How much pressure from the water on the hull?

BobnLesley

BobnLesley

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We have been trying and failing to remove a leaking diesel tank from the boat, which has been installed by simply wedging it tight into the bilge void (we''ve a long keel) behind the ballast area; I'm sure someone thought it a good idea at the time; there's no base/platform below it, nor any wedges/battens to the sides, just an 'interference fit against the fibreglass and ten years of pounding in big seas with a 100kg of fuel inside to wedge it ever tighter. After several hours work we managed to get enough access to the rear of the tank, to drill a hole into it, fit a hook into said hole and connect that with a rope to the mainsheet and that onto a sheet winch. Still no joy, we cranked it until the 8mm rope (admittedly an old one) snapped without achieving any movement whatsoever. We're hauling out in a few weeks time, so if we were to repeat the exercise once ashore, is the removal of the outside water pressure likely to have any significant effect on our efforts?
 
If it is that severely jammed, are you certain it has not had a load of epoxy adhesive between the tank and the hull ? I can't see how a tank can conform 100% to the hull shape and jam itself in place without adhesive. Is the tank steel or poly ?

It may be that there is a certain amount of hull flexing which is pinching and holding the tank.

In the worst case you may have to cut out the top and make vertical cuts in the front and back panels, and that might allow the sides of the tank to move inwards and become free.

Rather you than me....
 
The answer the question in you title

Approximately 1.5 psi per meter of water depth.

14.7 psi at sea level and for every 10 meters the pressure increases by 1 atmosphere.
 
Because you already have cut a hole - means you have no intention of returning tank to boat. Therefore I would flush the tank with water ... drain and then with a power grinder disc - cut the tank to lift out in sections.

You will then find probably that lower section is fixed someway preventing simple lift out.

Why flush tank ? Even diesel can ignite with sufficient heat ... and that grinder is going to be creating some heat !

On subject of items in bilges. My Father dried out his long keel Hilyard and I as a child watched his attempts to remove the pig-iron ballast ingots that had been placed in sand. That sand had set like concrete and those ingots never did get lifted out ! Even when he spoke to local boatyard about their doing it - they advised to leave well alone !!
 
If the tank was fitted with the boat in the yard and some of the weight on the keel the hull can bow outwards a surprising amount. Thus when it's put in the water the hull reverts to ts natural shape and might well pinch onto your tank.

Cut it outwith an angle grinder as suggested above is probably easiest, or park her on her keel on a drying or partly drying quayside or scrubbing posts and see if the hull releases it's grip.
 
If your problem is really the slight deformation of the hull inwards, it may be countered by making the tank to shrink a bit :-). There are some factors to consider, like the overall shape of that thing, material, way it is welded etc before you decide to try it - but under certain circumstances it may work.
Try seal all the openings and attach the business end of a vacuum cleaner to one. Switch the machine on and watch what will happen. There definitely will be some bucking and creaking - make sure you remove the suction hose immediately when there is any sign of the deformation becoming excessive. You may be surprised what atmospheric pressure applied in the wrong direction can do. Good luck!
 
jiris said:
If your problem is really the slight deformation of the hull inwards, it may be countered by making the tank to shrink a bit :). There are some factors to consider, like the overall shape of that thing, material, way it is welded etc before you decide to try it - but under certain circumstances it may work.
Try seal all the openings and attach the business end of a vacuum cleaner to one. Switch the machine on and watch what will happen. There definitely will be some bucking and creaking - make sure you remove the suction hose immediately when there is any sign of the deformation becoming excessive. You may be surprised what atmospheric pressure applied in the wrong direction can do. Good luck!
Click to expand...

OP does say the tank leaks ... and I would assume that if the leak is not from fittings - is the reason he's removing tank ?
 
Thanks Guys.
As noted, the leaks -and new hole - preclude the negative pressure idea. I think we'll just have to rebuild everything, get it ashore to see if that helps and try again, if not we'll have to go the angle grinder route; though the difficulty with the angle grinding idea will be in finding a suitable 'grinder', are there any child chimney sweeps still around nowadays?
 
A couple of ideas. When you next attempt it, get some serious tension on the tackle, but stop before you break something, then cleat it off (maintaining the tension). Next,
1) Fill the bilge with water to near the top of the tank. (I know the tank leaks, but presumably not that fast.) This will give you significant buoyancy adding to the upward force from the tackle. It may even tend to cause the sides of the bilge to ease outwards slightly. (Make sure your are not in the way in case the tank pops up suddenly as it releases!)
2) Once you've got the tackle tension and buoyancy in place, repeatedly bash a corner of the tank with a hammer. The vibrations will tend to help release the grip between the tank and the hull (assuming it's not fixed with adhesive). (Mind your ears. And did I mention keeping clear in case the tank comes up suddenly?)

Good luck with it, and let us know how you get on.
 
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I can relate to the frustrations of Bob and Lesley - we have a sister ship (Challenger 35, aft cockpit version of the Warrior).
When we first acquired the boat 22 years ago (after she had had a fire on board, and was in a bit of a state), we found a lot of very weird things.
As Bob says, there is a deep bilge at the aft end of the long keel, under the engine.
With our boat, we found that she had two S/S fuel tanks in this space, with one sitting on top of the other. We had been told that only the top one had been used by the previous owner so we decided to hoik them out to investigate.
This was still very difficult, even when they are not wedged in tight - they had to sort of slide diagonally up and forwards out from the glory hole.
Got the first one out, and then the second - this one was a tighter fit, but we eventually got it out.
And found that with the bottom tank sitting in a puddle of water in the bilge, there was extensive corrosion of the S/S (as in holes rusted through already) where it was sitting in the water - despite the boat being only a few years old then.

I built a sort of plinth out of greenheart timber to sit in the bilge to support the top tank at the right height, otherwise it would have just got terribly wedged in as per Bob's experience if we had just slung it back in without anything supporting it. Both tanks had been beautifully built to match (perhaps too well) the tapered shape of the bilge.
Although I remember that plinth went in and out many times, each time chopping a bit more off the height, until it fitted reasonably well.
And then a tight fit (with more swear words) getting the top tank back in, to sit nicely on the plinth.

Bob, re the access to the rear end of the tank - was this via the space under the cockpit, forward of the rudder quadrant? The keel is very narrow back there, and the propeller shaft / stern tube is above. Re your rope with the hook, did you have a suitable access hole / hatch in the cockpit floor, or did you have to lead it out via the starboard cockpit locker to the mainsheet?
 
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A small leak shouldn't be a big problem because of the large volume vacuum cleaner handles. It would have to be a serious hole to make some difference.
 
LittleSister said:
...Fill the bilge with water to near the top of the tank....
Click to expand...

I'm not sure we can access a corner to hit it, but I like the 'fill it with water' suggestion; upward pressure and lubrication, will definitely add that to the mix when we try again ashore.

Bajansailor said:
...the second - this one was a tighter fit, but we eventually got it out...I built a sort of plinth out of greenheart timber to sit in the bilge to support the top tank at the right height..

Bob, re the access to the rear end of the tank ...
Click to expand...

How did you get the second/lower tank out?

Our engine's a Yanmar and when that was fitted they drilled a 4" diameter hole through the engine well with an upstand around to allow a smooth/easy route for the exhaust pipe from elbow to waterlock, with the gearbox and exhaust hose removed and the waterlocked pulled sternward out of the way, that 4" hole lines up almost exactly with the back face of the fuel tank, so a rope from the hook fixed into the back face of the tank has a straight vertical run to the mainsheet/boom through the companionway; given how good that pull direction was, we were doubly disappointed when it didn't move at all.
 
How did we get the second / lower tank out? It was 22 years ago, and I can't quite remember, but it was with difficulty - there was not much clearance. But we did manage to haul it out in one piece.
I like how your exhaust was run downwards through the hole in the tray under the engine - at least I think this is how it is arranged? We have a Volvo 2003, and it was difficult to get a reasonable slope down from the exhaust to the waterlock.
LittleSister made an excellent suggestion re trying to use buoyancy to help you - I hope it works!

Re a new tank, will you be fitting another S/S tank, or will it be a plastic one?
If you are making it the same size as the previous tank it would be worthwhile building a pedestal / plinth that can sit in the bilge to support the weight of the tank, to ensure that it is not a 'jam fit' again.
 
I replaced a leaking fuel tank many years ago - a tip, Don't use an angle grinder, messy, sparks everywhere, I used a reciprocating saw with a metal cutting blade _ very quick and much quieter, Replaced with a standard rectangular tank, on a built up base and secure.d with ratchet straps.
 
PetiteFleur said:
I replaced a leaking fuel tank many years ago - a tip, Don't use an angle grinder, messy, sparks everywhere, I used a reciprocating saw with a metal cutting blade _ very quick and much quieter, Replaced with a standard rectangular tank, on a built up base and secure.d with ratchet straps.
Click to expand...

If you have access to such tool - totally agree.
 
Rogershaw said:
The answer the question in you title

Approximately 1.5 psi per meter of water depth.

14.7 psi at sea level and for every 10 meters the pressure increases by 1 atmosphere.
Click to expand...

The 14.7psi at water level is atmospheric pressure which has no effect on the hull as it is the same inside and outside the boat .
 
How much pressure from the water on the hull?
Weight of boat/Area of hull= calculation should be easy enough.
But as problem now solved it is irrelevant

However, it does make one wonder how little pressure is needed to distort a hull (in what should be a fairly strong point) over the keel or rudder. Enough to trap the tank & possibly crack the seams of a replacement tank as it crushes it. It may be that it was boat flex & not water pressure causing it.

Let us know when the keel falls off & the forum can have another " keel/rudder failure" topic-- Not wishing to worry you, of course :unsure:
 
Daydream believer said:
How much pressure from the water on the hull?
Weight of boat/Area of hull= calculation should be easy enough.
But as problem now solved it is irrelevant

However, it does make one wonder how little pressure is needed to distort a hull (in what should be a fairly strong point) over the keel or rudder. Enough to trap the tank & possibly crack the seams of a replacement tank as it crushes it. It may be that it was boat flex & not water pressure causing it.

Let us know when the keel falls off & the forum can have another " keel/rudder failure" topic-- Not wishing to worry you, of course :unsure:
Click to expand...

It initially seems a simple equation of surface area vs weight ... but the pressure exerted also changes with depth ...

But I would assume that given the hull draft rather than hull + keel - that the change is not great enough to make a simple calc too far out.
 
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