Actual Weight? Displacement ?

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richcray

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What is the actual weight of a yacht, its displacement figure derived by the designer is how much water it needs to displace and is therefore is the weight (law of floatation). But a yacht may have a displacement figure of 6 tons when it actually weighs 3 tons out of the water.

Could someone explain the disparity since people are taking the displacement value as the true weight. Can't be right can it??
 
Errrr - don't see how it can weigh (displace) 6 tons in the water, but only three tons out of it. That's agin the laws of physics. Most usual circumstance is it weighs (displaces) more than the designer reckoned, coz the owner stuffs loads of kit and whisky aboard.
 
That surely is not correct? The actual weight of a vessel, if it was weighed out of the water is the same as the weight of water it will displace when afloat. (Archimedes discovered this /forums/images/graemlins/grin.gif )

Where there is often confusion is that there are a number of different "measurement" systems applied to yachts.
One is "Thames measurement" which is an old rating system related to length and beam, but which uses Tons as it's unit, older yachts might be described as a 2 1/2 or 4 tonner, using this rating. (The actual formula was (L-B) X B X 1/2B divided by 94)

Then there is Registered Tonnage, which is what appears on your Part I registration. I am not sure how this is arrived at, but I understand that it is really more appropriate to commercial shipping and relates to cargo carrying capacity.

Thus my Halmatic 30 whose displacement is about 4.5 tons has a Registered tonnage of 7.69. ( her TM would be 9.6)


Hope this a) helps and is b) accurate
 
The boat will float as soon as it displaces a volume of water equal to it's weight. So they are the same figure. Eureka!
 
[ QUOTE ]
What is the actual weight of a yacht, its displacement figure derived by the designer is how much water it needs to displace and is therefore is the weight (law of floatation). But a yacht may have a displacement figure of 6 tons when it actually weighs 3 tons out of the water.

Could someone explain the disparity since people are taking the displacement value as the true weight. Can't be right can it??

[/ QUOTE ]

Put the correct names to the weights and you will see you are confusing various different derived tonnages.
It is suggested that a yacht will displace approx. 10% more than design weight when new due to owners additions. That displacement will increase over years as hull absorbs moisture, owner adds more rubbish. It is not unusual for a 3 tonne displacement yacht to end up nearer 4 tonne later in life.
 
Went round this one a couple of months ago in one of the fora. Just to be clear

Weight equals displacement. Measured in imperial tons (2240 lbs) or metric tonnes (1000kg or 2200lbs). Equal to the weight of water displaced.

Thames Tonnage (TM) commonly used to indicate size and also in good old days a basis for rating yachts for comparison is based on LBP (length between perpendiculars, sort of LWL but not quite, beam and draft. Very much affected by beam as it occurs twice in the formula so low rating is achieved by narrow beam, hence Victorian Plank on Edge yachts.

Registered Tonnage (actually Tunnage) A measure of volume of cargo capacity designed as a basis for charging vessels for such services as light dues and port charges. Derived from LBP, beam and depth of hull including deck houses with allowances for such things as machinery space, chart space and much to my daughters' delight, accommodation space for apprentices. The volume is referenced back to a Tun which is a standard sized container for transporting wine.

The last measure is arrived at through a survey and clearly there was every incentive to minimise the measure when costs were based on the registered figure. Now is only of interest to those who want to put their yacht on the Part 1 register. Anyone who has an old Blue Book registration document can see how the calculation is arrived at.

The number derived by the three measures rarely coincide for an individual yacht, although they all generally get bigger as the yacht gets bigger - but at a rate that depends on how you define "big"!

The point about displacement growing is valid. Most designers tend to quote "light displacement" in their literature, but as others have said, actual displacement grows in use as owners add more equipment, and in case of some construction methods, absosrb water.
 
No, the water is in the structure. If you fill your water tank, the weight of your boat increases as does the amount it displaces. Same with water in the structure - absorbed into the cells in the wood or in the GRP laminate is the same. Try weighing a sodden pice of wood then putting it in the oven to dry out and re-weigh it. Good school lab experiment.

Not sure how much difference it would be in practice though.
 
Sorry, I intended to put "Tonnes" in brackets to differentiate.

I am not sure in what circumstances a "Short" ton (2000lbs) is used. My understanding is that it is used in the US, but I can't verify.

Interestingly the OED defines Register tonnage as 100 cuft of capacity, but I cannot reconcile that with the calculations for my boat as there must be a factor to take into account the shape of the boat. I have length of 23.5, breadth of 8.1 and depth of 3.2 which gives a gross cube of 552, but a gross tonnage of 4.72 or 13.36 cu metres, which is approx 460 cu ft. What a lovely mixture of measures!

Maybe a tame surveyor can enlighten us.
 
[ QUOTE ]
What if it uses water as ballast

[/ QUOTE ] If it uses water as ballast then the total weight including the ballast is still the same as the displacement. BUT once the ballast is pumped out the weight is reduced by an amount equal to the weight of the the water ballast.
 
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