How high will it float.

Hi
Love the forum
I have a stationary floating office on a pontoon i have almost finished constructing.
The flat deck is aluminium 6m x 2.5m and weighs 500kg. Under this is 18 210 litrem plastic drums. On top of the deck is an office 4.5m long. 2.5m wide and 2.3m high. Weight of the office is 400kg.
So this gives me 900kg on floats that can supply just over 3700kg of bouyancy.
The bit I can't work out is that I want this dock to float level with the Marina arm it is tied up to. This is 40cm above the water. The thickness of the deck on the pontoon before the drum starts in it's cradle is 6cm.
Each drum is 94cm high by 48cm wide, and they are in 3 rows (6,6,6) on their side,

I' m trying to work out how much water, if any,I will need to put in each drum to lower the deck to 40cm above water level, making sure I still have plenty of flotation for the 900kg deck/office structue and a max of 8 people (700kg) who may visit the office at once. I may even need to add more space between the drums and the deck to raise the whole thing. I feel it would be good to have some water in the drums to counter the height of the office.
The office is aluminium construcion with eveything above 1m kept as light as possible.
Is there a formula I can use.
. Any input would be much appreciated.
Chris

 

How much do the floats weigh? Assuming the floats weigh 400kg, they will be 1/2 submerged supporting a total weight of 2000kg. Ergo, if the floats are 1 meter tall they will have a draft of 0.5 meters.

 

Drum weight

Each plastic drum weighs 9.5kg, and they are all on their sides.

 

Changed specs

Sorry about that, I had 2 too many drums in the equation. They are in 3 rows of 6 on their side about 10 cm under the deck.

 

Okay, so the total weight is 1771kg. Based on that total weight and floats all bearing the same amount of weight, only 44.2 percent of the drums will be under water. if the drum end has a circumference of 100 centimeters, 44.2 centimeters of the end will be submerged.

 

But will it float level? Where is the center of gravity of the pontoon and office?

 

Position of office

Good Call. Office starts .5m from one end and is 4.5m long,(2.4m wide) leaving 1m at the other end.

 

Dave and Hoyt: Free surface effect, if he ballasts all of the drums with water....

 

Does that mean ballast is a no go, as it can all move the same way at the same time.
Maybe I am better off just adding 2 more drums to give it an extra 400kg of lift, so my total lift is 4100kg and if I only use 45% of that as a safety, then with the following:
Base 500kg
Drums 190kg
Office 400kg
People Max 8 at 80kg each 640kg.
Total weight 1730kg

45% is 1845kg
Each drum has a circumference of 185cm (Drum width 59cm)
The height of each drum is 92cm.
2 outside rows of 6 with 5 across at each end and 4 across down the middle 4 rows. so the drum placement across the 2.5m would be 5,4,4,4,4,5 over the 6m (If I can find a way not to put 2 extra drums in it would make life easier, but I will put them in in a heartbeat if it's a safety thing.
Drums are laid lengthwise on their side.
The level deck is 10cm above the top side of the floats.
Office is moved to centre.

One 6m side is tied to the walkway.
The Marina is fully enclosed and swell is not an issue.
In the event of Cyclone the office unbolts to lie flat on the pontoon.

I really need this to float near level with the walkway at 40cm above the water.

Any hints.

 

Hi, Chris,
I would try to help you. But I dindn't understand your description of the arrangement of the drums.
Could you post a sketch?
Are the drums shaped like cylinders (oil drum) or more like old beer-barrels?

You are specially interested in the height of the deck. But you should also pay attention to heel and trim and the change in draft when people enter the deck/office! The relation of 640 kg mass of people and 1090 kg mass of pontoon without visitors shows, that mass of people will have a strong effect.

For this purpose the Positions of the drums also must be known for calculation.

 

Drum layout

The drums in the picture go over to the far left and there will be 6 in a row. 6 in the middle and 6 to the right. They will go to the edge at the front and rear.
We would never get 8 people at once, as they have to enter a door to get into the office.

 

Look at this calculating sheet (I modelled your pontoon roughly):

http://www.bootsphysik.de/rechner/bootxp.php

First click top right button "English", then "choose / alter frames"; in the last line you will find a cross section of your pontoon (if I understand ist right).
Click this image, if nessecary "back to calculations", "restart frame"; if nessecary, "English" again.

Top right you will find lenght, beam, displacement and draft (1730 kg, "all people on board"). Right column input box "Mass to Load / Unload"; first type 640 in and then click the button "unload" (4 rows under input box).

The draft will change from 225 mm to 161 mm.

 

Let 3 people enter the empty pontoon at its edge.
3 times 80 kg = 240 kg, transversal distance to CG is -1250 mm (half beam in starboard direction).

After click "load" you will see the shift of the new overall CG.

Click "find equilibrium"; the graph shows an heel angle of 3.5 deg.

That means, the edge of the pontoon on the starboard side will drop about 10 cm caused by entering 240 kg at this point (in the middle of the lenght).

Also should be calculated trim effect, but this is not possible with the linked calculation sheet.

Here is a roughly similar pontoon in 3 dimensions (draft, heel, trim):
http://www.bootsphysik.de/rechner/boot_engl_pont.php

Warning: a quick change from one calculation sheet to the other won't work, because the server uses the same session variables filled with "wrong" values. If possible, take another browser or close browser and try again half an hour later.

 

Extra drums.

If the draught is 225mm, then since the overall height of the pontoon out of the water is (drum width 587 plus deck width 100) 687mm then that gives us a deck height from waterline at 462mm, which is pretty close to where I want to be.
Does this sound correct to you?

Most people will enter the office from the 2.4m end.

Should i put an extra drum at 1 end (4 across instead of 3) to add a bit more bouyancy at the entrance end (the 2.4m end to the right, as shown in the outline sketch).

Thanks for your help.

 

There are differences between your pontoon and the modelling on the page.
The longitudinal spaces between drums are not considered.

The model has got 3 tubes, each 600 cm long and with 59 cm diamater.
6 of your drums: 6 * 92 cm = 552 cm. A "loss" of 8%.

Increasing weight of the pontoon with 8% (140 kg) leads to a draft of 24 cm, which will be nearer.

(I assumed fresh water, in case of sea water the draft will be a bit smaller.)

Another point.
In normal use of this drums they are filled with liquid content. This causes hydrostatic pressure from inside, which is a very different case to what you are going to do.

The drums are made of plastics and not constructed to withstand a pressure from outside. You must consider they could be deformed after immersing. This would mean a loss of buoyancy.

Take one of them and submerge it to test it. Enable the air to escape the drum without entry of water. Look at the shape resistance of the drum.