Parabolizing lines of existing hulls

[Guest625101138]

Originally Posted by Leo Lazauskas

.............
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...5319&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]

Originally Posted by 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?

[Leo Lazauskas]

Originally Posted by baeckmo

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 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]

Originally Posted by Rick Willoughby

Have you seen this test work on a planing hull:
http://nparc.cisti-icist.nrc-cnrc.gc...5319&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...

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]

Originally Posted by yipster

and if i read back well a wardoss 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]

Originally Posted by 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.

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]

Originally Posted by Ad Hoc

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]

Originally Posted by Leo Lazauskas

I was thinking about the trajectory a fly must take to land on a ceiling...

Geeeee, I loved this one!!!

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]

Originally Posted by daiquiri

Geeeee, I loved this one!!!

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/th...id/174854.html

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

Leo.

[Leo Lazauskas]

Originally Posted by 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!!

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.