bow wave question

bow wave question

Post by P.C. » Wed, 19 Feb 2003 23:20:53


Hi



Quote:



> > >Be one with the water, grasshopper...

> > Am I a wave or a particle of water?

> I've always thought of being a particle of water with some mass and
> frictional properties when I use this illustration.

Reasoable aproach, but this say much more about the water "particle" than the
develobment of forces round the hull surface.  Maby turbulence is producing the
wrong water spirals on the wrong hemisphere, then vaccum is maby not produced
the exact right place if the race is in the wrong place in the world -------
just joking.
P.C.
 
 
 

bow wave question

Post by P.C. » Wed, 19 Feb 2003 23:41:45

Hi



Quote:
> Interesting discussion on the merits of empirical vs numerical testing, and
> worthy of its own thread, but I thought I'd try to offer some thoughts on
> the original question about bow wave and waterplane angle.

> It sometimes is interesting to look at hydrodynamic problems, such as wave
> systems and drag, from the water's perspective. I know it sounds a little
> Zen, but bear with me. Be one with the water, grasshopper...

> As you are floating along, minding your own water-like business, some large
> floating body pushes you out of the way. For now, you don't know anything
> about the ship behind the this bow point. It could be 10 feet more ship or
> 1000 feet. Thinking along these lines, the bow wave system (in isolation) is
> independent of the waterline length.

> If you are resting on the water surface and are pushed out of the way, you
> may find the "path of least resistance" is to move up into the less dense
> air, rather than to move out into the water. Then gravity takes over and you
> are pull back into the water. This is what creates the bow wave. The nature
> of the wave - its shape and size - is more-or-less determined by the
> waterplane entrance angle (your initial question), but also by the sectional
> area distribution. The change in volume below the water is important as this
> helps determine how easily water is pushed out of the way. One parameter
> that that can give an indication as to how quickly the sectional area is
> changing is the prismatic coefficient. Larger prismatic coefs mean "fuller"
> ends, hulls with "fine" ends have lower prismatic coefs. Fuller ships get to
> their maximum sectional area quicker, which is important as energy relates
> to how quickly mass is moved out of the way. This also suggests that the
> rake of a bow has little influence on the bow wave.

> There is a similar wave system at the stern, and as you collapse back to
> centerline you also move up or down to follow the path of least resistance.
> As the ship moves along, the amount that your mass is moved and the friction
> between you and the hull are indeed dependent on the ship length. More ship
> means more friction and more energy needed to move the water out of the way
> and back together.

> Hope this is helpful.

These are very accurate descriptions of the way you calculate these
hings,  ------- but around the hull there often is a layer of water that hardly
move and even in spots will move reverse the expected direction. Some places
there are distrubuated preasure and some carry vaccum "arears" , and all this we
must calculate from 90 deg sections.
-------- There are no such sections in the real world, so what can we then
expect from the tradisional calculations, except a picture about how things are
or shuld be from measurements and calculations that do not reflect the real
world forces.
P.C.

 
 
 

bow wave question

Post by William R. Wa » Thu, 20 Feb 2003 00:43:33

Quote:

>...-------- the fact that 1000 years ago, the Vikings
> produced heavy displacement boats, that did _not_ produce the midship wave that
> is the vorse hassle for displacement boats. ------ this also relate to the bow
> wave as you can not part these.

I recently learned that the Celtic word for boat is "long" so naturally I
wonder if "long" was what the Celts called the Viking boats and it came
down to us, or is it "long" in the later English sense of length?

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bow wave question

Post by William R. Wa » Thu, 20 Feb 2003 00:53:43

Quote:

> The last time the America's Cup was done in 12 metre boats, the winner
> was modeled in a computer IIRC. Haven't heard anything about tank
> tests lately in that context.

I read that early on they had strain guages stuck all over them wired to
an onboard computer for real time analysis. I worked with agriclture
engineers who did the same with a tractor and its towed implements,
mounting a computer technician's seat beside the tractor driver. This is
another computer approach, sticking sensors all over a boat and monitoring
them on a computer screen. Probably only the government and America's Cup
billionaries have that kind of money. And the folks who build our mass
produced cars where the development cost can be spread over millions of
vehicles.

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bow wave question

Post by William R. Wa » Thu, 20 Feb 2003 01:15:37

Quote:

> Interesting discussion on the merits of empirical vs numerical testing, and
> worthy of its own thread, but I thought I'd try to offer some thoughts on
> the original question about bow wave and waterplane angle.

it seems intuitive that the boe half angle or prismatic coefficient would
make a differnce beween a theoritical wavelength, ie teh waterline length,
and the effective wavelength. I'm thinking of those longey arrow powerboats
with the knifelike stem and hollow sections just aft. They can't be
producing much of a bow wave but some designers have assumed it would
lenthen the waterline and thus the boat's speed at the same power.

So I assume there is no known rule of thumb for the effective vs the
theoretical wavelength as it relates to waterline length. Some adjustemnt
for bow half angle or prismatic coefficeint would seem to be in order.

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bow wave question

Post by P.C. » Thu, 20 Feb 2003 04:16:47

Hi



Quote:

> >...-------- the fact that 1000 years ago, the Vikings
> > produced heavy displacement boats, that did _not_ produce the midship wave
that
> > is the vorse hassle for displacement boats. ------ this also relate to the
bow
> > wave as you can not part these.

> I recently learned that the Celtic word for boat is "long" so naturally I
> wonder if "long" was what the Celts called the Viking boats and it came
> down to us, or is it "long" in the later English sense of length?

Now Celtics are even today proberly one of the few cultures who deliver the best
boat ever ; compleatly round, but extreamly stable, anyway safe in daily rutines
and as toy, then Vikings come along and you know how Celiic boatbuilders shuld
point and say "long" ?
Long long is no name beside these have the section cut of a flying saucer there
as far as I know, is a guy who still produce original round boats , ------- what
do you need on a lake or a small river guess a boat designer today can learn a
lot about Celtic boatbuilding , I never doubted ;))
Viking proberly would be very impressed about local hand crafts ,their boats was
very different , then on the other hand ; this was then and not now.
Today Celtics shuld study math. and check 3D-H as core for a vessel,  3D-H was
develobed with framework in mind somthing solid, cheap and easy for my sake they
could *** spacecrafts but this is becaurse I have the stand, that if this is
what you want this is the way for a third, four times as strong, ---------
emagine the options forming in detail like cheap cabins, and F22 one $ a cut
sheet. ready for engine install .
This is the option when you chose the choice are your's maby you do not want
everything then this subset making a boat long, add abilities unknown , even
make them fly made in sheet materials and metal but proberly never in form,
avaibility and abilities allway's is best in nature materials doing fast boats,
the most beautifull boat is not yet build, but it will be nice .
Celtics can be genious nomatter what they do, ------- maby the round boat I keep
refering, build as original very very local boatbuilders doing
roundboats, ------- maby google can find somthing about how Celtic boatbuilders
would say building only round boats, when they saw a Viking Longboat, still I
doubt, they proberly just would have said, "hey another Viking, they must grow
near by"  ;))
And these people would be able to design spacecrafts, in that inviroment.
What's wrong building a boat, when you are supposed to build a boat , then if
this is best of all, done in the avaible material no this is not 3D-H as that
just form the material for the formworks,  .
Celtic boats building right.
Fast boat ?
Nice talking to you.
P.C.

Nice talking to you.

 
 
 

bow wave question

Post by Rodney Myrvaagne » Thu, 20 Feb 2003 13:28:33

On Tue, 18 Feb 2003 07:04:11 -0500, "Thomas Bloomer"

Quote:

>>The last time the America's Cup was done in 12 metre boats, the winner
>>was modeled in a computer IIRC.

>So do we give the computer a trophy?

Huh? I guess so if earlier efforts earned a trophy for the towing
tank. I never heard of trophies being awarded to tools.

Rodney Myrvaagnes                        J36 Gjo/a

"If Brecht had directed 'Waiting for Godot,'  he would have hung a large sign at the back of the stage reading 'He's not going to come, you know. ' "   -- Terry Eagleton

 
 
 

bow wave question

Post by Rodney Myrvaagne » Thu, 20 Feb 2003 13:28:34


Quote:

>> The last time the America's Cup was done in 12 metre boats, the winner
>> was modeled in a computer IIRC. Haven't heard anything about tank
>> tests lately in that context.

>I read that early on they had strain guages stuck all over them wired to
>an onboard computer for real time analysis. I worked with agriclture
>engineers who did the same with a tractor and its towed implements,
>mounting a computer technician's seat beside the tractor driver. This is
>another computer approach, sticking sensors all over a boat and monitoring
>them on a computer screen. Probably only the government and America's Cup
>billionaries have that kind of money. And the folks who build our mass
>produced cars where the development cost can be spread over millions of
>vehicles.

They used the sensors to check on, and refine, the computer model.
They tweaked the model empirically to get it closer and closer to the
measurements.

Once they were close, that allowed them to test a lot of ideas cheaply
and quickly. Definitely cheaper than the older procedure, for the same
number of tests.

Rodney Myrvaagnes                        J36 Gjo/a

"If Brecht had directed 'Waiting for Godot,'  he would have hung a large sign at the back of the stage reading 'He's not going to come, you know. ' "   -- Terry Eagleton

 
 
 

bow wave question

Post by Rodney Myrvaagne » Thu, 20 Feb 2003 13:28:36


Quote:

>it seems intuitive that the boe half angle or prismatic coefficient would
>make a differnce beween a theoritical wavelength, ie teh waterline length,
>and the effective wavelength. I'm thinking of those longey arrow powerboats
>with the knifelike stem and hollow sections just aft. They can't be
>producing much of a bow wave but some designers have assumed it would
>lenthen the waterline and thus the boat's speed at the same power.

>So I assume there is no known rule of thumb for the effective vs the
>theoretical wavelength as it relates to waterline length. Some adjustemnt
>for bow half angle or prismatic coefficeint would seem to be in order.

That is quite an assumption. It doesn't follow from the previous
graph. The VPP programs of IMS take lots of things into account.

Rodney Myrvaagnes                        J36 Gjo/a

"If Brecht had directed 'Waiting for Godot,'  he would have hung a large sign at the back of the stage reading 'He's not going to come, you know. ' "   -- Terry Eagleton