Idiot 'displacement' question

[aitchw]

Idiot \'displacement\' question

Am I right in thinking that the stated displacement of a boat is the equivalent of the total weight of the vessel when laden to design capacity?
Thanks.
Howard

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[aztec]

Re: Idiot \'displacement\' question

it's the weight of the amount of water it displaces, but in essence it's the weight of the boat..

wait for the backlash... sea water's heavier etc....

why were you wondering then?

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[steve_cronin]

Re: Idiot \'displacement\' question

Not the backlash you were expecting but in seawater or in fresh the displacement is the same (in weight) but the VOLUME displaced will be different.

Archemedes cannot lie! (Now THAT might get a backlash!)

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[MedMan]

Re: Idiot \'displacement\' question

Agreed. So what has Archi got to say about this? A completely full glass with an up-standing meniscus has an ice cube floating in it. What happens to the level when the ice cube melts, and why?

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[Evadne]

Re: Idiot \'displacement\' question

It depends on how warm the water is and how big the ice cube is. Personally I never put ice into neat water, always make sure it is diluted with plenty of alcohol.


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[snowleopard]

Re: Idiot \'displacement\' question

it depends on the designer. it will generally be when the boat is sitting at its design water line. there is no fixed rule on what load will bring it down to that waterline, in some cases it might be virtually empty, in others a full crew and cruising inventory.

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[bbilly]

Re: Idiot \'displacement\' question

I always thought it was the number of sheep, in imperial volume std divided by length in feet or something to do with commercial rating, it's not actually the volume of liquid that it displaces - i fink

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[sailorman]

Re: Idiot \'displacement\' question

it is the weight of "water" it displaces

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[Guest]

Daft question actually ....

Given that temp and size of jar etc., influence the answer.

But in a situation where jar is wide enough for meniscus to be un-influenced by surrounding structure - in theory the water level will not change - IF water is fresh and literally same supply as the ice cube and temp is 0......

Why ? Water is denser than ice, ice weight will displace water to the volume commensurate to ice weight as long as it is free floating.

In practice the volume will change by a tiny amount due to slight differences in the water etc. especially if the container has a narrow opening and meniscus is being influenced etc. and also temp .....

I knew I shouldn't have got involved in this one ...... now I'm going to be flayed alive !!


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[aitchw]

Re: Idiot \'displacement\' question

Thanks, Aztec.
I have recently bought a little 16' day sailer/trailer sailer. She has, I am told, a displacement of 1080lbs. She carries ballast of 360lbs leaving 720lbs for payload, structure and fittings. Allowing around 400lbs for two adults and some gear (incl. o/b) would mean her structure can't be more than 320lbs which seems to me unlikely. My Albacore weighs around 270lbs rigged to race. She is considered light for her 15' length and has no superstructure.
The two issues that I am struggling with are what is her likely trailing weight and how the hell is she likely to behave with three or more people on board. I have pottered around on gentle days with 4 adults in the Alb without problems and can't believe the little cruiser would not cope better.


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[Guest]

Re: Idiot \'displacement\' question

doubt it - displacement is volume of water then expressed in weight displaced by the boat when freely floating without oustide influences.

The design capacity is another matter and for a designer to quote a displacement taking that into account is not sensible. As far as I am aware - unlike big ship practice, yacht displacement quoted on brochures / Bristows etc. is basically as sold from dealer / factory new..... without inventory other than basic necessity.

The fact of Sea or Fresh water has no influence other than on volume of water displaced .... the weight of wtaer is same as the density used to calculate the weight will have changed ....

ie. frssh water is 1000 kgs per cubic metre, standard sea water is 1025 kgs per cubic metre.

(Now before some bright spark starts quoting harbour densities etc. etc. - local, tidal, current and other factors such as rivers entering into harbour, hot summers / clod winters etc. will influence local water density .... and could range from 995 to 1035 kgs per cu.m ......)


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[williamshugh]

Re: Idiot \'displacement\' question

Displacement is the weight of water the vessel displaces and equals the weight of the vessel.

Thames tonnage is all about load carrying capacity (sheep may be involved!?).

There is no relationship between displacement and thames tonnage. The tonnage figure is often much higher than the weight (displacement) of the vessel.

Hugh

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[paulrossall]

Take the people out before you tow it

The displacement weight will be same as the weight out of water and on the trailer. I had a 17ft boat and always put the outboard in my car as it increases car weight and reduces trailer weight which is best thing to do when towing. What make of boat is it?Paul

<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".

 

[oldharry]

Re: Idiot \'displacement\' question

The 'displacement' quoted for most smaller boats is for the dry bare hull, mast and sails if any. It does not allow for the weight of the engine (unless specified), or the weight of fuel, water, and gear such as anchor and chain, cushions, gas bottles, and the wifes handbag. Crew weight is variable (in my case upwards) so also is not included.

Do not forget in your calculations to include the weight of the trailer itself.

If you are near the towing limits of your car, load it all up and go to the local weighbridge, and for a small fee get a weight certificate to wave at the benevolent fund collectors in blue if they stop you.

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[aitchw]

Re: Take the people out before you tow it

It is believed to be a Sharky, Paul. LOA 16 LWL 15, Beam 6', Draft 1'6". I've bought her to potter around Loch Torridon and similar water during the summer. Hoping that the availability of some shelter from the rain might tempt 13yr old son and SWIMBO to sail sometimes and if not I can sail her one up and do some fishing. She is not really big enough but should be easy to trail.

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[paulrossall]

Re: Take the people out before you tow it

Have you bought a trailer with the boat? If so look at the gross weight of the trailer and then the net weight. These should be stamped on a plate. The difference between the two is weight the trailer will carry. I would have thought a decent 2 wheel (1 either side) would carry the boat given the weight of 1000 od kg that you quote. Also look at the tyres. They should say on them what weight they will carry at a certain pressure. Multiply it by 2 and that should be same as or slightly more than the gross trailer weight.
When it comes to what the boat will carry, I would have guessed two adults and one or two early teenager kids, obviously depending upon them being normal weights. I carried about eight blokes on a river once on my 17ft boat but the water did tend to come back up the cockpit drains whenever another boat went passed.
Would have thought boat would carry more than your previous one.
Have some fun with it. I used to take my 17fter to Windamere and all the family had a great time.
Paul


<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".

 

[steve_cronin]

EXACTLY....

Nothing more or less. The volume of that water depends upon it's density. So how far up the sides of the boat it comes again depends upon the density of the water it's sitting in.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[steve_cronin]

Ahh!

Can you EVER take the schoolmaster out of the man!

Is this whole set-up above or below 4C?

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[MedMan]

Re: Ahh!

Is this whole set-up above or below 4C?

The ice, of course, is below zero. Let us say, -18 degC. The water is at a room temperature of 20 deg C. They are both from the same source, let us say Thames Water Board or Coca Cola's bottled 'Pure' Water - same thing! The ice will tend to cool the water down but both will be warmed by the surrounding air. Just to make life easier, let us assume that the second measurement is taken when the water temperature has stabilised back at 20 degC. Will the glass over flow or will the level inside go down?

P.S. Relative humidity of the room is 100% and SWMBO is wearing green slippers.

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[steve_cronin]

Ahh - Umm!

It's just that, in the dimness of my memories of O level physics, water contracts as it's cooled down to 4C and then it starts to expand. Have I got that right?

Just considering the ice itself though, that part of it below the surface of the water will contract and if that were our only concern then you would expect the level in the glass to reduce. However as it melts it lets the one ninth above the surface be included in the liquid, which would increase the level.

Question is, which has the greater effect on the level, the reduction in the volume of the 10/10ths of the ice now turned water or the inclusion of the 1/9th into the volume below the surface?

Since the melting of Polar Icecaps is supposed to be raising global sea levels, I'd have to go with that, the level rising and therefore overflowing.

Now IF you'd suspended this ice within the body of the water by a thread fixed to the bottom of the beaker, leaving the surface within the meniscus unbroken, that would be a different matter!

Steve Cronin



<hr width=100% size=1>The above is, like any other post here, only a personal opinion