How do motorboats sink?

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Mike k

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This may seem a stupid question but when you daydream sometimes the strangest thoughts come into mind.So here goes-- i was thinking about the layout of the bilge in my boat..the engine bay has a manual and auto bilge pump so thats ok. But below the forward cabin is the bilge space that houses the BT and battery and is a sealed space in so far as that if ever the boat was holed at that point the water would have no where to run back to the bilge pumps.I then started to think well what actually would happen would the water enter to the level of the water line and remain buoyant ( weed box approach?) or would that extra weight just keep lowering the bow and the water totally swamp and sink her.

I know i know its basic but i am happy to embarass myself as long as someone comes along with some answers/thoughts on the matter.

Thanks

Mike
 
I don't have any statistics to prove it but I suspect many more boats sink from failure of hoses/seacocks etc than becase of holes in the hull from a collision.

That is why the pump intakes are designed to catch this.

If the forward sapce is totally sealed, then in the unlikely event of a hole there, no water could get into the rest of the hull and you would just take a nose down attitude but remain afloat.
 
I would think the risk is the boat tilts and then water runs in through the air intakes.
I am not sure bilge pumps will do much against a hole though-more a trickle.
 
I'd guess that that space is sealed from the rest of the boat as a safety measure in case it does get holed.

Work out the approximate volume of the area up to water level and consider that 1 litre (1000ccs) of water weighs a kilo and 1 cu ft of water weighs 65lbs. Working out how much that would lower the bow, so how much extra would come in, so how much that would lower the bow so... is complex, but thinking about your weight and how much the boat sinks under it will give you an idea which may set your mind at rest - or give you a better basis for your concern!

However, as others have said, there are many more likely scenarios for your boat sinking. At least, if you hole the bow, you're probably on board!
 
When you say "totally sealed", do you mean the top is sealed as well, or just that there is a "wall" between that section of bilge and the next?

Pete
 
davidej said:
I don't have any statistics to prove it but I suspect many more boats sink from failure of hoses/seacocks etc than becase of holes in the hull from a collision.

That is why the pump intakes are designed to catch this.

If the forward sapce is totally sealed, then in the unlikely event of a hole there, no water could get into the rest of the hull and you would just take a nose down attitude but remain afloat.
Click to expand...

Sorry i think i have misled you all - not totally sealed but a wall between the space and the rest of the bilge and a removeable wooden cover on the top.
 
I would think more sink from neglect than anything else.
Flat batteries, so auto bilge pumps don't work.
Seacocks,hoses, prop shaft seals failing.
Rainwater, over time.
 
My boat (not of the mobo variety) has a sealed space under the forward berth. That is, sealed against the bottom and sides of the hull but completely open at the top. The top edge is well above the normal waterline, and since the hull is very shallow at the bow, not much water would enter even if she was holed there.

As I see it, this is a deliberate design feature which assumes that the bow is the most likely area to be holed (due to collision) so that is the area that should be isolated from the rest of the boat. Seems reasonable to me.
 
Limber holes

My 4 boats had linked bilges, with what I believe are called "limber holes" with the bilge pump pick ups at the lowest points.
stearman65
 
If you have a "wall" i.e. bulkhead that doesn't go right up to deck level or form a completely sealed watertight compartment then the one thing that jumps to mind is...

Titanic


If the forward compartment floods and the remaining buoyancy (and trim angle) allows water to flow over the top then the rest of the boat will fill with water. OP has probably realised this but calcs. may be complicated by need to work out the effect on the angle at which the boat will float with a flooded forward section. i.e. How much water is needed to come over the top of the compartment. The stern will lift as the bow sinks and have an effect on the final trim.
 
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Mistroma said:
If you have a "wall" i.e. bulkhead that doesn't go right up to deck level or form a completely sealed watertight compartment then the one thing that jumps to mind is...

Titanic


If the forward compartment floods and the remaining buoyancy (and trim angle) allows water to flow over the top then the rest of the boat will fill with water. OP has probably realised this but calcs. may be complicated by need to work out the effect on the angle at which the boat will float with a flooded forward section. i.e. How much water is needed to come over the top of the compartment. The stern will lift as the bow sinks and have an effect on the final trim.
Click to expand...

Exactly

Motorboats designed to have petrol engines dont have limber holes in front of the engine room. The reason is for this is to contain fumes from leaking/spilled petrol to the engine room.
Less room for the ventilation fan to empty and reduces the risk for fire in the forward section of the boat. Not intended as a way to make water tight compartments.
 
Sinking

As OP can see there are many variables to this question. The walls around areas may be of value but would be far better if they were fitted with a sealing lid. Perhaps a clamp and rubber seal. if the seal is not that good it would at least give you more time before sinking. If battery was not located there you could with benefit seal and screw down the lid. This would both seal this area if bow was holes but also provide buoyancy if water got into the main under floor area. Indeed you could end up with just the bow showing above water.... I hope not good luck olewill
 
Almost always hose or seacock failure. Favourite in the past has been the exhaust hose dropping off, double whammy as it lets water in and you pump it in with the engine. Make sure all hosee have double high quality clips on
 
Mike k said:
I then started to think well what actually would happen would the water enter to the level of the water line and remain buoyant ( weed box approach?) or would that extra weight just keep lowering the bow and the water totally swamp and sink her.
Click to expand...

If you make a hole below the waterline, the water coming in will make the boat heavier. It will gradually* sink deeper until the weight of the boat and contents is greater than the weight of water displaced and the boat sinks. How quickly this will happen depends mainly on the size of the hole. Even a hole of a few mm will sink your boat eventually, although it may take many hours, conversely a hole the size of your head might sink it in ten or twenty minutes, say.

There are three things you need to achieve for safe boating:
Keep the water OUTSIDE the boat
Keep the people INSIDE the boat.
Keep the boat from bashing into anything hard.
 
Mistroma said:
If you have a "wall" i.e. bulkhead that doesn't go right up to deck level or form a completely sealed watertight compartment then the one thing that jumps to mind is...

Titanic


If the forward compartment floods and the remaining buoyancy (and trim angle) allows water to flow over the top then the rest of the boat will fill with water. OP has probably realised this but calcs. may be complicated by need to work out the effect on the angle at which the boat will float with a flooded forward section. i.e. How much water is needed to come over the top of the compartment. The stern will lift as the bow sinks and have an effect on the final trim.
Click to expand...
Hi thanks for replies but....
do you mean 'deck level' or water line- the top of the compartment is above the waterline?
 
Having come close to sinking earlier this year, I can say it's a very alarming situation (no surprise there I guess!)

This was a failed hose on the intake pump (pressure side). As Fisherman says, it's a double whammy as the engine acts as a 150hp pump, filling the boat with water.

4 electric bilge pumps didn't put a dent in the problem. My only recommendation is to make sure that the intake and exhaust systems are in tip top condition, and that you have alarms in the bilge to make you aware of the situation.

Also, rather counter intuitively, I now realize that switching off an engine (the right one of course) can greatly improve the situation. This is not the natural thing to do in such a situation, as you rely on it for propulsion. The important thing is correct diagnosis - it happened to me in the dark - being tired, and unable to see in the poor light will not help...

There's also many distractions that don't help - I had exhaust alarms, gas alarms, engine alarms all sounding at once. Instinctively you block them all out, which doesn't help your diagnosis. Having persons assigned to operate the radio, manage the boat, and help prepare people to abandon the boat is important - I ended up doing most of this, and it takes away your valuable time from finding the root cause of the problem.


Cheers,
Paul
 
Like many boats mine has a triangular double berth in the forecabin, beneath which is a large locker which is divided into 2 compartments by a transverse half bulkhead and accessed via loose plywood covers which fit into recesses in the berth top under the mattress. I've often wondered whether it would be worth replacing the plywood covers with watertight access panels such as these. I would think that if the yacht is going to be at risk of sinking due to being holed, the impact is most likely to be in the forward part of the ship.
 
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