Export from Delftship to Michlet

[fredschmidt]

Apparently we have a problem in export offsets from Delftship to Michlet.
We noted that the offsets came from Delftship do not finish with a row of zeros.
The last line values are of the last submerged section and this indicate to Michlet that we have a transom submerged, that is not true.

Therefore the results from Michlet is not correct.

Anybody already have this problem? How solve?

[Guest625101138]

Originally Posted by fredschmidt

Apparently we have a problem in export offsets from Delftship to Michlet.
We noted that the offsets came from Delftship do not finish with a row of zeros.
The last line values are of the last submerged section and this indicate to Michlet that we have a transom submerged, that is not true.

Therefore the results from Michlet is not correct.

Anybody already have this problem? How solve?

Is the stern WL at zero?

The export does have a problem but it is when you have a submerged transom. The conversion will try to produce a canoe stern. There is an easy fix for this.

If you post the Delftship fbm file I can produce the correct in.mlt file.

Rick W

[fredschmidt]

Hi Rick

I sent the archive by e-mail.

Annex the profile.

Thanks for the help.

Best regards

Fred

[bdVlajko]

Originally Posted by Rick Willoughby

The export does have a problem but it is when you have a submerged transom. The conversion will try to produce a canoe stern. There is an easy fix for this.
...

Rick W

Rick, would you mind sharing this easy fix ?

Thanks
Vladimir

[marshmat]

Rick, would you mind sharing this easy fix ?

If the Delftship-generated Michlet file has zeros as the final row of offsets (ie, a canoe stern) when there is supposed to be a transom, simply replace the final row with a copy of the second-last row. This forces Michlet to recognize the presence of a transom. As long as you are using lots of stations, the error that is introduced by doing this is negligibly small (however, it will distort the hullform if you have used a small number of stations).

[fredschmidt]

Rick

When I add more two stations the distance between them was modified and the new table of offsets do not represent more the boat lines.

The problem is that Freeship or Delftship do a mistake on the exported archive.

Maybe the software take the Lwl in a par number of divisions and by consequence an unpaired number of stations. Seems me that the mistake is do in the adopted Lwl.

Apparently the software adopt as end of Lwl the last station on the drawing not the end of the real DWL, hence this problem

Fred

[Guest625101138]

Fred
You are correct about the slight hull distortion with the way I did it.

In the original file the transom is tiny and offers very little drag. You can see its contribution in the data file srh.mlt. The drag line does not even show on the chart.

If you are concerned about it then check that your hull is set with the aft waterline at x=0. When you export, go the the hull geometry and increase the number of stations and waterlines. You can increase both to 81. This will make the "transom" miniscule.

The need for precision with such a small hull is doubtful because the accuracy of the friction line at such small scale and speed is also doubtful. It is OK for comparison but may be somewhat out for absolute performance.

Rick W

[fredschmidt]

Rick

I do not understand "check that your hull is set with the aft waterline at x=0". X is a reference for longitudinal reference and wl have vertical position reference.

Other problem (big) with the offsets exported is the differents values obtained by Cp and wetted area.

In Freeship I have: Cp=0.57 WA=0.142 m^2
In Michlet the exported archive generate: Cp=0.60 WA=0.155 m^2

[Guest625101138]

Fred
The stern of your hull is further aft than where the waterline meets the hull. I suggest you move the model backwards in Delftship so the point where the waterline meets the hull is at zero x-coordinate.

Before you export the offsets make certain that the aft end of the waterline is at the zero position for the x coordinate. Also make sure the x coordinate of the waterline at the bow is the same as the hull length set for the project. This means the distance between the stations will be an exact fraction of the waterline length. Right now the waterline length in the export file is 919mm. I believe actual waterline length is 981mm. You need to make certain that the Michlet export file has the same waterline length as the Delftship model. If you do as I suggest it should be but check it when you create the file.

Michlet is only interested in the underwater portion. There is no sinkage or trim adjustment unless you preset values. If you set up as I have stated the stations will be the correct length separation.

If you cannot get Delftship and Michlet to agree on the basic ratios I will have a close look and get the correct export file. I believe Michlet takes equal spacing between the stations and waterline based on the project length and the project draft. (I normally have true canoe sterns so it is not something that bothers me. The waterline length is the hull length.)

Rick W

[Guest625101138]

Fred
I have attached the Michlet out.mlt file for your export file with the length set to 981mm instead of 919mm. I think you will find it is much closer to the Delftship data.

Rick W

[Guest625101138]

Fred
I do not think you actually need to move the hull but do it for now so you get it right. Once you set the project length to the actual waterline you should get the right result irrespective of where you locate the hull with respect to the reference point. Michlet must have the waterline length so check that in the export file.

Rick W

[fredschmidt]

Rick

Sorry by delay.

Why you think that the length is 981 mm?

The report from design hydrostatics confirm that the length is 919 mm.

I will move the end of wl to position 0 and say to you the result.

Regards

Fred

[fredschmidt]

Rick

The basic reference for the stations is the transom and move the WL to 0.0 do not change the exported archive.

Fred

[Guest625101138]

Originally Posted by fredschmidt

Rick

Sorry by delay.

Why you think that the length is 981 mm?

The report from design hydrostatics confirm that the length is 919 mm.

I will move the end of wl to position 0 and say to you the result.

Regards

Fred

Fred
I was basing that Lwl on the information provided earlier per the attached.

When you export to Michlet make sure that the Lwl in the exported file matches the Lwl of what Delftship has.

If the hull parameters do not agree then you can increase the resolution by increasing the number of stations and waterlines in the export file.

If the hull parameters still disagree then one or both of them is in error. You can make a very simple model and do a hand calculation to determine which one has the greatest error.

One point to note is that you are working at very small scale and might be suffering from rounding errors. Your displacement volume is exported to one significant digit. This could be the cause of the discrepancy.

Rick

[Guest625101138]

Originally Posted by fredschmidt

Rick

The basic reference for the stations is the transom and move the WL to 0.0 do not change the exported archive.

Fred

I was not certain because I usually use canoe hulls with vertical stem and stern.

Rick W