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- Thread starter ShipsDog
- Start date

the end will anchor the fender then letting other people know where you lost your life... a bunch of fenders may be good enough for you but get the crew a nice shinny life raft....

even an etap should not carry out a long journey without a life raft. i understand the point that the boat may be more robust than the life raft but the idear is to stay dry and as warm as possible.

do your crew a favour and get a liferaft...

If you calculate the volume of the fender in litres and then say 1 litre = 1kg of displacement then subtract the weight of the fender from the displacement that is very roughly the the weight it would support at the point of going under. What you have to do is displace enough water from within the hull to support the boat.

What's the trailing weight of yours? You might be a bit put off how many you need to do the job.

Let me have a go.

Assume your boat has a displacent of 8,000 kg - typical for a 30 and a bit footer.

So the boat weighs in air 8 tons but weighs nothing in water - because it floats.

So assume it is now holed and no parts of it have any bouyancy. (Fuel tank will have bouyancy - even if full because diesel is lighter than water) and assume that the woodwork inside has no bouyancy.

8,000 kilo is about 8,000 litres of seawater or 8 cubic metres.

Each fender is a cylinder 60cm by 20 cm (I guess) (Pi r sqd H ) gives 3.14x100x60 or 18,840 cubic centimetres. or 0.018 cubic metres.

To achieve 8 tons of displacement with fendres we need 444 fenders at 20 quid each. or 8 thousand pounds which is the cost of 6 to 10 liferafts- depending on specs etc.

BUT it was assumed that all the bits of the boat, which displace 8 tons (weigh 8 tons in air) would have no residual flotation or bouyancy in the water. The lead keel and the engine will have about the same result.

The hull skin might have a lot of bouyancy if an Etap, none if a steel boat but there will be insulation.

So I will go for a very rough guess and say about 250 fenders should do the job for a typical 32 footer - or about 3 liferafts in terms of cost.

I know someone will find a flaw in my arguement and object to the wild approximations, come on but don´t be excruciatingly pedantic.

[ QUOTE ]

Let me have a go.

So I will go for a very rough guess and say about 250 fenders should do the job for a typical 32 footer - or about 3 liferafts in terms of cost.

I know someone will find a flaw in my arguement and object to the wild approximations, come on but don´t be excruciatingly pedantic.

[/ QUOTE ]

Would be interesting to watch the deck pop off as the boat sunk. Somehow I doubt the deck to hull join was ever designed to take those sort of loads

I had a similar idea with 2L lemonade bottles. A colleague drinks them like a fish so I started collecting them, the idea being to stuff them under all my bunks etc.

Eventually got round to figuring out how many I would need. My boat is (say) 1 tonne (1000kg) which would need 1 cubic metre of bouyancy. So if each lemonade bottle displaced 2kg I would need 500 !!!!!!!

Eventually I started to bin my collection, my binmen must have thought I was a lemonade freak!

I wear a life jacket

Regards

Cameron

Assuming a 32 footer displacing (weighing in air) 8,000 kilos.

I guess;

Lead 2,000 kg (keel)

Steel (engine,winches etc) 300 kg

wood 600 kg

Aluminium 300 kg

GRP 4,900 kg

totalling 8,000 kg.

Recalculating each amount for its density in water (the wood produces a negative number) I come up with a "weight" in the water (that is totally submerged) of 3,850 kilos in terms of the bouyancy required to stop it accelerating to the sea-bed.

An XM Majoni fender (25 inches by 8 inches) from Pumpkin Marine has a volume of 0.0184 cubic metres at a cost of GBP 13

So - to keep your boat afloat when totally flooded you need 209 fenders at a cost of GBP 2,717 -

OR you could buy 3.4 XM 4man valise liferafts for the same cost (3.02 6 man liferafts)

So, the short answer is enough fenders to buy 3 liferafts.

(I made no allowance for bouyancy of mattresses,cushions etc - so the answer is probably closer to two and half liferafts)

Indeed-

The stanchions and wires were not designed to take 200 fenders and the upward load on a sunken boat.

You'll have to do your sums again to calculate it on incremental fenders rather than total fenders to get a proper cost to base a decision on.

Why not use Helium.. isn't that somewhat lighter than Oxygen / Air .. Or use the eberspacher vent and use hot air...

Ive often had the same thought but when conditions are bad i cant imagin how i would get in it, same goes for a life raft.

Another point, my tender goes on deck when im on passage its quite heavy but still relies on the sucktion of the water to keep it there,Once i had it unlashed on deck when sqalles arrived picked it up and launched it for me and it weighs about 150kgs. Happly its painter was attached,

The point is for the dingy to be a life boat it would have to be towed or very very solid on a very big boat.

For small boats at least mine when ive been towing my tender and conditions have picked up it either crashes into my transum reminding me its there or the towing line goes slack then tightens up with a jerk risking breaking somthing and loosing the tender.It dosent matter how long i make the tow line.

As in other posts you fail to read or understand the question. If an Etap or Sadler is unsinkable due to the bouyancy built into the hull, then by adding bouyancy then surely a "normal" craft can also be made unsinkable. As in bouyancy bags or tanks in a sailing dinghy. An easy way to do this would be by adding fenders. Now how do I work out how many I need?

[/ QUOTE ]

I think the "formula" is dry weight of boat (plus gear) in tonnes times 1½ to give volume in cubic metres of bouyancy needed to keep her afloat when swamped. so a boat weighing say 5 tonnes needs around 7½ cu metres of bouyancy added - that is one hell of a lot of fenders!

As a point of interest my Sadler does (or did ) actually float when totally swamped / sea cocks opened to the sea. - seems she was one of the test boats used for the DTI testing way back in '86. I hope teh foam is still O.K. but the little I have exposed seems to be as good as the day it was put in.

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