theoretical displacement hull shape for min drag

[Padava]

What differential ( ordinary ODE or partial PDE ) equation describes a hull shape for minimum resistance and/or mimimum fuel consumption of displacement boats/ships operating at around the 1.34 sqrt (loa or lpp) ? I like to look at fundamentals to compare "pure" form with hull softwares now available to see how or which design parameters characterize different power displacement curves. Regards

[Rick Willoughby]

What you seek is not available.

The lowest drag hull for calm water is primarily a function of both speed AND displacement not just speed.

A common parametric hull that is used for drag analysis is the Wigley hull. It is simply a function of LWL (L), BWL (B), and draft (D). The equation for beam at any point on the hull is:

y = B/2 * (1 - (2x/L)^2) * (1 - (2z/D)^2)

You can find tow test data on these hulls if you search around the web.

Rick W

[Ad Hoc]

Padava

You need to define a bit more clearly what you're objective is...is it mathetical, ie equations, or hydrodynamic, for example, since these two are not always compatible.

[mydauphin]

A canoe... a very long light canoe...

[Ad Hoc]

Is that a "whafer thin" infinitely long hull in an inviscid fluid? ;)

[mydauphin]

Is that a "whafer thin" infinitely long hull in an inviscid fluid? ;)

Might as well make it a vacuum

[Leo Lazauskas]

What differential ( ordinary ODE or partial PDE ) equation describes a hull shape for minimum resistance and/or mimimum fuel consumption of displacement boats/ships operating at around the 1.34 sqrt (loa or lpp) ? I like to look at fundamentals to compare "pure" form with hull softwares now available to see how or which design parameters characterize different power displacement curves. Regards

You should get hold of:
"The Wave Resistance of Ships"
John V. Wehausen
and read Chapter H.3:
Ships of minimum resistance.

A few other references that might be useful...

"Ships of Minimum Total Resistance"
Lin, Wen-Chin, Webster, W.C. and Wehausen, J.V.,
College of Engineering,
Uni. of California, Berkeley,
Report No. NA-63-7
Aug. 1963.

"Optimal Ship Forms of Minimum Wave Resistance"
Chi-Chao Hsiung
College of Engineering,
Uni. of California, Berkeley,
Report No. NA 72-1
Aug. 1972.

Some formulations are just quadratic programming exercises,
but there are others available that require calculating Mathieu
functions. Unless you know some numerical analysis, they will
have you tearing your hair out! Hunt through Wehausen's work
for more information.

Good luck!
Leo.

[Ad Hoc]

Leo

You have been quoted as saying "...Leo advises that there are two lowest drag hulls but I have never seen this....", in reference to hull shapes and drag, would you care to elaborate on this please?

[Leo Lazauskas]

Leo

You have been quoted as saying "...Leo advises that there are two lowest drag hulls but I have never seen this....", in reference to hull shapes and drag, would you care to elaborate on this please?

In some cases, and at some Froude numbers, it is possible to get more than one "optimal" hull. Think of it this way: a short beamy hull has low skin-friction and high wave resistance. At the same speed (and for the same displacement), a long thin hull can have high skin-friction and low wave resistance. Theefore it is possible to have two hulls with the same total drag, but different proportions of wave resistance and skin-friction.

For more, see:
http://www.cyberiad.net/library/rowing/misbond/misres.htm

Leo.

[Ad Hoc]

Leo

Ok, so the statement is referring to the obvious aspects, as you have noted, when designing a hull-form, theoretically, both ends of the spectrum as such. Not some "holy grail" as appear the poster was eluding too! But I'm skipping over the obvious, the the difference in actual hull forms of such a theoretical exercise renders both hulls somewhat impractical in reality and hence, is just an "interesting" theoretical debate.

Since a short beamy hull will have poor length displacement ratio and all the consequential knock on affects.

[Leo Lazauskas]

Leo

But I'm skipping over the obvious, the the difference in actual hull forms of such a theoretical exercise renders both hulls somewhat impractical in reality and hence, is just an "interesting" theoretical debate.

Since a short beamy hull will have poor length displacement ratio and all the consequential knock on affects.

I don't agree that they are impractical.

I gave an example of two extremes as an illustration. Instead, think slightly shorter and beamier, and slightly longer and thinner (instead of the extremes) and the conclusion is the same: there can be two hulls with the same total drag but different proportions of skin-friction and wave resistance.

[Ad Hoc]

Leo

Interesting article. But, this is related to long thin rowing boats, of sorts.

A rowing boat is basically a skin, to keep the water out, a seat and method of "moving the boat" oars/rowlocks etc, that is all.

A real boat, in the sense that it is for more than just going up and down in a straight line for several thousand meters - that is to say a design brief of one line, is somewhat different to a real boat. A real boat in terms of what the hull form must provide has a design brief that runs into many pages such as outfitting, compliance with class requirements, payloads, propulsion, structural arrangement etc. This is where the "theoretical debate" breaks down between "hull-forms" and real hulls used for a working design.

[Leo Lazauskas]

Leo
This is where the "theoretical debate" breaks down between "hull-forms" and real hulls used for a working design.

The same "trick" can be done with real hulls.
Keep the shape the same but scale one hull so it is slightly shorter and beamier, and make another that is slightly longer and narrower. Now all three hulls have the same total drag, different proportions of wave drag and skin-friction, and, of course, different performance characteristics, seakindliness, etc.

[Ad Hoc]

Leo

But your study is based upon volumetric Froude numbers, yes?..so there are 3 variables for any given volume and hence 3 solutions not 2.

So if the theory is correct, why are the assumptions for an "idealised hull", what is wrong with "any shape", a true "ship shape" becomes irrelevant for your study, just any shape is applicable, that is moving through a fluid.?

You even acknowledge this to an extent viz:

"..In this study, we present results for one hullform only - a canoe body defined by parabolic waterlines, elliptical cross-sections, and a parabolic keel-line. Although this form is an obvious idealisation.... Clearly this is a much finer type of hull than that of a typical merchant ship, but is relevant to sporting canoes and hulls of special high-speed vessels..."

"..When there are length restrictions, and hence a greater contribution of wave resistance to the total drag, multihulls can have less total drag than monohulls of the same length..."

"...In any case, the hulls resulting from the optimisation process also have the property that their wave resistance is generally only about 10% of the total, so that the absolute accuracy of the wave resistance measure is not critical..."


So, as i stated, it is a nice theoretical exercise for a 3D shape and finding a low drag shape, at low speeds for that 3D shape. But for a real hull ship shape, not so.

Since i don't see any reference to sinkage and trim, hence change of LCG/LCB, nor transom sterns etc, as one finds on higher froude number vessels etc.

The rowing boat study you provided is fine for one type of "hull form" at low Fn's, (which in itself i rather interesting) but i fail to see how this is applicable to real hull shapes given the endless caveats required to arrive at a conclusion in your study.

[apex1]

Ahh, nice to see that finally they found each other (Ad Hoc and Leo).

[Leo Lazauskas]

Leo

But your study is based upon volumetric Froude numbers, yes?..so there are 3 variables for any given volume and hence 3 solutions not 2.

So if the theory is correct, why are the assumptions for an "idealised hull", what is wrong with "any shape", a true "ship shape" becomes irrelevant for your study, just any shape is applicable, that is moving through a fluid.?

You even acknowledge this to an extent viz:


No argument there. That paper was from 1996 and is, admittedly, for an idealised shape. Since then I have written other programs (e.g. Michlet, Flotilla, Flotsm, Godzilla) that can take real hulls and pressure distributions and do the same thing. I can account for sinkage and trim, reduce waves made by the ship, estimate bottom pressure signatures, and (for monohulls at least) estimate their performance in waves.

There will always be a gap between the predictions made, the "optimal" forms found, and what happens out in the real world. I have never tried to hide from that fact. Nor do I think that an automated design code will ever replace the common sense of an experienced naval architect. I just write tools for other tools ;-)

Leo.

[Ad Hoc]

Leo

What are the parameters of the "hull-form" inputs that 'define' the applicablity?

Is it applicable across a wide range of length/displacement ratios and froude numbers, say from 0.1 to 2.0 (Fn in the classic sense)? for example.

How does it treat the waves, is it mere a function of amplitude caused by the pressure distribution or an energy based function?

"..There will always be a gap between the predictions made, the "optimal" forms found, and what happens out in the real world. I have never tried to hide from that fact. Nor do I think that an automated design code will ever replace the common sense of an experienced naval architect. I just write tools for other tools ;-).."

Nicely said, sadly too few appreciate the simplicity of your statement
Unfortunately there are many who use your programs and seem to be under the impression it provides the holy grail, since they are not as you put it, experienced naval architects!

PS..i co-authored a paper on hydrodynamics some 10 years ago, generally on higher Fn's...we found some very interesting results. If you PM me your email addy, i would be happy to send you a copy, if you're interested.

[Leo Lazauskas]

Leo

What are the parameters of the "hull-form" inputs that 'define' the applicablity?

Is it applicable across a wide range of length/displacement ratios and froude numbers, say from 0.1 to 2.0 (Fn in the classic sense)? for example.



No, they are generally for thin ships, say L/B > 6 or so.

Leo

How does it treat the waves, is it mere a function of amplitude caused by the pressure distribution or an energy based function?



Energy-based, but I have started looking at ship routing problems, e.g. "optimal" paths around storm systems etc. It's a hobby interest (like most of my work) so I haven't got far yet.

Leo

Unfortunately there are many who use your programs and seem to be under the impression it provides the holy grail, since they are not as you put it, experienced naval architects!



Schmucks, like the poor, will always be with us.

Leo

PS..i co-authored a paper on hydrodynamics some 10 years ago, generally on higher Fn's...we found some very interesting results. If you PM me your email addy, i would be happy to send you a copy, if you're interested.

my surname (in lower case) at gmail.com will find me.

[Ad Hoc]

if i pressed all the right buttons in the right places...a copy should squirted down the line to you as i type...i hope!

[Leo Lazauskas]

if i pressed all the right buttons in the right places...a copy should squirted down the line to you as i type...i hope!

I got your paper. Thanks!
I'm more interested in mathematical approaches and in physics-based methods rather than the empirical approaches that you use. That said, I also have to resort to empirical models for some transom calculations, estimating boundary layer effects etc. so I'm not being critical. It's purely a matter of taste.

It was good to see that you were interested in wash effects all those years ago. I also had a look at the effects of viscosity on wave decay in a recent work. See:
http://www.cyberiad.net/leo.htm

I'm not sure how well the theory will work in the real world. Measuring wave decay rates kilometres from a ship is not an easy thing!

Regards,
Leo.

[Ad Hoc]

Leo

Non taken. You're approach is from a 'numbers' based environment, whereas mine is from a 'real world' environment, based upon 'numbers'. Each are unique but also require each others input for validation and understanding. Since numbers are just..well,...numbers without validation. And shapes are just...shapes, without know what affects them.

The tank testing we did of various hull forms though, in regards to shape, cannot be ignored - has been proven by many since, theoretically too!

You should the research report 457 by MCA in the UK..very good stuff. Also the obvious stuff by Dand and Whittaker, excellent work on wash. Leaders in the field I would say.

[Ad Hoc]

Leo

"..I can account for sinkage and trim, reduce waves made by the ship, estimate bottom pressure signatures, and (for monohulls at least) estimate their performance in waves..."

I'm interested how you do this. What methods do you use and what do you use to validate the results, and over what range is the validation applicable?

Also, I'm not aware of any reliable methods that can take into account the Stern transoms at both low and high speeds, how do you account for this, and what method of validation do you use for these over a speed range?

I read many papers either as personal interest or asked to critique, so I'm wondering about your method of validations and their applicability.

[Leo Lazauskas]

Leo

You should the research report 457 by MCA in the UK..very good stuff. Also the obvious stuff by Dand and Whittaker, excellent work on wash. Leaders in the field I would say.

I know of Whittaker's work. His student Max Osterried used an old version of Michlet in his Master's Thesis of 2002.

Recent work work on wakes by Macfarlane and Renilson, and Doctors and Day might be of interest to you. The first pair use a lot of experimental data; the second pair use a more theoretical approach.

A more interesting problem for me is: given a wave pattern, what is the shape of the ship (or fleet of ships) that made that pattern? Godzilla can "solve" a simple form of the problem. Other approaches have been tried by some excellent hydrodynamicists, including Art Reed, Carl Scragg, and Jerome Milgram.

I'd love to get some data that gives the wave elevations over a fairly large patch of otherwise calm water, but I haven't been able find anything that goes back far enough from the ship. Know of anything like that?

[Leo Lazauskas]

Leo

"..I can account for sinkage and trim, reduce waves made by the ship, estimate bottom pressure signatures, and (for monohulls at least) estimate their performance in waves..."

I'm interested how you do this. What methods do you use and what do you use to validate the results, and over what range is the validation applicable?

Also, I'm not aware of any reliable methods that can take into account the Stern transoms at both low and high speeds, how do you account for this, and what method of validation do you use for these over a speed range?

I read many papers either as personal interest or asked to critique, so I'm wondering about your method of validations and their applicability.

I have no way of validating many results because that is not my field.

I have a couple of methods to estimate bottom pressure signatures. Some use line sources, some use assemblies of Havelock sources.

Transom sterns are, as you note, very difficult.
If you want to see how CFD performs, look up papers by Kevin Maki, L.J. Doctors and Robert Beck. As you said, there is nothing reliable at very low speeds or very high speeds, however there are some empirical methods and a couple of theoretical results (that I know of) that can be used to get reasonable agreement with experiments. Sometimes.

Doctors and Day did a lot of work on transoms, and haven't found anything that I consider consistent yet. I have dozens of variations on their theme and others and I always feel that I am shoe-horning results into experimental data by tweaking a variety of parameters.

[Ad Hoc]

Leo

I know Renilson well, we have worked together before. Doctors work is interesting, but his work has been, at times, too much theoretical work. By that i mean just research for the sake of research and has no real application, save for a thesis or a paper presentation. But there is some good stuff in there, just ahve to wade through the unusable stuff, from a practicing naval architecture point of view, not a academics/theoreticians.

I have given some advice recently to a post-doc research student looking at weather systems ie sea spectrum's in relation to parametric rolling in terms of weather routing models...a possible link up with your far field waves?

The MCA 457/ work by Dand have measurements up to 600m from the vessel track. Not perfect, for your work/area of interest, but it is a start!

"...I have no way of validating many results because that is not my field..."
Without any validation, i could only take the results with a pinch of salt!
This is not to say it is totally meaningless, but just "on hold" so to speak until the results/theories are validated.

PS..Molland and Hudson i recall have some some interesting work on transoms and wake fields, i know them too and could contact them to see what they have been up too recently, unless you already know?

[Leo Lazauskas]

Leo

"...I have no way of validating many results because that is not my field..."
Without any validation, i could only take the results with a pinch of salt!
This is not to say it is totally meaningless, but just "on hold" so to speak until the results/theories are validated.



Sorry, I meant verify. I don't have access to facilities to compare my results to experiments in every case.

Doctors work is often fairly theoretical, but in nearly every paper I have read he does try to compare results to experiments. I find that alone useful.

I prefer the theoretical and mathematical because it can lead to unexpected places, whereas experiments with a hull, or series of hulls, just gives you info about, not surprisingly, more hulls.
The work I started on in identifying ships from their wave patterns morphed into all sorts of weird stuff and, for example, prompted my colleague Ernie Tuck to use the techniques to investigate the Riemann Zeta function. That's a very long way from ship hydro!

[Ad Hoc]

That's the beauty of "pure" research, takes you into unknown and unplanned territory.

But research is only any good if the results can be validated and verified qualitatively. Otherwise it just remains 'numbers' for the sake of numbers sake. There is plenty of that here in Japan...most of it excellent work, but has no application and/or next to impossible to verify.

"...Sorry, I meant verify. I don't have access to facilities to compare my results to experiments in every case..."

So the work/results you obtain from your research and your programs, without verification, how do you know where the limitations are and hence its own applicability for use in modelling the real world environment?

[Ad Hoc]

Just thought about this one, nice idea.

"...A more interesting problem for me is: given a wave pattern, what is the shape of the ship (or fleet of ships) that made that pattern?.."

What you're after is a 'hydrodynamic' signature that relates to a unique 3D immersed body moving at the water-air interface. Hmmm...given the fact that waves, especially far field waves, from various hull forms at varying speeds and varying depths, whilst "appearing" unique, all follow pretty much 'set patterns'; despite great pains to model such "simple" behaviour, especially in terms of wash. How you get a signature out of divergent, transverse and solitary waves, when most of this is still in its infancy anyway, very tricky...but interesting!

You're probably better looking down the quantum physics route, just as those studying rogue waves, have found very similar correlations.

[Leo Lazauskas]

"...Sorry, I meant verify. I don't have access to facilities to compare my results to experiments in every case..."

So the work/results you obtain from your research and your programs, without verification, how do you know where the limitations are and hence its own applicability for use in modelling the real world environment?

I try to compare results to experiments whenever they are available. Even when they are available they are not always useful. For example, the NPL series of hulls were studied in great detail by Molland Couser and Wellicome. However all the hulls were only 1.6m long which raises a number of issues to do with viscous effects. Shame, it's one of the best pieces of experimental work on catamarans there is.

I read through your paper last night. There is a huge amount of good work summarised in there! The only criticism I have is that there are no error bars on any of the experiments. You aren't alone there - hardly anyone does that any more but it makes verifying predictions all the more difficult.

Cheers,
Leo.

[Ad Hoc]

Leo

"We", my colleague and I, were often approached by universities for verification/validation of their work, with the boats we designed, at our previous company. We ended up with a good working relationship with one in particular. May be worth approaching a shipyard, or even as we had once, a ship operator, and ask if you could do some 'collaborative' work, which benefits both parties. Downside is that almost always it is confidential data that you obtain, hence limited to what you can demonstrate publicly.

But without verification, it will be hard to "punt" your theories/research to anyone other researchers and academics. I have to justify and verify everything I do when designing, if i can't it comes down to experience and judgement...if a software program looks great, but has not been verified, wont use it...unless for just simple sensitivity analysis.

Thanks for the comments, we got very favourable reviews when i presented it. Prof Molland actually said he'll make it essential reading for his 1st year students!..praise indeed.

Yes, we didn't put error bars on just the trend lines, since the paper is not a real 'academic' paper as such, would have bored the audiance too!

[Ad Hoc]

as a thought, is the Series 89 catamaran hull series done by Burkhard Muller-Graf at Berlin Model Basin of any use for your verification?

[apex1]

Well, Mr. Willoughby
did that brighten your horizon?





.

Now it really started to go professional here. Thanks mates!

[Leo Lazauskas]

as a thought, is the Series 89 catamaran hull series done by Burkhard Muller-Graf at Berlin Model Basin of any use for your verification?

I have several series of cats that were in FAST proceedings and I haven't had time to digitise them all yet. I have enough for my resistance and squat predictions.

What I am really waiting on are the results for the DDG61 destroyer hull that is being tested at more than 30 towing tanks around the world. The complete set of data will allow some interesting comparisons of theoretical models and their capabilities, or lack thereof.

I'd also like to see more comparisons of drag predictions using the (IMO dodgy) ITTC line and physics-based methods for skin-friction.

Leo.

[Ad Hoc]

The series 89 is in the true tradition of German hydrodynamacists...lots of very useful data there.

I presented a paper at a RINA conference in 1995. Was awarded a RINA medal for well...anyway..but we were pipped for the silver medal. It was a paper on just such a subject of skin friction, was areal break through...unfortunately, can't remember the title nor who wrote, would have to dig around for it.

[Leo Lazauskas]

The series 89 is in the true tradition of German hydrodynamacists...lots of very useful data there.

I presented a paper at a RINA conference in 1995. Was awarded a RINA medal for well...anyway..but we were pipped for the silver medal. It was a paper on just such a subject of skin friction, was areal break through...unfortunately, can't remember the title nor who wrote, would have to dig around for it.

Probably Grigson. I know his work pretty well.

[DMacPherson]

Let me jump into the loop here, if I may.

We use a couple different in-house implementations of linear wave-making code for slender ship calcs, and I find the one big empty place is the viscous addition of form and the consideration of displacement thickness (i.e., boundary layer). For example, Leo pointed out his concerns when using the small models of the Insel/Molland cat tests. (We also have used these for validation, with the same concerns.) In many cases, the magnitude of the viscous addition of form exceeds the wave-making (as would be the case for most of the Insel/Molland tests below 0.4-ish FN) - so accurate prediction of form factor would seem to be something that gets too little attention.

When model tests are available, you can often derive some sense for the viscous form addition (as was done for the cat tests), then add your wave-making calc. Sometimes, though, you are guessing at a form factor, if the test speeds are too high to determine where wave-making degrades to zero (and then you have to guess-timate if there is any immersed transom pressure drag to contend with). I can get great correlation by fitting it with an arbitrary form factor, but this does not validate the wave-making code, of course.

When doing a prediction for a new ship, you're left determining your own form factor, and any speed dependent correction you might want to use. (We often use Holtrop's 1988 form factor equation with his speed-dependency correction. It's generally a workable solution.)

I'd love to hear your comments on getting the right form factor and viscous addition. Has any one done any work on predicting form factor using the same distributed hull data that goes into the wave-making code? Something like form factor from a sectional area curve rather than simple hull parameters?

Ad hoc: I'd love to read your paper, as well. Thanks.

Very interesting thread...

Don MacPherson
HydroComp

[Ad Hoc]

I'll also ask a few Prof's here at the Uni i occasionally lecture at too, they are very helpful. They have endless data.....bit of a hidden gold mine really.

Grigson...ahh sounds familiar! Can't recall the title though.....still fair play they got the higher RINA medal than we did.

I've just dug out a short paper called "Short review of semi-displacement series for defining hydrodynamic characteristics of ships (boats)" by Antun Gamulin, have you seen this one?

Trouble is, i have around 2000 papers all stacked away in various files on a range of subjects....I've forgotten what half of them are about now some of them so old, and some highly confidential too, unless i start digging them all up i don't know what I may have lurking on my book shelf..

[Ad Hoc]

D.MacPherson

Yes, form factor has always been the "problem". as you nicely put it ".. I can get great correlation by fitting it with an arbitrary form factor, but this does not validate the wave-making code, of course..."..

Every systematic series I've ever read all use different 'fudge' factors, which speaks volumes about the difficulty. Perhaps it requires a different approach, since we are all "naval architects/hydrodynamacists" we end up thinking in pretty much the same vein. Perhaps input from say theoretical quantum physics may assist. Their work on Rogue waves, for example, pointed the way forward.

I hate all this management speak (its all bollocks), but can't help saying though, thinking out of the box is required...hence maybe a totally unconventional and different approach to the problem is needed, from an unlikely source.

If you PM me your email address, id be happy to pop over the paper.

[DMacPherson]

Leo, I'm sure you've seen this site, but I'll post it anyway just in case.

www.iihr.uiowa.edu/gothenburg2000/ (http://www.iihr.uiowa.edu/gothenburg2000/)

I've always wanted something like this for smaller ships and all of the other test data that's available in the public domain.

Don MacPherson
HydroComp

[Leo Lazauskas]

Leo, I'm sure you've seen this site, but I'll post it anyway just in case.

www.iihr.uiowa.edu/gothenburg2000/ (http://www.iihr.uiowa.edu/gothenburg2000/)

I've always wanted something like this for smaller ships and all of the other test data that's available in the public domain.

Don MacPherson
HydroComp

Thanks, Don. I know the Goth2000 papers well. I love the estimates of surface area by some of the codes - some couldn't even get that right to within 5%!

I addressed the horrible form factor a bit in the first paper in the list on my site at http://www.cyberiad.net/leo.htm

But what I found isn't anywhere near the final word. I would have liked to have done more, but I just couldn't find enough good experimental data and eventually I ran out of time and a supervisor.

All the best,
Leo.

[Padava]

You should get hold of:
"The Wave Resistance of Ships"
John V. Wehausen
and read Chapter H.3:
Ships of minimum resistance.

A few other references that might be useful...

"Ships of Minimum Total Resistance"
-- Aug. 1963.

"Optimal Ship Forms of Minimum Wave Resistance"
--- Aug. 1972.

Some formulations are just quadratic programming exercises,
but there are others available that require calculating Mathieu
functions. Unless you know some numerical analysis, they will
have you tearing your hair out! Hunt through Wehausen's work
for more information.

Good luck!
Leo.

Many Thanks to you and Ad Hoc. I am quite a novice here, am given to methods that are sometimes called " Form Finding ", but not so much interested in shapes defined a priori (standard parabolic waterlines/transverse frames or fourth degree buttocks) for hydrodynamics that follow as a consequence. Before I can read the above topics, would appreciate your comments. If the unknown shape is expressed i.e., the offsets are geometrically given in Monge's form y = f(x,z), fluid dynamic and mathematical formulation can be therein incorporated, or so I think, keeping wave drag, skin friction as parameters which can be modelled into a simulation numerical program representing all forces acting on each element and then integrating for full hull having minimum total resistance using a combination of algebraic, pde (or ode). That could be solved later numerically using standard mathematical softwares like Matlab, Maple or Mathematica. As a start we may assume symmetry longitudinally about midship, ignoring transom/bow dissimilarity as a crude starting mathematical model, assume port/starboard mirror imaging and disallowing sudden changes at body line bilge chines.

To illustrate the methods by two examples: a) Finding a stable position of a string of constant linear density hanging between two points, the integral (z ds) potential energy is minimized resulting in a catenary shape. b) Finding the profile of a rocket with minimum aerodynamic drag: Isaac Newton had arrived at the surface of revolution shape by using calculus of variations for the first time. In both cases differential equations describe needed optimal shapes.

Could a reasonably simple program be written defining simplified hull shapes with various parameters for given displacement and speed ? I was expecting F(p,q,r,s,t) = 0 sort of pdes for complicated and odes for simpler cases...

Best Regards

[Leo Lazauskas]

Could a reasonably simple program be written defining simplified hull shapes with various parameters for given displacement and speed ? I was expecting F(p,q,r,s,t) = 0 sort of pdes for complicated and odes for simpler cases...

Best Regards

I think yes, and I urge you to look at Wehausen's work. G. Weinblum also looked at the problem in the 1950's. As I said, depending on the assumptions you make in your model regarding wave resistance and skin-friction, you might end up with (at best) having to evaluate a number of Mathieu functions. I'm not sure how well Matlab and Mathematica handle those nasty functions for a wide range of parameters. It might be better to get specific C or Fortran code.

In a more general case you will have to solve the problem numerically.

The "optimal" hull forms found in the 1960's were very strange wobbly shapes. One, named Ward's Optimum Symmetric Ship was bundled with Michlet 8.07 as an example. Aslo see:
Ward, Lawrence W., Wave resistance surveys on a ship model of minimum resistance, Webb Inst. of Naval Arch. August 1965, pp. 16. (+ errata).

Experiments on these wobbly hulls showed that their wave resistance at the design speed was very low, as predicted, but the skin-friction was high because of the large surface area, and the form drag was high because of possible early separation.

Other people who have looked at optimal shapes include Ada Gotman of U. Novosibirsk, Russia and Doctors and Day, e.g.
Day, Alexander H. and Doctors, Lawrence J., Resistance optimization of displacement vessels on the basis of principal parameters.

Day, Alexander H. and Doctors, Lawrence J., Design of fast ships for minimal resistance and motions, (submitted to) Sixth Int. Marine Design Conf. IMDC-97, June 23-25, 1997.

I'm not exactly sure how where to find pdes for hull shapes that you will need. Ward and some others used Fourier series if my memory is not completely gone.

Have fun! You have a lot of work ahead of you!
Leo.

[Rick Willoughby]

.......

Could a reasonably simple program be written defining simplified hull shapes with various parameters for given displacement and speed ? I was expecting F(p,q,r,s,t) = 0 sort of pdes for complicated and odes for simpler cases...

Best Regards

Padava
Are you familiar with Leo's Michlet/GODZILLA software?

Rick W

[apex1]

Padava
Are you familiar with Leo's Michlet/GODZILLA software?

Rick W

His question makes clear he is not.

But Rick, are you familiar whith the theories behind?


Your statements in the past made obvious you´re not! Now let the real pro´s discuss the issues, and let´s stay back a while.

Regards
Richard

[Ad Hoc]

"..Padava
Are you familiar with Leo's Michlet/GODZILLA software?

Rick W.."

:rolleyes:

[apex1]

"..Padava
Are you familiar with Leo's Michlet/GODZILLA software?

Rick W.."

:rolleyes:

What did I miss???? Has RW gained some real world knowledge I did not mention?

[Rick Willoughby]

I have no way of validating many results because that is not my field.

......

Leo
Rowing shells have evolved over a period of about three hundred years to be close to the most efficient hull form for their power and displacement - whether they be 1, 2, 4 or 8. The fact that GODZILLA can improve on their shape to produce a lower drag hull in a matter of minutes is powerful validation.

Even if GODZILLA was only to get close to the optimum it reduces years of effort in trial and error to arrive at the lowest drag hull. My own experience in the single person class, probably the most used form of craft, indicates it produces not just close to but, indeed, the lowest drag hull for the design power and displacement.

Rick W

[Ad Hoc]

No, sadly "normal service" is resumed...sigh!

Was an intelligent thread, for once....oh well, good things don't last for ever!

[Ad Hoc]

Rick

If you have read and if you have actually understood any of the postings above, you would not make such a vacuous statement as thus:
" ..The fact that GODZILLA can improve on their shape to produce a lower drag hull in a matter of minutes is powerful validation..."

You have no concept of validation and verification, this is clear.

Without verification all work is meaningless, until otherwise quantitatively proven and verified independently.

Leo quiet rightly acknowledges this, with his own work, yet for some reason you ignore this and claim it to be something it is very clearly not.

Leo's work is very interesting and potentially exciting, but in the hands of those that do not understand, very dangerous!

[yipster]

good read Leo Lazauskas
have the patience attaching file Ward's Optimum Symmetric Ship here?
loads in 8 and 20 stations version rite?

[Leo Lazauskas]

good read Leo Lazauskas
have the patience attaching file Ward's Optimum Symmetric Ship here?
loads in 8 and 20 stations version rite?

I think this should work in version 8.x

1. Rename the file to in.mlt
2. Run Michlet
3. Look at the wave drag curves and marvel at how low the drag is at 1.223m/s
4. Think about why the wave drag is so low :-)

Optimum symmetric ship shapes at higher Froude numbers have smaller bulbs at the ends of the ship, but sometimes have large midship bulges... like some of the older readers here.

All the best,
Leo.

[yipster]

works and with the 8.5 manual next to the keyboard marvelling, thank you

[Leo Lazauskas]

Leo
Rowing shells have evolved over a period of about three hundred years to be close to the most efficient hull form for their power and displacement - whether they be 1, 2, 4 or 8. The fact that GODZILLA can improve on their shape to produce a lower drag hull in a matter of minutes is powerful validation.

Even if GODZILLA was only to get close to the optimum it reduces years of effort in trial and error to arrive at the lowest drag hull. My own experience in the single person class, probably the most used form of craft, indicates it produces not just close to but, indeed, the lowest drag hull for the design power and displacement.

Rick W

Hi Rick!
I'm not sure that Godzilla results are really a validation of
anything, but I'm not that worried about confirming results
for the thin smooth hulls and speed ranges you usually deal with.
Michell's theory does a pretty good job with the wave resistance
and that's all that really concerns me.
Form drag, of course, is tough to estimate, but everyone has that
problem. I'm not sure what CFD guys do - maybe they just wave
their hands around like the rest of us.

I can get pretty good agreement with experimental rowing data,
but I have to make a few biomechanical assumptions along the way.
However, with those assumptions I can get good agreement for Froude
numbers between about 0.2 and 0.75. As far as I am concerned, there's
no real need to repeat the verification for every "suitably thin"
hull. (I doubt that you, personally, will ever get much over F=0.75 in
your human-powered events!)

Another point to remember is that there are some very subtle features
of rowing shells that aren't captured by the mathematical series I use
as examples in Godzilla. The human designer I worked with was much
better than Godzilla at fitting two 2metre tall brutes into a 10m shell,
and making the hull efficient and stable for its design speed range.

The graphs in the attached pdf document illustrate a few of the factors
I used to model the velocity and acceleration of a rowing stroke.
From what I remember of your human-powered events, you tend to move at
a more constant, slower speed, so some of the following is probably more
than you need to consider.

Measured data comes from a trial crewed by Olympic Gold medallists.
I've used data for 5 strokes at 36.1 strokes per min for sake of this
example.

The first page shows the rowers' body segment angle regimes and the
trajectories of their segment centres of mass.

We also need to know the oar angle regimes and the forces exerted by
the rowers. Good measurements are usually available for these
quantities as shown on the second page.
I'd guess that your pedal drives etc are probably easier to model than
oars, but then we both have difficulties with the propulsors - me with
what happens at the oarblades, you with propellers.

I need to know the hydrodynamic forces and moments on the hull
that induce sinkage and trim (aka "squat") because these change the
underwater shape of the hull. The location of the bow and stern of
the hull during a single stroke are shown in the top plot on the
third page.
These curves (red bow, green stern) were calculated using the
combined effect of the rowers' centres of mass and the hull squat.
Of course, these results depend on the location in the shell of the
crew members, and the exact shape and proportions of the shell.
Matching the crew, the forces they are capable of producing, and several
other factors, make for a very tricky design problem, moreso given that
races are sometimes won by a bee's dick.

Once we have all that, we can estimate the hydrodynamic drag as shown
at the bottom of the third page. We must also burn some offerings
to the air drag gods. After collecting some empirical drag coefficients
for the riggers, oars, human bodies and hull we might get a reasonable
estimate. Or maybe not, air gods can be fickle.

Having assembled that morass, (and a few things I've left out) we can
now estimate the instantaneous hull acceleration and velocity as shown
on the last page.

For long distance events, squat is not going to be an issue, and for
fixed-seat events you aren't going to bob the hull around much, so
Michlet should give reasonable estimates for your applications.
If you are actually going faster than Fr=0.45, squat might become
important. On the other hand, if you are moving at a constant
Froude number, you can move the cg a bit to reduce adverse effects.
It's not so easy with rowing shells that operate over a wide range of
Fr.

In short, I wouldn't be surprised if the predicted performance of the
Godzilla hulls you found agreed as well with your experiments as I have
shown here for my more complicated problem. Whether your hulls are truly
optimal (whatever that means) for your "missions" is another matter
completely.

All the best,
Leo.

[Rick Willoughby]

..........
For long distance events, squat is not going to be an issue, and for
fixed-seat events you aren't going to bob the hull around much, so
Michlet should give reasonable estimates for your applications.
If you are actually going faster than Fr=0.45, squat might become
important. On the other hand, if you are moving at a constant
Froude number, you can move the cg a bit to reduce adverse effects.
It's not so easy with rowing shells that operate over a wide range of
Fr.

In short, I wouldn't be surprised if the predicted performance of the
Godzilla hulls you found agreed as well with your experiments as I have
shown here for my more complicated problem. Whether your hulls are truly
optimal (whatever that means) for your "missions" is another matter
completely.

All the best,
Leo.

Leo
As you point out I have simplified the analysis significantly by using steady thrust.

Knowing what the lowest drag hull looks like for any set design conditions is a good start in optimising the design. Whether the end result is an optimum boat is a function of many variables.

The pedal powered boats are primarily my large model boats but give me endless fun. I am moving on to electric powered now with the aim of building a boat for coastal cruising using solar and wind power. The target is design cruise of 8kts with 9.5m hull using energy collection from both sun and wind so well within Michlet range.

Although squat is not an issue for what I am currently doing I am still interested to know if you intend to make Flotilla publicly avaialable. I am sure there are a few other tools like me who wouls appreciate it and be prepared to pay for it.

Rick W

wand .

[Ad Hoc]

Leo

".. As far as I am concerned, there's no real need to repeat the verification for every "suitably thin hull"..."

But for verification to show the solution converges consistently, a large sample is required. Just one or two samples statistically isn't enough validation.

What B/T ratios are these thin hulls?

For better correlation, you really need 'naked' resistance value. Since there are far to many variables, in the example given, that could under or over estimate your values, or even introduce some other unknown quantity. But therein lies the difficultly, getting a 5m model to be "propelled" in someway where you can reduce the variables to just 1 or 2 and have these measured, such as via strain gauging and cavitation expts for the power delivery and prop characteristics, at your budget.

Then to also repeat this on large hulls, say in the 50m range, just to confirm that there are no real scale effects or, your 'model' is able to predict the scale effects with sufficient accuracy.

Since from a naval architecture point of view, those rowing hulls, are not much larger than some models i've used for ship/boat expts.

[Ad Hoc]

Rick

"..Knowing what the lowest drag hull looks like for any set design conditions is a good start in optimising the design..."

You're missing the point, again. The results are not verfied in any form, so to claim as you consistently do that you ahve the perfect hull, is extremely misleading at best, and in competence at worse.

Incase you misread it, like you appear to have continuously done so far , here it is again.

"Hi Rick!
I'm not sure that Godzilla results are really a validation of
anything, but I'm not that worried about confirming results
for the thin smooth hulls and speed ranges you usually deal with..."

[Leo Lazauskas]

Leo

What B/T ratios are these thin hulls?

For better correlation, you really need 'naked' resistance value.

B/T is about 2 to 3

Unfortunately, hardly any published experimental data gives error bars on resistance results which makes them almost useless for validation and verification purposes. Futhermore, the small model sizes are a real worry, as you and Don M have noted.

If there was more good published data it would make life easier for us academic types.

As I said earlier, I'm happy with how my model works for "sufficiently" thin hulls. The rowing shell used in the comparisons on the attached Excel sheet was measured at full-size and the agreement is good enough for my purposes.

Leo.

[Leo Lazauskas]

Leo
Although squat is not an issue for what I am currently doing I am still interested to know if you intend to make Flotilla publicly avaialable. I am sure there are a few other tools like me who wouls appreciate it and be prepared to pay for it.


Unfortunately I don't have time to put out a new version of Michlet, let alone other codes I have that are scattered around the garage floor.

I'm only on here because I am doing some computer runs that each take about 30 minutes. That's too short to do much else except to make a few moves in CivIV on another computer and post crap to the internet.

All the best,
Leo.

[Rick Willoughby]

Leo
I note your earlier comment about running out of a supervisor. Have you made progress on your PhD being awarded.

My son submitted his thesis two years ago and is still waiting for the formal recognition. Having put in all that work it is unreasonable to let it slide.

Life takes people in interesting directions. In some cases formal qualifications count for little but in others they can be critical to opening doors. I know people who have not pressed the last little bit to get the formal recognition and they have come to regret it at certain times in their career.

Rick W

[Leo Lazauskas]

Leo
I note your earlier comment about running out of a supervisor. Have you made progress on your PhD being awarded.


Yes, I finished it and had it marked just before Ernie Tuck died in March. It was a very sad and quite sudden loss for his family and the hydro community. We still had a lot of interesting and useful stuff to do. Maybe, like him, I'll find a nerdy clot to do some of the work.

Leo.

[Leo Lazauskas]

Leo

".. As far as I am concerned, there's no real need to repeat the verification for every "suitably thin hull"..."

But for verification to show the solution converges consistently, a large sample is required. Just one or two samples statistically isn't enough validation.



I no longer see that as a great issue. Michlet allows the user to easily, for example, double the number of stations and waterlines so that surface area estimates are within 1% or less. Similarly, the number of "theta" intervals in the wave resistance calculations can be increased until the desired accuracy is required. As a further check, the spectral functions for a Wigley hull, although calculated numerically, are exact because of the transforms I use. This can be checked using as few as 3 stations and 3 waterlines.

I take your point about checking convergence. As I said to Don Mc, some of the high-powered CFD codes showcased at Gothenburg 2000 couldn't estimate surface area to within 5%.

Personally, I'd love to see more (for want of a better phrase) "blind competitions" where good experimental results are available for several vessels and competitors are given only the hull dimensions and shapes. The Wake-Off of a few years ago produced some interesting results, and had some code writers to quickly re-submit predictions after the experimental results were released.

So, got some good model and full-sized data that would be up to that sort of test?

[Rick Willoughby]

Yes, I finished it and had it marked just before Ernie Tuck died in March. It was a very sad and quite sudden loss for his family and the hydro community. We still had a lot of interesting and useful stuff to do. Maybe, like him, I'll find a nerdy clot to do some of the work.

Leo.

Leo
My only knowledge of Ernie Tuck is through reading various papers he has authored or coauthored. I am personally very appreciative of the result you have achieved with Ernie and making a portion of those results so readily available through Michlet.

It is sad news. It is not an uncommon experience for me to lose relations and colleages to cancer.

I think I told you before that my eldest son is doing cancer research. Essentially looking for the magic bullit. At this stage they have now verified their process for drug evaluation. The real work will require more funding to get going by the end of this year. It is a long slow process.

Does having your thesis marked mean it is now Dr Leo Lazauskas or is it going through expert review?

Rick W

[Ad Hoc]

Leo

"...So, got some good model and full-sized data that would be up to that sort of test?.."

We've got around 20~30 years worth of tank test data and full scale sea trails. Some sensitive/confidential, some not. Some with loads of data some not so. Not sure if the data we have can assist, but no harm in asking/trying....just let me know what is the type of exact data that would really suit you.

"..I no longer see that as a great issue. Michlet allows the user to easily, for example, double the number of stations and waterlines so that surface area estimates are within 1% or less. Similarly, the number of "theta" intervals in the wave resistance calculations can be increased until the desired accuracy is required..."

Hmm...calculating surface area, is just down to the accuracy of the method employed, i don't see how this makes the program any better, simply because the WSA is now more accurate. I could manually measure every square millimeter or more, does that make 'me' suddenly able to say "I've cracked it".? It still relies on form function to obtain a decent frictional resistance. Or does the program say, ahh, very accurate WSA no need for form function, for example?

Narrowing down the WSA % errors,to within minimal values, of course helps, but it doesn't mean the results are more accurate, just means the input data is more accurate. Since whether the WSA is calculated to within an accuracy of 1% or 10%, the percentage error on the other variable (form factor) in the result remains. I've read many papers, as I'm sure you have too, where the form factor is different for different models and ratios and speeds etc, which is correct?. How would this aspect be improved simply by having a more "accurate" WSA, does the program say, ooo this hull has 99% accurate WSA i'll use a different formula, via a set of 'set' parameters?

Again, not sure i follow how increasing the number of 'theta' intervals increases the accuracy. Again, just reduces the % error, which does not equate to increasing the accuracy of the residuary resistance, which is based upon other variables. Since how does the program know what is accurate/correct and what is not, other than a series of algorithms and parameters?..or is this where the "accuracy" lies, in selecting a more appropriate set of algorithms.

Good luck with your PhD.

[Leo Lazauskas]

Leo

"...So, got some good model and full-sized data that would be up to that sort of test?.."

We've got around 20~30 years worth of tank test data and full scale sea trails. Some sensitive/confidential, some not. Some with loads of data some not so. Not sure if the data we have can assist, but no harm in asking/trying....just let me know what is the type of exact data that would really suit you.


I was half joking. As I said, I wish that someone would organise more blind competitions to see how well CFD and other algorithms perform.

Leo
I've read many papers, as I'm sure you have too, where the form factor is different for different models and ratios and speeds etc, which is correct?. How would this aspect be improved simply by having a more "accurate" WSA, does the program say, ooo this hull has 99% accurate WSA i'll use a different formula, via a set of 'set' parameters?


Sorry, I misunderstood what you were on about before. I didn't realise you were talking about code that somehow also estimates form factors.
I concede defeat - I can't predict form drag. When I feel the need to use form factors, I either use parameter versions, or I grudgingly use Prohaska's method. I guess that the only real concession I make is to use a more physics-based skin-friction line instead of the ITTC line.


Good luck with your PhD.

Thanks. I'm over it now.

[Leo Lazauskas]

Leo
Does having your thesis marked mean it is now Dr Leo Lazauskas or is it going through expert review?
Rick W

The former, although I prefer "Your Serene Highness" in written correspondence.

[DMacPherson]

Ad Hoc:

I think Leo is talking about the calculation of the dynamic (speed-dependent) WSA. As has been mentioned, wave-making is just part of the puzzle. Viscous drag needs to be properly considered. The current practice of using static WSA is reasonable, but in the context of "academic" interest perhaps leading to better solutions, using the dynamic WSA makes sense.

Is it worth the effort? Depends on your acceptable error band. Our company worked on model expansion and VPP development for one of the AC syndicates back in the early 90's. We did a sensitivity study of the difference in expanded results using dynamic and static WSA, and given the large models and well-drafted test program, we had the data to determine that there was a measurable effect - one that we could use to better achieve full-scale results. As an aside, we also found negative K-values in the form factor (i.e., FFs less than 1), which of course is physically incorrect, but a result of using the ITTC CF line which contains a bit of form correction. This speaks to Leo's interest in the Grigson line. (We ultimately used the Hughes line for the AC work for better correlation.)

So, the fact that the CFD developers chose to try and calculate dynamic WSA - and then got it wrong by 5% - is an important part of any discussion regarding validation of codes. Validation to empirical results can be tricky. You first must validate the testing itself. I've seen CR vs FN curves for different tests of a standard Wigley hull that have a pretty broad scatter. So you have to take all of this with some reasonable attention to the ultimate objective.

I care about "what is", and much less about "what should be". I'm interested in validating the underlying code only in so far as it leads to a better treatment of the whole picture. In other words, the calculation of CW is only as good as the final calculation of CT, which is the real objective. I personally believe that final validation of hull codes should be to CT and nothing else, including calculation of all of the components that make up CT - CW, CF, WSA corrections (if any), and form factor. Arbitrary selection of any of these makes any conclusion about the "validity" of a particular code somewhat deceptive.

Regards,

Don MacPherson
HydroComp

[Ad Hoc]

Leo

I must confess, i ahve no idea about what your programs are about however, this

"...I concede defeat - I can't predict form drag. When I feel the need to use form factors, I either use parameter versions, or I grudgingly use Prohaska's method. I guess that the only real concession I make is to use a more physics-based skin-friction line instead of the ITTC line..."

does this mean Michelt, is just a more accurate way of calculating WSA (as also noted by Don re: dynamic) than existing hull modellers?..hence a more "accurate" result, since everything else is the same?

Don

"..I care about "what is", and much less about "what should be"..."

Fully concur there.

[Ad Hoc]

Don

We have had some interesting debates with Tank test houses regarding the way they calculate the Ct in recent years.

Our way is slightly different, (we do our own in-house tank testing) but yields the same results. In so far as we "assume" a position for the laminar flow portion and hence the laminar WSA and apply the studs in said region, and thus also obtain the turbulent WSA. It seems some modern test houses no longer take this approach.

But as you say:
"..I personally believe that final validation of hull codes should be to CT and nothing else, including calculation of all of the components that make up CT - CW, CF, WSA corrections (if any), and form factor..."

That is indeed the goal.

[Leo Lazauskas]

Don,
Some of the codes weren't able to esimate the static WSA accurately.
Some omitted the bulb from the destroyer hull, but that's not always clear from the published results. None of the results at Goth2000 showed dynamic WSA, IIRC.

I also found negative k_f for very fine hulls when using the ITTC line which prompted me to look at Grigson's work more closely.

IMO, the ITTC line doesn't contain "a bit of form correction".
I used to believe that, but:
1. The ITTC formula is not physics-based. It was proposed by R. Newton(?) at the 1957 conference and accepted by delegates who wanted something a little steeper than Schoenherr's line at lower Reynolds numbers. Hardly a scientific approach, but it meant the delegates got home earlier than if they were deadlocked.

2. As Grigson has noted, since 1978 the ITTC line is the recommended formula to be used in estimating the form factor: therefore,
"...there can be no form factor implicit in the ITTC57 2-D friction line, that would be a contradiction in concept".

I.e. the ITTC line is accepted as a "pure" skin-friction line.

What worries me is that non-physics based methods are sometimes used to pass judgement on prediction methods when they themselves are highly questionable.

The method of estimating form factors is also often really dodgy.
For example, as much as I love the work on the NPL series by Molland et al, I find their method of raising the transom out of the water at low speed in order to get consistent results very unusual. The form factor so derived is then applied to situations where the transom is immersed, and often considerably so when the hulls are free to squat. I'd hate to have my work judged without qualification against something that hairy.

Cheers,
Leo.

[Leo Lazauskas]

Leo
does this mean Michelt, is just a more accurate way of calculating WSA (as also noted by Don re: dynamic) than existing hull modellers?..hence a more "accurate" result, since everything else is the same?


No. It is primarily a code that estimates wave resistance and far-field wave patterns of thin ships.

[Leo Lazauskas]

Don

We have had some interesting debates with Tank test houses regarding the way they calculate the Ct in recent years.

Our way is slightly different, (we do our own in-house tank testing) but yields the same results. In so far as we "assume" a position for the laminar flow portion and hence the laminar WSA and apply the studs in said region, and thus also obtain the turbulent WSA. It seems some modern test houses no longer take this approach.



Do you use the ITTC line? Or something else?

[Leo Lazauskas]

Ad Hoc:

I think Leo is talking about the calculation of the dynamic (speed-dependent) WSA. As has been mentioned, wave-making is just part of the puzzle. Viscous drag needs to be properly considered. The current practice of using static WSA is reasonable, but in the context of "academic" interest perhaps leading to better solutions, using the dynamic WSA makes sense.

Is it worth the effort? Depends on your acceptable error band. Our company worked on model expansion and VPP development for one of the AC syndicates back in the early 90's. We did a sensitivity study of the difference in expanded results using dynamic and static WSA, and given the large models and well-drafted test program, we had the data to determine that there was a measurable effect - one that we could use to better achieve full-scale results.

When you (and AdHoc if you are reading this) measure the trim and sinkage of a vessel, do you also measure how much the water level around the vessels trims and sinks?

[DMacPherson]

Leo:

Can't calculate static WSA within 5%? That's really, really simple... T'would certainly make me wonder about the rest of the calcs.

Ad Hoc: FWIW, I've had the chance to look over a number of publications that Leo has authored and co-authored. His focus is, and always has been, on developing the wave side of the problem. (Leo, correct me if I'm wrong here.) The wave-making drag calculation is just part, but the work is thorough and well-developed. The Michell integral calculation is not my particular preferred approach for calculating CW for a variety of reasons, but given that as a starting point, I have no criticisms. My only comment is that when traveling from London to Rome, it nicely gets you to Paris. Then you need to get the rest of the way.

Don

[DMacPherson]

Leo:

Ah, the old "moving dish". Do we measure it? Well, the best answer is "somewhat". We measure sinkage and trim off the heave post, of course, but in the case of the AC work, we also photographed the hull and geometrically calculated the WSA from the photographed running WL on the model. Then we did a systematic series type of statistical regression for future prediction of dynamic WSA. We never correlated the sinkage with the mean WL from the photos to get the depression, however.

Don

[Leo Lazauskas]

The Michell integral calculation is not my particular preferred approach for calculating CW for a variety of reasons, but given that as a starting point, I have no criticisms. My only comment is that when traveling from London to Rome, it nicely gets you to Paris. Then you need to get the rest of the way.


And what mode of transport do you use to get the rest of the way? With CFD you might get a little closer to Rome, but only by digging downwards as you go until you hit magma and are forced to give up. :p

[Leo Lazauskas]

Leo:

Can't calculate static WSA within 5%? That's really, really simple... T'would certainly make me wonder about the rest of the calcs.


I shouldn't have used 5% from memory. Most are a bit better than that.

From Gothenburg 2000...
Code: S/L^2
Measured: 0.149
FinFlo: 0.1558
MGShip: 0.1485
CFDSHIP: 0.1550
ICARE: 0.1486
UNCLE: 0.1520
CFX: 0.1517

Mean: 0.1519

And, yes, most of my work with Tuck was concerned with the wave-making of thin ships and moving (over-)pressure distributions.

Leo.

[Rick Willoughby]

To me the most interesting aspect of arriving at the lowest drag hull for given speed and displacement constraints alone is that the wave drag ends up being very low.

Although the evolved hull shape is highly dependent on the wave drag, its actual value is almost negligible in determining the total drag of the vessel. Hence the accuracy of determining the friction component plays a more significant role in the total drag than wave drag.

It is enlightening to watch the evolution of a hull or hulls during an optimising run in Michlet. It inevitably makes sense when you see the output and reason it out but the result is sometimes surprising. Those so enlightened realise the benefit of analytical methods over empirical methods. Extrapolating from existing forms simply does not match what analytical based approach offers.

Maybe one day there will be sound analytical methods for determining the viscous component of hull drag and some will find smart ways to lower it. For now I am happy that Michlet provides an accurate estimation of wave drag from fundamental physics and GODZILLA has the power to minimise the overall hull drag; albeit relient on the in-built empirical based determination of viscous drag.

Rick W

[Ad Hoc]

Leo

As far as I am aware Molland et al, described 2 methods for determining form factor, One being the raising the transom clear of the water, as you noted, but is based on Prohaska’s work. They didn’t actually suggest doing this. He indicates that it is only suitable for conventional ships below 0.4Fn, and hence inappropriate for Higher Fn numbers. Also that it introduces more complexities.

Ahh, ok, so Michlet is estimating the Cw. And how is this verified to obtain a degree of accuracy or convergence?

Yes, we use the ITTC line.

We have sometimes, when time and money allows, measure the sinkage of the waterline around the model/ship…these in reality have only been done when funded by EU projects, since we are not really interested in this, only the end result. However we did do so for a client where wash was a prime objective in securing the contract.

[Ad Hoc]

Rick

"..Extrapolating from existing forms simply does not match what analytical based approach offers..."

and what analytical methods do you use to validate and verify this?

[Leo Lazauskas]

To me the most interesting aspect of arriving at the lowest drag hull for given speed and displacement constraints alone is that the wave drag ends up being very low.


I tend to treat the whole thing as some sort of weird computer game with artificial life-forms. For me the most interesting point was Godzilla's "discovery" of the hulls and diamond arrangement that almost completely eliminated waves at one speed:
http://www.cyberiad.net/waketet.htm

That sort of thing used to really bug Tuck until he could prove mathematically why it should be so.

I don't think that we ever satisfactorily explained the strange "optimal" pressure distributions shown in:
"Free-surface pressure distributions with minimum wave resistance"
http://www.cyberiad.net/library/pdf/tl01.pdf

At low speeds they are a bit like the weird and wobbly Ward Optimum Sym. Ship Shapes, but acting on top of the water. At high speed there is definitely something like hydrofoils at the ends.

That's the great thing about wave-making theories. They might not always give perfect real-world answers, but they can provide interesting insights. (Or to use Don's analogy, they might not get you all the way to Rome from London, but they can get you as far as the middle of the English Channel.)

Leo.

[Rick Willoughby]

....That's the great thing about wave-making theories. They might not always give perfect real-world answers, but they can provide interesting insights. (Or to use Don's analogy, they might not get you all the way to Rome from London, but they can get you as far as the middle of the English Channel.)

Leo.

From my own experience I am heading up the Thames at least in calm water. May be in the Channel when the wind driven waves need to be accounted for.

Actually in sheltered water with short fetch distances the windage is much more significant than the wind waves.

Rick W

[Leo Lazauskas]

Leo

As far as I am aware Molland et al, described 2 methods for determining form factor, One being the raising the transom clear of the water, as you noted, but is based on Prohaska’s work. They didn’t actually suggest doing this. He indicates that it is only suitable for conventional ships below 0.4Fn, and hence inappropriate for Higher Fn numbers. Also that it introduces more complexities.


I just noticed that they tried the bow down test for only one hull.

I still find the use of form factors as large as those shown in Molland et al (i.e. from 1.2 to 1.7) very worrying.


Ahh, ok, so Michlet is estimating the Cw. And how is this verified to obtain a degree of accuracy or convergence?


By running the code and looking at the results. You might change your mind about declaring Michlet "Dangerous" after you see what it does and under what assumptions. :p


Leo
Yes, we use the ITTC line.


Is that because it is easy to use on a calculator? Or because it is still the accepted "standard"?

Cheers,
Leo.

[Ad Hoc]

Leo

I've read their papers quoting as high as 1.45, but not 1.7. Which paper shows this as a result?
He has said though that the 1.45, whilst still being on the high side and is still unclear why it is high, does however, suggesting it may be owing to using different towing points compared to the original experiments.

Any software is "dangerous" until it is fully verified. To suggest otherwise is fool hardy. It may be ok to suggest as such when the only thing to loose is face amongst ones peers. But try telling that to a client when their 40m, 400 passenger, 40 knot, $10million ferry only goes 35knots, and the only explanation one can give is, well, the software has yet to be fully verified but we're told it's close enough. That's all.

Until another "standard" comes along that is proven to be more effective and consistent and verifiably so, then we shall continue to use the ITTC.

[Rick Willoughby]

I will need to sleep on the hydrofoil pressure distribution and its significance.

I had never even thought of hovercraft making waves. I might drag out one of the models I have.

Rick W

[Leo Lazauskas]

Leo
I've read their papers quoting as high as 1.45, but not 1.7. Which paper shows this as a result?
He has said though that the 1.45, whilst still being on the high side and is still unclear why it is high, does however, suggesting it may be owing to using different towing points compared to the original experiments.


Report 71, 1994

Monohulls: 1.23 - 1.45
Cats: 1.40 - 2.18

Leo

Any software is "dangerous" until it is fully verified.

Fair enough. But so strongly criticising other people's attempts without knowing what they are actually doing is a bit over the top.


Until another "standard" comes along that is proven to be more effective and consistent and verifiably so, then we shall continue to use the ITTC.

It's a very versatile beast. I've seen NASA papers where it is used (after correcting for compressibility effects) to predict skin-friction on re-entry vehicles.

[Ad Hoc]

Ahh..that’s the only paper I don’t have no.71. So can’t comment further on that. Do they state why they get higher coef’s and how they arrived at them?

As for verification and the likes.
You have slowly given details of the “caveats” and hence limitations of the programs. Even to the extent where you say you have no way of knowing if the results are a validation of anything, and that you’re not validating them anyway, as it is not your field, to mention a few. In light of this and other comments, outside of the world of pure research and theoretical field, which you reside in, once you cross over into using such software for real hulls used in anger all those caveats come into play, rather ‘seriously’ too I is have noted, hence your comment. This is not to say it is not a useful tool, as it is, especially in the theoretical fields for others. It also one hopes, shall eventually lead to furthering the science in such areas, as this is always welcomed.

But it is just a tool and not an absolute. When someone who is not a professional naval architect uses this ‘tool’ to supplement their gaps in knowledge, or inexperienced novices or students etc, it becomes dangerous. Owing to the fact that there is just unconditional acceptance of the results without any hint of “is this real and believable” when there are so many caveats to start with. (Unless of course one stays within the cevats -always!). If it were me, I would not be so laissez-faire about how it is used so wantonly; since it reflects on you early on in your career. But that is my own personal opinion.

In doing my own PhD, the ideas, language and words I use, and have to use, are totally different to the “real world” in which I make a living. The two are more often than not incompatible.

If any of the naval architects I’ve had under me and trained up came to me with colour plots of wave trains and ‘optimal’ hulls and suggested we redesign the hull as to the output, I would have to seriously question their analytical ability in the professional world. One must always ask questions “why” and “how” and obtain independent verification as much as possible. When designing vessels that cost millions of dollars, that I do, I’m not interested in personal feelings, just quantitative results and a professional attitude that demonstrates the ability to critique results, regardless where they come from to justify their bold statements. I’ve worked with some highly intelligent naval architects; some just could not work in the professional field, owing to being constantly questioned, they preferred the esoteric field…and so left. Sad.

It’s a simple as that.

To paraphrase Don, I would rather a naval architect I have trained get to Rome methodically then get to Paris in a fast and brilliant unique way, but get lost afterwards or stuck in never ending traffic and never seen again.

[Leo Lazauskas]

Ahh..that’s the only paper I don’t have no.71. So can’t comment further on that. Do they state why they get higher coef’s and how they arrived at them?

As for verification and the likes.
You have slowly given details of the “caveats” and hence limitations of the programs.

The caveats and limitations of thin-ship theory are very well known. I assumed you knew the limitations of the program which is why there was some confusion.

Here's the first two main paras of the manual just so we are on the same page :p

MICHLET IS RESEARCH CODE.
Please check all estimates generated by Michlet against
experimental results before committing any time or funds
to your project ...

and

Michlet is a computer workbench that can be used for investigations into some aspects of ship hydrodynamics. Although it is not a ship design program, Michlet can be used for preliminary design work such as estimating the resistance, wave elevation patterns and bottom pressure signatures of monohulls, multihulls and submarines. GODZILLA, the optimisation module of Michlet, uses artificial life algorithms to search for hulls of minimum resistance, or for hulls with other specific characteristics.

I agree with you that some people will stray outside the caveats despite warnings, but that's their look-out IMO. If someone blows themselves up because they designed a pressure mine using Michlet without knowing about what they are doing, I can hardly be held responsible. You might as well blame Mr Simpson for making his well-known integration method public.

As to the large form factors in Molland et al, in their favour they do stress that the factors "may not necessarily be used directly for design or resistance scaling purposes". Part of the reason (for cats) could be because of wave-breaking between the demihulls, or viscous interference.
For thin demihulls, where skin-friction is the main component of drag, the magnitude of the factors is particularly worrying. On the other hand, the models are only 1.6m long, so maybe they should only be used as a rough indication of what might happen at full scale. Like some mathematical models, I guess ;-)

It will be interesting to see what comes out of the DTMB5415 destroyer model tests around the world. At least they are being performed on larger models.

Leo.

[Leo Lazauskas]

To paraphrase Don, I would rather a naval architect I have trained get to Rome methodically then get to Paris in a fast and brilliant unique way, but get lost afterwards or stuck in never ending traffic and never seen again.

And I'm just happy to fly them to Paris safely, and let them find their own way from there.

[Ad Hoc]

"..I agree with you that some people will stray outside the caveats despite warnings... I can hardly be held responsible.."
With all the caveats stated, that is fair comment. So long as the users has the intelligence to understand the limitations and caveats, he shouldn't blow himself up!


Ahh..the effects of wave breaking and spray et al...that is a perennial problem that affects the Cw, along with far filed waves, not to mention transom sterns. Although the viscous effects are generally more pronounced when hull separation (for catamarans) is low.

Still, without work like yours and others, the science and understanding wont progress...keep plugging away.

[Leo Lazauskas]

"
Ahh..the effects of wave breaking and spray et al...that is a perennial problem that affects the Cw, along with far filed waves, not to mention transom sterns. Although the viscous effects are generally more pronounced when hull separation (for catamarans) is low.


There are all sorts of messy issues when the demihull separation is low, including single breaking bore waves at some speeds in shallow water which I most definitely do not model yet.

"
Still, without work like yours and others, the science and understanding wont progress...keep plugging away.

Thanks.
I agree that the 15 or so comparisons of Michlet predictions with experiments you have seen is still not enough to verify the code yet. The real problem for me is getting hold of good data (with error bands) for a wide range of hull types and then collating the results. Maybe some other people could submit comparisons in return for the free code.

Cheers,
Leo.

[yipster]

before the thread started i was watching rowboat racing and saw how deep the bow burries during strokes
and was impressed from the quik speed reduction between strokes ( on 4 "engine" boats even more )
i wondered if the sinking could have anything to do with slender ships like those fast boats from the thirty's

thanks again for the wardoss hull form witch satify's my curiosity, i'm trying to get a shaded visual out of the .mlt file
to get a better idea of the, i gues dolfin bulbnose, this without succes so far

like to input a dented bottom hull with a volume keel sticking out of the dent checking on area rule but thats a nono eh?

have to learn a bit more NA to quiker compare out files but started using Dr Leo's acronym
planty very intersting hull shapes and must say the program is a gem.

i'm still looking for a virtual tuft becouse gee, how did i make those batch files long ago?
and sorry, no cant help with verification i'm affraid :(

[Leo Lazauskas]

y's

thanks again for the wardoss hull form witch satify's my curiosity, i'm trying to get a shaded visual out of the .mlt file
to get a better idea of the, i gues dolfin bulbnose, this without succes so far



Thanks for the kind comments, Yipster.

Some of these minimum wave resistance hulls have very unusual bows
as shown in (attached) fig29 and fig30. The hull on the left of
fig29 has a fairly normal bow, but side bulbs.

The measured resistance of Ward's Optimum Symmetric Ship
is shown in fig37 from Wehausen's monograph, "The Wave
Resistance of Ships". The drag curve you calculated using
Michlet should be very similar to the one shown in that graph.
The low drag seems to be confirmed by experiments, which is
a pretty good result for Michell's theory. More so because
the bulbs are so large that Michell's fundamental
assumption (small longitudinal hull slope) is violated at
some places on the hull.

Of course the residuary resistance (wave + form drag)
is much higher, as expected.

It is a funny sort of result for mathematical hydrodynamics.
Once again, mathematicians have shown themselves to be clever,
but not very practical! On the other hand, there are some features
of the weird bulbs that are similar to the sonar domes on some
naval vessels.

I think it's one of those research projects that just had to be
done, even though the ultimate outcome was not particulary useful.

Regards,
Leo.

P.S. fig29 and fig 30 come from:
"Optimal ship forms for minimum wave resistance"
Chi-Chao Hsiung,
Report No. NA 72-1
College of Engineering
Uni California, Berkeley.
August 1972.

[yipster]

thank you, wobly hull and bulb visualised, i'm on it and comparing but playing chess (level7 on vista) with a frend as well :D
old drop out that i am i was in berkely at that time but for a rock concert. btw, resistance in kN i asume, wikipedia say's

1 N is the force of Earth's gravity on an object with a mass of about 102 g (1⁄9.8 kg) (such as a small apple).

On Earth's surface, a mass of 1 kg exerts a force of approximately 9.80665 N [down] (or 1 kgf).
The approximation of 1 kg corresponding to 10 N is sometimes used as a rule of thumb in everyday life and in engineering.

The force of Earth's gravity on a human being with a mass of 70 kg is approximately 687 N.

1 kN equals 101.97162 kilograms of load, but multiplying the kN value by 100 is a good rule of thumb.

[johnhazel]

.......

I don't think that we ever satisfactorily explained the strange "optimal" pressure distributions shown in:
"Free-surface pressure distributions with minimum wave resistance"
http://www.cyberiad.net/library/pdf/tl01.pdf

.... At high speed there is definitely something like hydrofoils at the ends.

Leo.

I looked at the hydrofoil wakes shown in a NACA report from:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930087415_1993087415.pdf

It seems the min resistance condition for the high speed case might be a result of the trailing pressure bar (or hydrofoil of a dual hydrofoil assisted cat) could be surfing down the face of upward moving wake from the leading pressure bar (hydrofoil).