Michlet

Hi Rick!
In some applications I find it easier to compare the performance of hulls on the basis of equal displacement.

Leo.

 

Hi Fred!
Sorry, but it is NOT a problem with Michlet so I can't help you. It's a known problem with Freeship and Delftship.

In fact, I have written a lot of code (e.g. for prediction of squat, ship motions, added resistance, and wave patterns) that I tried to make directly usable by Freeship and Delftship. Unfortunately, those two codes don't easily accept a simple table of offsets (and they don't export accurately) so I have abandoned the project. I also tried to create a table of offsets from Michlet that could be used by Freeship, but gave up on that too. It's a bit of a shame really - the community could have had a very powerful suite of free programs to go with a pretty good, free (or inexpensive) hull design program.

As for your hull...
If you input the correct offsets to Michlet it should give you the correct results. If you put garbage in from another program, you will get garbage out

Good luck!
Leo.

 

Leo

Is a mathematician vision or naval architect vision?

For me is curious that I needing know the resistance of my hull a software change my table of offsets. Is not my hull.

 

Fred,
According to the manual, Michlet does not regard the offsets as absolutes. The offsets defines the hull shape, but does not inform Michlet on length, draft or width. The unit (scale factor) for the offsets are therefore irrelevant for Michlet.

One way to write the code could have been to have you define length, draft and width separately, together with the non-dimensional offsets. Leo choose to instead let you define length, draft and displaced volume. Width will then be a result of the length, draft, displacement and hull shape you have defined.

In my opinion, one way is as good as another. Just adjust to it.

When I calculate hull volume from offsets by integrating over the volume (in a simple Excel program) my results on beam and displacement agree with Michlet. I therefore believe that the beam Michlet calculates provides the displacement you have defined for the hull shape you have provided. Why not correct the displacement you feed Michlet until you get the expected beam from Michlet?

The problem could be that the offsets you have don't represent the hull you expect. Maybe the stations are too few. Have you calculated the displacement yourself from those offsets to check what is correct?

A last word. I'm neither an expert on Michlet nor on FreeShip. I just had to learn how Michlet wants the input file when I tested a hull shape where I generated the offsets myself.

Erik

 

For resistance comparisons, displacement is a logical value to hold constant among multiple runs. Displacement, after all, is the true measure of "how much boat" you are dealing with.

If you specify length, beam, draught, displacement, and offsets, you have over-specified the problem: any computer trying to interpret it will encounter internal conflicts. One of the above *must* be left open for the code to adjust to suit the rest. Freeship leaves displacement open, Michlet leaves beam open- both ways are appropriate for their respective cases.

Getting accurate data in and out of Freeship/Delftship takes practice. Get used to it. Once you understand how, exactly, the modelling software computes offsets and key parameters, it is fairly easy to get the different programs to work together quite well.

 

Fred
To make the point on how meaningless the Cp value is that FreeShip produces, have a look at the attached data for hydrostatics from your hull. This is exactly the same hull shape you sent me. To get this result I moved it 0.4m longitudinally. So the Freeship Cp value has nothing to do with what is actually immersed.

You are going to get next to meaningless results if you use the data from Freeship to do drag analysis based on a Cp value it produces.

Michlet gives you accurate results for what is in the water to achieve the specified displacement.

Rick

 

Eric818 and Marshmat

Right.

Leo say what Freeship and Delftship export is garbage. I agree. I discovered now, he is right.

Forget all said in this thread.

My problem is:

I have a hull with offsets and thanks to Leo I can use Michlet for know each resistance my hull have.

The offsets are composed by numbers that represent stations and waterlines.

The program receive length and draft.

Well, I think that with this I have the hull. Or not?

With these inputs I have some physicall characteristics of my hull, including volume, Cp, sectional areas, waterplanes area, etc.

But the input request the volume. OK.

Well I am a very happy naval architect that will have in few seconds all resistances.

Came the result.

I see different B, different Cp and naturally all beautifuls results.

Well, will be my hull?????

I do not want compare nothing. I want my hull resistances. The software is not for this?

This happened thanks to Freeship and Delftship that export garbage. How the offsets are not correct, according to the volume that come with the exported archive, what the software do? Correct the offsets.

Thanks to the wrong input from Freeship and Delftship we see that the software do this correction,The software need work with these inputs - volume - table of offsets.

If Freeship export correctly we do not see this operation.

But this operation is strange for me. As naval architect. Modifie the offsets?

If I do a software I think: Well, if the input are discording I take a deviation and give a alert: The inputs are discording - CIAO

Leo say, come garbage - you have garbage. And he is correct.

The problem is only this, like Leo say. The problem is solved.

Or better, If all input is correct we do not have the correction.

For me it's OK. Naturally if I give corrects inputs the software do all right. Leo say this, and for me he is wright, BUT........

Curiosity is a great man property.

Why the software do this correction?

I do not know if Leo someday have a problem like this.

For me Leo, without time, say this:

Hi Rick!
In some applications I find it easier to compare the performance of hulls on the basis of equal displacement.

If Leo said: This happen when the inputs are conflicting, is a operation not normal. Is not expected wrong inputs.

But leo's response activate my curiosity.

Up to now, I do not have more doubts. I will arrange right inputs and see the results. I think that is all right.

But, curiosity is curiosity.

 

Rick

Ok. For me is OK.

What Leo say, for me is OK.

I go to do right input and see the result.

But our conversation is not finished. Much things cleared for me about freeship/delftship that I will say you when my calculations finished. The problem is time.

regards

 

Fred:
In Part 7 of the the Michlet manual I warn the user that offsets will be scaled.
7. HULL OFFSETS
It does not matter whether the offsets describing each hull are in dimensional or non-dimensional form. Michlet will automatically scale the offsets to the individual hull displacements.


I do this because it means I can use the same table of (non-dimensional) offsets for a family of shapes. For example, look at the NPL examples in Michlet. You can see that you can use the same offset table for NPL3B, NPL4A, ... , NPL 6C, and for catamarans too.

Sorry that it is confusing for you as a beginner to Michlet, but believe me, there is some logic behind it.

If you think Michlet is confusing, wait until you see my program "Flotilla for Surface Effect Ships". There you have to specify the offsets of the hulls AND the pressure distributions of the air cushions, as well as the initial trim and sinkage of the hulls, and whether to use experimental values of squat or calculate it. After I release it into the public domain I will disappear so nobody can email me

All the best,
Leo.

 

Leo
Have you thought about setting up a prerelease with applicants providing a donation for the privilege of getting first use.

Rick W

 

Hello people,
How about involving Martjin and Viktor in this discussion? Since many people use both Michlet and Delftship/Freeship, wouldn't it be more beneficial to ask them to act in a coordinated way as to resolve this issue, for the benefit of everyone? They are possibly not aware of this problem, so I will try to invite them - let's hope this will lead to something good. Fredschmidt, it is a great thing you have noted this flaw!
Cheers!

 

Freeship's Michlet export does leave a bit to be desired. Apart from the transom vs. canoe stern thing, though, I find it works pretty well most of the time, if you're careful with your inputs. Specifically: use maximum waterlines, maximum stations, make sure you're measuring draught properly, and your design length and beam in the FBM file properties have to match the actual boat. And always, always manually check the length, draught and displacement in the actual in.mlt file. If this is all done properly, the values in Michlet tend to match those in the model to at least two, usually three significant digits.

At some point, I want to put together a Rhino script or plugin that will get my hulls from there to Michlet.... for the time being, I have to take them through ACIS to Freeship first, which is a nuisance.

Leo - have you decided if / when / for how much $ you'll make the Flotilla codes available? You keep teasing us about it

 

Daiquiry:

Thanks for the invitation. I wasn't aware of this particular problem.
Personally I haven't used the Michlet export feature anymore since I started developing DELFTship, so I hadn't noticed the problem myself.
The bug was easy enough to fix though once you know the problem.

I'm very close to releasing version 4.0, so it will be fixed with the new version.
I also increased the number of decimals for the volume to 5, which should be enough for even the smallest models.

Leo:
Wouldn't it be a good thing to display a warning/notification if the offsets are scaled, or even better prompt the user and wait for a confirmation?
I understand why you are doing this, but this can be confusing and potentially misleading, particularly to novice users.
If a user doesn't want the offsets to be scaled you could still the volume as calculated by Michlet instead of the specified volume.

 

Leo

OK Leo. Thanks. I understand and I am very thankful for your reply.

Daikiri

I hope that Freeship/Delftship correct the export file.

Martijn_vE

Great. Thanks very much. The IOM class, Rg 65, Marblehead and many others RC control boat designers are thankful to the initiative in increase the digits.

Who could think that design little incredible boats are a new obsession and a newest market for softwares and hardwares?

Freeship and Delftship are great softwares. We work in grams 6 are welcome.

Will be a help for designers if a advice is show in the software indicating that Delft series formula for resistance is not compatible for little boats like RC boats.

Great suggestion for Leo. Is a obvious suggestion.


For who read my blog:

That posts generate this thread. I was searching this problem in the occasion. Now I can give the better notices.

 

The program will be free when I release it here on boatdesign.net, but the manual and training might be expensive!

I'll try to upload a version that produces wave patterns as soon as I get some time later this week.

Leo.