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Author Topic: Buoyancy, displacement & Eureka!  (Read 2717 times)

hopeitfloats

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Buoyancy, displacement & Eureka!
« on: January 10, 2017, 06:41:47 AM »

if a boat crosses over a viaduct /aquaduct (not sure which term is correct), does the weight of the boat get transferred to the viaduct or is it dispersed in the water
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Martin [Admin]

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Re: possible silly question.
« Reply #1 on: January 10, 2017, 07:04:30 AM »


Weight is 'transferred'(?)  by the displaced water.

  See:
  http://www.modelboatmayhem.co.uk/forum/index.php/topic,41275.msg414785.html#msg414785

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hopeitfloats

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Re: possible silly question.
« Reply #2 on: January 10, 2017, 07:33:28 AM »

thanks martin. my next question was going to be about a large ship crossing but your thread link explained that to. :-))
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rickles23

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Re: possible silly question.
« Reply #3 on: January 10, 2017, 08:01:09 AM »

Hi,


The only silly question is the one not asked.


Regards
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Tug Hercules Fireman

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Re: possible silly question.
« Reply #4 on: January 10, 2017, 06:17:09 PM »

As they say;

      "Better to be foolish for 5 minutes, then foolish for a Lifetime."
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jarvo

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Re: possible silly question.
« Reply #5 on: January 10, 2017, 07:02:59 PM »

A fool is the one who wont ask!!!
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malcolmfrary

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Re: possible silly question.
« Reply #6 on: January 10, 2017, 07:25:03 PM »

While the barge displaces its own weight of water, so that the total weight is not affected, if it is a close fit and moving a bit rapidly, the barge will push its bow wave ahead of itself.  This extra lump of water must increase the weight supported by the structure underneath itself as it passes.   If the boat is a sloppy fit and slow moving, the displaced water has a path round the boat to the water behind it. The chambers on the Anderton and Falkirk boat lifts, however, remain at the same weight since the displaced water is shoved back where the boat came from, into the feeding canal section.
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derekwarner

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Re: possible silly question.
« Reply #7 on: January 10, 2017, 08:56:06 PM »

All of the theorist's ....are missing one point  :o

The original scenario is also a little vague....consider the viaduct /aquaduct  to be a limited volume of water [as opposed] to the open sea and also negates in considering hull speed and the increase in the velocity of water due to the limited volume of water under examination....

Ask any Master of a 100,000 ton vessel entering a harbour or river mouth of a river......

Courtesy of Doctor Rick......
_________________________
Date: 07/07/99 at 12:09:48
From: Doctor Rick
Subject: Re: hull speeds of displacement hull boats

Hi, Colin, thanks for writing!

The question and answer you refer to has been there a while and I
guess the original writer must not have written back saying that
this was not what he meant. I have heard of the relation you
state, between length of the waterline and "maximum" speed of the
boat, and I'm pretty sure this is what the writer meant, not a
relation between speed and _height_ of the waterline.

It is my recollection that this hull-speed limitation is
determined by the wavelength of the bow wave - that the distance
between crests of the wave increases with boat speed until there is
a crest at the bow and another at the stern, and the centref the
boat is essentially unsupported. It would thus sink lower in the
water relative to the wave height at bow and stern.

_____________________________________________________________________

There is a following mathematical proof .......but wont bore you with it...

Derek
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Martin [Admin]

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Re: possible silly question.
« Reply #8 on: January 10, 2017, 10:00:16 PM »


I was was thinking of vessel when stationary, at rest, otherwise we need to take into account viscosity, aeration, Morton's number, Eötvös' number, variable buoyancy, the Deborah number, Stokes flow, Reynolds number, Differential drag & possibly Bernoulli's principle!    :P

 
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NFMike

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Re: possible silly question.
« Reply #9 on: January 10, 2017, 11:53:09 PM »

.... a crest at the bow and another at the stern, and the centref the
boat is essentially unsupported. It would thus sink lower in the
water relative to the wave height at bow and stern.

Yes, this is true. But the reason is obvious if you look at a picture of a fast moving vessel - tugs are good as they have lots of power and are short. The water level in the centre is lower than the normal waterline - it's the trough between two waves. Obviously having moved water from the center of the boat to the ends and somewhat beyond the hull there is less under the boat, so it will sink a bit. The laws of displacement must still be in effect though - anything floating must displace it's weight of the liquid it is in.

rickles23

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Re: possible silly question.
« Reply #10 on: January 11, 2017, 07:57:04 AM »

Hi,
I have remembered where this question could be answered:



The wheel has an overall diameter of 35 m (115 ft) and consists of two opposing arms extending 15 m (49 ft) beyond the central axle and taking the shape of a Celtic-inspired, double-headed axe.[30][31][32] Two sets of these axe-shaped arms are connected to a 3.8 m (12 ft) diameter central axle of length 28 m (92 ft). Two diametrically opposed water-filled caissons, each with a capacity of 250,000 litres (55,000 imp gal; 66,000 US gal), are fitted between the ends of the arms.

The caissons or gondolas always carry a combined weight of 500 tonnes (490 long tons; 550 short tons) of water and boats, with the gondolas themselves each weighing 50 tonnes (49 long tons; 55 short tons). Care is taken to maintain the water levels on each side, thus balancing the weight on each arm. According to Archimedes' principle, floating objects displace their own weight in water, so when the boat enters, the amount of water leaving the caisson weighs exactly the same as the boat.This is achieved by maintaining the water levels on each side to within a difference of 37 mm (1.5 in) using a site-wide computer control system comprising water level sensors, automated sluices and pumps. It takes 22.5 kilowatts (30.2 hp) to power ten hydraulic motors, which consume 1.5 kilowatt-hours (5,100 BTU) per half-turn, roughly the same as boiling eight kettles of water.

Each of the two caissons is 6.5 metres (21 ft) wide, and can hold up to four 20-metre-long (66 ft) canal boats. Watertight doors at each end match doors located on the upper structure and lower dock pit. Due to space concerns, where a normal hinged door would dramatically reduce the useful length of the caisson, vertically rising doors were chosen.The doors are raised from a recess in the base of the caisson and powered by a hydraulic lance when docked.

Regards
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kinmel

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Re: possible silly question.
« Reply #11 on: January 11, 2017, 08:49:27 AM »

if a boat crosses over a viaduct /aquaduct (not sure which term is correct), does the weight of the boat get transferred to the viaduct or is it dispersed in the water

The weight of the boat is displaced at the point the boat enters a body of water and remains displaced whilst it's in that body of water, it's current position is irrelevant.  The weight on the aqueduct columns remains constant.

The boats in the photo are moving at about 4mph
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BFSMP

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Re: possible silly question.
« Reply #12 on: January 11, 2017, 09:13:12 AM »


ahh, Pontcysyllte, Many a year has passed since I was a passenger on a boat sailing on that modern wonder of the world.


Jim.
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rickles23

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Re: possible silly question.
« Reply #13 on: January 11, 2017, 02:20:20 PM »

Oh yes!


 The Archimedes principal.


Regards


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boat captain

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Re: possible silly question.
« Reply #14 on: January 11, 2017, 04:16:19 PM »

So Rickles I take it like me you have visited the Falkirk Wheel.
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Martin [Admin]

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Re: possible silly question.
« Reply #15 on: January 11, 2017, 05:03:07 PM »


The boats in the photo are moving at about 4mph


Pontcysyllte Aqueduct ?
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BFSMP

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Re: possible silly question.
« Reply #16 on: January 11, 2017, 05:06:35 PM »


Pontcysyllte Aqueduct ?



yes indeed...........unless it's Crick just north, to far back to remember, lol.


Jim.
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Netleyned

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Re: possible silly question.
« Reply #17 on: January 11, 2017, 05:15:47 PM »

Think you mean Chirk Jim,
Crick  is a tunnel on the Liecester
Arm of the Grand Union.


Ned
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Steve Dean

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Re: possible silly question.
« Reply #18 on: January 11, 2017, 05:27:59 PM »

Ah great memories ...... we once went across the aqueduct by boat 3 times in one day (with diversions to the pub) just for the fun of it.
To stand on the non towpath side of your boat and look down is not for the faint hearted !!!!!
Good job it was built a very, very long time ago ......... the elf & sarfty police wouldn't allow it now.
If you've never been there, then put it on your bucket list. By boat or on foot it is STUNNING!
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John W E

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Re: possible silly question.
« Reply #19 on: January 11, 2017, 05:29:29 PM »

This silly question reminds me of "the question" if  Concorde was flying at Mk 2 speed, and a passenger at the back of the plane threw a ball to a passenger sitting at the front of the plane - how fast would the ball be going? %) %) %)
john
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BFSMP

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Re: possible silly question.
« Reply #20 on: January 11, 2017, 05:34:24 PM »


Think you mean Chirk Jim,
Crick  is a tunnel on the Liecester
Arm of the Grand Union.


Ned


thanks ned.........yes you are right.......its no wonder when I google earthed it I couldn't find Crick aquaduct.........mind you it was 40 years or so I had the pleasure of sailing across them on the way to Llangollen.......I don't even know the name of my street sometimes these days.


Jim.
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aeronut

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Re: possible silly question.
« Reply #21 on: January 11, 2017, 06:24:32 PM »

This silly question reminds me of "the question" if  Concorde was flying at Mk 2 speed, and a passenger at the back of the plane threw a ball to a passenger sitting at the front of the plane - how fast would the ball be going? %) %) %)
john


The ball would be travelling at Mach 2.0 + the speed at which the ball was thrown.  IIRC, the Ryder Cup golf team made a putt the length of the cabin whilst at M 2.0 and thus hold the record for the fastest putt ever.
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kinmel

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Re: possible silly question.
« Reply #22 on: January 11, 2017, 06:36:12 PM »

It's really a matter of relativity.

In the plane it is travelling at just it's own speed.

From a point outside the plane it is travelling at M2 + it's own speed

From the sun it is travelling at the speed of the Earth in orbit +/- speed of plane and ball !
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Colin Bishop

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Re: possible silly question.
« Reply #23 on: January 11, 2017, 06:40:27 PM »

Any question of this sort has to specify the baseline of the measurement. At sea, it is either the speed over the ground or the speed through the water which can be very different.

When I had a 1:1 scale boat it was possible to whiz past Cowes on the ebb at 8 knots although the boat was only doing 4 knots through the water.

Just the same with airlines, when you have a tailwind you will reach your destination a lot faster. Not rocket science but many people simply don't seem to appreciate the difference.

Colin
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hopeitfloats

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Re: possible silly question.
« Reply #24 on: January 11, 2017, 11:36:09 PM »

The weight of the boat is displaced at the point the boat enters a body of water and remains displaced whilst it's in that body of water, it's current position is irrelevant.  The weight on the aqueduct columns remains constant.

The boats in the photo are moving at about 4mph

that is the aquaduct that prompted the question from me. wouldn't get away with that here. our health and safety would have 6ft high fences all around that. they would  totally ruin the view though. has anyone actually fallen over the side of that structure.
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