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Author Topic: Trouble getting ballast right  (Read 3863 times)

Stewpo28

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Trouble getting ballast right
« on: July 11, 2007, 11:55:17 AM »

Hi
I have just completed a Model slipway Clyde puffer, Lovely little model. BUT I cannot get the ballast weight right to get her to float without  tipping over, I have tried lead sheeting, and little sand bags but still having problems, never had this problem with my other boats.

Is there anyone out there that can give me some advice on how to get an even ballast weight

cheers

Stewart
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HS93 (RIP)

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Re: Trouble getting ballast right
« Reply #1 on: July 11, 2007, 11:58:57 AM »

are you putting it as low as possable.? because if its low the boat should not tip. post a picture of the inside if you can, so we can get an idea of where the weight is

Peter
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Bunkerbarge

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Re: Trouble getting ballast right
« Reply #2 on: July 11, 2007, 12:18:08 PM »

Not a lot of use to you at the moment but people should all be aware that you do all your ballast calculations and design before you make the model and not after you have finished.

Ballast should always be as low as possible to ensure the best possible righting moment of the model and hence the best stability.  Even if you  eventually get this one to sit in the water right it soulds like it may be unstable and roll excessively.

You need to get the ballast as low down in the hull as your finished model will allow and your best bet now would be to use something like lead shot from a fishing suppliers.   This will flow into the lowest nooks and crannies and give you the best chance.  Once it it moved around to where you want it it can be fixed in place by pouring some resin over the top of it.

I would do a little test though with this.  When you have it ballasted to the line you want in the bath get hold of the top of the mast and push it over to one side.  When you let go you want it to return to the upright quickly.  If it only returns slowly you have a problem that needs sorting otherwise you may loose this one on a choppy day in the middle of a turn, with a gust of wind, etc...etc..

Next time make sure you do your ballasting first.  You should do the experiments in the bath with the bare hull, work out what you need, subtract the internal components and the model itself and then you know how much ballast you need to lay in the bottom of the hull.  Give yourself a bit of leeway so the final trimming can be put in at the end.
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Stewpo28

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Re: Trouble getting ballast right
« Reply #3 on: July 11, 2007, 02:43:42 PM »

 ;D
Thanks very much for your help tried lead shot from fishing tackle box works a treat

Problem solved

regards

Stewart
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Bryan Young

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Re: Trouble getting ballast right
« Reply #4 on: July 17, 2007, 09:58:23 PM »

Not a lot of use to you at the moment but people should all be aware that you do all your ballast calculations and design before you make the model and not after you have finished.

Ballast should always be as low as possible to ensure the best possible righting moment of the model and hence the best stability.  Even if you  eventually get this one to sit in the water right it soulds like it may be unstable and roll excessively.

You need to get the ballast as low down in the hull as your finished model will allow and your best bet now would be to use something like lead shot from a fishing suppliers.   This will flow into the lowest nooks and crannies and give you the best chance.  Once it it moved around to where you want it it can be fixed in place by pouring some resin over the top of it.

I would do a little test though with this.  When you have it ballasted to the line you want in the bath get hold of the top of the mast and push it over to one side.  When you let go you want it to return to the upright quickly.  If it only returns slowly you have a problem that needs sorting otherwise you may loose this one on a choppy day in the middle of a turn, with a gust of wind, etc...etc..

Next time make sure you do your ballasting first.  You should do the experiments in the bath with the bare hull, work out what you need, subtract the internal components and the model itself and then you know how much ballast you need to lay in the bottom of the hull.  Give yourself a bit of leeway so the final trimming can be put in at the end.
This problem has intrigued me for a number of years. Why should a model boat have to be "bottom heavy" when real ships do not.  I understand about CGs, CBs and all that stability guff including metacentric heights and righting levers...but none of them work on models. I think it may have something to do with the "unvariable" in the equation..the water.(Not forgetting the effect of wind). It is possibly helpful to have some knowledge of ship stability, but sticking the weight in the bottom is as good an answer as you can get.
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Colin Bishop

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Re: Trouble getting ballast right
« Reply #5 on: July 17, 2007, 10:05:45 PM »

Bryan, I suspect it's to do with the differing effects of wind and water when scaled down. I think wind has a disproportionate effect upon models. Also the difference between centre of gravity and metacentric height on a model must be tiny compared with real vessels yet the medium in which the model floats is "full size". In other words, all the parameters are cock a hoop compared with full size practice. Same thing applies to scale speed and viscosity of "water".
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PSSHIPS

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Re: Trouble getting ballast right
« Reply #6 on: July 17, 2007, 10:17:46 PM »

Then there is surface tension! ;)
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Bryan Young

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Re: Trouble getting ballast right
« Reply #7 on: July 17, 2007, 10:25:02 PM »

Then there is surface tension! ;)
Only surface tension? See the responses I get and it goes way beyond the surface!!!!!
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Colin Bishop

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Re: Trouble getting ballast right
« Reply #8 on: July 17, 2007, 10:50:59 PM »

Needs consideration in depth Bryan! I've never been sure if you should be floating a scale model in syrup or light oil.

Then there is the timescale factor. In movies they always slow the action down when using models. Still never looks right though.
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malcolmfrary

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Re: Trouble getting ballast right
« Reply #9 on: July 18, 2007, 12:04:12 PM »

With lead ballast as low as possible in the hull and the hull sitting at the appropriate water line, which could vary enormously on a puffer depending on the load, the model should ride as the original in the same conditions.
You have to remember that on a model scale speed is proportional to the sqare root of the scale - this works the other way with sailing conditions - the wind is multiplied by the square root of the scale.  What is a gentle breeze to a 1:1 sailer is about 8.5 times as strong to a 1:72 sailer.
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Bunkerbarge

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Re: Trouble getting ballast right
« Reply #10 on: July 18, 2007, 01:24:37 PM »

The way I have always understood it is, as you say related to the scale of the waves. 

Imagine the movement of a real vessel at sea.  How long does it take a wave to pass and the swing of the ship from one side to the other to occur?  Heavy weather, North Atlantic, say 5 seconds.

Now imagine you model boat sat on the pond with a fresh breeze lifting up a nice ripple on the surface or your mates fast electic kicking up a wake.  How long does it take one of these waves to pass your model?  A small fraction of a second.

So now how would your model fare hanging over to one side as the next ripple hits it on the same side?  Not very well!!

Our models need to respond very quickly to the wave conditions on the pond so they are ballasted very low.  In real terms we would call the ship very "stiff" and a very stiff ship can actually be very uncomfortable and even more hazardous because it is responding very quickly to waves due to it's high "righting moment".  You are basically being thrown from side to side.

On your model your only consideration is to make it as stable as possible and the best way to achieve this is to keep the ballast as low as possible. 

To see this in action put a bare hull in the bath with the appropriate ballast in the bottom and attach a mast.  Push the mast to one side and see how fast the mast returns to upright.  Now put the ballast on top of a block of polystyrene so you are effectively reducing the GM or righting moment.  Now push the mast and notice how much slower the model returns to upright.  You would initially think that it actually looks more realistic but the models slowed response will make it far more susceptable to the next wave going over the deck.
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Bryan Young

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Re: Trouble getting ballast right
« Reply #11 on: July 18, 2007, 08:08:43 PM »

Bryan, I suspect it's to do with the differing effects of wind and water when scaled down. I think wind has a disproportionate effect upon models. Also the difference between centre of gravity and metacentric height on a model must be tiny compared with real vessels yet the medium in which the model floats is "full size". In other words, all the parameters are cock a hoop compared with full size practice. Same thing applies to scale speed and viscosity of "water".
Just as an afterthought on "real" ships and wind...a 15knot breeze on the beam of an "OL" class ship would be the equivelant of a 30 ton push against the hangar sides. I guess wind is wonderful when you need it...not so good when you don't. BY.
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Bryan Young

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Re: Trouble getting ballast right
« Reply #12 on: July 18, 2007, 08:13:56 PM »

Needs consideration in depth Bryan! I've never been sure if you should be floating a scale model in syrup or light oil.

Then there is the timescale factor. In movies they always slow the action down when using models. Still never looks right though.
I still think that models should be floated in some gloop that has the viscosity of paraffin!
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Bunkerbarge

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Re: Trouble getting ballast right
« Reply #13 on: July 19, 2007, 12:57:14 AM »

You want to see the effects of a 25 knot wind against the side of a cruise ship when the combined effects of five side thrusters at 2 1/2 thousand horsepower each plus a sideways thrust from the main propulsion can't hold her.
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Colin Bishop

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Re: Trouble getting ballast right
« Reply #14 on: July 19, 2007, 10:25:05 AM »

Quote
You want to see the effects of a 25 knot wind against the side of a cruise ship when the combined effects of five side thrusters at 2 1/2 thousand horsepower each plus a sideways thrust from the main propulsion can't hold her.

Is that where the discussion of symmetry comes in....?  ;)
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Bryan Young

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Re: Trouble getting ballast right
« Reply #15 on: July 19, 2007, 06:53:09 PM »

You want to see the effects of a 25 knot wind against the side of a cruise ship when the combined effects of five side thrusters at 2 1/2 thousand horsepower each plus a sideways thrust from the main propulsion can't hold her.
Point taken...my friendly tug skipper says they have mega problems berthing the big car carriers...not dissimilar to liners as far as windage goes.
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Bunkerbarge

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Re: Trouble getting ballast right
« Reply #16 on: July 19, 2007, 09:57:15 PM »

Another side to this little story is that cruise ships nowadays are designed with as little draught as possible to enable them to get into as many ports as possible.  Consequently they are built with all the weight low down, such as engines and machinery, and as light as possible up top, such as fibre glass funnels, fan rooms and even gel coated foam filled masts!

The down side of this is that there is no resistance to the windage effects from the draught so they simply slide sideways.
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malcolmfrary

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Re: Trouble getting ballast right
« Reply #17 on: July 19, 2007, 11:44:27 PM »

But a loaded puffer should sit in the water like a current in a bun.  The centre of gravity and the roll centre should both be well below water line.  It should sit upright if sufficiently and properly ballasted, provided that the topsides have not been built too heavy.
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farrow

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Re: Trouble getting ballast right
« Reply #18 on: August 16, 2007, 09:10:48 PM »

This has happern before. The chief officer or master not doing thier sums right. The vessels gm becomes zero, the vessel is held upright by its tight mooring warps on the jetty, when the the weight is taken on the cargo hook, the weight is transferred to the derreck head, when the dereck swings across the beam the gm now in negative and is pulling the vessel against it,s warps . Will overpower the warps and the vessel lists excessively. As the vessel is working cargo, it,s hold is open and allows engress of water into hold and vessel sinks. This is the most likely cause and has happened before.
David Farrow, mariner (deck) retired.
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malcolmfrary

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Re: Trouble getting ballast right
« Reply #19 on: August 17, 2007, 12:32:36 PM »

Very true, but this should not nornally be a problem with a model puffer at 1:72, if only because working cargo handling is unlikely at this size.
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Re: Trouble getting ballast right
« Reply #20 on: September 29, 2007, 01:37:29 PM »

Needs consideration in depth Bryan! I've never been sure if you should be floating a scale model in syrup or light oil.

Then there is the timescale factor. In movies they always slow the action down when using models. Still never looks right though.

when a 53ft big rig passes the camera at a range that it fills the screen, and takes 2 seconds to pass the camera, im sure that they sould place a model at a proportianate range and make it pass the camera in the same time-gap, so indeed why the slow down?

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malcolmfrary

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Re: Trouble getting ballast right
« Reply #21 on: September 29, 2007, 02:19:22 PM »

Needs consideration in depth Bryan! I've never been sure if you should be floating a scale model in syrup or light oil.

Then there is the timescale factor. In movies they always slow the action down when using models. Still never looks right though.

when a 53ft big rig passes the camera at a range that it fills the screen, and takes 2 seconds to pass the camera, im sure that they sould place a model at a proportianate range and make it pass the camera in the same time-gap, so indeed why the slow down?


To get a correct looking reaction in water, the speed of the model is the speed of the original divided by the square root of the scale.  Assuming a video frame rate of 25/sec, if this was filmed at 25 times (square root of scale), and then played back at 25, it would look as right as possible, given that the surface tension of the water cannot be scaled, so the drops and bubbles formed will remain full size and be the give-away.  The same scale/time/motion considerations apply to any other type of model.
F'rinstance, if you take a 100th scale boat for a 1 mile walk and it takes an hour, the boat has just done 100 scale miles at 10 mph.  If you then apply the time scaling yo get a 10 hour cruise.  My brain now hurts.  I am going for a lie down in a dark room.
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