Michlet 9.20 Released

[Leo Lazauskas]

Attached is Michlet 9.20.
The program is free to all users but please consider making a donation
at www.cyberiad.net/michlet.htm

To install the program, unzip into a convenient subdirectory.
Documentation is in the docs sub-directory.
To uninstall, delete the entire directory.

New features include several hull series (20,32 and 42), hull
constraints, and new objective functions. For example, Objective 100
allows users to make a mathematical approximation of existing hulls by
trying to match the principal dimensions and also their section and
waterplane area curves.

Michlet and Godzilla input files for version 8.x will need to be
changed by adding a couple of extra lines to accommodate the new
features.

Have Fun!
Leo.

[DCockey]

Thanks! Donation made.

I have a project which should be a good use for it, analysis of a series of slender double-end rowing boats.

[pavel915]

Thanks a lot Honorable Leo,,,

[rapscallion]

I would like to study a series of dory hull shapes for a low cost trimaran design, would michlet be a good application for this task? I'm using freeship 2.89+rc1.

[Leo Lazauskas]

Originally Posted by rapscallion

I would like to study a series of dory hull shapes for a low cost trimaran design, would michlet be a good application for this task? I'm using freeship 2.89+rc1.

The main restriction is that the longitudinal slope of the hulls should be small. That's usually Ok for hulls with length-to-beam ratios greater than about 7.

Can you give us a little more information about the size of the hulls, e.g. length, beam, draft and displacement?

Leo.

[rapscallion]

The length of the amas and the main hull are the same length. The beam is about 6' on the top of the shear and 2.6' at the bottom of the shear. 26' long. 1450 lbs estimated weight.

[Leo Lazauskas]

Originally Posted by rapscallion

The length of the amas and the main hull are the same length. The beam is about 6' on the top of the shear and 2.6' at the bottom of the shear. 26' long. 1450 lbs estimated weight.

If the bottom of the shear is close to the waterplane, then the underwater portion of the hull is certainly thin enough for Michlet.

Good luck!
Leo.

[rapscallion]

It is.

[nukisen]

Holy macaroni!
I have watched this program for several times still not getting it.
Hmm I think I still have a lot to learn.

[Remmlinger]

To get familiar with Michlet I compared it with the results presented by Kuhn & Scragg at the 11th CSYS in 1993. They used a panel method with Havelock singularities to calculate lift and drag of a surface piercing foil. Since their simplifications are equivalent to the thin ship assumptions, their resulting theoretical drag-curve is practically identical to the results obtained from Michlet. Nevertheless, the theory does not match the measured wave resistance. The attached diagram is a copy of Fig.7 in Kuhn&Scragg's paper.
The measured wave resistance was calculated by K&S from the measured wave elevations using a longitudinal cut method.
Now the question is: do I trust the measurements or are the two independent theoretical methods more reliable? K&S argue that agreement between theory and measurements is "good".

Leo, what do you think?
And thank you for sharing this wonderful michlet program.
Uli

[Leo Lazauskas]

Originally Posted by Remmlinger

To get familiar with Michlet I compared it with the results presented by Kuhn & Scragg at the 11th CSYS in 1993. They used a panel method with Havelock singularities to calculate lift and drag of a surface piercing foil. Since their simplifications are equivalent to the thin ship assumptions, their resulting theoretical drag-curve is practically identical to the results obtained from Michlet. Nevertheless, the theory does not match the measured wave resistance. The attached diagram is a copy of Fig.7 in Kuhn&Scragg's paper.
The measured wave resistance was calculated by K&S from the measured wave elevations using a longitudinal cut method.
Now the question is: do I trust the measurements or are the two independent theoretical methods more reliable? K&S argue that agreement between theory and measurements is "good".

Leo, what do you think?
And thank you for sharing this wonderful michlet program.
Uli

Thanks, Uli.

I have only seen the Abstract, not the actual paper.

There are several possible sources of differences between predictions and measurements.

1. Human error in non-dimensionalising.
K&S's "Drag Area" was obtained by dividing the wave resistance by 0.5*rho*U^2. You do the same with Michlet results but they then must be converted to feet squared.

2. If the foil is at an angle of attack, then Michlet is not an appropriate tool. It does not calculate lifting effects. For that, vortices as well as sources are needed.

3. The rounded nose of airfoils do not have small longitudinal slopes which is essential for thin-ship theory. The inaccuracy is most pronounced at low Froude numbers (where the wavelength of the waves is comparable to the extent that the small slope assumption is wrong). At high Fr, the wavelengths are very much longer than the small region where the slope is large, and results are usually quite Ok.
I'm not sure of the length of K&S's foil, so I'm not sure what Fr we are looking at in your attached graph.

There is some discussion of K&S and the problem in general in a later CSYS paper by Binns et al titled "Hull-Appendage Interaction of a Sailing Yacht, Investigated with Wave-Cut Techniques". It can be found at:
http://cmst.curtin.edu.au/local/docs...esapeake97.pdf

All the best,
Leo.

[Remmlinger]

Sorry for the late answer (I have still another family, besides boatdesign.net)
My comment on your 3 points:

Point 1. and 2. can be ruled out.
3.) The Froude number at the lowest measured speed (2 kts) is 0.5 based on the chord length and 6.5 based on the nose-radius. I thought thin-ship theory would not be far off in this case.
May be the measured values of wave resistance are suspect. I do not know, how accurate the wave cut method is.
If I had plenty of time I would try to get acquainted with the free OpenFoam CFD code. In their tutorial under the LTSInterFoam-solver they publish the simulation of a wigley hull. It would be interesting to see, how CFD performs on the surface piercing foil. I have not seen any publications in the open literature yet.

I know the AMECRC paper by Binns that you mention. To me it was of little practical use, because there is no information about the sinkage and trim of the 6 models. The funny influence of the rudder on the viscous resistance can only be explained if the trim changes dramatically.

best regards
Uli

[aggogle]

Donation made. Thanks.

[Leo Lazauskas]

Originally Posted by aggogle

Donation made. Thanks.

Thanks, Peter!

[Leo Lazauskas]

Originally Posted by Remmlinger

Sorry for the late answer (I have still another family, besides boatdesign.net)
My comment on your 3 points:

Point 1. and 2. can be ruled out.
3.) The Froude number at the lowest measured speed (2 kts) is 0.5 based on the chord length and 6.5 based on the nose-radius. I thought thin-ship theory would not be far off in this case.
May be the measured values of wave resistance are suspect. I do not know, how accurate the wave cut method is.
If I had plenty of time I would try to get acquainted with the free OpenFoam CFD code. In their tutorial under the LTSInterFoam-solver they publish the simulation of a wigley hull. It would be interesting to see, how CFD performs on the surface piercing foil. I have not seen any publications in the open literature yet.

I know the AMECRC paper by Binns that you mention. To me it was of little practical use, because there is no information about the sinkage and trim of the 6 models. The funny influence of the rudder on the viscous resistance can only be explained if the trim changes dramatically.

best regards
Uli

There is some work on surface-piercing foils at the IIHR:
http://old.iihr.uiowa.edu/~shiphydro/efd.htm

I am still very wary of CFD results for Wigley hulls. Some cherry-pick experiments that give the best "agreement", others tweak the parameters in their codes to get good agreement with experiments. Others are just not very good, e.g. as shown in:
Michell v CFD and EFD: Round 2007
That Wigley hull has L/B = 0.8 and it can be seen that the CFD code is pretty poor, as is thin-ship theory. It also took the CFD code about 7 days on several processors to calculate the drag at one speed!

I hope Openfoam is better, but I'd like to know how many parameters they tweaked to get the agreement. When I know that, I'll know how many I can adjust in some of my codes, e.g.
Michell-Lamb Wave Resistance Integral for Thin Ships

There I choose the value of one parameter, a non-dimensional "viscosity".

Leo.