Swamped monohull

[JD-MDR]

I have a question. How do you figure out how much floatation would I need to keep my boat afloat . I'm wondering about inflating the dinghy (10'6 merc) inside the cabin. Would that keep my 10,000 lb afloat. I hope this isn't a dumb question but I would hate to abandon the boat.

[StuM]

VERY roughly and disregarding lots of factors :
10,000lbs of water is about 1000 gallons (@ 10lbs per Imp gallon), which is about 160 cubic feet.
Assume 25ft of tubing in your dinghy with a diameter of 12"
Volume of air would be about 78 cubic feet, so you'd need roughly two of them.to provide 10,000lb of bouyancy.

[jimbunyard]

25' x 12" diameter tubing equal 5000 lbs displacement?

36 (6 squared) x 3.1415 = 113.094 sq in x 300 (25 ft in inches) = 33928.2 cu in divided by 1728 (cu in/cu ft) = 19.634 cu ft x 62.34 lbs/cu ft = 1224.0069375 lbs displacement per tube.


So I come up with 8.16987 for 10000 lbs flotation, which doesn't seem right either...

But someone else should check the math.

[Don C L]

I am just wondering how many decks and cabins are strong enough to support the ballast weight before they get ripped off from the hull. Do you think you could lift your boat by the deck and cabin?
Interesting idea though. Never thought of it.

[Anders]

You should not need to offset the entire displacement since a lot of it will float.
I guesstimate that if you offset the keel the boat should float high.

[LittleWing77]

Remember options like watertight (flotation) bulkheads. Some offshore boats are specifically fitted with sections that, when the section is closed off (sometimes they are permanent, sometimes temporary) seals air in and water out, providing emergency flotation.

I've only ever heard of them on offshore race boats, never seen them myself.

Anyway, I'm thinking something like that might be more reliable than trying to inflate a dinghy under extreme conditions, considering what a PIA inflating a dinghy is, just sitting on a flat dock on a fine day...

Fair winds,
LittleWing77

[Tetepare]

I’m thinking if the decks are awash I’d rather be in the dinghy.

[Don C L]

It's pretty tough to put a hole in a Cape Dory in any case.

[Orion Jim]

Originally Posted by Don C L

It's pretty tough to put a hole in a Cape Dory in any case.

That’s the truth but I always carry my life raft and an EPIRB offshore just for ha ha’s. Don’t want the kids to get their inheritance too soon….

[StuM]

Originally Posted by jimbunyard

25' x 12" diameter tubing equal 5000 lbs displacement?

Doh, diameter/not radius
So it's only about 20 cubic feet, not 78
So yes, you would need 8, not 2 !


(Sorry about that, chief)