Weight of a 100ft wooden boat?

[eitc]

Hi. I'm trying to help my son work out the dead weight of a wooden built boat measuring theoretically 100ft long x 11ft wide x 9ft high keel. I know there are lots of variables but the maths so far is doing my head in. Anyone here have an educated guess? I estimated 30-40 tonnes but might have gone wrong!?

[Jen-in-Wellies]

It will weigh the weight of the water it displaces, so you need to work out the volume of water that the hull takes up under the water line, then use the density of water. Easiest to do it in cubic metres, then one tonne equals one cubic metre. You'll need the shape of the hull under water and the draft at the fore and aft end. It doesn't matter if it is wood, or steel, or plastic, or how much or little ballast, or load is in it. Just shape and draft under the water line.

Jen

[ditchcrawler]

Well if its made of 1/4 ply probably a little under a ton

[Alan de Enfield]

1 minute ago, ditchcrawler said:

Well if its made of 1/4 ply probably a little under a ton

 

 

If it is waterlogged it could be 100 tons.

 

Its actual weight can only be done by weighing it or by using Jen's method.

[ditchcrawler]

2 minutes ago, Alan de Enfield said:

 

 

If it is waterlogged it could be 100 tons.

 

Its actual weight can only be done by weighing it or by using Jen's method.

Or knowing the density and volume of the materials know to build it.

[LadyG]

You could make up a scale model and multiply by something .......

One ton per metre?

It is, after all, like a very long narrowboat, there will be barges of that sort of dimension, I would think.

Edited March 4, 2021 by LadyG

[ditchcrawler]

2 minutes ago, LadyG said:

You could make up a scale model and multiply by something .......

One ton per metre?

It is, after all, like a very long narrowboat, there will be barges of that sort of dimension, I would think.

But unless you know the density of the wood used even it was the same volume you would have no chance  one made of balsa wood would be approximately a quarter of the weight of one made of ash

[Alan de Enfield]

1 minute ago, ditchcrawler said:

But unless you know the density of the wood used even it was the same volume you would have no chance  one made of balsa wood would be approximately a quarter of the weight of one made of ash

 

There again, there are a lot of NBs that can be used as a comparison

 

100' x 11' with a 9' draft (and manufactured in wood) are commonly found NB characteritics

[eitc]

13 minutes ago, Alan de Enfield said:

 

There again, there are a lot of NBs that can be used as a comparison

 

100' x 11' with a 9' draft (and manufactured in wood) are commonly found NB characteritics

......That was what I was thinking (ie a 50ft wooden NB as comparison) but only kept coming across examples with steel hulls which I presumed would be different to wood? 

[David Mack]

1 hour ago, ditchcrawler said:

Well if its made of 1/4 ply probably a little under a ton

That depends. Does the OP want to know what the wooden hull and superstructure alone weighs, or does he want to know the all-up weight in boating condition, including engine and gearbox, other equipment, internal fitout, ballast, contents of water, fuel and black water tanks, crew's personal possessions, etc.?

[eitc]

2 minutes ago, David Mack said:

That depends. Does the OP want to know what the wooden hull and superstructure alone weighs, or does he want to know the all-up weight in boating condition, including engine and gearbox, other equipment, internal fitout, ballast, contents of water, fuel and black water tanks, crew's personal possessions, etc.?

Just wooden hull and superstructure I think.

[DandV]

If every thing is scaled up, 4x the weight of a 50ft wooden boat of the same proportions.

[Detling]

13 hours ago, DandV said:

If every thing is scaled up, 4x the weight of a 50ft wooden boat of the same proportions.

Not a bad ball park, which would make it 48 to 60 ton range. As Jen said the material used for the hull is irrelevant it is the amount of water displaced that matters, a 50 foot narrowboat will displace 12 - 15 tons depending on its draft.

 

[IanD]

1 hour ago, Detling said:

Not a bad ball park, which would make it 48 to 60 ton range. As Jen said the material used for the hull is irrelevant it is the amount of water displaced that matters, a 50 foot narrowboat will displace 12 - 15 tons depending on its draft.

 

Don't forget that's including the engine, water/fuel, internal fittings, and ballast -- a bare hull weighs (and displaces) less. Especially with no superstructure (cabin).

 

And the difference can be even bigger for wooden hulls where the bare hull tends to be lighter than steel -- depending on thickness, obviously. Empty wooden hulls ride very high in the water...

[ditchcrawler]

The OP is asking how he can work out the dead weight of a boat, Unless he can weigh it, float it or have the full specification and amounts of materials used its impossible. All this talk of scaling up doesn't help. Its just an impossible task. 

Edited March 5, 2021 by ditchcrawler

[Murflynn]

if we knew WHY you need to establish the weight it would avoid all the confusion.

 

it is impossible to predict the WEIGHT of a wooden boat unless you specify all the scantlings and the type of timber.

 

if you need to know the displacement (i.e, the amount of water displaced by the boat in the conditions that you choose), you need to specify the draft of the hull in water.  You stated 9ft high keel.  Do you mean that the type of boat is a Humber Keel ?  In which case the displacement (in tonnes) will be approximately 90% x L x B x D where D is the hull's draft at the mid-point between bow and stern and all the dimensions are in metres.   L=30, D=3.3.

[Tacet]

18 hours ago, DandV said:

If every thing is scaled up, 4x the weight of a 50ft wooden boat of the same proportions.

 

4 hours ago, Detling said:

Not a bad ball park, which would make it 48 to 60 ton range. As Jen said the material used for the hull is irrelevant it is the amount of water displaced that matters, a 50 foot narrowboat will displace 12 - 15 tons depending on its draft.

 

Except it should be 8x

[Tam & Di]

22 hours ago, eitc said:

Hi. I'm trying to help my son work out the dead weight of a wooden built boat measuring theoretically 100ft long x 11ft wide x 9ft high keel. I know there are lots of variables but the maths so far is doing my head in. Anyone here have an educated guess? I estimated 30-40 tonnes but might have gone wrong!?

People have quite correctly said that the best way is to put it in the water and measure how much it displaces. Unfortunately from your description I can't even begin to imagine what the boat looks like, and without that there is no other option - what do you mean by '9ft high keel'? Without futher information it is not even possible to have an informed discussion.

Edited March 5, 2021 by Tam & Di

[Tam & Di]

22 hours ago, eitc said:

Hi. I'm trying to help my son work out the dead weight of a wooden built boat measuring theoretically 100ft long x 11ft wide x 9ft high keel.

 As you are talking in pre-metric 'old money' here are some equivalences which might help: 1 cu. ft = 6.24 gallons, and 62.4 gallons of water weigh 62.4 pounds. From the dimensions you give and that it is made of wood I assume this is some sort of yacht, so even the calculations of area before taking account of the depth will be quite difficult - it will not be a slab sided 'skip' shape. Another point is why does your son want to know the weight? If the weight is a critical factor, for cranage perhaps or load on some trailer, you will have to be much more accurate than if it is idle interest.

 

Tam

[DandV]

6 hours ago, Tacet said:

 

Except it should be 8x

You are right. I thought I had posted a correction but missed out pushing the submit button!