How much does my 40Kg anchor weigh when in sea water?

[tudorsailor]

A level physics was a long time ago, so can someone help me out?

I have a 40kg anchor (no mention of design). When in sea water, the load on the windlass will be less than 40kg owing to the bouyancy of the water. The specific gravity is 1.028. Can someone tell me how to calculate the load of the anchor when in sea water? Obviously I need to do the same calculation for the chain. There will be a additional load for the huge amount of sea bed that the anchor usually brings ups (still no mention of design!)

Thanks

TudorSailor

 

[rogerthebodger]

Take a bucket or sea water that is full to the brim and place the anchor into the bucket. The water will overflow. Then remove the anchor and measure the depth from the rim of the bucket to the top of the remaining water. Measure the top of the bucket to get the area and multiply by the depth bucket rim to water level. This will give you the displacement of the anchor . Multiply by the density of sea water then deduct this weight from the 40Kg and this will be the weight of the anchor in sea water.

IMHO the difference will be next to nothing.

PS never did A level physics.

 

[Minchsailor]

The density of water is 1 (for the sake of argument). The density of steel is c7.8. So the weight of you anchor in water is 40 x (7.8-1)/7.8 = 38Kg

QED. Archimides. c260BC

PS never did A level physics.

O level.

 

[rogerthebodger]

The density of water is 1 (for the sake of argument). The density of steel is c7.8. So the weight of you anchor in water is 40 x (7.8-1)/7.8 = 38Kg

QED. Archimides. c260BC



O level.

That assumes the anchor is solid. Some have hollow roll bars or even shanks.

Never did O level physics either.

 

[Mistroma]

If the anchor is 40kg and made of steel or something close to it then it should displace around 5kg water.

Steel has a density of around 7,850 kg/m3 vs. figure of 1028 kg/m3 you gave for seawater. Using these figures, gives 40/7,850 m3 for volume of anchor and that 's around 5 litres.

I should imagine that's close enough but friction in bow roller will probably compensate for the small difference and take force needed back to almost same needed to lift a 40kg anchor in air.

 

[rogerthebodger]

If the anchor is 40kg and made of steel or something close to it then it should displace around 5kg water.

Steel has a density of around 7,850 kg/m3 vs. figure of 1028 kg/m3 you gave for seawater. Using these figures, gives 40/7,850 m3 for volume of anchor and that 's around 5 litres.

I should imagine that's close enough but friction in bow roller will probably compensate for the small difference and take you back to almost 40kg.

See edited post above #4

 

[Spi D]

To calculate the water displaced you need to decide on material to know the specific gravity.

Here are some. Blended materials will add an extra dimension.

Metal	kg. / cu. m.
cast iron	6800 - 7800
iron	7850
lead	11340
steel - stainless	7480 - 8000

Edit: Minchsailor beat me to it. Bulding tables in this forum software takes a little time

 

[macd]

How much does my 40Kg anchor weigh when in sea water?

The difference is so small as to be well within the range of tolerance that any prudent sailor would demand of any system on board.
So if it seems marginal...that's because it's marginal. No need for CSE sums.

 

[Mistroma]

The density of water is 1 (for the sake of argument). The density of steel is c7.8. So the weight of you anchor in water is 40 x (7.8-1)/7.8 = 38Kg

QED. Archimides. c260BC

O level.

Just a thought, is 40 x (7.8-1)/7.8 = 34.9kg rather than 38kg. That's in line with my hastily estimated figure of approx. 5 litres of displaced water. That's for a solid anchor of course, not one of Roger's hollow ones .

Not very relevant though, the point I was making was that other factors will outweigh (no pun intended) the effect of displaced water, unless you are using an air filled drum as an anchor.

 

[Neeves]

I think the only anchor with a 'sealed' hollow chamber would be the Ultra. Rolls bars are 'open' - so that they can galvanise and the Spade shank is open at the bottom and if it lands on the seabed on its side such that water can enter - it will also fill with water. Hopefully there was a little hole at the shackle end of the Spade shank to allow the interior to also be galvanised.

So unless TudorSailor won the jackpot and has bought an Ultra I think the hollow section in an anchor is a bit of a red herring.

I thought the educated advise was that the windlass should be comfortably capable of lifting all the chain (and the chain is the heavy part) and the anchor hung freely - (the assumption being: the windlass auto deploys the lot when you are in the middle of the Atlantic - when this happens your anchor, of unknown design, might not be full of mud or weed). If your windlass is going to struggle with your anchor, some mud and a short bit of chain, say 10m, I have this nasty feeling you have a very small windlass!

Anecdotally - it is said that replacement windlass sales have increased since use of concave roll bar anchors and the windlass makers have made mention the increased loads on windlass, from retained mud, might be a factor. This appears to be nonsense - if the windlass has been sized to lift all the chain freely.

Jonathan

 

[tudorsailor]

I thought the educated advise was that the windlass should be comfortably capable of lifting all the chain (and the chain is the heavy part) and the anchor hung freely - (the assumption being: the windlass auto deploys the lot when you are in the middle of the Atlantic - when this happens your anchor, of unknown design, might not be full of mud or weed). If your windlass is going to struggle with your anchor, some mud and a short bit of chain, say 10m, I have this nasty feeling you have a very small windlass!

Jonathan

Point well made. So, I have 100m of 10mm chain which I think weights 200kg. I have a 40kg ***** anchor. The Lewmar Ocean 2 windlass has a working load of 180kg but a maximum of 760kg. So it should cope with anchoring and retrieving. It may struggle with lifting the whole length of chain + anchor as this is above the working load but within the maximum. (Note to self, don't let the anchor deploy mid-Atlantic.)

TudorSailor

 

[Minchsailor]

I have found that of there a lot of weed around, the anchor comes up with water saturated weed clinging to it. This weighs a LOT. My 1000W winch does struggle a bit. OK if the weed ball is recovered slowly, so that the contained water drains out of if as it lifted past the surface.

 

[mjcoon]

The Lewmar Ocean 2 windlass has a working load of 180kg but a maximum of 760kg.

It may be that 180kg is continuous rating but 760kg briefly before burn-out. (So long as plenty of current.) But in mid-Atlantic you should have lots of time for brief bursts...

Mike.

 

[[3889]]

Is Archimedes relevant? A cubic meter of lead displaces the same volume of water as a cubic meter of rock i.e. a cubic meter, but they have very different 'weight' in water and neither accord with the mass of water displaced.

 

[RichardS]

Is Archimedes relevant? A cubic meter of lead displaces the same volume of water as a cubic meter of rock i.e. a cubic meter, but they have very different 'weight' in water and neither accord with the mass of water displaced.

That's what I remember from school.

Both the lead and the concrete displace the same volume and therefore same weight of water so the loss in weight of each in water is that amount displaced and is the same. It's a much bigger proportion of the rock than the lead's total weight of course.

If the weight of water displaced is equal to the weight of the object ........ it floats!

Richard

 

[VicS]

Is Archimedes relevant? A cubic meter of lead displaces the same volume of water as a cubic meter of rock i.e. a cubic meter, but they have very different 'weight' in water and neither accord with the mass of water displaced.

Yes Archimedes principle states that when a body is wholly, or partially, immersed in a fluid it suffers a loss in weight equal to the weight of the fluid it displaces.

 

[Mistroma]

Is Archimedes relevant? A cubic meter of lead displaces the same volume of water as a cubic meter of rock i.e. a cubic meter, but they have very different 'weight' in water and neither accord with the mass of water displaced.

The point would be that a 40kg steel anchor would displace less water than a 40kg rock (assuming it is a non-metallic mineral). So there would be a lower "pull" to lift an immersed rock than an immersed anchor.

40kg of steel would displace ~5 litres of water
40kg of rock would displace ~ 15 litres of water (guessing a density around 2,600kg/m3)

Quite a noticeable difference in the "pull" required to lift from the bottom of a container, so Archimedes principle does apply if you want to calculate it.

i.e. That when a body is partially or completely immersed in a fluid, it experiences an apparent loss in weight which is equal to the weight of the fluid displaced by the immersed part of the body.

But anchor pull is affect by other larger effects (e.g. Breaking suction in mud) and these are probably much more important in real life.

EDIT: Beaten to it by RichardS & VicS (they probably were not watching TV and also being distracted by their wives whilst trying to type, well that's my excuse).

 

[mjcoon]

Is Archimedes relevant? A cubic meter of lead displaces the same volume of water as a cubic meter of rock i.e. a cubic meter, but they have very different 'weight' in water and neither accord with the mass of water displaced.

The point (which other answers have not made prominent) is that we were not asked about a concrete mud-weight which might be of known cubic dimensions and unknown weight. But an anchor of known weight (assumed correct!) but unknown volume. So it is all to do with input data...

Mike.

 

[blackbeard]

It's about time to consider that celebrated invention, by the excellent Des Sleightholme, of the Water Ballasted Anchor.
Apparently the YM office had to field enquires from potential manufacturers who wished to market it.

 

[Buck Turgidson]

Probably good to remember that density is mass/volume. So your anchor made of rock if it weighs the same as your anchor made of steel will have a smaller volume and therefore will displace a smaller mass of water. However if your rock anchor is the same size (volume) as the steel one it will weigh much more.

I did 0 level physics, but it was a long time ago.