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Old 14 September 2014, 20:19   #1
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RIB Floatation

So I was asked by a friend and I didn't have the answer.

So RIBs supposedly have more floatation than a hard boat and common lore is they can float when swamped. I have a 7m ex-military zodiac, how much water can I take? Are we talking about the top deck being swamped or can the whole fiberglass hull below deck fill up?

Thanks
Jason
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Old 14 September 2014, 23:23   #2
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Not sure about all ribs but on my SR4 (with the self flooding hull), I had water on the deck almost to the top of the tubes and still floated fine.

On my Zodiac Hurricane 533, I left the plug out on accident and once water filled the inside of the hull, it pulled the boat down enough to have water leak in between the tube and transom, then water filled the boat but it still floated fine.

On my SRMN, I had A LOT of water inside the hull that leaked in through some cracks in the knees. Then the boat got flooded with water almost to the top of the transom and it still floated fine.
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Old 15 September 2014, 01:53   #3
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Same here. I had a foot and half section of the lower tube mount fail; water came in and up onto deck at the front and rear of the deck. Water was knee high at the back, and about a foot or more at the front; floated fine. Didn't motor worth a damn, but kept plodding along.

Found out later that slowing down and dropping the trunks would probably have made headway against the water ingress.

Another way of looking at this is from working loads: My boat (roughly the same as a Boston Whaler Montauk, which is also known for great flotation) has a max load of 4000lbs. The Whaler is 1760. If you kick the Whaler up a model to a 19 Montauk, max load goes to 2500 lbs; and 2800 lbs for a 21'.

jky
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Old 15 September 2014, 03:47   #4
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Measure the capacity of the inside of the RIB hull - that will give you the weight of water it will hold. (Volume in liters)

Measure the capacity of air in the tubes - thats what will offset the water in the hull if it floods.

If the volume of air is larger, then that will offset the volume of water and you will float.
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Old 15 September 2014, 05:35   #5
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Well many RIBs are built with the ability to purposly flood the hull to add ballast for extreme rough conditions.
so if a bigun should break over you, and fill above deck level, then at least the boat won't be instantly top heavy as the hull is already full of water. and the tubes will at least keep you floating.
Water getting into fuel would be the biggest worry at that point, no problem with a fuel tank snorkle though.
so as long as your still upright and driving ahead all the water above deck should/will pore over the stern.
I would imagine it would take a lot of weight to completely sink a fully inflated RIB?
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Old 15 September 2014, 12:52   #6
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Quote:
Originally Posted by Trimix View Post
Measure the capacity of the inside of the RIB hull - that will give you the weight of water it will hold. (Volume in liters)

Measure the capacity of air in the tubes - thats what will offset the water in the hull if it floods.

If the volume of air is larger, then that will offset the volume of water and you will float.

Not exactly, as the water in the rib would be not part of the mass, as the deck will be open, and at the transom the water will come and go.

As a rule of thumb, find out how much your boat weighs when out of the water, and for every tone you will need approx 1cubic meter of tube volume.

A circular tube of 500mm dia has as near as damn it 1 cubic meter of air in it, so will support 1 tonne of boat, irrespective of the boat being holed (or bilged to use it's technical term) as long as the tubes are intact they are reserve buoyancy.
To look at it in stability terms, your hull is not watertight, as it is open at the top, and the shape keeps it afloat when it is empty. Fill it with water so it overflows, and the tubes only need to hold up the weight of the hull material, engine and gear.

To get technical, you also need to see how much your fibreglass displaces, so if you know what the manufacturer says is the weight of the hull and tubes, you can work out how much it will need to keep it afloat.

For example, my OP 6.5 M according to Humber weighs 600 kilo, just hull and tubes.
Now fibreglass has a density of 2 tones per meter cubed so that hull will take up 300 liters of physical space, which will give 300 kilos of lift when submerged. Therefore to stop the whole lot going to the bottom you would need the tubes to hold only 300 liters of air, and completely flood the hull, and it would still float.
Add all the extras such as engines, tanks, batteries, gear etc etc. and you need extra buoyancy.

I know my boat weighs in at around 1500 kilos, and with 300 kilos of hull boyancy, I would need only 1200 liters of air in my tubes to stop it going under.
The tubes are 500mm, so I need a total of about 6 meters of tube to keep it afloat, even if I took a big drill and cut holes all through the hull deck and transom, it would still float.
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Old 15 September 2014, 12:58   #7
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Originally Posted by Bigtalljv View Post
So I was asked by a friend and I didn't have the answer.

So RIBs supposedly have more floatation than a hard boat and common lore is they can float when swamped. I have a 7m ex-military zodiac, how much water can I take? Are we talking about the top deck being swamped or can the whole fiberglass hull below deck fill up?

Thanks
Jason

So if you have a 7 m military rib, and it has a bit of gear on, let's say your all up weight is 2000 Kg.
You might have a heavy layer up hull, so let's say the fibreglass is about 800 Kg, which will give you 400 Kg of lift. ( upward thrust )
So you would need 1600 Kg of reserve bouyancy or 1600 liters of air.
If you have 500 mm tubes, you will need 8 meters of tube to keep it from Davey jones locker.

Sea water will give more boyancy, but not much, and the heavy bits on the boat will also displace water, but don't worry too much as that is it gives you a bit to play with.

Bottom line is that your tubes probably have enough on each side to support your boat if fully flooded.
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Old 15 September 2014, 14:35   #8
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Swamped

They really are incredible load carriers
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Old 15 September 2014, 15:22   #9
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I've been swamped to the top of the tubes, completely full of dive gear too. Floated and even motored, albit slowly.

The fuel vent is the weakest link in mine. I got about a cup of water into the full through there that I was later able to siphon out. Extending running probably would have sucked all of that into my fuel/water separator and overloaded that and that would have been the end of motion in the ocean

As pointed out, you need to figure out the total mass of your boat, divide by the average density of fiberglass/stainless (2000 kg/m3 for GRP or thereabouts, 2700 kg/m3 for stainless so on average your boat is probably 2200 kg/m3) relative to fresh water (1000 kg/m3). So net you are about 1200 kg/m3 negative if your tubes were deflated. Based on the dry weight of the boat how many m3 are you?

Now figure the total tube volume, do you have more or less buoyancy in the tubes than you need to support that water weight of fiberglass and steel? Whichever is greater determines whether you sink or float. I would guess you probably float like me.
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Old 15 September 2014, 16:38   #10
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I've been a diver on an inflatable like that in college but it was a SIB not a RIB. Lots of good info here, I hope to never actually need it.

Official specs on my boat say 3432 liters for tube volume. With fuel specs say approx 3700lbs lifting weight, I'm one engine and 65 gallons of fuel lighter than that, 500-600lbs lighter. 4400lbs capacity including outboard and fuel, actually more like 5000 with the underweight motor and fuel. 1000lbs of engine and fuel gives me a 4000lbs working capacity. That's about 500 gallons of seawater and I'm still within working limits.

I'll venture to say that sinking is not my biggest concern.

Jason
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