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Author Topic: Ballast Question  (Read 1386 times)

Yoshic48

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Ballast Question
« on: August 26, 2017, 11:13:35 AM »

Hi Guys and Gals and (Tugers)  Question Is there a formula to denote How much Ballast any
boat can have before it is too much? and I don't mean (Waterline) I mean before it
Sinks?  I may have a problem with Weight? I know the recommended  overall weight
(Approx) they say? should take it to (waterline?) I haven't got a Bath?
Will have to wait until I take it to ballast tank?  It is something to do with
(Displacement) but What? any info at all would be Appreciated Thanks. :-))
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Colin Bishop

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Re: Ballast Question
« Reply #1 on: August 26, 2017, 11:27:25 AM »

If you know what the finished model should weigh to take it to the intended waterline then you simply need to weigh the model and all the fittings etc. you intend to install in it.

However, you also need to consider stability. The boat might be the right weight but if too much of that weight is high up then the model will be top heavy and fall over!  There is no magic formula for that I'm afraid.

Colin
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boneash

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Re: Ballast Question
« Reply #2 on: August 26, 2017, 11:38:12 AM »

A sheet of plastic and some bricks and or wood makes a ready pond for floatation checks.
It only needs to be a few inches bigger than the length,  breadth and draught.
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Yoshic48

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Re: Ballast Question
« Reply #3 on: August 26, 2017, 11:40:38 AM »

If you know what the finished model should weigh to take it to the intended waterline then you simply need to weigh the model and all the fittings etc. you intend to install in it.

However, you also need to consider stability. The boat might be the right weight but if too much of that weight is high up then the model will be top heavy and fall over!  There is no magic formula for that I'm afraid.

Colin
Hi Colin Thanks for the quick reply only I have fitted two heavy batteries plus two motors in a 1/48 scale tug boat and it may come out over the Approx
 waterline weight IE: 13kg     I may be approaching 14kg but my bath scales are not to bright? what do you Think?? Roy
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Yoshic48

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Re: Ballast Question
« Reply #4 on: August 26, 2017, 11:42:13 AM »

A sheet of plastic and some bricks and or wood makes a ready pond for floatation checks.
It only needs to be a few inches bigger than the length,  breadth and draught.
Nice Idea but don't have room but thanks anyway got to go out now Thanks again
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Brian60

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Re: Ballast Question
« Reply #5 on: August 26, 2017, 12:19:12 PM »

If you don't have a bath, simply buy a cheap kids inflateable paddling pool and put some water in it. I've seen them on sale for as little as a tenner.

plastic

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Re: Ballast Question
« Reply #6 on: August 26, 2017, 01:02:36 PM »

If you have a bare hull, all you need to do is block up any holes with duct tape or similar, put the hull on a support so it is level and then fill the hull with water up to the waterline.

The number of litres of water will be the total amout of water the hull will displace - and as 1 ltre = 1 kg, you now know the total finished weight.

Subtract all the running gear, battery, structure & fittings weight and you now know how much ballast you will need.
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bfgstew

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Re: Ballast Question
« Reply #7 on: August 26, 2017, 01:36:03 PM »

To add to the discussion.


I need approx 20lb to get my Granit down to the water line, it has 2 SLA batteries in just forward of midships, now how would the best way to distribute the extra 20lb be? 10lb on the stern and 10lb on the bow? Spread evenly? All at miships under the batteries? I would hate for it to bob about like a cork in storm so can you point me in the right direction please kind mayhemers.


Regards


Stewart
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Buccaneer

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Re: Ballast Question
« Reply #8 on: August 26, 2017, 02:21:12 PM »

Hello Stewart

At 1:48 and 13Kg I assume you are building a modern Ocean Going Tug. My Amsterdam, 1:50 and 106 cm long will come in at 9-10 Kg. My smaller harbour tugs are nearer 2-3 Kg when scaled down to 1:48. I use the same method as Plastic when I get a new boat. For additional likely displacement take length (cm) x beam (cm) x 0.8. This will give you approximately the area of the deck in sq. cm. Every 1000 sq. cm will increase the weight by 1 Kg. for every cm of extra displacement. 

For volume up to the water line I use length x beam x draft x 0.7. The general rule is to keep the weight as low as possible in the boat, especially if adding extra ballast.

John
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poll

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Re: Ballast Question
« Reply #9 on: August 26, 2017, 02:27:36 PM »

  Hi Roy. I have a Portgarth hull the tug is Svitzer Maverick  with all the bits & peaces fitted.the ballast is 1x12.12  2 12. 7s
  I don't have any problem.


   John
                 Balne moor mbc.
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Buccaneer

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Re: Ballast Question
« Reply #10 on: August 26, 2017, 02:31:28 PM »

Sorry! Sent my last post to bfgstew, should have been for Yoshic48.

For bfgstew I would suggest trying to get the extra weight about a quarter of the length aft and three quarters of the way aft. This should give you a fairly even distribution along the length of the boat and hopefully enable you to keep to the waterline along the length of the boat. It is however a bit of try it and see as there is more hull up forward to take the extra displacement than there is further aft.

John
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bfgstew

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Re: Ballast Question
« Reply #11 on: August 26, 2017, 02:39:09 PM »

Thanks for that John, have got plenty of room to disribute it over, appreciate your help and advice.


Stewart
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roycv

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Re: Ballast Question
« Reply #12 on: August 26, 2017, 02:55:25 PM »

Hi if you think of the boat as a dumbell which has all the weight divided between the two ends, distributing the weight like this will make the boat more difficult to manoevre than if all the weight is in the centre.
As a full size example Joshua Slocum in his round the world voyage commented that he had a very rough trip until he reached S. America.  When this came out in conversation he was advised to move his (tins) storage from the centre of the boat to more forward and aft, this he did and the boat travelled much better after that.  (this from memory).
So a model boat will bob up and down looking a lot less scale when the weight is all located in the centre of the boat.  It needs to be spread around as Buccaneer suggests.
Hope this helps,
regards Roy
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Butts

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Re: Ballast Question
« Reply #13 on: August 26, 2017, 03:23:10 PM »

Yoshic48, others have mentioned dimensions and this is the crucial issue.  How long and how wide is your tug?


I've a tug which probably weighs as much if not more than you quote, but it is 107cm long and 36cm beam.  I'm guessing you are building freelance as you do not quote a kit name?

malcolmfrary

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Re: Ballast Question
« Reply #14 on: August 26, 2017, 08:49:34 PM »

Hi Guys and Gals and (Tugers)  Question Is there a formula to denote How much Ballast any
boat can have before it is too much? and I don't mean (Waterline) I mean before it
Sinks?  I may have a problem with Weight? I know the recommended  overall weight
(Approx) they say? should take it to (waterline?) I haven't got a Bath?
Will have to wait until I take it to ballast tank?  It is something to do with
(Displacement) but What? any info at all would be Appreciated Thanks. :-))
When a hull floats in water, it displaces its own weight of water, hence "displacement".  Usually, the volume of water displaced is less than the volume of the hull.  If it isn't, the hull continues settling below the surface of the water, and has then sunk.  If it is right on the borderline, the chances are that it will ship water, become heavier, and sink.
If a weight has been recommended for a finished model, that will usually take it to the waterline.  Because there is an infinite variety of hull shapes, there is no formula that will give displacement.  If you really want to, you can do a lot of measuring to determine the cross sectional area at as many points along the hull as you can, then work out the sum of the volumes between the sections, and that will, after a lot of work, give a sort of guess.  Or do as suggested above with a paddling pool.  Or, if no fitting out has been done, fill to the waterline and weigh it.
On a model, you don't want more than the recommended weight unless you feel the need to have it sit deeper than the real thing would.
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Bob K

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Re: Ballast Question
« Reply #15 on: August 26, 2017, 10:21:21 PM »

If it is a scale model of an actual ship then you can look up the actual displacement for that ship.
You then take the full-size weight and multiply it by the cube root of the scale for your model.
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tigertiger

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Re: Ballast Question
« Reply #16 on: August 27, 2017, 04:17:37 AM »

Ref. distribution of ballast, fore and aft.


I think I read, but stand to be corrected, the following:
If the weight is centered, the the boat will rotate quickly, both port/starboard (very maneuverable) and bow (up) to stern (down) (and vs vs). The latter being the rough ride in the tins example above.
If the weight is split and put at both ends, the dumbbell effect means it will rotate slowly port/starboard (slow to turn), and bow to stern (move steadily through the water).
The balance you want for a tug, may be different to the one you want for a model oil tanker.




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Brian60

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Re: Ballast Question
« Reply #17 on: August 27, 2017, 08:53:35 AM »

THERE IS A FORMULA AND NOBODY HAS MENTIONED IT YET.

Not even me in my first post! %%

As long as you work in metric this works for all builds near as dammit.

Measure your boat as a rectangle, forget the pointy bits - thats why I said near as dammit, you can get it perfect if you want to triangulate as well but its not worth it.

So bow to stern the ship is 800mm long and lets say 150mm wide. Now you need to know how high the waterline is, lets say 60mm.

So you cube 800 x 150 x 60 = 7200000, then divide by a 1000 = 7200. The 7200 figure is the weight of water in millilitres that it would take to fill the ship with water to float it to the waterline.

Now because of the way the metric system works, they said 1 square metre of water weighs 1 metric tonne, so you can extrapolate that the measurement above 7200 millilitres must weigh 7.2 kilo's, simple :} This works for all measurements, I just used simple figures for length and width here.

roycv

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Re: Ballast Question
« Reply #18 on: August 27, 2017, 09:17:15 AM »

Oh well if we are going to get technical!!!  I recently built a small cargo boat.  It is virtually a block of wood with some round corners. and a pointy bit  I deduct the length of the triangle that the bow makes, assume that if this volume is added to the stern section it will equal a 'block'  do the arithmetic above and you are there.
For more accuracy the block co-efficient for cargo boats is about 0.9.

If this was a yacht then measure the area of the immersed centre section and multiply by the length of the waterline.  Then as this is a very 'fine' hull the prismatic co-oefficient is about 0.55, multiply by that and you have the immersed volume of the hull.  If you are measuring in centimetres then the answer is in grammes divide by 1000 and you have kilograms.
 If doing it properly in inches multiply by 0.62 and the answer is in ounces divide by 16 for pounds.
Nice little mental exercises to send you off to sleep at night, we never had calculators when I was at school!

The difference between the block and prismatic co-efficients is that the prismatic one is the actual area of the centre section.  The block is just the beam times the depth.  (Have I got that right)?
regards Roy
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Mark T

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Re: Ballast Question
« Reply #19 on: August 27, 2017, 09:24:32 AM »

Hi Brian


Thanks for that information  :-))   If thats correct my Anteo tug is 890x220x10 which when worked out gives me a figure of 11.7kg  :o


Now using just a bit of common sense and due to the shape of the hull obviously not being square I would have though that the probable weight will be more like 10kg.


Thats great to know and will help with my build but I guess I had better start saving for some lead shot!
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Jerry C

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Re: Ballast Question
« Reply #20 on: August 27, 2017, 09:42:55 AM »

Hi mark, you have a good eye for hull form. There is a coefficient called block coefficient (Cb). If you multiply the value you obtained by LxBxD by the block coefficient for the type of vessel then you will get a better approximation of the displaced volume of your hull when "at her marks". I've had a quick search on the web but have been unable to find a Cb for a tug hull. My best guess however (50 years at sea, 25 in tugs) would be Cb=0.8. Multiplying your block answer by 0.8 gives 9.36kg. I stress this is approximate only. When you do eventually find your total displacement weight you can work out Cb accurately. If your plans show hull form drawings you can apply Simpsons Rules but it's hard work and in your case not really necessary.  Hope this is of use to you.
Jerry.

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Re: Ballast Question
« Reply #21 on: August 27, 2017, 09:43:42 AM »

Yoshic48, others have mentioned dimensions and this is the crucial issue.  How long and how wide is your tug?


I've a tug which probably weighs as much if not more than you quote, but it is 107cm long and 36cm beam.  I'm guessing you are building freelance as you do not quote a kit name?
Thanks Buddy For your reply I am advised that the simple way is to ( before doing anything with the hull IE putting any holes in it is to either fill it with water! or fill a receptacles that it will fit in no matter size or shape! and proceed to push it under water! at water line ! mark it ! then keep pushing until it flows over the top ) Now weigh the water either way you do it and metric ( I believe 1000g = 1kg and by weight water is calculated the same way 1 Litre of water = 1000g I still have a question because I forgot to do this at the start!!*** but because I think I may be over weight for the water Line I'm trying to calculate (lets just say another 1 kg over weight ? how much higher would the water go up and would it be to much and sink it.
 this would help a lot of builders of any boat including me? and Yes the Kit is a (Shipway) yorkshire man although I wont be calling it that? and I'm trying to run 3 heavy batteries? Thanks for all your replies  Roy
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Yoshic48

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Re: Ballast Question
« Reply #22 on: August 27, 2017, 09:50:05 AM »

Yoshic48, others have mentioned dimensions and this is the crucial issue.  How long and how wide is your tug?


I've a tug which probably weighs as much if not more than you quote, but it is 107cm long and 36cm beam.  I'm guessing you are building freelance as you do not quote a kit name?
Thanks Buddy I have posted an answer myself but could still use some of the info as I forgot to do it with this (I should have known) but been out of it for awhile? I have to wait now till all eletrics done then before super structure (weigh that then try get to local test tank for trial and to check motors, and the rest before doing superstructure Roy
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Butts

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Re: Ballast Question
« Reply #23 on: August 27, 2017, 09:58:43 AM »

OK, Slipway's Yorkshireman is 875cm by 249cm with a recommended displacement of 10kg.


Given that, I reckon you will be too heavy. Perhaps fewer batteries?

Mark T

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Re: Ballast Question
« Reply #24 on: August 27, 2017, 11:16:32 AM »

Hi mark, you have a good eye for hull form. There is a coefficient called block coefficient (Cb). If you multiply the value you obtained by LxBxD by the block coefficient for the type of vessel then you will get a better approximation of the displaced volume of your hull when "at her marks". I've had a quick search on the web but have been unable to find a Cb for a tug hull. My best guess however (50 years at sea, 25 in tugs) would be Cb=0.8. Multiplying your block answer by 0.8 gives 9.36kg. I stress this is approximate only. When you do eventually find your total displacement weight you can work out Cb accurately. If your plans show hull form drawings you can apply Simpsons Rules but it's hard work and in your case not really necessary.  Hope this is of use to you.
Jerry.


Hi Jerry


Yes that is definitely helpful and now gives me confidence in my first thoughts that my hull will need plenty of ballast even once the steam plant is in.  In fact it gives me lots of leeway to make a nice base for the plant without worrying about the weight too much and that will also put most of the weight right at the bottom of the hull too


Mark
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