Sunk

[Naughty Cal]

 

And hope somebody notices ?

 

A better idea would be to put in a float switch on the bilge pump with a permanent supply and keep the batteries fully charged.

And then still check your boat on a regular basis. Float switches and bilge pumps can and do fail.

 

Owning a boat comes with some responsibility.

[John V]

And then still check your boat on a regular basis. Float switches and bilge pumps can and do fail.

 

Owning a boat comes with some responsibility.

 

in spades

[Neil2]

My boat is a Liverpool Boat and they probably built more cruiser stern narrowboats in the last 15 years than any other builder, so watertight bulkheads separating the engine room from the cabin really can't be that uncommon.

 

I also think that the second part of what you said is based on a very odd assumption. I don't know if it reflects a somewhat limited degree of experience? I know it's called a canal forum, but some of us do venture out of the ditches and don't have such a "canal-centric" attitude.

 

There are loads of narrowboats on rivers all over the country. My last mooring was at least 15ft deep and this one is about 10ft. Anyway I don't agree that most builders assume the boat will almost always be in shallow water. The builders I've met at boat shows certainly didn't assume that.

The vast majority of boats on the network today will have a watertight bulkhead, most will have been built that way and many "wet bilge" boats will have been modified as well. But they will vary in height so in a lot of cases it won't take much water to overcome the bulkhead and spill into the cabin.

 

Now can someone help me out here as I'm having trouble thinking this through.

 

Suppose the boat has a full height bulkhead, completely sealing the engine room. The stern gland starts to leak, filling the bilge with water. Let's assume the engine room has a vent, so the boat will continue to take on water until it reaches the vent, then water starts to come in through the vent but it is still being contained behind the engine room bulkhead.

 

What's going through my mind is in this situation depending on the size of the engine room, even if it fills with water there is still likely to be enough overall buoyancy to keep the boat afloat - yes?

[Tony Dunkley]

In the dark, with a jetty between you and the boat?

I wasn't suggesting that looking to see if the usual 'above water' deposits at the waterline had gone under was something to be done in the night, but if you particularly wanted to check a boat in the dark then you'd use a torch, wouldn't you ?

[DHutch]

 

What's going through my mind is in this situation depending on the size of the engine room, even if it fills with water there is still likely to be enough overall buoyancy to keep the boat afloat - yes?

I think it very much depends on the length of boat, size of engine room, and height of bulkhead.

 

However I have certainly seen some that looked, without calculation, quite dubious. Obviously as the water taken on is at the extreme end, not only with the stern lower, the bow will rise, and a lot of engine rooms would likely take several tons/1000l's of water before being full.

 

 

Daniel

I wasn't suggesting that looking to see if the usual 'above water' deposits at the waterline had gone under was something to be done in the night, but if you particularly wanted to check a boat in the dark then you'd use a torch, wouldn't you ?

My previous reply was maybe a little short, but my point is that while if you where performing a daily check on a boat what you say is right, if you where just going about your business passing it to get to your own, the chance of spotting it is in my mind minimal. At this time of year most will be coming/going outside the limited hours of daylight, and most marinas are basically unstaffed.

 

 

 

Daniel

[Tony Dunkley]

The vast majority of boats on the network today will have a watertight bulkhead, most will have been built that way and many "wet bilge" boats will have been modified as well. But they will vary in height so in a lot of cases it won't take much water to overcome the bulkhead and spill into the cabin.

 

Now can someone help me out here as I'm having trouble thinking this through.

 

Suppose the boat has a full height bulkhead, completely sealing the engine room. The stern gland starts to leak, filling the bilge with water. Let's assume the engine room has a vent, so the boat will continue to take on water until it reaches the vent, then water starts to come in through the vent but it is still being contained behind the engine room bulkhead.

 

What's going through my mind is in this situation depending on the size of the engine room, even if it fills with water there is still likely to be enough overall buoyancy to keep the boat afloat - yes?

Yes, if the bulkhead was watertight to the level of the top edge of the hull sides, and dependent on the hull volume in the engine compartment being sufficiently less than the hull volume forward of that bulkhead, the boat would stay afloat, subject to the watertight integrity of the hull to a sufficient height throughout the length of the accomodation.

[Tony Dunkley]

I think it very much depends on the length of boat, size of engine room, and height of bulkhead.

 

However I have certainly seen some that looked, without calculation, quite dubious. Obviously as the water taken on is at the extreme end, not only with the stern lower, the bow will rise, and a lot of engine rooms would likely take several tons/1000l's of water before being full.

 

Daniel

 

When considering the effects of flooding it's necessary to keep in mind that the water finding it's way inside the boat is not weight added, it's buoyancy lost. Water entering an engine compartment (via side vents) will only rise to the level of the water outside of it when the vessel has reached the new state of trim caused by the loss of buoyancy that had been provided by the portion of the hull, below the normal waterline, comprising the engine compartment. It must also borne in mind that, due to the swim, and the counter overhanging the sternpost by a considerable amount , the below water volume per lineal foot of length of a stern engine compartment is much less than that per lineal foot of hull along the accommodation space.

 

Edited to add : (via side vents).

Edited February 1, 2015 by Tony Dunkley

[Neil2]

When considering the effects of flooding it's necessary to keep in mind that the water finding it's way inside the boat is not weight added, it's buoyancy lost. Water entering an engine compartment will only rise to the level of the water outside of it when the vessel has reached the new state of trim caused by the loss of buoyancy that had been provided by the portion of the hull, below the normal waterline, comprising the engine compartment. It must also borne in mind that, due to the swim, and the counter overhanging the sternpost by a considerable amount , the below water volume per lineal foot of length of a stern engine compartment is much less than that per lineal foot of hull along the accommodation space.

That's a very useful explanation, it's what I thought but I couldn't quite figure out the principles involved.

[DHutch]

...water finding it's way inside the boat is not weight added, it's buoyancy lost...

Same diffrece. It doesn't matter if its a ton of water or a ton of bricks.

 

As you say, it will only come up level with the water outside, all bit it the new state of trim, but although I did not work it out at the time I suspect even given the shape of the swim the volume is considerable.

 

##

 

Calcs

 

If you assume the boat is 2m wide (just under 7ft), the swim 2.3 long (8ft) , the sides of the swim straight.

- 0.5m (18inch) draft boat, that gives 1.15 cubic meters volume. Just over a ton

- 0.75m (2ft6) draft boat, would give 1.85 cubic meters volume. Nearly 2 ton.

 

That's excluding the volume of the engine, but also excluding the fact the sides are curved, but also the added volume due to the waterline rising relative the boat. And of cause at the point, your flooding over the uxter plate right to the back of the boat.

 

Calculating the effect that would have on the trim of a boat would take longer, but given most boats are really quite light at the box anyway, the centre of mass probably starts off quite far back.

 

Using the broad brush 'ton per inch' for level loading on a 70ft boat, I have no doubt at all that a ton right at the back could push the boat down 3-4 inches, which as your now flooding a larger area, would be another 3-400kg, which itself would lead to another inch or so, and hence another 100kg or so...

 

Based on the above fag-pack sums if I was 'rule of thumbing it' I would want the bulkhead to come up to gunnel level at least!

 

 

Daniel

[Tony Dunkley]

Same diffrece. It doesn't matter if its a ton of water or a ton of bricks.

 

As you say, it will only come up level with the water outside, all bit it the new state of trim, but although I did not work it out at the time I suspect even given the shape of the swim the volume is considerable.

 

In fact it does make a big difference . . . . whether it's water flooding a hull compartment which is open to the water outside the hull ( loss of buoyancy), which adds no weight at all to the vessel concerned, or an quantity of bricks is loaded into that compartment instead ( increased displacement), which most certainly does add weight. The flooding does not increase the displacement, the bricks do.

Edited February 1, 2015 by Tony Dunkley

[John Williamson 1955]

In fact it does make a big difference . . . . whether it's water flooding a hull compartment which is open to the water outside the hull ( loss of buoyancy), which adds no weight at all to the vessel concerned, or an quantity of bricks is loaded into that compartment instead ( increased displacement), which most certainly does add weight. The flooding does not increase the displacement, the bricks do.

If you're talking about a compartment which should be empty (As in the engine bay) filling with water, then the effect on the trim of the boat of letting a ton of water in as a whole is identical to putting in that amount of weight as bricks, steel or anything else. A ton of water in the engine bay has the same effect a ton of bricks (or people) on the back deck. It helps if you think of the air the water displaces as having negative weight.

[Tony Dunkley]

If you're talking about a compartment which should be empty (As in the engine bay) filling with water, then the effect on the trim of the boat of letting a ton of water in as a whole is identical to putting in that amount of weight as bricks, steel or anything else. A ton of water in the engine bay has the same effect a ton of bricks (or people) on the back deck. It helps if you think of the air the water displaces as having negative weight.

What I've been referring to are compartments open to the water outside (the hull), through such as ventilation grills, and the condition(s) takes effect only after any side hull openings are submerged. Only if you flood a stern compartment that's closed off from the outside does the weight of the water affect both the trim and the displacement. If the flooded compartment is open to the outside and water can flow freely in or out, there is no increase in displacement but there is a loss of buoyancy, the boat's C of G remains fixed and the Longitudinal Centre of Buoyancy moves forward, increasing the stern trim. If a closed off stern compartment is flooded there is an increase in displacement, the C of G moves aft and so does the LCB. The effect on the trim won't be very much different, but the boat will be deeper in the water due to the increased displacement.

Edited February 1, 2015 by Tony Dunkley

[Hair Bear Bunch]

An isolated circuit of small gel batery - float switch set just above bilge switch - horn.

Someone will soon notice, specially at night.

[John Williamson 1955]

What I've been referring to are compartments open to the water outside (the hull), through such as ventilation grills, and the condition(s) takes effect only after any side hull openings are submerged. Only if you flood a stern compartment that's closed off from the outside does the weight of the water affect both the trim and the displacement. If the flooded compartment is open to the outside and water can flow freely in or out, there is no increase in displacement but there is a loss of buoyancy, the boat's C of G remains fixed and the Longitudinal Centre of Buoyancy moves forward, increasing the stern trim. If a closed off stern compartment is flooded there is an increase in displacement, the C of G moves aft and so does the LCB. The effect on the trim won't be very much different, but the boat will be deeper in the water due to the increased displacement.

A leaking stern gland will fill the engine compartment with water in exactly the same way as if you had used a hosepipe or opened a seacock with no pipe attached. When the trim gets to the point where the water outside is level with the bottom of a vent, then water will flow in through the vent, and attempt to equalise the levels inside and out. If there is enough reserve bouyancy in the rest of the hull, the boat will stay afloat, albeit with a lower stern. The only time water entering through a vent will have no effect on the bouyancy, trim or displacement is when the levels inside and out are the same.

[DHutch]

In fact it does make a big difference . . . . whether it's water flooding a hull compartment which is open to the water outside the hull ( loss of buoyancy), which adds no weight at all to the vessel concerned, or an quantity of bricks is loaded into that compartment instead ( increased displacement), which most certainly does add weight. The flooding does not increase the displacement, the bricks do.

 

What I've been referring to are compartments open to the water outside (the hull), through such as ventilation grills, and the condition(s) takes effect only after any side hull openings are submerged. Only if you flood a stern compartment that's closed off from the outside does the weight of the water affect both the trim and the displacement. If the flooded compartment is open to the outside and water can flow freely in or out, there is no increase in displacement but there is a loss of buoyancy, the boat's C of G remains fixed and the Longitudinal Centre of Buoyancy moves forward, increasing the stern trim. If a closed off stern compartment is flooded there is an increase in displacement, the C of G moves aft and so does the LCB. The effect on the trim won't be very much different, but the boat will be deeper in the water due to the increased displacement.

 

Buoyancy is upward force, mass/weight is downward force. Even forces cancel. So loss of buoyancy and gain in weight have the same outcome.

 

 

There is a free surface effect with partially contained water, that would you would get with a solid mass, but other than that there is no difference at all.

 

 

Daniel

[Nightwatch]

Right, I'm going to hijack this thread for a little question and request for opinion.

 

A friend of mine has a boat on the K&A which sadly sank a week or so ago. With the help of an 'ex' forum member it is now afloat.

 

He,due to financial restraints, only had third party insurance cover. However his insurance are interested.

 

My view is, he ought to go hell for leather and get quotations for the rain fit out etc. is this the way to go. My friend is the type who will make do and possibly tell the insurance company that he will do the job. He is no youngster,even older than I, it's an old boat and to be honest not in A1 condition but a nice old boat. They have had it for fifteen plus years.

 

Any comments.

 

Martyn

[Tony Dunkley]

 

 

Buoyancy is upward force, mass/weight is downward force. Even forces cancel. So loss of buoyancy and gain in weight have the same outcome.

 

 

There is a free surface effect with partially contained water, that would you would get with a solid mass, but other than that there is no difference at all.

 

 

Daniel

Loss of buoyancy(in the after part of a vessel) has no effect on displacement (volume) or the position of the C of G, but it moves the Longitudinal Centre of Buoyancy forward, on the other hand, the addition of weight(in the after part of a vessel) increases displacement (volume) and moves the C of G and the LCB aft . . . . not the same outcome at all.

Free surface effect and the resulting reduction in transverse or longitudinal GM are not relevant to this, because we're not considering stability, and in any case, only the Moment of Inertia of the free liquid's surface area affects stability, the volume does not.

Edited February 2, 2015 by Tony Dunkley

[Neil2]

Loss of buoyancy(in the after part of a vessel) has no effect on displacement (volume) or the position of the C of G, but it moves the Longitudinal Centre of Buoyancy forward, on the other hand, the addition of weight(in the after part of a vessel) increases displacement (volume) and moves the C of G and the LCB aft . . . . not the same outcome at all.

Free surface effect and the resulting reduction in transverse or longitudinal GM are not relevant to this, because we're not considering stability, and in any case, only the Moment of Inertia of the free liquid's surface area affects stability, the volume does not.

 

Surely in a scenario like this the centre of buoyancy must move backwards to create a righting force.

 

As for Dan's point, if the weight of the "flooded" water is exactly the same as the weight of any "added" water, surely the effect on overall displacement is the same? It's the effect on the list ,or trim that would be different, depending on the (underwater) design of the boat.

[DHutch]

As for Dan's point, if the weight of the "flooded" water is exactly the same as the weight of any "added" water, surely the effect on overall displacement is the same? It's the effect on the list ,or trim that would be different, depending on the (underwater) design of the boat.

As far as I am aware it is.

 

For instance, if you put a container of water either side of the engine, open top, with the water line level with the outside, connecting a pipe from the container to a skin fitting or existing hole on/in the hull would not change the position of the boat and there would be negligible flow.

 

Either way, I maintain the overall point, that if you flooded the engine compartment of a modern rear underdeck-engined narrowboat with bulkhead in front of the engine compartment, the trim and attitude of the boat would change significantly due to that.

 

 

Daniel

[Tony Dunkley]

 

Surely in a scenario like this the centre of buoyancy must move backwards to create a righting force.

 

As for Dan's point, if the weight of the "flooded" water is exactly the same as the weight of any "added" water, surely the effect on overall displacement is the same? It's the effect on the list ,or trim that would be different, depending on the (underwater) design of the boat

In the situation we're looking at the LCB does move forward initially as buoyancy is lost from the after compartment, causing an increase in stern trim. If that after compartment is sufficiently open to the outside, as it would be once any side ventilation openings were submerged and water can flow freely in or out, then it can neither contain added weight in the form of water, and nor can it contribute any buoyancy because it's no longer capable of displacing any water. As the water levels inside and outside the open compartment equalize the boat settles lower at a new waterline, with increased stern trim but still displacing the same volume and weight of water as it did before flooding started, and at this stage, as you rightly say, the CB does move aft again, into vertical alignment with the C of G, to restore equilibrium. Any vessel taking on water is subject to a complex and constantly changing set of conditions, and what may seem to be happening can be an over simplification of what's really going on.

The point I was making originally, perhaps not very well and with too little explanation, is that a boat with a flooded aft engine compartment, with large and submerged open side vents, does not undergo an increase in displacement ( weight) due to the flooding, and therefore floats higher than it would if the displacement had been increased.

This can be demonstrated by two different workings of a force diagram of a beam on a moveable pivot, in one case subtracting the smaller of the two buoyancy figures, and in the other case adding the same quantity to the displacement. The beam serves as the boat and the pivot point begins coincidental with the C of G and the initial LCB ( to be strictly correct the Centre of Flotation, about which any vessel rotates longitudinally when undergoing a change of trim). Buoyancy is represented by (say) 2 upward forces proportional to the lengths of the engine and accommodation spaces and positioned appropriately, displacement (weight) acts downward from the fixed C of G and equals the sum of the buoyancy forces, except in the second case working where the increased displacement is acting through a C of G that has been moved aft by the added weight (only occurring in the circumstances of a 'closed off' flooded compartment).

As far as I am aware it is.

 

For instance, if you put a container of water either side of the engine, open top, with the water line level with the outside, connecting a pipe from the container to a skin fitting or existing hole on/in the hull would not change the position of the boat and there would be negligible flow.

 

Either way, I maintain the overall point, that if you flooded the engine compartment of a modern rear underdeck-engined narrowboat with bulkhead in front of the engine compartment, the trim and attitude of the boat would change significantly due to that.

 

 

Daniel

I think you've misunderstood what I've been trying to say.

[Alf Roberts]

Right, I'm going to hijack this thread for a little question and request for opinion.

 

A friend of mine has a boat on the K&A which sadly sank a week or so ago. With the help of an 'ex' forum member it is now afloat.

 

He,due to financial restraints, only had third party insurance cover. However his insurance are interested.

 

My view is, he ought to go hell for leather and get quotations for the rain fit out etc. is this the way to go. My friend is the type who will make do and possibly tell the insurance company that he will do the job. He is no youngster,even older than I, it's an old boat and to be honest not in A1 condition but a nice old boat. They have had it for fifteen plus years.

 

Any comments.

 

Martyn

 

If he's Basic Boat, they'll pay for the salvage, they won't pay for any restitution.

 

What he needs to do is get a whopping invoice from the salvers and get that met by the insurers.

He can then use that to fund the refit.

 

 

It's vital he gets his ducks in a row before talking to them.

Edited February 3, 2015 by Alf Roberts

[Nightwatch]

Thanks for that Alf. I'll try to find a way of putting that to him.

 

Martyn

[kienik]

Incidentally I crusied Lizzie Anne back to her home mooring yesterday and noticed that said sunk boat is now out of the marina basin and on hard standing next to the slipway, gawd knows the state of the inside but it's good to see they have raised her and she is getting some attention

 

Nik

[John Williamson 1955]

Good. Hopefully somebody has done the necessary after salvage work to the engine already.

[Neil2]

In the situation we're looking at the LCB does move forward initially as buoyancy is lost from the after compartment, causing an increase in stern trim. If that after compartment is sufficiently open to the outside, as it would be once any side ventilation openings were submerged and water can flow freely in or out, then it can neither contain added weight in the form of water, and nor can it contribute any buoyancy because it's no longer capable of displacing any water. As the water levels inside and outside the open compartment equalize the boat settles lower at a new waterline, with increased stern trim but still displacing the same volume and weight of water as it did before flooding started, and at this stage, as you rightly say, the CB does move aft again, into vertical alignment with the C of G, to restore equilibrium. Any vessel taking on water is subject to a complex and constantly changing set of conditions, and what may seem to be happening can be an over simplification of what's really going on.

The point I was making originally, perhaps not very well and with too little explanation, is that a boat with a flooded aft engine compartment, with large and submerged open side vents, does not undergo an increase in displacement ( weight) due to the flooding, and therefore floats higher than it would if the displacement had been increased.

This can be demonstrated by two different workings of a force diagram of a beam on a moveable pivot, in one case subtracting the smaller of the two buoyancy figures, and in the other case adding the same quantity to the displacement. The beam serves as the boat and the pivot point begins coincidental with the C of G and the initial LCB ( to be strictly correct the Centre of Flotation, about which any vessel rotates longitudinally when undergoing a change of trim). Buoyancy is represented by (say) 2 upward forces proportional to the lengths of the engine and accommodation spaces and positioned appropriately, displacement (weight) acts downward from the fixed C of G and equals the sum of the buoyancy forces, except in the second case working where the increased displacement is acting through a C of G that has been moved aft by the added weight (only occurring in the circumstances of a 'closed off' flooded compartment).

 

 

But... If we take our boat that has lost some of its buoyancy and block up the hull side vents, it will regain its buoyancy but just because we have turned the water into a displacement force doesn't mean the boat will sink any lower. The amount of water hasn't changed and the displacement force at that moment will be the same as the loss of buoyancy was ie the boat will remain in the same position.

 

There has to be a position of equilibrium between these two states.

 

But the difference now is that adding more water will increase the displacement, move the CoG and CoB backwards and the stern will start to drop.

 

If we had left the vents open adding any more water (or removing it) won't make any difference because it can't affect the displacement of the boat and hence the CoG or the CoB.

 

Maybe this is what Tony was trying to explain and I've been a bit slow on the uptake.