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Removing Ballast


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1 hour ago, Neil2 said:

I'd suggest you talk to someone who actually does overplating rather than going by what a surveyor says.  

A 70 foot boat will stand a lot of extra weight compared to a "short" vessel I wouldn't worry about removing ballast until the work's done and she's back in the water.  Unless of course you're contemplating replating which is a mammoth job especially on a boat with ballast laying direct on the baseplate.  

I've removed ballast from our boat which admittedly did have 2' square concrete slabs so I didn't have the option of the sort of keyhole surgery you're thinking of, but the idea of cutting holes here and there in the floor to try and extract bricks sounds nightmarish to me. You are effectively constructing an inspection hatch with all the carpentry that involves every time you take a saw to the floor.  Taking the whole floor up (in sections) might sound more disruptive but it's the best way IMHO.  If you're lucky the fitter out put the floor down after the partitions/bulkheads but It's probably not that straightforward.

Talking to a yard experienced in overplating sounds like a plan. 

Unfortunately the idiot who fitted it out put the partitions and sides directly on the floor. Still, I wasn't thinking about overplating it 18 years ago, thats why I did it like that:)

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37 minutes ago, Bargebuilder said:

... glueing/screwing battens up from underneath is a pain.

Indeed, but only necessary if the floor is visible. If the floor is covered then you can attach the battens from the top. 

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1 hour ago, system 4-50 said:

Trend used to do a jig that let you make circular holes with a router with a lip for which you could get standard "lids" but whilst anything made by Trend is high quality it is also expensive. I haven't checked to see if they still do. (Search Trend & router as there is another company called Trend.)

Trend routabout? 

https://www.trenddirectuk.com/rbt-2?gclid=EAIaIQobChMI7ZDimuHl1QIVimwbCh05qwCoEAQYAyABEgI1KfD_BwE

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7 hours ago, Mike the Boilerman said:

Ok, some here seem capable of making this incredibly complicated when it isn't.

If you add an 8mm baseplate you'll need to remove the exact same weight of ballast as the steel you're adding, to keep the same displacement and waterline.

Mild steel weighs 8 tonnes per cubic metre near as dammit, so a new 8mm baseplate will weigh approx 2.5 tonnes. So you'll need to remove 2.5 tonnes of ballast. Engineering bricks are usually around 3kg each (weigh one of yours to check), so that means taking out 833 bricks. 

Do you actually have than many under there?

Reason at last, but I didn't check your maths :captain:

 

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7 hours ago, Mike the Boilerman said:

Ok, some here seem capable of making this incredibly complicated when it isn't.

If you add an 8mm baseplate you'll need to remove the exact same weight of ballast as the steel you're adding, to keep the same displacement and waterline.

Mild steel weighs 8 tonnes per cubic metre near as dammit, so a new 8mm baseplate will weigh approx 2.5 tonnes. So you'll need to remove 2.5 tonnes of ballast. Engineering bricks are usually around 3kg each (weigh one of yours to check), so that means taking out 833 bricks. 

Do you actually have than many under there?

 

7 hours ago, rusty69 said:

Doubt it. 

You almost certainly do, and definitely if you have two layers, which is likely.  

But you won't be overplating a 6mm bottom with 8mm.  6mm is enough and the extra mass is nothing on a 70 footer, you probably won't feel it necessary to lose excess weight. 

Here's a funny thing - we had our boat overplated on the base before we had it galvanised.  We cruised it down the Soar and back up the trent & mersey before I had an opportunity to take out some of the ballast, though I was in two minds about it as the boat didn't seem to mind the extra weight at all.  After taking out about 3/4 tonne I swear the boat doesn't seem to cruise as fast as before.  This makes no sense to me but I sort of regret now that I rushed in and took out the ballast, it was a pain of a job too. 

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1 minute ago, Neil2 said:

 

You almost certainly do, and definitely if you have two layers, which is likely.  

But you won't be overplating a 6mm bottom with 8mm.  6mm is enough and the extra mass is nothing on a 70 footer, you probably won't feel it necessary to lose excess weight. 

Here's a funny thing - we had our boat overplated on the base before we had it galvanised.  We cruised it down the Soar and back up the trent & mersey before I had an opportunity to take out some of the ballast, though I was in two minds about it as the boat didn't seem to mind the extra weight at all.  After taking out about 3/4 tonne I swear the boat doesn't seem to cruise as fast as before.  This makes no sense to me but I sort of regret now that I rushed in and took out the ballast, it was a pain of a job too. 

Pretty sure there is only one layer. Im only suggesting 8mm because that is what the surveyor mentioned. I have just had the weed hatch increased in height and the cockpit drains welded up because they were apparently too close to the waterline. 

If any extra plating causes it to sit too low, I guess the other option is to raise the outlets. 

Perhaps I should just leave it, and worry about it in six years time:)

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20 minutes ago, Neil2 said:

 

You almost certainly do, and definitely if you have two layers, which is likely.  

But you won't be overplating a 6mm bottom with 8mm.  6mm is enough and the extra mass is nothing on a 70 footer, you probably won't feel it necessary to lose excess weight. 

Here's a funny thing - we had our boat overplated on the base before we had it galvanised.  We cruised it down the Soar and back up the trent & mersey before I had an opportunity to take out some of the ballast, though I was in two minds about it as the boat didn't seem to mind the extra weight at all.  After taking out about 3/4 tonne I swear the boat doesn't seem to cruise as fast as before.  This makes no sense to me but I sort of regret now that I rushed in and took out the ballast, it was a pain of a job too. 

I thought 2 ton on a 70 footer would drop it about 2"

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2 hours ago, Neil2 said:

6mm is enough and the extra mass is nothing on a 70 footer, you probably won't feel it necessary to lose excess weight.

Second time you have said it, but I'm not getting it.  If you add 70 foot of steel plate bottom to a 70 footer, the increase in draught is not going to be radically different to adding 50 foot of steel plate bottom to a 50 footer, (although I concede it will be a bit less, if swim lengths are the same on both).

2 hours ago, ditchcrawler said:

I thought 2 ton on a 70 footer would drop it about 2"

Exactly so.  70 foot boats draw about an extra inch for each ton loaded.  Add 2 tons of overplate, and it will go down about 2".

That's fact, and no other wild theories cam change that fact.

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As you say:

If steel is 8 tonnes per cubic metre and water is 1 tonne per cubic metre, then for every 1mm thickness of steel added, the draft of a floating vessel will increase by 8mm.

6mm plate - 48mm additional draft.     

8mm plate - 64mm additional draft.

This assumes a flat bottom with vertical sides, so submerged sides that slope outwards (as might exist at the bow) will float a little higher in the water.

This is the same for any width or length of vessel that is floating in water.

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On 20/08/2017 at 23:41, Bargebuilder said:

As you say:

If steel is 8 tonnes per cubic metre and water is 1 tonne per cubic metre, then for every 1mm thickness of steel added, the draft of a floating vessel will increase by 8mm.

6mm plate - 48mm additional draft.     

8mm plate - 64mm additional draft.

This assumes a flat bottom with vertical sides, so submerged sides that slope outwards (as might exist at the bow) will float a little higher in the water.

This is the same for any width or length of vessel that is floating in water.

 

What an elegant yet obvious observation. But only obvious now you've said it1

 A valuable addition to forum knowledge I suggest, which will be repeated in future whenever this subject crops up. 

To summarise this new 'rule of thumb': "For each 1mm of steel baseplate overplating added, the boat will sit approximately 8mm deeper in the water". 

 

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4 hours ago, Mike the Boilerman said:

What an elegant yet obvious observation. But only obvious now you've said it1

I agree but it took me a while to 'get' why. Once I grabbed hold of the twin aspects of it in my mind I had the eureka moment.  1 tonne of steel displaces 1 tonne of water. Steel is 8 times as dense as water so must displace 8 times the volume of water. Then I got it :)

Nice one, Bargebuilder!

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21 hours ago, Bargebuilder said:

As you say:

If steel is 8 tonnes per cubic metre and water is 1 tonne per cubic metre, then for every 1mm thickness of steel added, the draft of a floating vessel will increase by 8mm.

6mm plate - 48mm additional draft.     

8mm plate - 64mm additional draft.

This assumes a flat bottom with vertical sides, so submerged sides that slope outwards (as might exist at the bow) will float a little higher in the water.

This is the same for any width or length of vessel that is floating in water.

 

21 hours ago, Mike the Boilerman said:

 

What an elegant yet obvious observation. But only obvious now you've said it1

 A valuable addition to forum knowledge I suggest, which will be repeated in future whenever this subject crops up. 

To summarise this new 'rule of thumb': "For each 1mm of steel baseplate overplating added, the boat will sit approximately 8mm deeper in the water". 

 

That ignores the buoyancy of the bow shape, as Bargebuilder says, and the buoyancy of the uxter plate section.  Both these factors are more significant in longer boats, I reckon.  That rule of thumb assumes a straight sided shape of which the entire bottom surface is overplated.  Narrowboats might be relatively crude in design but they aren't that simple.  

 

 

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On 21/08/2017 at 22:03, Neil2 said:

 

That ignores the buoyancy of the bow shape, as Bargebuilder says, and the buoyancy of the uxter plate section.  Both these factors are more significant in longer boats, I reckon.  That rule of thumb assumes a straight sided shape of which the entire bottom surface is overplated.  Narrowboats might be relatively crude in design but they aren't that simple.  

 

 

Well obviously the uxter plate will need overplating too so the rule holds good. And it s a 'rule of thumb', not an accurate calculation as already explained. Broadly speaking, the rule of thumb will be a good approximation.

Further, I disagree about your 'longer boats' comment. The shorter the boat the more impact the bow and stern shapes have on the calcs. 

In fact I disagree with pretty much everything you said!

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On 20/08/2017 at 09:55, alan_fincher said:

Don't understand that.  If it is 6mm currently, it can't end up less than 10mm, as 4mm overplate would surely be a minimum.

 

When a boat is overplated the original steel thickness is disregarded in any subsequent measurement. So for example we don't say 3.4mm original + 6mm overplate = 9.4mm because the original 3.4mm thicknesses was too thin which is why it was overplated in the first place. As you said, ideally the thin area should have been cut out and replated so why would you include it in any overplated measurement? No, the new overplated area is now simply 6mm.

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1 hour ago, blackrose said:

When a boat is overplated the original steel thickness is disregarded in any subsequent measurement. So for example we don't say 3.4mm original + 6mm overplate = 9.4mm because the original 3.4mm thicknesses was too thin which is why it was overplated in the first place. As you said, ideally the thin area should have been cut out and replated so why would you include it in any overplated measurement? No, the new overplated area is now simply 6mm.

Which is, of course, why a surveyor would recommend an over-plate of no less than 8mm.

Once the over-plate is rusted through to below 4mm, you will have two inadequate layers as far as a surveyor is concerned, making comprehensive insurance difficult to get.

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26 minutes ago, Bargebuilder said:

Which is, of course, why a surveyor would recommend an over-plate of no less than 8mm.

Once the over-plate is rusted through to below 4mm, you will have two inadequate layers as far as a surveyor is concerned, making comprehensive insurance difficult to get.

Why no less than 8mm? I thought 6mm overplating was also commonly used?

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6 minutes ago, blackrose said:

Why no less than 8mm? I thought 6mm overplating was also commonly used?

Because, a surveyor will offer what is in his professional opinion 'best' advice.

The over-plate is, as far as the surveyor is concerned, the ONLY plate, and if you were in the market for a brand new narrowboat, would you specify a 6mm bottom plate?

6mm doesn't give you much 'leeway' for oxidation or galvanic corrosion, so reducing steel thickness is life limiting and may reduce resale value.

Could it be that 6mm is commonly used to save money? 

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2 minutes ago, Bargebuilder said:

Because, a surveyor will offer what is in his professional opinion 'best' advice.

The over-plate is, as far as the surveyor is concerned, the ONLY plate, and if you were in the market for a brand new narrowboat, would you specify a 6mm bottom plate?

6mm doesn't give you much 'leeway' for oxidation or galvanic corrosion, so reducing steel thickness is life limiting and may reduce resale value.

Could it be that 6mm is commonly used to save money? 

I  suggested 6mm.The surveyor suggested 8mm.

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Once you know the additional cost of an extra 2mm, you will be able to decide if the extra is worth it for you. Thickness is only one factor that will affect the final cost, as dry dock/lifting and labour will remain the same irrespective of plate thickness.

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