displacement = weight?

[itinerant baker]

Does a figure given for displacement = weight? or not (or sort of?) I'm specifically looking at a yacht at the moment but would be interested in hearing about other craft as well.

 

 

My specific query is can the figure given for displacement be used to work out weight so I know what to tow it with from a legal/not breaking the vehicle standpoint.

[Chris Pink]

if displacement = weight, boat is sunk.

Edited April 3, 2008 by Chris Pink

[norman.r]

Displacement is in fact volume of water displaced by a vessel when it is freely floating and is the actual weight of the vessel as represented by the weight of the water displaced.

Norman

[Chris Pink]

Displacement is in fact volume of water displaced by a vessel when it is freely floating and is the actual weight of the vessel as represented by the weight of the water displaced.

Norman

 

"Displacement is a measure of the weight of water that a vessel displaces"

 

from http://en.wikipedia.org/wiki/Displacement_%28fluid%29

[chris w]

The boat will float at a level in the water at which the weight of water displaced equals its own weight. So YES weight = displacement. This is simply "Archimedes Principle" which states that the volume of water displaced exerts an upward force equal to its weight. The boat will float at a depth at which this upward force exactly balances the weight of the boat.

 

Let's take, for example, a 60' x 7' cruiser stern boat with a 2' draft. Not all of the whole 60' will be floating (eg: the swim) so let's say that the length submerged is 50'

 

So the volume of water displaced is 50 x 7 x 2 = 700 cubic feet.

 

1 cubic ft of water weighs 62.5lbs so the weight of the water displaced is 43,750lbs or 19.5 tons

 

A 50' boat would weigh around 16 tons and so on based on the same draft.

 

Interestingly, if you run the maths, it takes about 0.8 tons of additional ballast to move the draft down 1 inch for the 60' boat (ie: from 2' to 2' 1").

 

Chris

 

PS: I remember debating with a guy whom I met on the cut who claimed his 40' narrow boat weighed 40tons because it was made of extra thick steel. I told him he meant 14 tons not 40 tons. He insisted he was correct. I explained that the maths doesn't support that (excuse the pun) but he wouldn't have it.

 

The maths: say 33' actually floating and a 2.25' draft. 33 x 7 x 2.25 = 520cu ft which weighs 14.5 tons

Edited April 3, 2008 by chris w

[Keeping Up]

Displacement is in fact volume of water displaced by a vessel when it is freely floating and is the actual weight of the vessel as represented by the weight of the water displaced.

Norman

Or whatever else the vessel was floating in when the measurement was taken.

 

You may have a problem if it was floating in treacle at the time the measurement was taken.

 

OK, I'll get my coat

[ChrisPy]

some people have a way of complicating the answer. Displacement = weight. Full stop.

 

PS: the need for the word displacement probably came about because other types of 'tonnage' weight measurements were/are used, particuarly to describe the capacity of cargo vessels.

Edited April 4, 2008 by chris polley

[WJM]

Displacement is the amount of water that is displaced (shoved out of the way) when you put your boat into the water. This displaced water weighs exactly the same as your boat.

[NB Alnwick]

some people have a way of complicating the answer. Displacement = weight. Full stop.

 

PS: the need for the word displacement probably came about because other types of 'tonnage' weight measurements were/are used, particularly to describe the capacity of cargo vessels.

 

Yes it is as simple as that but, as someone who rarely carries mathematical formulae in my head, I found Chris W's explanation very useful. I will now calculate the displacement of 'Alnwick' although estimates will need to be made to calculate the area of the bows and swim.

[andywatson]

Yes it is as simple as that but, as someone who rarely carries mathematical formulae in my head, I found Chris W's explanation very useful. I will now calculate the displacement of 'Alnwick' although estimates will need to be made to calculate the area of the bows and swim.

 

Make life easy for yourself and do it in metric.

 

Work in meters for lengths to get volumes in cubic metres. This will give you weight in tonnes.

 

This assumes the density of (fresh) water is 1 which will be as close as your measurements.

Edited April 4, 2008 by andywatson

[Keeping Up]

You will be surprisingly close if you work on a basis of one inch per ton.

 

For example if your boat draws 2 feet (average of bow and stern) the weight is 24 tons.

 

This applies to 70-foot narrowboats of conventional shape (typical bow and stern swims, flat bottom, straight sides, no keel, etc)

 

If your boat is a different length, just multipy by the fraction of 70 foot that it is. For example if it is 50 foot long, for every inch it weighs 5/7 of a ton.

[NB Alnwick]

You will be surprisingly close if you work on a basis of one inch per ton.

 

For example if your boat draws 2 feet (average of bow and stern) the weight is 24 tons.

 

This applies to 70-foot narrowboats of conventional shape (typical bow and stern swims, flat bottom, straight sides, no keel, etc)

 

If your boat is a different length, just multipy by the fraction of 70 foot that it is. For example if it is 50 foot long, for every inch it weighs 5/7 of a ton.

 

That makes us around 30 tons - that should help when meeting 'Baldock' for a tug-o-war!

[magnetman]

You should come to the Ricky festival sometime i'm sure Baldock would be glad to oblige.

[Bullfrog]

I just thought someone should say "EUREKA", so I just did.

[magnetman]

But was he floating in the bath or sitting on the bottom?

[Chris J W]

That makes us around 30 tons - that should help when meeting 'Baldock' for a tug-o-war!

 

I'll put £50 on Baldock!

[Bones]

I had no idea Baldock was so heavy. He doesn't look it, but then I suppose he is quite tall, and hairy (at times) perhaps it is the hair.

[magnetman]

Putting 50 pounds on him helps quite a bit :-D

[chris w]

You will be surprisingly close if you work on a basis of one inch per ton.

 

For example if your boat draws 2 feet (average of bow and stern) the weight is 24 tons.

 

This applies to 70-foot narrowboats of conventional shape (typical bow and stern swims, flat bottom, straight sides, no keel, etc)

 

If your boat is a different length, just multipy by the fraction of 70 foot that it is. For example if it is 50 foot long, for every inch it weighs 5/7 of a ton.

 

That's a handy rule-of-thumb Allan, so I checked it out mathematically - well it is me!!!

 

So we have L x W x D X62.5/2240 = weight in tons where L =length, W = width and D = draft (all in feet) and 1 cubic foot weighs 62.5 lbs and 2,240 lbs = 1 ton

 

For your formula to work, the weight in tons has to equal 12 x D ie: the draft in inches.

 

So, for a narrow boat,

 

L x 7 x D x 62.5/2240 = 12 x D

 

Solving for L gives L = 61 feet, but this is the length actually floating. so a boat of overall length of 70 feet will fit the bill pretty accurately.

 

Chris

[andywatson]

That's a handy rule-of-thumb Allan, so I checked it out mathematically - well it is me!!!

 

So we have L x W x D X62.5/2240 = weight in tons where L =length, W = width and D = draft (all in feet) and 1 cubic foot weighs 62.5 lbs and 2,240 lbs = 1 ton

 

For your formula to work, the weight in tons has to equal 12 x D ie: the draft in inches.

 

So, for a narrow boat,

 

L x 7 x D x 62.5/2240 = 12 x D

 

Solving for L gives L = 61 feet, but this is the length actually floating. so a boat of overall length of 70 feet will fit the bill pretty accurately.

 

Chris

 

 

 

I can see some wet and dirty tape measures followed by heavy boat claims coming this weekend.

 

So far Alnwick is leading the fat boat contest but I might get my (ex!) mother in law onboard and go straight in at number 1

Regds A

Edited April 4, 2008 by andywatson