maxsurf the process is automated so you do not specify a trim angle for Savitsky method. In fact, you do not specify anything else except the methods and speeds. Then the software also gives a chart of trim angle v/s speed. What it means is that the software changes the trim angle on its own. I do not know the detail functioning of the software. But the result which I get for my hull is like I have posted.

I am still in the process of finalizing the weight and cg for the vessel. I do not have much time here to work on the manual Savitsky method. But as soon as I am in a position to do that I will try.

Also I have to get comments from my Professor on all the data.

 

I do not believe that any program is able to guess when a hull begins to plan or how it does if the position of the center of gravity is not known. Therefore, before doing power/speed studies and comparing the results of two programs it is imperative to calculate the cog, and total weight of the boat.
And, of course, forcing the trim to have a predetermined, fixed value, can give totally erroneous results.

 

Sir,

Yesterday when I run the hull in "Maxsurf Resistance" module, the software assumed the lcg value by itself. In my case it was about 6.9m from aft (nearly 40% of lwl). Then based on that it calculated speed vs resistance and speed vs trim and also the wave surface for specified speed. I am trying to keep the lcg of the vessel at the same location so as to replicate the same results in the model.

@Rabah Sir, could you please verify your values in this online calculation so that we have some idea?
http://illustrations.marin.ntnu.no//hydrodynamics/resistance/planing/index.html

Best Regards,
Devu

 

Attached is a preliminary weight calculation. Does it look okay to you. Or do I need to change something here? Also, how can I move the LCG, which is now at 8.08 to the 6.92 required by Maxsurf? Is it better if I switch to transverse framing so that I get my LCG aft.

 

The revised structure is somewhat like this. Will it be better to change it to transverse framing so as to get the LCG aft?

 

I see no reason why the ship should change its longitudinal structure to transverse in the bow. In any case, there should be a transition zone between the two zones.
I also do not see the reason for trying to put the c. of g. where Maxsurf has supposed it. I would like to know the algorithm that Maxsurf uses to calculate the position of the c. of g. I do not know to what extent that estimate, regardless of the procedure used to calculate it, is reliable.

 

Absolutely agree. Structure in the bow look's very strange. Mixing longitudinal and transverse structure is used mostly for long river gouging vessels.

 

Kindly find attached the calculation:

Structural weight (current) = 12.92 t
Outfit weight = 28.27 t
Lightship weight = 41.19 t
Dead weight = 03.23 t
Fully loaded displacement = 44.42 t

Does the weight calculation look okay to you for this size of a vessel built in aluminum? I do not have data for any existing vessel. Just working in the dark. Can someone please give me the closest estimate of weight based on the weight calculation I have provided or some existing data.