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Author Topic: Calculating buoyancy?  (Read 4241 times)

Martin [Admin]

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Calculating buoyancy?
« on: December 18, 2009, 03:27:28 PM »


OK, I've got a small hull here and want to fit a big motor;

1. How do I calculate the buoyancy of the hull to the waterline?
2. The hull is flat bottomed, if I need to add flotation, what would the effect be if I added a layer of polystyrene under the hull.

( This is in reference to:  http://www.modelboatmayhem.co.uk/forum/index.php?topic=21112.msg205332#msg205332 )

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HS93 (RIP)

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Re: Calculating buoyancy?
« Reply #1 on: December 18, 2009, 03:37:27 PM »

I have a brushless I took out the mini fire boat that will prob do  {-) O0 :-))  nice and light

peter
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DickyD

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Re: Calculating buoyancy?
« Reply #2 on: December 18, 2009, 03:49:39 PM »

Link doesn't open Martin, but you being an it eggspurt would know this.

Polystyrene would increase the drag considerably and fall apart with wear and tear.
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Martin [Admin]

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Re: Calculating buoyancy?
« Reply #3 on: December 18, 2009, 04:40:46 PM »


Link fixed.

The Polystyrene with be just the buoyancy aid, it will be encased in styrene etc.
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malcolmfrary

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Re: Calculating buoyancy?
« Reply #4 on: December 18, 2009, 05:39:59 PM »

If it fits in a washing-up bowl, put enough water in the bowl to float it, put hull in water, push down to waterline, mark the water level on the bowl side, take boat out, then get scientific.  Very carefully, pour measured amounts of water into the bowl until the mark is reached.
The poly under the hull will raise the centre of gravity.  Raise it enough and it will fall over.  (Watched some Round Tablers rehearsing for a raft race with a bath modified like that.  NPML  {-) )
For a rectangular boat, its just the floor area times the depth to get the volume.  The bit at the front has a triangular cross section, so the volume of that is easy enough.Its just a question of sticking formulas for each chunk into a spreadsheet and adding the answers together.
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Martin [Admin]

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Re: Calculating buoyancy?
« Reply #5 on: December 18, 2009, 06:04:10 PM »

 :P
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HS93 (RIP)

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Re: Calculating buoyancy?
« Reply #6 on: December 18, 2009, 06:17:55 PM »

what is the hull made off and have you got to close the hole up if so do that first then just load it up in the sink with rolls of solder batteries till you reach the desired depth then just weigh the ballast on the kitchen scales, as long as you keep and idea of the weight as you build there should be no problem .it is a wide hull so will prob be very buoyant .

peter


ha someone left the Handbreak on the web site seems slow today ?   %%
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DickyD

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Re: Calculating buoyancy?
« Reply #7 on: December 18, 2009, 06:28:49 PM »

I think there is clear plastic in the bottom of the hull.

But your thinking is the same as mine Peter.
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Martin [Admin]

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Re: Calculating buoyancy?
« Reply #8 on: December 18, 2009, 07:03:19 PM »


Does anyone know of a small powerful 'slow' 400 size motor or should I just fit a standard 'slow' 540... any ideas?
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DickyD

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Re: Calculating buoyancy?
« Reply #9 on: December 18, 2009, 09:07:25 PM »

What about the Deans Marine Kyte motor ?

Cant give you a link, their hosting company have taken them offline.
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derekwarner

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Re: Calculating buoyancy?
« Reply #10 on: December 18, 2009, 11:42:35 PM »

Martin.....just remove all 'removable' bits from the hull......place the hull in a water tank....bath or what ever & gently add water to the inside  :(( of the hull until you get to the desired waterline immersion.......then  {-) just decant the water to a measuring container & hey presto ....each litre = 1 kg of ballast required  :-)) .....+ do the add & subtract thing for the bits you had previously removed         ......Derek
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peter.dwight

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Re: Calculating buoyancy?
« Reply #11 on: December 19, 2009, 01:54:54 PM »

Er, you could also try Simpsons Rule. Which is definately beyond Martin and my ability.
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Bryan Young

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Re: Calculating buoyancy?
« Reply #12 on: December 19, 2009, 03:56:11 PM »

Why do you want to calculate "buoyancy" in the first place? It's "displacement" that you should be looking at. BY.
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Martin [Admin]

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Re: Calculating buoyancy?
« Reply #13 on: December 19, 2009, 11:09:16 PM »


 I guess I'll use Heath Robinson's HS93 Hypothesis, stick it in the bath, load up all the gubbins and see if it floats!
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HS93 (RIP)

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Re: Calculating buoyancy?
« Reply #14 on: December 19, 2009, 11:11:51 PM »


 I guess I'll use Heath Robinson's HS93 Hypothesis, stick it in the bath, load up all the gubbins and see if it floats!


then paint it WHITE   <:(

Peter   >>:-( <*<
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Martin [Admin]

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Re: Calculating buoyancy?
« Reply #15 on: December 19, 2009, 11:45:58 PM »

I'll paint it white as soon as I see a flying horse!  ok2
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HS93 (RIP)

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Re: Calculating buoyancy?
« Reply #16 on: December 19, 2009, 11:56:13 PM »

I thought white was your favourite colour ?? {:-{ {-)   you have done plenty of it   >>:-(



Peter <*< >>:-(
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Bryan Young

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Re: Calculating buoyancy?
« Reply #17 on: December 20, 2009, 06:14:18 PM »

Er, you could also try Simpsons Rule. Which is definately beyond Martin and my ability.
Simpsons Rules are basically quite simple....but long winded. They involve you in (mathematically) slicing up a hull and then aggregate out the volume of the slice. The more slices you have, the more accurate the result. And the more slices you have, then the more sums you have to do. No matter what "deck level" you wish to stop and calculate the end result is a measure of volume. For model making ....forget it. But also forget stuff like buoyancy. Archimedes got it right a few years ago, and his idea hasn't been bettered or proven wrong since he shouted out "Eureka" to a startled populace. BY.
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