Displacement

B

Baggywrinkles1

Well-Known Member
Joined
1 Jul 2009
Messages
70
Visit site
Does anyone know the formula for displacement? I am looking at a barge for sale and it is 30 cm too high for some bridges and my idea was to pump in water into ballast tanks in the stern to lower the boat for a temporary navigation of a bridge. The boat weighs 60 metric tonnes so how much water do I need for each centemeter? Lowering of the wheelhouse is not an option.
 
Baggywrinkles1 said:
Does anyone know the formula for displacement? I am looking at a barge for sale and it is 30 cm too high for some bridges and my idea was to pump in water into ballast tanks in the stern to lower the boat for a temporary navigation of a bridge. The boat weighs 60 metric tonnes so how much water do I need for each centemeter? Lowering of the wheelhouse is not an option.
Click to expand...

Try working this out:

http://www.ehow.com/how_5697769_calculate-ballast-weight-displacement.html

Good luck - I have a feeling you will find the idea unworkable as I suspect the amount of ballast needed will be greater than easily available tank space, not to mention the filling and emptying issues.
 
boatone said:
Try working this out:

http://www.ehow.com/how_5697769_calculate-ballast-weight-displacement.html

Good luck - I have a feeling you will find the idea unworkable as I suspect the amount of ballast needed will be greater than easily available tank space, not to mention the filling and emptying issues.
Click to expand...

The Tuesday Night Club of narrowboaters have weighed their boat down with bricks anddustbins full of water to get through particulalry tight places but ISTR it was not somehting done lightly but needed pre planning. Ballast tanks would need to be pretty large, might there be some mileage in adding permamnet ballast if you would need to "duck under a bridge" often and live with the extra draught?
 
A rule of thumb for this forum's full length favourite boat is:-

1 ton of ballast = 1 inch displacement, so for a bigger, wider boat it's got to be an awful lot of ballast.

A while ago there was a series of pictures of a smaller dutch barge being ballasted to get under Osney bridge.

Was it on this forum??
 
Immersion

You need to calculate the TCM, ie tonnes per centimter immersion, which is a formula involving the displacemnent, length, breadth, and draught.
There is a very good little book 'Kemp and Young stability' which is used by Merchant Navy officers as a basic 'crib book'. It explains it all as simply as possible. Maybe available via Amazon or similar.
I've forgotten it all now - summink to do with age !!
 
Re Displacement

Thanks for all the comments! and advice. After much thought having stood with 6 friends on a bow of a yacht to raise the prop out of the water, my thoughts are I am NOT trying to lower the whole boat in the water only one third (aft) When I see these cargo barges go down in the water by several metres and I am only asking for 30cm (1 foot) at one end of the boat. How about a large collapsable bag that could hold 2/3 tonnes of water, pumped up by a pump, 3 tonnes of water = 3000 kilo (thats 30 blokes!) so guys work this one out !!!
 
Last edited:
Are you seeking this information in order to make a purchasing decision? If so, you really should be taking advice from a naval architect rather than making casual enquiries of us gentlefolk.

As for the 6 guys on the bow of a yacht thats a completely different ball game - completely different underwater shape with thin pointy end and blunt back end - probably a fin keel or maybe a couple of bilge keels etc etc.

In the words of the Dragons - I'm out !
 
Re Displacement

boatone said:
Are you seeking this information in order to make a purchasing decision? If so, you really should be taking advice from a naval architect rather than making casual enquiries of us gentlefolk.

As for the 6 guys on the bow of a yacht thats a completely different ball game - completely different underwater shape with thin pointy end and blunt back end - probably a fin keel or maybe a couple of bilge keels etc etc.

In the words of the Dragons - I'm out !
Click to expand...

Looks as I might have to! Just trying for free first!!
 
I'd be very carefull doing this especialy with a barge, as B1 has intimated its not quite as simple as standing a couple of blokes on the bow of cruiser. The water needs to be contained and managed, if for instance you flooded the bilge with a few tonnes of water but then you and your crew all walked to the same side the water will move with you, and then it will continue to move until over you go, a flat bottomed narrowboat only needs one tonne of water to become unstable then this happens:
IMG_2077.jpg
 
If you are going to ballast with water, only do so with properly subdivided purpose made ballast tanks. Free surface is not nce in a narrow and shallow hull, see Herald of Free Enterprise. Your freeboard will be, potentially, seriously compromised (HMS Captain) and even on the non tidal bit of the river if you have only a foot or so of freeboard that can increase the downflooding risk with only a little heel (and remember that possible free surface) As said by others I would be talking to a naval architect about this one. I used to be able to do the sums, and there will be others here far slicker than I. For a one foot height difference that represents a susbtantial change in draught and you may find that the cost of reducing your normal air draught is cheaper, or safer, than ballasting.
 
further thoughts

Take a 12m x 4 m boat with a hull draught (not including stern gear) of 0.6 m, and a block coeficient (box ratio) of 0.6.

The displacement would be 12 x 4 x 0.6 x 0.6 = 17.28 tonnes.
Increase the draught to 1m and you get 12 x 4 x 1 x 0.6 = 28.8 tonnes
Thats an increase of 11.52 tonnes - a whole lot of ballast. It gives a 'Tonnes per Cm' immersion of .23. thats nearly a quarter of a tonne for only 1cm sinkage.
Just 'ball park' figures but gives some idea.
 
Google is your friend.


Tonnes per centimetre

Tonnes per centimetre (TPC): This is the mass required to increase or decrease a ship's mean draft by 1 cm. This value varies only according to the waterplane area (Aw), and the waterplane area can vary according to the ship's draft. Therefore, the TPC can vary according to the ship's draft.

TPC = Tonnes per centimetre immersion
TPI = Tonnes per inch immersion



3.2.4.a. Solved problems

3.2.4.a.(i) Example 1

A rectangular barge having a length of 20 m and breadth of 12 m is floating in salt water (ρ = 1,025 kg/m3).

What is its waterplane area?
What is its TPC?
Find the waterplane area:
Aw = (length × breadth) at the waterline = 20 m × 12 m = 240 m2
Find the TPC:
TPC = (Aw×ρ) ÷ 100 = (240m2 × 1.025) ÷ 100 = 2.46 tonnes
This means that loading 2.46 tonnes on the ship will increase the draft by 1 cm. You can also say that unloading 2.46 tonnes from the ship will decrease the draft by 1 cm.
 
Baggywrinkles1 said:
Does anyone know the formula for displacement? I am looking at a barge for sale and it is 30 cm too high for some bridges and my idea was to pump in water into ballast tanks in the stern to lower the boat for a temporary navigation of a bridge. The boat weighs 60 metric tonnes so how much water do I need for each centemeter? Lowering of the wheelhouse is not an option.
Click to expand...

Wouldn't it be easier to modify the wheelhouse by making the roof removable and folding the windows down?
 
You must log in or register to reply here.

Other threads that may be of interest

T
Notices to Mariners - Week 3
Replies
0
Views
445
tillergirl
T

Share this page

Top