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How much does a narrowboat weigh?


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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 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.

Thanks for that.  Its nice to understand the originating reason for some of our seemingly weird laws and regulations.  

Not getting het up,  but yes the vat would buy an awful lot of extra's and fittings if I do go the route of getting a boat built.  I dont know anyone who would willingly and knowingly spend far more than they need especially when it likely to be the second biggest purchase of their life.

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5 minutes ago, Alan de Enfield said:

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 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 'weight' means the actual KG of your boat, and not a formula based volume extrapolation.

or, to put it another way, the word 'tonnage' in use today to define the size of a ship is misused.  The correct word is 'tunnage', which quantifies the number of tuns a ship could carry in its cargo hold.  A tun was a standard sized barrel used as a container in medieval times.

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1 minute ago, Murflynn said:

or, to put it another way, the word 'tonnage' in use today to define the size of a ship is misused.  The correct word is 'tunnage', which quantifies the number of tuns a ship could carry in its cargo hold.  A tun was a standard sized barrel used as a container in medieval times.

Thanks.  Even better explanation.

 

 

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Ships Tonnage (alt - Tunnage)

The capacity of a merchant ship expressed in tons, for which purpose a ton is considered as 40 cubic feet of freight or 100 cubic feet of bulk cargo, unless such an amount would weigh more than 2000 pounds in which case the actual weight is used.

 

Note : TONS, and not TONNES

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This question pops up periodically on the forum and on a couple of occasions I have outlined a very different approach.

 I've never heard of anyone trying this method, but I'll post it here again in case anyone is interested.

Years ago I saw the following method for weighing a narrow boat using only a spring balance, a stopwatch and a measuring tape. I'm not a mathematician, so please don't ask me to explain the details. All my part in the experiment was to provide a narrow boat while a colleague of mine and his A level maths class took all the measurements and made the calculations. The method was for me to tie a line from the boat to a spring balance which itself was tied to a bollard. I started the engine until a thrust of about 50kgs was measured on the spring balance and the boat was released. For the next 50 metres I drove the boat at a constant speed past markers at 10 metre intervals, then cut the engine until the boat drifted to a stop past further markers. This gave the acceleration and deceleration rates.

The results were as follows:

Thrust (T) = 46 kgs; Accel. (a) = 0.0346m/sec/sec; Decel. (d) 0.0231 m/ sec/sec; Gravity (g) 9.8 m/sec/sec

My colleague used the formula W=Tg/(a+d) and the weight was 7.8 tonnes. I reckoned this was spot on because at the time I had a 38 ft Springer.

The full explanation which my mathematical friend wrote up is several pages long. We submitted the article to the waterways press, but none of them could understand what we were saying so the article has languished in a drawer for nearly 30 years.

No, I don't understand it either - but it did seem to work!!

 

Edited by koukouvagia
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15 minutes ago, koukouvagia said:

This question pops up periodically on the forum and on a couple of occasions I have outlined a very different approach.

 I've never heard of anyone trying this method, but I'll post it here again in case anyone is interested.

Years ago I saw the following method for weighing a narrow boat using only a spring balance, a stopwatch and a measuring tape. I'm not a mathematician, so please don't ask me to explain the details. All my part in the experiment was to provide a narrow boat while a colleague of mine and his A level maths class took all the measurements and made the calculations. The method was for me to tie a line from the boat to a spring balance which itself was tied to a bollard. I started the engine until a thrust of about 50kgs was measured on the spring balance and the boat was released. For the next 50 metres I drove the boat at a constant speed past markers at 10 metre intervals, then cut the engine until the boat drifted to a stop past further markers. This gave the acceleration and deceleration rates.

The results were as follows:

Thrust (T) = 46 kgs; Accel. (a) = 0.0346m/sec/sec; Decel. (d) 0.0231 m/ sec/sec; Gravity (g) 9.8 m/sec/sec

My colleague used the formula W=Tg/(a+d) and the weight was 7.8 tonnes. I reckoned this was spot on because at the time I had a 38 ft Springer.

The full explanation which my mathematical friend wrote up is several pages long. We submitted the article to the waterways press, but none of them could understand what we were saying so the article has languished in a drawer for nearly 30 years.

No, I don't understand it either - but it did seem to work!!

 

 

At first sight I'd have expected some factor to take into account the viscosity of the water but on reflection, it probably cancels itself out.

The one thing I'm not sure about is the thrust. You started with a bollard thrust of 50Kg and it looks to me from your notes as though this was assumed to be a constant force during the acceleration. It isn't. The prop thrust reduces as the speed of the boat increases, assuming your instructions were to leave the throttle setting alone. This would, I think, lead to an overestimation of the mass accelerating and stopping.

Somehow this feels rather basic though, and I imagine your friend took it into account. I just can't see how though.

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1 hour ago, Mike the Boilerman said:

 

At first sight I'd have expected some factor to take into account the viscosity of the water but on reflection, it probably cancels itself out.

The one thing I'm not sure about is the thrust. You started with a bollard thrust of 50Kg and it looks to me from your notes as though this was assumed to be a constant force during the acceleration. It isn't. The prop thrust reduces as the speed of the boat increases, assuming your instructions were to leave the throttle setting alone. This would, I think, lead to an overestimation of the mass accelerating and stopping.

Somehow this feels rather basic though, and I imagine your friend took it into account. I just can't see how though.

I've just dug out the notes made by my colleague and his A level maths class in 1988.  They run to twelve pages of formulae, calculations and methodology.  They are completely incomprehensible to me, a mere classicist.  If anyone would like a copy of the notes PM me and I'll send you a PDF.

The class calculated that my Springer weighed 7.84 tonnes.  Pretty accurate, I thought.

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21 minutes ago, koukouvagia said:

I've just dug out the notes made by my colleague and his A level maths class in 1988.  They run to twelve pages of formulae, calculations and methodology.  They are completely incomprehensible to me, a mere classicist.  If anyone would like a copy of the notes PM me and I'll send you a PDF.

The class calculated that my Springer weighed 7.84 tonnes.  Pretty accurate, I thought.

Was the colleague called Archie?

 

2 hours ago, rusty69 said:

With Ulreka! 

Ka ka ka ka.

 

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On 04/06/2017 at 21:53, sharpness said:

Our 45ft clonecraft sailaway weighed 9 1/2 tones according to the builder & the chappy on the crane so that's before I started adding anything!

Steve

 

Seeing as all the heavy bits (bow, stern, engine) are already built into the shell, I'd say the extra 13ft of plain steel shell and ballast would weigh in the order of a couple of tonnes, so my estimate of 10.4 tonnes is not that far out. Your data suggests a weight of 11.5 tonnes for a 58ft cruiser stern.

How deep is the swim on yours?

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4 hours ago, Mike the Boilerman said:

 

Where does the error lie in my calculations which I explain every step of the way, then?

 

A summary of the weights :

Ditchcrawler @ 57' = 18 tons

System 4/50 @ 60' = 18+ tons

Still Earning @ 55' = 18 tons

D Hutch 'Guess' = 18 Tons

Matty 40 'Guess' = Near 20 tons

MtB's calculation = 10.5 tonnes

 

I'd suggest that one of the quoted figures is out of line with known actuals.

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On 04/06/2017 at 22:14, Alan de Enfield said:

A summary of the weights :

Ditchcrawler @ 57' = 18 tons

System 4/50 @ 60' = 18+ tons

Still Earning @ 55' = 18 tons

D Hutch 'Guess' = 18 Tons

Matty 40 'Guess' = Near 20 tons

MtB's calculation = 10.5 tonnes

 

I'd suggest that one of the quoted figures is out of line with known actuals.

 

Again, I ask you to point out where the error in my calculation lies.

I suggest you can't, and your list above is just blokes willy-waving. Blokes love to claim their boats are heavier than they actually are, as we all well know.

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40 minutes ago, Mike the Boilerman said:

 

Again, I ask you to point out where the error in my calculation lies.

I suggest you can't, and your list above is just blokes willy-waving. Blokes love to claim their boats are heavier than they actually are, as we all well know.

I think your estimation of draft is too shallow and you have probably rounded down your calculation of the surface area in the water a little too much.

I have a construction drawing of Vulpes which shows the intention for the boat to sit level in the water with 2' of hull side below the water line and 1' above it. That tells me the intended fully fitted and ballasted mass of the boat was at least 12 tonnes. It's only 35' long so 18 tonnes for a ca.60' boat sounds about right.

What is so heavy about the construction of your own boat that makes you think a boat 10' shorter would only be about half the 20 tonnes plus mass you calculate for your boat?

JP

Edited by Captain Pegg
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On 04/06/2017 at 23:18, Captain Pegg said:

 

What is so heavy about the construction of your own boat that makes you think a boat 10' shorter would only be about half the 20 tonnes plus mass you calculate for your boat?

JP

 

Details set out in my previous post, copied here:

"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."
 

I note no-one seems able to find a flaw in my calculations. They only ever make comparisons e.g. this boat weighs X (without producing evidence) so my calcs must be wrong.

Unless the OP tells us what his boat draws at bow an stern, your guess can be no more accurate than mine

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On 04/06/2017 at 23:18, Captain Pegg said:

 

I have a construction drawing of Vulpes which shows the intention for the boat to sit level in the water with 2' of hull side below the water line and 1' above it. That tells me the intended fully fitted and ballasted mass of the boat was at least 12 tonnes.

 

I'm struggling with this bit. HOW does it tell you 12 tonnes? Can you show your calculations please?

Thanks.

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Just now, Mike the Boilerman said:

 

I'm struggling with this bit. HOW does it tell you 12 tonnes? Can you show your calculations please?

Thanks.

10 metres long x 2 metres wide x 0.6 metres deep x density of water @ 1 tonne per cubic metre = 12 tonnes

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