Maxsurf stability problem

[sn1987]

Hi don92,
In this case the only keel part without gaps is the aft rounded one.the problem is I can't bond the trimmed surface (longitudinal view) with the transversal surface (body view). Do you have any suggestions?

 

[devudegoa]

Hi,

I was able to open your model in Maxsurf as well as Hydromax. By default it shows the design waterline at 2.2 meters in your model. Whereas the results given by you earlier are for a draft of 0.5 m.

I have not modified anything you model. I am attaching here the results from Maxsurf and Hydromax.

It appears the baseline for the model is set at the lowest point of the model (i.e. skeg) and not the lowest point of hull. So when you specify a draft of about 0.5 from baseline, that's the hydrostatics you will get. For example have a look at the Cb value at 0.5 draft (0.624) and at 1.0 m draft (0.296).

Also, for Maxsurf, as far as possible try to use the minimum number of surfaces. That will reduce the headaches. Model only what is required for the purpose of "what Maxsurf does".

 

[devudegoa]

As far as your other query about trimming is concerned, if your model is simple enough, even trimming the surfaces is not required. A skeg for example is best modeled simply by intersecting it with the hull surface, i.e. just keep the top controlpoints of the skeg within hull (i.e. say about 10 mm above the hull surface) and the lower control points on the center line, Maxsurf will do the rest. Maxsurf has the ability to automatically close the shape.

Best way to do it is to model the most important surface first and check if everything is okay with it. Then start adding other surfaces which are required. Unfortunately, I do not have any personal model to provide you for an example here.

 

[TANSL]

I also was able to work with the model in Hydromax but have obtained negative values for KMt. I have not gotten to know why. Something in the model is not good.
In my opinion the baseline placed at the lowest point of the keel, should be no problem.
I have the same opinion as devudegoa: the simpler the model and less surfaces, the more reliable and faster results will be

 

[sn1987]

Thanks a lot guys,
I repeated the calculations and now kmt is not negativeAs you said there is some problems in the model probably on the skeg.
Anyway could you show to me how to create this skeg from the attached model?

 

[Rabah]

Hi sn1987,
You should know with an absolute accuracy what you want to receive at the end of designing.
Length of your model Loa=17,453m. Total displacement about 9t so that is impossible to be.
Model which you was made, precisely responds on calculations made devudegoa for specified DWL-that is at draft=2,2m and displacement=48,82t.
But if the boat will be from plastic, I think that total displacement is necessary to be less 49t. Approximately 34,28t at a draft =1,95 m. At VCG=2,2m it is received GMt=1,168m.
I do not see eny problems in the skeg!
________________________________
NA Razmik Baharyan

 

[TANSL]

Sure is a slip, because vessels of this size are not built in plastic (just plastic).

 

[Rabah]

Hi All,
I disagree with the above-stated!
Soon on this site was discussion -Pilot vessel L16,5m. There the vessel prototype Interceptor 55 it is whole built from plastic. Total displacement 25,87t. Draft 0,923m
<http: // www.boatdesign.net/forums/boat-design/creating-maxsurf-model-existing-design-52575.ht>
________________________________
NA Razmik Baharyan

 

[TANSL]

Hi All,
I disagree with the above-stated!
Soon on this site was discussion -Pilot vessel L16,5m. There the vessel prototype Interceptor 55 it is whole built from plastic. Total displacement 25,87t. Draft 0,923m
<http: // www.boatdesign.net/forums/boat-design/creating-maxsurf-model-existing-design-52575.ht>
________________________________
NA (I don't think so) Razmik Baharyan

Perhaps they are built with fiber reinforced plastic.

 

[Rabah]

Hi,
Very wise ascertaining which helps to straighten out the problem!
But I think that is better to keep the designer himself to pick the material for the hull and superstructures.
If I projected this boat likely would pick an aluminum alloy.
_________________________
NA Razmik Baharyan

 

[TANSL]

That ship, by its appearance, coulod be a fishing boat. If this is true, build aluminum hull would not be convenient at all. So, Rabah, as you very well say, let the designer to define the SOR of his boat, select the most suitable material, and not to give him advice which may be totally wrong.
Another issue that has amazed me. How can you talk about the total displacement of the ship, if you do not know what kind of boat is nor, for example, equipment (machinery) that carry on decks?. What payload wants the designer?. You have a ball fortune teller?

 

[Rabah]

Hi everybody,
Whence we shall be sure what this motor boat there will be a trawler ship for industrial fishing? It will be possible it is a motor yacht for walk. The author speaks nothing about it.
About the total displacement there is reference enough technical publications on weight loads of the different built motorboats with different ship dimensions and from different materials.
______________________
NA Razmik Baharyan

 

[TANSL]

That is, you consider that only with "reference to technical publications" you are able to know the full load of a ship which you do not know what is going to be used, or its payload, or the material on which is to be built, the accommodation it has, etc.... Rabah friend, congratulations, you've solved one of the most difficult problems that a boat designer faces. I am amazed, congratulations, again. But not to tell me your method, please, I can not waste my time learning it. I have no doubt that you are soothsayer.
I also advise you not to waste any more time with me, that you take your time to study much more about what the ship design. You'll thank me.
I will not continue this meaningless conversation. Congratulations.

 

[sn1987]

Hello guys,
I think to have found the problem After I tryed many times, I changed one option in "Section Calculation Option at maxsurf stability start. In particular I uncheked the option Include skin thickness in Calculate new section sub menu and everything work now. Anyway it is more clear in the attached snapshot.
Instead about the material construction of the boat I would to build it in plywood's mahogany and epoxy. The boat is a fishing vessel and I 'm at the early stage of the design, then the real draft is unknown at the time. It should be approximately 2 m but I don't know.
Thanks a lot to everybody

 

[TANSL]

sn1987, I'm glad you've solved the problem.
Note the portion of the transom is submerged once you have determined the full load draft and the resulting freeboard. To facilitate the work of the crew aft, it may be advisable to lower the bulwarks in this area, but should have a minimum height of 1 m, following the sheer of the deck.