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The weight of my boat?


man1nvan

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I am trying to work out the weight of my boat as follows

 

1 cubic meter of water is very close to a ton

A boat displaces the volume of water equal to it's mass

so 

If i measure the depth of the boat from the baseplate to the water level as close to the front and the back of the boat as i can . then average them to give me depth in meters  (X)

Then work out the foot print of the boat in meters (Y)

then X x Y = metric tons

 

Is this right or am i just an idiot?

Sorry . i know the swim isnt full depth  but that is just a bit of contingency....

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Very, very roughly ( and gives an 'over estimate') 1 metre of boat = 1 tonne.

(Have you got full / empty water tanks, pump-out tank, fuel tank ? That can give you over a tonne of difference. Is it fully fitted or a sailaway ?)

So, for example a 57 foot boat would be about 17 tonnes

Edited by Alan de Enfield
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When I did this displacement calculation I took the length of the pointy bits at each end and divided those volumes by two and added the volume of the full width middle bit. Reckoned these could be approximated as triangles in area. Otherwise as you describe Ignored the counter as this is only in the water a couple of inches. Came to around 16 ton if I remember correctly. The crane driver said it was 12.5 ton when it went in the water as a sailaway with full water and fuel tanks, so about 3.5 ton of fitout inside.

Jen

Edited by Jen-in-Wellies
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divide up the straight parts and the curved parts.

an circle or ellipse have the area of a X b X pi, or A X B X 0,785

A triangle A X B X 0,5

so a curved pointy part not round at the end and not a triangle A X B X 0,64

A long ton is 1016 kg or 2240 lb

fresh water density is 1 (0,99995 kg/dm3 ) at +4C and 0,998 at +20C

Edited by Dalslandia
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why are peeps discussing 'long tons', and 1 cu.m. of water being approximately 1 ton?

 

in case anyone missed it, all principal industries have been using tonnes for decades, and 1 tonne can be defined as the weight of 1 cu.m. of fresh water (subject to standard conditions of course).

any crane driver or truck driver will automatically use tonnes.

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and ships in tons, volume that is, register ton, netto, brutto ... that is 100 cubic foot, 2.83 m^2

and feet fot and inch, lb's pint gallon ... as long as it stated what unit the numbers is in, I don't care. 

but you are right metric system is coming to the imperial countrys also, inch by inch. :) 

Edited by Dalslandia
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10 hours ago, koukouvagia said:

This topic has been discussed many times before.  

 

If anyone wants a copy of the full method PM me.  You have to be a mathematician to understand it.  I certainly don't.

That's a neat approach. It does of course measure the mass of the boat, not its weight.  The maths in the formula quoted is fairly straightforward, though measuring the acceleration and deceleration (which won't be constant) is a bit tricky. The combination allows  you to eliminate the drag force from the answer. 

If you move your boat to the canals on Mars then the mass will stay the same, but the weight will not. (But most scales eg bathroom scales are calibrated in kg, not the more correct Newtons) .

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14 hours ago, Dalslandia said:

The Thrust is not constant either, can't use static thrust to calculate dynamic thrust, and the dynamic thrust is reduced with speed, T=P*Eff/V

I'm going to have to some searching, most of the articles on internet are about aeroplane props which are not relevant. What is V in your equation? Presumably the speed of the water past the propeller (which is not zero even if the boat is stationary)? I think turbulence factors will also be important, and reduce efficiency when rpm are suddenly increased when the boat is moving slowly or is stationary.

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Yes, V is speed or velocity, 

thrust is Power * efficiency / speed

and you see that thrust is reduced with speed if Power and efficiency is held constant.

efficiency is increased slightly with speed normaly untill cavitation come in.

efficiency is the result of boat speed not the water.  

propellers have there openwater efficiency

and hull its hull efficincy, with the propeller doing its job at the back.

Imperial Power is a horse lifting 550 lb-ft/sec or 33000 lb-ft/min (76,04 kg-m/sec)

metric horse lift 75 kg one meter in one sec.

5 hp will then make 5*75*0,5/1,8= 104 kg of thrust at 3.5 kts (4 MPH) with an efficiency of 50%

thats for theory.

in that case of have 50 kg of thrust at zero speed, then let go. in practise the prop will let the Engine rev more at higher speed, if perfectly governend (sp) the rpm will be held constant but fuel and Power will be reduced with higher speed due to unloaded prop, and the prop will probably have a better bite at some speed then static.

to make that test correctly to get the mass of the boat, the thrust need to be monitored and held constant.

 

Ps, I do calculate propellers for airplanes.

 

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Hm. My problem with that formula (which is an energy or power equation) is that as V tends to zero it would give you an infinite thrust. But we know from observation that the thrust at very low speed isn't very different from the thrust when the boat is held stationary. So I don't think it makes sense to hold efficiency constant ....

To put it another way, at zero speed the energy from the engine goes into pushing the water backwards, with equal and opposite thrust forces on the water and the boat. No power is transmitted to the boat.  As the boat speeds up more of the power from the engine goes into the boat (force x speed), as it accelerates, until offset by drag, with rather less going into the water.

The other issue (related to overall efficiency?) is how much of the power is converted to bulk kinetic energy (whether of the boat or the water) and how much is converted into noise and heat (via water turbulence).

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correct if the efficiency would be constant, at zero speed the efficiency is zero if we see to the work of moving the boat and not the mass of water.

What I meant is that there is so many variables that in practis it will not workout to give an correct number of the mass of the boat.

We can also use the THP, Thrust HP instead of shaft HP*eff. 

THP is thrust*V, so if thrust is held constant, THP goes up with speed, or if THP is constant, thrust is going down with speed.

Edited by Dalslandia
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On ‎24‎/‎01‎/‎2018 at 09:59, Alan de Enfield said:

Very, very roughly ( and gives an 'over estimate') 1 metre of boat = 1 tonne.

(Have you got full / empty water tanks, pump-out tank, fuel tank ? That can give you over a tonne of difference. Is it fully fitted or a sailaway ?)

So, for example a 57 foot boat would be about 17 tonnes

Mine's a 57ft boat but it's closer to 30 tonnes

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