capnsensible
Well-Known Member
Of great interest to Submariners......
Weight vs. Displacement
- Thread starter GrandadPig
- Start date
ylop
Well-Known Member
If you are taking it to / from a weigh bridge then you will be over the limit for that journey (if it weighs over). It may be useful to note that journeys to/from the nearest public weigh bridge are exempt (burden on you to prove that’s where you were going - an appointment may be useful, at the least it programmed in your satnav). it doesn’t mean you’ll get away with being clearly dangerous (the same as you wouldn’t get away with being clearly unroadworthy on way to an mot). When returning from one - you have to go to the nearest place that would not cause an obstruction to lighten the load (that might be the lay-by outside the weighbridge and mean making multiple trips with the contents of your boat!).
many years ago there was a method in PBO (sketchbook?) to work out the weight of the load using some clever maths and physics which you could do at home with bathroom scales and moving the bot back n forth on the trailer.
many years ago there was a method in PBO (sketchbook?) to work out the weight of the load using some clever maths and physics which you could do at home with bathroom scales and moving the bot back n forth on the trailer.
Bouba
Well-Known Member
Some people say that fiberglass absorbs a third of its weight in water...which might be why some boats weight more than their owners were expecting
geem
Well-Known Member
From what I can find in scholarly articles the absorption limit on polyester resin 0.2-0.5%. More likely failed cores causing water retention in cored hulls. Since the cores like balsa can hold lots of moisture and they are often a considerable percentage of a cored hulls thickness
Bouba
Well-Known Member
It’s a controversial subject...which is why I prefaced my post with ‘some people say...’ but if your boat weighs more now than when it was new and you can account for everything carried in or on...then you have to think
Slowboat35
Well-Known Member
I'm surprised this even comes up for discussion. The answer is unequivocable and proven these 2500 years or so.
"In fluid mechanics, displacement occurs when an object is largely immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, and from this, the volume of the immersed object can be deduced: the volume of the immersed object will be exactly equal to the volume of the displaced fluid."
Thus a boat displaces it's current weight, no more, no less. Whatever that weight is at the current time. This includes everything on board. It bears no relationship to what the manufacturer stated for an empty boat except that their figure is most unlikely to be less than present reality.
How much more depends on how much toot owners have placed on board since build.
If your scales say you weigh 16 stone you can't rationally argue that you were once 12 and the rest is in the lockers, tanks and bilges and somehow doesn't count. (Though I've heard many such attempts over the years)
What about when the 'manufacturer's agent' slapped your arse and declared "Seven pounds four ounces..."? What is your 'displacement' now aged 50?
You displace what you weigh.
End of...
"In fluid mechanics, displacement occurs when an object is largely immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, and from this, the volume of the immersed object can be deduced: the volume of the immersed object will be exactly equal to the volume of the displaced fluid."
Thus a boat displaces it's current weight, no more, no less. Whatever that weight is at the current time. This includes everything on board. It bears no relationship to what the manufacturer stated for an empty boat except that their figure is most unlikely to be less than present reality.
How much more depends on how much toot owners have placed on board since build.
If your scales say you weigh 16 stone you can't rationally argue that you were once 12 and the rest is in the lockers, tanks and bilges and somehow doesn't count. (Though I've heard many such attempts over the years)
What about when the 'manufacturer's agent' slapped your arse and declared "Seven pounds four ounces..."? What is your 'displacement' now aged 50?
You displace what you weigh.
End of...
Tranona
Well-Known Member
But that is not the issue here. The OP wants to know if the published displacement is a good indicator of the actual weight when calculating his towed weight. The answer is NO, because you don't know what was included when the displacement(weight) was calculated (or actually measured). therefore the adv ice is to take the rig to a weighbridge to get the actual weight of the boat and trailer. If it is above the limit for the towing vehicle then it may be possible to reduce it by removing some objects from the boat.
ridgy
Well-Known Member
If you have a lot of towing in your future then for an investment of £170 one of these will help. Just drive over it with each trailer wheel and add the nose weight.
You can do your car with it too if worried about the overall weight..
Reich Portable Caravan Weight Control - Yellow 1000kg - Homestead Caravans
At the lower end of the scale for (pun intended) of if you just want to weigh the trailer these are cheap and may have other uses:
300kg Heavy Duty Digital Postal Parcel Scales Platform Postage Shipping Weighing 732030806313 | eBay
You can do your car with it too if worried about the overall weight..
Reich Portable Caravan Weight Control - Yellow 1000kg - Homestead Caravans
At the lower end of the scale for (pun intended) of if you just want to weigh the trailer these are cheap and may have other uses:
300kg Heavy Duty Digital Postal Parcel Scales Platform Postage Shipping Weighing 732030806313 | eBay
GrandadPig
Well-Known Member
Looks like I might have to go down that route as the missus refuses to tow it. It’s just twice a year from lift out and spring lift back in. She lives in a swinging mooring during the season. About 25 miles each way but no means no. I have some vision but am registered blind so driving myself is not allowed
Bouba
Well-Known Member
If you told the Post Office that you want to send it Parcel Post, they would be obliged to weigh it for you
GrandadPig
Well-Known Member
Brilliant idea!
geem
Well-Known Member
We did quite a major refit in 2020. New paint in the hull and deck. New aluminium toerail, windows, etc, etc. We have a blue stripe at the waterline that was gelcoat but the paint was last sprayed with two pack paint in 2012. So we had a painted blue stripe. Once the paint was all sanded back it was evident that the blue gelocoat had failed. It was cracked in numerous places. We had it all removed with a router. This was then filed with epoxy and glass before the whole boat was resprayed in Awlgrip. As part of the work, we had the hull moisture reading checked. The boat was dry. Pretty remarkable considering its age of 40 years at the time. The hull is Airex cored. 6mm outer skin, 25mm Airex core, 6mm inner skin. No moisture problem. We do carry a huge amount of gear onboard but we float above our original marks. Pretty happy with that.
I once sailed on a 70ft sail training boat that was a cored hull. Apparently that was way heavier than its original build as the core was saturated. It does happen.
Last edited:
davidej
Well-Known Member
Archimedes would be turning in his grave if he could read this!
Refueler
Well-Known Member
Oh Dear !!
This is true but is not related to OP's question. The immersion you are talking about is FULLY immersed ... NOT just floating.
If a body is floating then VOLUME of water displaced will equal the VOLUME of the object in the water - with Density - you can then calculate its weight in the water.
If you fully immerse the object so its totally in the water = this does not allow you to calculate its weight - it only gives you VOLUME of the object.
For nigh on 50yrs I have been providing Draft Survey Services to the Shipping Industry and this is a fundamental fact.
So much so that there was a Company in Europe trying to convince Terminals to buy their Load Sensors and literally outlaw Draft Surveys. They sent out videos of a heavy block weighed .... then placed in a container of water. The displaced volume was then read and they used Density 1.000 of FW to say the weight of the block. Their claim was that the Terminal then lost cargo ..... they ignored the fact that the block did not float ...... Total rubbish !! Even a child can see the error.
ONLY when the body or object FLOATS.
Don't believe ? OK - Take a container of water .... known volume of water. Now place a brick in the water. Now see new level of water indicating a new total volume. Subtract first from second ... to get volume of brick. If FW multiply that volume by 1.000 to get weight. Oh Dear .... where has the real weight gone ??
Now take an object that floats ... do same again ... This time the volume is related to the weight because Physics says the water is supporting the object equal to its weight.
Apologies to OP ...... I could not let this go uncorrected.
Bouba
Well-Known Member
That is very interesting. Just a quick question…can you also get the weight of a floating object when placed on a sea? Since instead of supporting the object the water could just spill off somewhere like a stream or river a thousand miles away? And therefore are you just measuring buoyancy or is that the same thing?
Refueler
Well-Known Member
I have special Hydrometers that give me the Water Density. Its a special Glass version designed specifically for the job, unlike the metal Load Line Hydrometers.
Of course if the body of water cannot be viewed in terms of changed level or volume - then it comes to using the Draft Marks of the ship, along with Hydrostatic data for that vessel. Therefore any random large object without markings such as draft marks cannot be determined.
Its why a yacht cannot be calculated by draft ... unless you have full Hydrostatics and draft marks etc for that yacht. We could only estimate and TBH - I wouldn't do it.
Slowboat35
Well-Known Member
Dear me! Where to start!
"You displace what you weigh".
d'uh oh! Isn't that exactly what I said? So why the haughty 'correction' to something that wasn't in error?
We are talking about boats you know, and they do tend to float...
Oh dear indeed!
A pretty uncharitable and excessive response to an incorrectly chosen cut and paste - as my text below it made amply clear and accurately described the floating situation, so it is perfectly obvious that I wasn't incorrect, merely (carelessly) pasted the wrong quote...
"You displace what you weigh".
d'uh oh! Isn't that exactly what I said? So why the haughty 'correction' to something that wasn't in error?
We are talking about boats you know, and they do tend to float...
Oh dear indeed!
Chiara’s slave
Well-Known Member
There’s something seriously wrong with one if it doesn’t float. However, that isn’t the OPs problem. All he needs to know is how much his rig weighs.
Refueler
Well-Known Member
Your initial was in error. If you think my post correcting you was rude or such ... that was not intended - its your reading and interpretation.