Parabolizing lines of existing hulls

[Leo Lazauskas]

Here is a link to an unusual study done on "parabolizing" the
lines of vessels with extensive parallel middle bodies.

http://www.journalofoceantechnology.com/getFile.asp?file=AWaterlineParabol.pdf&article=True&vol=4&issue=3

If that link is too long, the article is in the latest edition
of the Journal of Ocean Technology. See:
http://www.journalofoceantechnology.com/CurrentIssue.asp

I really don't know what to make of the claims made in the paper. In my (mostly theoretical) optimisation work I have found that parabolic waterlines are often "optimal" from a total resistance aspect unless there are severe constraints on length, beam or draft for a given displacement.

When would it make economic sense to make such changes to existing vessels?

Is it really feasible to retrofit such changes on anything except small craft?

All the best,
Leo.

[yipster]

The waterlines became locally parabolic with
these add-ons. In order to determine the effect
of increased beam on resistance, wave
resistance was predicted theoretically using
Michell’s integral. The computer program
Michlet by Tuck et al. [1999] was used for
most of these calculations.
The experimental method used was costeffective
and less time-consuming compared to
the cost of testing different new full models.
Some discontinuity in water line slopes was
unavoidable. The retrofits extended the
waterlines continuously and eliminated the
parallel middle body. Test results showed that
the retrofits reduced residual resistance while
increasing the payload capacity and improving
ship stability for a range of Froude numbers.
they mention 15% optimising is possible but compared to what?
parabolising fluid dynamics helps but again, from to what?
awhile ago we discussed such an optimal hullform here
on ships i think the straight sides are worth something too
article mentions Michlet was used so guess you know best ;)

[Ad Hoc]

Had a scan through. The authors may have studied naval architecture, but clearly do not practiced it.

Their model #3, is running in a hump, (n=3) of the resistance curve. As such any modification would be beneficial.

But the main parameter for design, length displacement ratio, is shockingly low. If they changed the L/D ratio they would get significantly better results.

Their conclusion #2 and 3 is nothing new, just repeating what is well known in the 'faster vessel' naval architecture field. These two factors are beneficial whether one is using parabolic lines or not.

It says they used Michlet for their results. Correct me if I am wrong Leo, but Michlet is not for such low L/D ratio hulls shapes, nor transom style sterns. It surprises me they would used Michelt, knowing such limitations.

[daiquiri]

Hello Ad Hoc,

I understand that the paper talks about the experimental work on parent hulls in towing-tank (pages 63-67). Michlet was used for preliminary calculations before the realization of the final optimized model (pages 67-69). The Authors claim that Michlet has indicated a possible resistance reduction of about 25%, which then turned out to be 20% during the tests (page 68), which is not bad (imho), considering that uncertainity of measurements in repeated tests was about 3%.

This hull form at first appears not to be very suitable for analysis with Michlet, because the waterlines of the rear part of underwater hull are blunt-shaped and because of the wet transom, but maybe it is not really so. After having read the paper "The Wave-Resistance of a Ship" by J.H. Michell, I believe that Michlet software's applicability is not so much related to the general geometrical parameters of the hull, as it is to the form of the flow streamlines around it. If the angle between streamlines and the undisturbed flow is sufficiently small in every point of the computational domain, the error produced should be small, or at least acceptable. The error should be proportional, among other things, to the number of points (or to the extension of areas) which do not follow this rule.

In this particular case (hull lines at the page 71), the critical areas appear to be at the bow and at the transom. The angle of entrance at the bow should be considered in the horizontal plane, and it doesn't appear to be high (just don't ask me to define "high" and "low"... I don't know, it is a matter of feeling in this case :) ). It is a pretty fine bow actually. At the rear portion of the hull, though the waterlines are blunt the streamlines will get curved mainly in the vertical plane imho, and if you look at them from that perpective they again appear to be sufficiently flat relative to the undisturbed flow.
Remains the problem of the wet transom, which is another story. I would like Leo to confirm or deny this intuitive consideration of mine.

And Ad Hoc, thank you very much for the very informative papers you have sent me yesterday. You have a pretty impressive library of technical papers over there. :)

Cheers!

S.

[Ad Hoc]

Hi D.

I see what they have done and what they 'appear' to be saying (although I've only scanned it quickly). My point is, that have changed the hull form, rather than just changing the line curvatures.

Since in their table 7, the original hull has a Cb of 0.6, whereas B11p has 0.54. Keeping all 'things' the same; the length is the same, the draft is the same and the displacement is the same, but the beam has increased, the ratio of Cb's to maintain constant displacement, the Cb must be reduced.

So the form factors are different. It is not a case of just creating parabolic hull lines and hey presto. The parabolic lines created a new form factor. If they used curves lines or straigher lines etc, with the same form factors as a result, rather than parabolic, they would get the same result!

Hope you enjoy the read....i have some 1500~2000 tech papers scattered about in my library and on CD, so i have a few, half of which, i have forgotten i have them! :)

[daiquiri]

Yes, I see your point and it does seem very reasonable.
It would be really interesting to hear what would Authors have to reply on that valid objection, if only they had left some address for contacting them...

[Ad Hoc]

D
All they have really done, is what any naval architect all ready knows, or should know. That changing form factors has an effect....but they have gone, "wow", look what happens when you do this, as if no one knows. Only because they don't design. All naval architects understand this manipulation of form factors, since it establishes trends, that is all a NA does, looks to see what 'factors' have an effect and in what way. They just seem surprised...!!

As such, yes, would be interesting to hear what they have to say!

[Leo Lazauskas]

Thanks for your answers and comments!

1. Yes, Yipster, parallel middles have some advantages: for a start, they are probably cheaper to fabricate, especially if more than one vessel is being built.

The possible 15% optimisation mentioned in the paper is relative to their unmodified baseline hull.

2. Calisal is a very experienced experimentalist and has used Michell's integral for over 30 years. He would be well aware of its limitations.

That said, I personally wouldn't trust absolute predictions of the wave resistance for such low L/D ratios, but I would be fairly confident of relative rankings predicted by Michlet.

On the other hand, as Daiquiri mentioned, the experimental confirmation of the estimated potential improvement is reasonable, but a bit of a fluke, IMO.

3. I agree that there is nothing really new in the observation regarding parallel middle body. If a hull has extensive parallel regions, it often has a less sharp bow and stern and it is well known that "shoulders" in the hull can kick up large waves. I seem to recall that this issue was even mentioned in early editions of PONA.

Incidentally, the energy lost by these "shoulders" should be evident in the free-wave spectrum. It is a very useful exercise to see at what wave propagation angles most energy is lost, and to modify the hull shape accordingly to reduce peaks in the spectrum. It's not easy to do manually, but Godzilla can be set up to do a lot of the work.

3. Prediction of the drag of transom sterns is very difficult. Daiquiri is correct that Michell's integral can be used for hulls with transom sterns, but care needs to be exercised.

Michell's integral depends on the longitudinal slope of the hull. If this is small (in most regions), then predictions should be reasonable.

What is small? A good measure is if the sine of the slope is less
than the slope in radians, i.e. sin(alpha) < alpha.

So, yes, Daiquiri, you could also say that the streamlines should be smooth, and their longitudinal slope should be small.

Michlet uses the offsets of the hull rather than the slope (as in Michell's paper) because inputting slopes is less intuitive and small errors in slope specification can produce large errors.

In early versions of Michlet I allowed users to choose a variety of transom stern resistance models but I removed nearly all of the choices because they are inconsistent. In their mathematical models, Couser et al, and Doctors and Day add small extensions behind the transom to simulate the effects of the hollow cavity behind the stern. They claim better agreement with experiments, but I would add, "sometimes". Sometimes they agree better, and sometimes they don't. In other words, they are not consistent.

I am completely unconvinced by those methods, as I have said a few times on this forum. I cannot see how a cavity ventilated to atmospheric can sustain a transverse pressure. It can't create waves in the same way as a solid body.

Wet transoms are even more problematic.

A blunt wet transom can be modelled, but it would be very inaccurate at low Froude numbers. I have seen several papers that use the "wet transom" formulation of the integral, but I sometimes suspect that the authors were either too lazy, or couldn't do the required (slightly tricky) integration
by parts to get the infinite hollow formulation of Michell's integral. On the other hand, at high Froude numbers the difference between the "wet" transom results and those using the "infinite hollow" results are not large. Maybe some people figure that the extra drag predicted by the wet
transom formulation accounts for some otherwise unexplained non-linear effects.

In recent versions of Michlet, the transom is assumed to be completely wet at rest. I also define a Froude number based on the draft at the stern, Frt, and a (user-specified) critical transom depth Froude number, Frtcrit.
Frtcrit = 2.23 corresponds to the only theoretical result available at present.

In my model, if Frt >= Frtcrit then the transom is presumed to be running completely dry; if Frt < Frtcrit, it is partly wet and partly dry. A linear interpolation is used to estimate how much of the transom is wet. The wave resistance of the wet portion is estimated using the "wet" formulation. An infinitely long parallel-sided hollow (which makes no waves) is assumed to trail behind the dry portion.

In other words, it's a complete hack!

Michlet also includes the so-called transom stern hydrostatic resistance which is due to the loss of pressure on the dry portion of the transom.

Personally, I prefer using the infinite hollow method plus the hydrostatic resistance. If there are significant differences with experiments then I assume that they are due to a variety of effects that Michlet cannot hope to predict, such as wave-breaking, splash and spray, and boundary layer separation.

4. And now a question for you "practical" types...

When a heavily-laden planing hull with a big transom accelerates from rest, it
often creates a very large wave behind it. Is this primarily due to the wet
transom? Is it more to do with the wake created by the props? Or is it
because the vessel is in shallow water when it starts?

All the best,
Leo.

All typos and errors are due to loss of sleep during the hottest November
in Adelaide on record. And 41C for the next couple of days!

[Leo Lazauskas]

Yes, I see your point and it does seem very reasonable.
It would be really interesting to hear what would Authors have to reply on that valid objection, if only they had left some address for contacting them...

Calisal was last at the University of British Columbia (after stints at Berkeley and the US Naval Academy). Goren and some others were from Turkey.

I've attached an example of some of their other work.

Personally, I have no real interest in clarifying the points in their paper. I'll leave that to you "practical types". I have mathematical hierogyphics to play with :)

Cheers,
Leo.

[Ad Hoc]

Leo
"..When a heavily-laden planing hull with a big transom accelerates from rest, it often creates a very large wave behind it. Is this primarily due to the wet transom? Is it more to do with the wake created by the props? Or is it
because the vessel is in shallow water when it starts?.."

Hmmm...when you say heavily laden, does this imply too heavy to climb over the hump, or just a 'colourful' turn of phrase? (ie just a low L/D ratio)
Monohull...what L/B ratios are you thinking..since this influences the answer
Are you only considering props?...such as surface props or submerged props?
Are you assuming the 'wave' system is from the stern only..or are you trying to separate each wave system?
When you saying the transom is wet at the beginning, you imply this is not because it is at rest, but already moving?
Finally...are you thinking in shallow water to start with..or deep water to start with?...or not fussed, since each has a different effect.

More questions than answers...sorry :(

[Guest625101138]

....
When would it make economic sense to make such changes to existing vessels?

Is it really feasible to retrofit such changes on anything except small craft?

All the best,
Leo.

Leo
I doubt if there is any merit beyond relatively small craft. Larger cargo vessels are essentially a box with somewhat streamline ends.

Loading and unloading terminals are gradually increasing in size of vessel that they can handle as there are savings associated with economy of scale however the increasing size is gradual. (Melbourne has recently endured the pain of dredging Port Phillip to maintain its ability to handle modern cargo vessels)

The cost of dredging berth pockets and channels to accept deeper vessels is a factor that limits draft.

The reach of loaders and unloaders is a factor as increasing reach requires heavier machines, which, in turn, require heavier wharf structures. Hence there is significant cost associated with increasing beam. It would be wasteful to have one short section of the vessel setting the reach of machines rather than most of the full length. Also think of what would need to be done to the quay line to accept vessels of varying length and curvature. The fenders on exposed water berths would need to have significant adjustment so breasting loads could be distributed.

Other major factor is the canals. I know there is talk of widening the Panama Canal but even if it does happen there will still be a desire to maximise the volume within the available dimensional limits.

So I do not see any merit in considering the lower drag lines for anything but relatively small vessels. Certainly not up into the size of the smaller bulk carriers and upwards. Oil tankers could be an exception as these can have more adaptable loading and unloading set ups - but is the benefit going to be noticeable in their operating regime!

Actually it is interesting that you bring this question up because I was recently asked to determine the best hull shape for an origami pedal boat with parallel sides - so reverse of the topic in question. Aim is to do an infusion layup in one go for the entire hull as a single flat panel with four folding lines and taped ends. I can get drag within 10% of a hull with parabolic lines so speed difference of 3%.

Rick W

[baeckmo]

Leo, on your accelerating boat: Without diving into the math of it, I think it is mainly a result of the transient state. We have an analogy in pipe flow. Here we see pressure waves set up by a change of state; f.i. a valve closing generates a positive pressure pulse travelling upstream and a negative going downstream, both propagating with the respective speeds of sound in the fluid.

If the rate of change is faster than the time for the wave to reach a substantial area change, being reflected there (positive or negative, depending on status of reflecting point), and back to its origin, then it will produce its maximum pressure peak amplitude [pmax=density*(speed change)*(speed of sound)]. A "slower" change of speed, ie taking longer time than the critical travelling time, will result in a lower pressure wave.

With gravity waves, as you guys are working with, the surface amplitude is a manifestation of static energy, just as pressure will be in a closed system. There is also a typical length factor; WL length, so the necessary determining ingredients are certainly present. The difference is that the critical propagation velocity within a pipe is varying with sqr(elasticity/density), while the surface waves change with a length factor.

If the hull is accelerating at, or above the critical rate, it will be "locked" into the solitary wave's max amplitude until the rate of speed increase is levelling off as it reaches its final speed.

You also said "highly loaded"....If that means that the forward hull sections are full and buttocks increasingly convex, there will be a negative pressure on the hull surface, pulling it "nose down", increasing the total amplitudes. We have a fleet of planing pilot vessels in this region, all characterized by high bottom loading. They all show this nose-down attitude at about 8 to 10 knots. We studied this phenomenon long ago when looking into an accident where one of these vessels was trapped within the wave system of a bigger vessel, when separating after having picked up the pilot. The crew was not aware of the attraction forces and applied the wrong departing procedure, resulting in the loss of one crew member.

A slight off topic (?) note BTW: Speed of sound in clean water is ~1500 m/s, but even a small mix of free gas will reduce this speed substantially. 2 % free volume (as recorded from oceans, will result in a propagation speed of 110 m/s if the bubbles contain air, and 69 m/s if they contain a mix of air and water vapour, as in the state of incipient cavitation. With 10 % by volume, as when ocean water is accelerated into a propeller or water jet, the speeds are 53 and 33 m/s respectively. This means that the machine is partially operating in a high subsonic or transsonic state, where compressibility effects have to be dealt with. Simple numerical calculation methods will fail due to this, their basic application range beeing below Mach 0,5.

[daiquiri]

A slight off topic (?) note BTW: Speed of sound in clean water is ~1500 m/s, but even a small mix of free gas will reduce this speed substantially. 2 % free volume (as recorded from oceans, will result in a propagation speed of 110 m/s if the bubbles contain air, and 69 m/s if they contain a mix of air and water vapour, as in the state of incipient cavitation. With 10 % by volume, as when ocean water is accelerated into a propeller or water jet, the speeds are 53 and 33 m/s respectively. This means that the machine is partially operating in a high subsonic or transsonic state, where compressibility effects have to be dealt with. Simple numerical calculation methods will fail due to this, their basic application range beeing below Mach 0,5.

Damn! :eek:
I feel like a dumbass. I have never thought of such an obvious thing. Knowing (from readings on ecology) how important ocean waves are for the aeration of sea water and for the support of sea-life, it becomes pretty obvious that the considerations you have done are very important when treating both slamming loads on hull and flow around prop blades in rough seas. Yet, I've never thought of it, dammit...
I'm an ignorant. It is such a great and priviledged condition - lets me learn so many new things every day. Guess that's the beauty of life! ;)

The figure n.1 in this work looks impressive:
http://www.iahr.org/publications/assets/jhr38-5/Zhao_Li.pdf

P.S.: "You must spread some Reputation around before giving it to baeckmo (or Leo L.) again." is very unfair.

[baeckmo]

Calma, calma per favore!! The beauty of life is the intelligent interaction that makes 1+1=3!

When talking about slamming, there is some russian research on the consequenses of free gas on bottom slamming, sorry I lost the guy's name, it began with a K.... . I'll see if I can find the writer for you if you are into the subject.

As an example, we have managed to modify the inverted V hull in order to reduce slamming, by deliberately introducing forward hull lines that produced a liberal volume of foaming spray into the tunnel section.

[baeckmo]

Aah, D, I think the name is Korobkin. I'm in a rush right now, but will search more later.

[Guest625101138]

.............
When a heavily-laden planing hull with a big transom accelerates from rest, it
often creates a very large wave behind it. Is this primarily due to the wet
transom? Is it more to do with the wake created by the props? Or is it
because the vessel is in shallow water when it starts?

All the best,
Leo.

.....

Leo
Have you seen this test work on a planing hull:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=0
Figure 15 shows some interesting data for the aft mounted velocity measurement. This hull is propelled externally so the reverse flow has nothing to do with propellers. It is simply the water escaping from a high pressure region beneath the hull to a low pressure zone behind the hull.

The consequence of this escaping water will be to lower the pressure at the transom so it will tend to sink the stern even further so a compounding condition as a heavy boat gets on the plane. The water has gained momentum rearwards and this may contribute to the size of the wave behind the transom.

They went on to use these measurements to validate a CFD model of the hull and got good correlation. You would expect that they can predict the resulting stern wave with some accuracy but it is not something I have looked for in subsequent papers.

Rick W

[wardd]

im no expert but after decades of advanced hull design is this a new discovery?

or has it been done before and not given this name?

[daiquiri]

im no expert but after decades of advanced hull design is this a new discovery?
or has it been done before and not given this name?

:D :D :D

[Leo Lazauskas]

Aah, D, I think the name is Korobkin. I'm in a rush right now, but will search more later.

If it is about slamming, then it is Korobkin.
If it is about ships in waves, it is Korvin-Kroukovsky.
If it is related to ship wave resistance, it is Kostyukov.
No wonder you might have been confused.

Amusing sideline...
When the proceedings of the first conference on sea-keeping were sent to the publishers, the printers changed every instance of "Sea-keeping" to "Bee-keeping" because they had never heard of the term.

Must have made very interesting reading before the editors slapped down the printers!

Leo.

[yipster]

and if i read back well a wardoss (http://images.google.com/images?hl=en&source=hp&q=wardoss+hull&btnG=Search+Images&gbv=2&aq=f&oq=&aqi=) hull shape was tested rite?
as for waves getting on plane ricks article seems to confirm my thoughts that
most waves come from the running delta having a bad angle of attack
stepping on plane. always thought airation was good but it gives sinkage as well
and there is more involved, come to think of it my head is to small
must be good reads by Korobkin, Korvin-Kroukovsky and Kostyukov

[baeckmo]

Confused, hmmm? Reread your question, realized I misinterpreted it. Not about the distorted travelling wave but the one left behind..... sorry, linguistic messup!

Anyway, the wave left behind must be the hydraulic jump occurring when the propeller wash velocity is reduced from supercritical to subcritical froude depth number. That will be seen with an accelerating double-ender as well, so it has nothing to do with the wet transom area, just the jet seeking its way up and spreading near the surface.

[daiquiri]

Have you seen this test work on a planing hull:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=0
Figure 15 shows some interesting data for the aft mounted velocity measurement.

That is an interesting article, but it deals with masurements during the costant-speed regimes, while I understand Leo that was asking about the wave left behind the hull during the acceleration of the boat from the rest. Transient dynamics, in other words. However, the article is informative about the development of the boundary layer and illustrates well the concept of resistance hump and how the flow velocity increases in the aft half of the hull, which is not so obvious thing.

Leo, to be honest I've never observed some particularly huge waves behind the transom during the initial moments of the acceleration, but maybe I just wasn't too vigilant... Or, better said, no huge waves which could be attributed to acceleration only and not to the usual wave-train seen in constant-speed navigation... :confused:

That said, there still remains an uncertainity about the distance from the transom we are talking about.
Baeckmo's answer appears logical to me. First because the powerboat case is considered here and second because we are talking about the moment when propeller accelerates the flow against the mass of still water behind the transom. The high velocity difference between the penetrating jet and the surrounding still water causes the equally high energy transfer from the jet to the still water, partly in the form of kinetic energy (induced backwards flow, plus turbulent eddies) and partly in the form of potential energy (rise of the water free surface).

Since the hull doesn't remain motionless in the process, there might be some limited reinforcing contribution of the hull too, if it has a wet transom. The accelerating boat tends to dig the transom somewhat into the water, probably creating the recirculating vortex behind the transom and the consequent rise of the free surface. But then again, if the speed is low and the boat is of heavy-displacement type, the real contribution of the hull dynamics can be pretty disputable (or should I say: unsustainable...) and I certainly have no numerical data to corroborate this reasoning. If the boat accellerates slowly in spite of the prop pushing hard, then only the effect of propeller jet remains.

Water depth is an issue only if you want to consider water depth too, imho...
The question which comes up to my mind is: would the same phenomenon be observable in case of a boat equipped with an air propeller? The air prop would create a trim-down moment and would not create the above mentioned jet energy transfer. Any air-boaters present here to say few words on the subject? :)

[Leo Lazauskas]

and if i read back well a wardoss (http://images.google.com/images?hl=en&source=hp&q=wardoss+hull&btnG=Search+Images&gbv=2&aq=f&oq=&aqi=) hull shape was tested rite?

Yipster:
Ward's hull is just a mathematical curiosity that someone
thought should be built and tested. It was tested and it did
produce small waves at its design speed, but the skin friction
and form drag were high.

There are several other low wave drag vessels, and all are
impractical in some way.

1. Krein's caravans are described in Kostyukov's monograph
"Wave Resistance of Ships". I used to have an option to generate
these weird vessels in some old versions of Michlet.

Basically they are a string of hulls placed exactly end-to-end
that cancel nearly all waves at some Froude numbers.
Krein's original version was infinitely long (don't you love how
impractical some mathematicians are!). The Michlet version were of
finite length. Again, the wave drag is very small, but the viscous
drag is unreasonably large.

2. Diamond tetrahulls can be made to have very low wave rersistance
at a single speed. Of course, the skin-friction of four hulls
is very large.

3. A "Weinblum" dihull can produce small waves on one side of the vessel.
Like the diamond tetrahull, it is usually impractically long.

There are examples of Weinblums and diamond tetrahulls in Michlet.

4. Tuck's finned spheroid has zero wave drag, but the lift of the
fins pulls the object downwards, i.e. it has negative buoyancy.
No free lunch!

The reason that the objects are interesting (to some of us nerds)
is that they provide insights into wave-making properties.
Another reason is that they are a good check for blind optimisation
or search routines like Godzilla. If you don't constrain hulls or
collections of hulls, the program should find one of the low drag
configurations. If it doesn't then there should be a good reason
why it didn't.

All the best,
Leo.

[Leo Lazauskas]

Confused, hmmm? Reread your question, realized I misinterpreted it. Not about the distorted travelling wave but the one left behind..... sorry, linguistic messup!

Anyway, the wave left behind must be the hydraulic jump occurring when the propeller wash velocity is reduced from supercritical to subcritical froude depth number. That will be seen with an accelerating double-ender as well, so it has nothing to do with the wet transom area, just the jet seeking its way up and spreading near the surface.

As Ad Hoc noted, there are many factors.

I was alerted to the question by E.O. Tuck who noticed the phenomenon with some river boats in China. I have seen it mostly behind small boats with large outboard motors (and several Australians standing at the stern drinking beer) near beaches, so I think your explanation is probably correct.

Best reagrds,
Leo.

[Ad Hoc]

Leo

If you're referring to just shallow water effects -of this observation- then fig.9 (in our research, and others), shows the effects of shallow water, being a direct relationship with depth, nothing new here. But, also, we noted that changing the LCG at certain shallow depths reduced wash even more, which was not expected at all....most odd. We could not find any correlation or trends to establish what mechanisms influence this.

As for accelerating away, in fig.12, you can see the effects of accelerating on trim from rest. The greater the trim, the greater the transom immersion which there is a direct correlation to wash too.

So you have the combined effect of shallow water, wave generation, going from subcritical to supercritical froude depth, in a short distance, coupled with an increasing trim angle.

It is probably a superposition of these two conditions that create a larger wave than one would normally expect; when accelerating from rest in shallow water, if i understand you correctly.

Assumption is that you, or rather Tuck, was referring to the transverse wave too.

PS...just saw the weather forecast on the news, you've got 43c tomorrow!...nice, reminds when i used to live in Perth :)

[Leo Lazauskas]

Leo
So you have the combined effect of shallow water, wave generation, going from subcritical to supercritical froude depth, in a short distance, coupled with an increasing trim angle.

It is probably a superposition of these two conditions that create a larger wave than one would normally expect; when accelerating from rest in shallow water, if i understand you correctly.

Assumption is that you, or rather Tuck, was referring to the transverse wave too.

PS...just saw the weather forecast on the news, you've got 43c tomorrow!...nice, reminds when i used to live in Perth :)

Yes, I was referring to the transverse wave only.

A further complication is that the exact bathymetry is unknown. I have only seen the effect in shallow water, but who knows whether there was a ridge or hollow in the sand under or behind the stern when the boat took off.

Yes, 43C is unexpected this early in the year. I even had to sit inside a shop instead of outside while my wife browsed. There was an upside, however. I was thinking about the trajectory a fly must take to land on a ceiling, when I saw one do it on a shelf inches from where I was staring. I think it's the first and only time a man has been right in a women's shoe shop :)

[daiquiri]

I was thinking about the trajectory a fly must take to land on a ceiling...
Geeeee, I loved this one!!! :D :D :D

Now, if you think about it... You might start a bibliographical research on that topic, developing a big bunch of mathematical formulas based on unsteady aerodynamics of laminar flows, added masses, control systems etc. And then all the computational alghoritms to be developed... You will soon find out that it is not a trivial problem at all, to make that mathematicaly modeled fly land on the ceiling... It will take a big amount of time and of CPU power if you decide to dig deeper into the topic. And the expertize gained in the next few years could bring you new clients and jobs in the field of flying micro-UAVs, maybe a new academic title, the fame in the scientific society, who knows...

Yet, that tiny fly has landed perfectly on that ceiling in a fraction of second, without hesitations and without thinking too much about it. And she new how to do it ever since she took-off for her first flight.
Don't you find it amazing? These little things can impress me immensely sometimes. :)

[Ad Hoc]

Just going back to the original paper for a mo.

I thought i recognised their names and work. I have dug out another paper by them, done in 1993 (A resistance study on a systematic series of low L/B vessels)....they didn't fully analyse all their data, just gave a precise, same stuff. However they did also look at varying Cb.....they only presented the data graphically and gave no further mention to it, as other findings appeared more interesting to them.

However, low and behold, increasing the Cb decreased the resistance!!

[Leo Lazauskas]

Geeeee, I loved this one!!! :D :D :D

Now, if you think about it... You might start a bibliographical research on that topic, developing a big bunch of mathematical formulas based on unsteady aerodynamics of laminar flows, added masses, control systems etc. And then all the computational alghoritms to be developed... You will soon find out that it is not a trivial problem at all, to make that mathematicaly modeled fly land on the ceiling... It will take a big amount of time and of CPU power if you decide to dig deeper into the topic. And the expertize gained in the next few years could bring you new clients and jobs in the field of flying micro-UAVs, maybe a new academic title, the fame in the scientific society, who knows...

Yet, that tiny fly has landed perfectly on that ceiling in a fraction of second, without hesitations and without thinking too much about it. And she new how to do it ever since she took-off for her first flight.
Don't you find it amazing? These little things can impress me immensely sometimes. :)

The question was asked on a radio station here, but I didn't hear the answer. I remembered that flies face the opposite way after they land so they probably do a back-flip instead of rolling.

There are some answers on the net:
http://answers.google.com/answers/threadview/id/174854.html

In any case, I hope you can now appreciate what an exciting place Adelaide is.

Leo.

[Leo Lazauskas]

Just going back to the original paper for a mo.

I thought i recognised their names and work. I have dug out another paper by them, done in 1993 (A resistance study on a systematic series of low L/B vessels)....they didn't fully analyse all their data, just gave a precise, same stuff. However they did also look at varying Cb.....they only presented the data graphically and gave no further mention to it, as other findings appeared more interesting to them.

However, low and behold, increasing the Cb decreased the resistance!!

I have their FAST 93 paper on fishing boats. I guess that's the one you mean.

Another problem I have with their assertion is that there are sometimes extensive regions of low resistance in design spaces. At its crudest, think of trading off wave drag against viscous drag. It is relatively easy to find two hulls with almost identical total resistance at a particular Froude number, one hull with high Rw and low Rv, and the other with low Rw and high Rv. One hull might have parabolic lines, but the other might be quite different and perform better on other criteria.

They do make one good point - one should not restrict the design space too much in optimisation problems so that good solutions are not immediately rejected because they violate hard constraints. But that's hardly earth-shattering - similar advice is in the Michlet manual for a start.

Their choice of journal is also interesting. I haven't read any other papers in that (private?) journal so I'm not sure who the papers are pitched at.

Cheers,
Leo.

[daiquiri]

In any case, I hope you can now appreciate what an exciting place Adelaide is.
Yeah, you guys are having some great times down there. :D
Think that, from my geographical point of view, that fly is landing perfectly leveled and with her head up. If she decided to do some back-flips for the audience then it's probably just her exhibitionist nature... It is you guys in Australia who walk around turned upside-down. I don't want to think about the mess when you have to pee... ;) :P

[apex1]

Yeah, you guys are having some great times down there. :D
Think that, from my geographical point of view, that fly is landing perfectly leveled and with her head up. It is you guys in Australia who walk around turned upside-down. I don't want to think about the mess when you have to pee... ;) :P

Haah........ now some things become clear to me.http://www.imgbox.de/users/apex1/bug.gif

[marshmat]

I hope you're not getting cooked too badly there, Leo. (I'll gladly trade half of your weather for half of mine- we're down below freezing at night now, so your weather plus mine should give us a nice 22 C....)

Perhaps I'm missing something here, but the paper in question seems to be a lot more limited in its scope than the introduction and conclusion would suggest. That one particular fishing boat hull could have its resistance reduced by 15% in the speed range 0.3<Fr<0.4 says, to me, that this particular fishing boat was not optimized for efficiency alone (at this speed) when originally designed. (What will a wider beam do for motion and seakeeping, for example?) But since so few parameters were held constant, I don't see how the conclusion of parabolic waterlines being better can be extrapolated to parallel-middle-body hull forms in general.

It is, of course, never wise to artificially restrict the design space too early in an optimization problem. However, there may be artificial but essential constraints that help to push the design in a particular direction. The fishing boat hull in question was Canadian-designed, thus quota regulations were likely considered in the selection of its main dimensions. To use another example, a Great Lakes ore freighter (square bottomed, and wall sided for 80% of its length) is a long way from an 'optimal' shape from an efficiency standpoint, but since it must traverse canals with strict draught and beam limits, it is an economically acceptable solution.

[Ad Hoc]

Leo
"..I have their FAST 93 paper on fishing boats. I guess that's the one you mean..."
In my fast 93, their paper is not included, says oral presentation only! I was referring to their paper in Marine Technology.

I agree, it is odd to have a paper in such an obtuse publication.

And, as i noted previously, they are not practising naval architects. As you and Matt both rightly note the obvious:
"..They do make one good point - one should not restrict the design space too much in optimisation problems so that good solutions are not immediately rejected because they violate hard constraints.."
"..It is, of course, never wise to artificially restrict the design space too early in an optimization problem..."

One must always do the 'what if' scenarios, as a check, just in case and helps to validate the final solution when questioned by the client!

[Ad Hoc]

Have you ever noticed how you can slowly, very very slowly bring your finger from behind the fly's head and actually touch a fly. If you do this from the front, it flies away immediately. Neat :D

[apex1]

Try this with the one above John.............

[Leo Lazauskas]

Have you ever noticed how you can slowly, very very slowly bring your finger from behind the fly's head and actually touch a fly. If you do this from the front, it flies away immediately. Neat :D

I disagree. If you approach slowly from directly in front of them you can flick them right between the eyes. I think they must have a blind spot there. When you get good at it, and I fancy myself a bit at this sport, you can even flick them a little sideways in much the same way as cutting a billiard ball. This sort of thing can really impress girls, BTW.


Leo.

[Ad Hoc]

I've never got them directly from the front....hmmmm....sounds like an "ashes" style challange to me.!! :D

[wardd]

i have no desire to catch and eat flys

[marshmat]

Being married to a biologist, I have learned that our current crop of apparently identical fruitflies are in fact at least two different species, with different habits and behaviours.... I'm half expecting the genetic tracking experiments to begin any day now.

[apex1]

Now it gets really scientific here.
I like you peers...:D .......... http://www.imgbox.de/users/apex1/bug.gif

Dunno how she thinks about it, though.

[yipster]

gettin confused, o yes me to have some old schoolbooks on fruit fly's
If it is about slamming, then it is Korobkin.
If it is about ships in waves, it is Korvin-Kroukovsky.
If it is related to ship wave resistance, it is Kostyukov.

hydrodynamic impacts (http://cmrt.ec-marseille.fr/hydrodynamique/presentations/korobkin.pdf)by A. Korobkin and Korvin-Kroukovsky wave papers (http://en.scientificcommons.org/b_v_korvin-kroukovsky) that should open somewhere and thanks for your explanation Ad Hoc
amazing how much can be found on the net, here books (http://books.google.com/books?q=related:ISBN0521808537&id=iLx6AHNRJRYC&source=gbs_similarbooks_r&cad=2) related to Kostyukov resistance
doing some drawings but should an will get godzilla on screen, dont have a batch switching file huh? (or a free visual basic version :cool::P)

[yipster]

with nice big glossy buttons? have to see to what can be optimised and almost forgot godzilla is the optimiser
(i only used once or twice) and dont need new optimised hulls to import to check


looked at several waterjets at the marine equipment trade show yesterday
one reply was the boat would start to pulsate as well when the jet somehow would pulse
and reading a bit Korobkinon hydrodynamic impacts i dont find if the idea has any merrit at all
underway you may argue the jet hits new surface anyway etc still i'll ask you experts

[baeckmo]

Sorry,Yipster, I don't follow you. Exactly what are you asking??? Is it about jet performance in a "foaming/aerating" environment, or what? And what is the connection to parabolic lines?

[yipster]

baeckmo, no excuses, i know i'm a wild imaginer and there's no connection to parabolic lines but to mentioned Korobkin hydrolic impact(s) and waterjet drives
way above my head i know but imagine having a shower head taped and facing down on a scale, mine sprinkles, fine or rough, 2 aerating positions but no pulse
would the scale give various readings ? btw, not so shure i agree with propwash in your post 21, props or jets can be trimmed to step out smoothly
or to put the wet delta in a deep hole making big waves due to transit of deadrise to blunt bow trying to break froude nr is my belive

[rxcomposite]

[QUOTE=Leo Lazauskas;315893]


4. And now a question for you "practical" types...

When a heavily-laden planing hull with a big transom accelerates from rest, it
often creates a very large wave behind it. Is this primarily due to the wet
transom? Is it more to do with the wake created by the props? Or is it
because the vessel is in shallow water when it starts?

Leo,

I have noticed that while playing on the beach, I could kick up a larger traveling wave in ankle deep water than in deeper water.

In deeper waters, is it possible the propellers are kicking up the waters before the boat accelerate? Similar to a person starting to run on a long carpet. He will ruffle up the end (start) before he starts accelerating.

Rx