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How much does a narrowboat weigh?


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Hi,

Out of interest, approximately how much does a narrowboat weigh?  We've just completed the purchase of our first boat, a 2005 58' cruiser stern (10, 6, 4), and I'm curious to know approximately what it will weigh. I've guessed wildly at 8 tonnes, but I don't know what I'm talking about! It doesn't matter really - we're not planning to have it craned or shipped by road, but I'm just interested. TIA

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You're probably about right.

You can arrive at a more accurate figure by estimating the volume of the submerged hull (you know the dimensions!) in cubic metres. A cubic metre of displaced water weighs one tonne. 

On 04/06/2017 at 10:45, matty40s said:

Try nearer 20.

20 tonnes for a 58ft clonecraft?

Really?

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On 04/06/2017 at 10:51, Stilllearning said:

As MtB said, measure the draught at the front and the back, take an average depth, assume the beam to be about 2 metres, do the multiplication. Then add on a tad for the front and back swims. My last 55footer was about 18tons, if that helps.

The draft at the stern can be measured from inside the hull. Lift a deck board and measure the hight of the swim. Often 450mm on a modern cruiser stern. And the draft at the bow will usually be about 250mm. An average of 350mm or 0.35m.

Now the length underwater. A 58ft boat is 17.7m long. The waterline at the bow will be 300mm less, roughly. The swim at the back will taper to a point about 600mm from the stern. So the underwater shape displacing water will be about 16.8m long and 2.0m wide, assuming a 2m wide baseplate.

Knock off an allowance for the fact that both ends taper to a point and we have a notional length 15m underwater. 

So 15m x 0.35m x 2.0m = 10.5m3 displacement.

I estimate your boat weighs 10.5 metric tonnes.

On 04/06/2017 at 10:56, dor said:

Roughly a tonne a metre.

No. Precisely a tonne per cubic metre!

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Yeah, broadly speaking from a 40 Springer or Sea Otter to a 72ft loaded historic boat your talking a range of about 6 to 40 ton.

But in your case, for the age and length, somewhere around 18ton would be my guess too. 

However as said, if you really want to know, measure the draft front and back, as well as ideally the length of the tapering section at each end, and you can work it out fairly accurately from the displacement just like archamedis!

 

Daniel

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On 04/06/2017 at 11:07, DHutch said:

 

But in your case, for the age and length, somewhere around 18ton would be my guess too. 

 

I think that's pretty unlikely.

My 68ft boat drawing 2'8" at the stern and about 20" at the bow weighs between 16 tonnes and 22 tonnes, depending on which crane gauge I choose to believe. and the OP has a 58ft boat far more lightly constructed.

My displacement calcs suggest my boat weighs a little over 20 tonnes.

Edited by Mike the Boilerman
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On 04/06/2017 at 11:28, Murflynn said:

I think he means a tonne per metre length of boat, which is near enough for most purposes.

 

Ah I see.

I hold that that might be good for choosing the crane, but is wildly inaccurate if you actually need to know the weight fairly accurately. (Not sure why you ever would though.)

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The precise density of water here. So at 10 deg C it's 999.7 kg per cubic metre.

http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

[And strictly speaking kg and tonnes are measurements of mass, not weight. So the weight of your boat will vary according to  where you are - at least six different effects described here:


http://geol-amu.org/notes/m10-1-1.htm

 

  • Greenie 1
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59 minutes ago, rusty69 said:

So in answer to your question. A rough answer is anywhere between 10 and 20 tonnes depending on where your boat is and how much crap it has onboard:)

I help out with the craning team where we live and that is about as accurate a guess as it gets.  We do see everything within that range for a boat that size (according to the crane's scales).

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5 hours ago, Mike the Boilerman said:

..............nd the draft at the bow will usually be about 250mm. An average of 350mm or 0.35m

350mm being (in real money) about 14", and at the deepest point only 450mm (18") ?

 

I'm not sure that many steel NBs will have an average 14" draft.

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this is very strange to me.  Most already built  boats that I have considered, or the sail away design that I still have in mind are in excess of 15 tonnes true weight.

Yet Her Majesties extortion officers (otherwise known as the HMRC) seemingly have their own concept on what a boat weighs.  A boat that might actually weight 18 tonnes when launched could according to the HMRC formula only weigh 12 tonnes or less.

the formula they use is L (m) x B (m) x D (m) x 0.16

Has this ever been challenged?  It strikes me that that's a significant chunk of money to be save if you are going to spend £45,000 or more on a new boat.

 

But that aside,  when giving the weight of a boat is it normal to use the weight at launch, and is this the also the dry weight or weight including fuel and water?

 

 

 

 

Edited by efanton
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1 hour ago, Alan de Enfield said:

350mm being (in real money) about 14", and at the deepest point only 450mm (18") ?

 

I'm not sure that many steel NBs will have an average 14" draft.

Yes. My first two clonecraft narrow boats each drew 18" at the stern. I know this because the uxter plate sat level on the water and the swims were 18" tall. 

Therefore, draft at the stern of each was 18". 

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25 minutes ago, efanton said:

Yet Her Majesties extortion officers (otherwise known as the HMRC) seemingly have their own concept on what a boat weighs.  A boat that might actually weight 18 tonnes when launched could according to the HMRC formula only weigh 12 tonnes or less.

It is absolutely nothing to do with the 'actual' weight of a boat - a boat of certain size could be manufactured from wood, GRP, Steel, Concrete etc etc and each would have a different 'weight' - therefore a formula was decided upon based on one used in the Merchant Shipping Acts - it goes back to something like how many tea-chests of tea, of a certain weight, could be stowed in a certain volume.

 

You are getting het-up because you are assuming 'Gross Tonnage' means the actual KG of your boat, and not a formula based volume extrapolation.

 

Other measurements such as crane-weight, displacement and dead weight are all measurements of weight and not gross tonnage. These are not measurements of the same physical properties and one cannot be substituted for the other.

 

 

 

Edited by Alan de Enfield
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