Deadweight v Displacement

[boomer]

The main thing wrong with my education is that it was all too long ago! The result is that whilst I can remember what Archimedes said about bodies immersed in fluids and upthrusts etc I have difficuties with the practical application. So..... can the experts here explain the relationship between deadweight and displacement?
Do the final figures work out the same, albeit, perhaps for different reasons? More paticularly, if the boat hoist weighs our boat (motor-cruiser) at 20 tonnes, how do I calculate the displacement?

Grateful for any advice.

 

[oldharry]

Same thing, Boomer. Displacement is the weight of water displaced by the boat, and is therefore the same as the all up dead weight of the boat. Therefore if the hoist says your boat weighs 20 tons, it is displacing 20 tons of water.

Where it gets complicated is that the deadweight quoted by the manufacturer is not the same as the weight of the boat in cruising trim - i.e. with all fuel, water and gin tanks topped up, plus weight (not inconsiderable in my case!) of skipper, crew etc, plus all the extra gear you have installed or brought aboard.

Also manufacturers can have a nasty habit of - er - economising on the total weight quoted in the glossy brochure, by not taking into account certain basics which may vary from the standard - like engines, which can make a substantially bigger difference to displacement than the skippers waistline!

I recently weighed a 20 foot trailer sailor, whose builder quoted 18cwt as the deadweight. After allowing for the trailer, but adding odd incidentals, like the engine, mast and sails, which the builder conveniently forget to include in his weight estimate, that weight had increased to 26cwt - which was over the legal limit for both the trailer and the towing vehicle.

And that was before I climbed aboard!

 

[Trevor_swfyc]

Yes it is the same thing but only if it floats. Metals sink because they are not capable of displacing their mass in water. Wood floats because if you weigh the wood and then weigh the water it displaces when totally submerged, the weight of water will be the greater. When an object floats in water it is in a state of equilibrium gravity is trying to sink it, the upforce due to displacement is keeping it afloat the two are cancelling out.
The neat trick is a submarine were the mass of the vessel and the total max displacement are in fine balance increase the mass (take on water) and she sinks, reduce the mass (blow the tanks) and she comes up. Expect this is all controlled by computer now ?
Bloomer you are not the first to ask this question a club member would not accept that the weight of his boat was the displacement, in that case I gave up trying to convince him, hope you are happier with this concept?
Good post
All the best
Trevor

 

[boomer]

Totally convinced - thank you both for the succinct explanation. Now my memory has been jogged, it all comes flooding back to me (figuratively, that is ;-) )

 

[oldharry]

Hope the flooding is only figurative, Boomer - otherwise the mass increases and down she goes - like my fifth pint....

 

[trev]

There seems to be some confusion here over what is displacement and what is deadweight.
You have a 'light' displacement which is the weight of water displaced when yer boat is empty.
You have a 'loaded' displacement which is the weight of water displaced when yer boat is full.
Deadweight is the difference between the two, ie the amount a vessel can carry.
That's basically it !

Trev

 

[byron]

Sorry Kev., I hate being at odds with others but unless they have changed the rules since I was at sea, Deadweight is the actual weight of the vessel and bears no relationship to the cargo carrying capacity.

ô¿ô

 

[onenyala]

Sorry Byron Kev is correct.
Stuart Jones

 

[trev]

To End It !!
'Deadweight' - the total weight which a ship can carry, including cargo, provisions,fuel, stores, bunkers, crew, spares etc. up to her plimsol line or marks.
ie the difference between light and loaded displacements.
Where did you go to sea Byron ??!!

Trev

 

[oldharry]

There - I KNEW the skippers waistline came into the equation somewhere....!