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Author Topic: another question on ballast  (Read 1292 times)

class37

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another question on ballast
« on: October 27, 2009, 02:46:33 AM »

Hi all,

afraid I have another coupls of questions on ballast.

the PSV I am building will have both free flooding and pumped ballast tanks, with extra batteries to assist with ballasting as well as giving longer play - sorry - sailing time!

however by my calculations I will still need to add some ballast in the bottom of the hull, and so the questions are :

1. with the hull being flat bottomed should the fixed ballast be spread low across the whole width of the hull or

2. should it be concentrated down the centre line, possibly being higher.

as the hull is being built with basically a double hull, with the sides filled with foam sheet for bouyancy, it would seem best to fit the fixed ballast in the bottom double section.

cheers

alan
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derekwarner

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Re: another question on ballast
« Reply #1 on: October 27, 2009, 05:41:39 AM »

Hi Alan.......if the is the same  '1/72 platform supply vessel' it is best to keep the string together.....sometimes it is best to think of the real platform supply vessel & ask what is displayed in the water?

1) add the ballast in one central hunk amid ships & on axis ...& you create a pendulum effect centered around the axis of the mass...the bow wants to ride over the wave...but this is countered by the stern local buoyancy
2) add the ballast in one continuous length on axis......the vessel will roll from port to stdb with a higher frequency & possibly provide an out of scale perception
3a) add the ballast in a flat sheet shall we say evenly over the full hull area?......as the bow approaches a wave...the bow wants to slice through the wave as opposed to ride over the wave
4b) add the ballast in a flat sheet shall we say evenly over the full hull area...the vessel will roll from port to stdb gently ......Derek  :-))
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Derek Warner

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Illawarra Live Steamers Co-op
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www.ils.org.au

class37

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Re: another question on ballast
« Reply #2 on: October 27, 2009, 06:31:12 AM »

Hi Derek,

yes its the same one.

from all the replys from my other post it seems that I need to keep weight [a] central and low .

to this I'm hoping to put the batteries flat in the engine room, just forard of the centre of the hull, with the free flooding tank aft of that.

the pumped ballast tanks then go forard of the engine room, and aft of the free flooding tank.

I guess what I was asking, though not too  clearly, was whether the fixed ballast would be more stable spread across the bottom of the hull, or down the centre.

from your 3a and 4b it would seem that for the best stability it should be across the whole width of the ship, although this may produce dramatic bow scenes as it heads through the waves !!

the hull I'm building now is basically being used as a learning project, and for trying out ideas, and now I have received the plans from the shipping company, and got them scaled, I can begin to plan how to build the proper model, using the experience gained from this first hull.

anyway, must start clearing up and getting ready to get off home, so thanks for your comments, and I'll see what happens when I finally get the beast into the water [SWMBO keeps coming up with other things to take up my time !]

cheers

alan
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derekwarner

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Re: another question on ballast
« Reply #3 on: October 27, 2009, 10:02:08 AM »

......"from your 3a and 4b it would seem that for the best stability it should be across the whole width of the ship, although this may produce dramatic bow scenes as it heads through the waves"

no........ across the width [beam] of the ship [port to stdb]  + also evenly distributed along the length axis [fwd to aft]........ O0

Again as in real life vessels....any large brick like hull structure will attain the floating characteristics of a BRICK...so your free flow ballasting must be completed in 'calm waters'....... {-) ....Derek
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Derek Warner

Honorary Secretary [Retired]
Illawarra Live Steamers Co-op
Australia
www.ils.org.au

class37

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Re: another question on ballast
« Reply #4 on: October 27, 2009, 07:51:15 PM »

Hi Derek,

sorry for the confusion, I meant that the fixed ballast would be over the whole of the base of the hull, ie full beam and length.

the comment re the dramatic bow scenes was following on from your comment in 3a about the bow wanting to go through the waves, rather than over.

anyway, am on four nights off now, so hopefully will get some more done to the hull, and now I know where best to put the ballast.

free flooding tank wil have a top at the loaded w.l. level so as to stop it continueing to fill if things start to go wrong.

thanks for the advice,

cheers

alan
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Colin Bishop

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Re: another question on ballast
« Reply #5 on: October 27, 2009, 08:03:43 PM »

This seems to have got overly confusing.

In a model the ballast needs to be as low as possible (not always the case with a full size ship).

Generally speakiing the best distribution wiill be to confine it to the amidships 50% of the vessel. to give optimum handling characteristics. ( if you were to concentrate the weight at the ends of the hull you can introduce "twitchy" steering.

Colin
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class37

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Re: another question on ballast
« Reply #6 on: October 27, 2009, 09:48:51 PM »

Hi Colin,

so for the slow ones like me, as low as possible, within the middle 50% lengthwise and across the whole beam of the vessel ?

hope I've got this right.

cheers

alan
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Colin Bishop

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Re: another question on ballast
« Reply #7 on: October 27, 2009, 09:52:53 PM »

That's it Alan, you can't go far wrong with that. When you think about it you are concentrating the weight low down for stability and distributing it so that the mass is in the centre of the model which will facilutate manoeuvering.

Colin
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