How much water on board would it take....

damo

damo

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....to lower the water line a certain amount?

eg a fairly traditional hull, without an integral keel so with shallow bilges.

Is there a rule of thumb that could indicate that if say there was a foot of water inside, the boat would be down by say 9"?

The reason I'm asking is that I would like to have some idea how long it would take for a leak of a certain rate to lower a boat enough before downflooding started.
 
No easy rule of thumb. Its a question of relative volumes. If the boat is pushed lower in the water by say one inch, it will displace more water.
As an example a beamy 50 foot yacht might have an area at the waterline of 400 square feet, requiring 33.33 cubic feet of water inside the hull to push it one inch lower. Initially that volume of water might be several inches deep in the lowest part of the bilges, but as the boat begins to fill and the area inside the hull at the level of the water inside approaches closer to that of the area at the waterline outside, the boat may sink lower almost inch for inch.
My advice is, don't try to test this theory.
/forums/images/graemlins/laugh.gif
 
Another factor is the position of the hole.
The deeper down, the greater head of pressure and therefore flow.
But as the boat fills up, the head of pressure decreases and correspondingly the flow.
As the level rises inside the boat, it will settle further in the water maintaining a difference in water levels such that the volume of the difference equals the weight of the boat.
The difference in water levels required to maintain that volume will decrease (generally) as the boat settles in the water because the surface area is increasing and the flow will get weaker.
Until finally the water overflows or there isn't sufficient volume of air left to float the boat.

Short answer - it varies, too many factors, no rule of thumb is possible.
 
I somehow expected it would be like that /forums/images/graemlins/frown.gif

I've done a little test this evening and timed how long it took for a bucket to fill from a tap at the flow rate in question - 100lt/hour. So if the rate was constant that is 1 ton in 10 hours.

You are saying that the rate should DECREASE as the boat fills, but even so any guesses how long an 8m shallow bilge yacht would stay afloat? /forums/images/graemlins/smile.gif
 
[ QUOTE ]
Is there a rule of thumb that could indicate that if say there was a foot of water inside, the boat would be down by say 9"?

[/ QUOTE ] It depends entirely on the shape of the hull. At one extreme a perfectly flat bottomed hull with vertical sides ie a box would sink by the same as the depth of water inside. (A foot in your example.) At the other extreme a boat with a deep V shaped bilge will sink very little for the same <u>depth</u> of water

In both cases the boat has to sink to displace an extra volume of water equal to the volume on board. That will be a maximum in the case of a square box but could be a very small volume in the case of a deep V bilge.
 
8 m boat at 100 lt/hr.
Ignoring all other factors and treating the volume required to float the boat as a half cylinder 2 m wide:
Volume = 1/2 pi r squared x length
1/2 x 3.14 x 1 x 8
12 & 1/2 m3 = 12,560 litres
@ 100 lt/hr = 126 hours
approx 5 days. (cylinder is a VERY crude approximation for a hull)
 
My 8m boat has an immersion rate of 101kg/cm at the waterline according to the designer, so at 100l/h it would sink 1cm in one hour. The immersion rate obviously varies according to depth but the crude approximation would be 4 days (100h) to reach the companionway (~1m above WL)...
 
That's a better approximation than mine, assuming the boat to be vertical sided.
But if we refine that slab sided shape and consider the hull an inverted cone stretched along one horizontal axis...
the volume of a cone (if I remember right) is a 1/3 of that of a cylinder of the same base dimensions, so your boat will fill in a third of the time of the "slab sided" model.
Plus a bit because its a cone with "bulged sides"
1 day 8 hours plus a bit.
 
Thank you both - my rough guess seems to be in the right ball-park /forums/images/graemlins/smile.gif I reckoned it would be about 12-24 hours to accumulate a foot of water in the boat.

(There has been an argument about when it would be reasonable to notice a that boat was taking on water, when it was one amongst many others)
 
[ QUOTE ]
(There has been an argument about when it would be reasonable to notice a that boat was taking on water, when it was one amongst many others)

[/ QUOTE ]

When it is very far gone I suspect. Put my boat next to a Contessa 32 and the Contessa looks half-sunk already. By the time mine got that low in the water I should think that the engine would be under and there would be 3 or 4' inside the boat.
 
[ QUOTE ]
Thank you both - my rough guess seems to be in the right ball-park /forums/images/graemlins/smile.gif I reckoned it would be about 12-24 hours to accumulate a foot of water in the boat.

Damo This is assuming the water is flat calm and the boat stays level. In a sailing dingy if you have shipped 6 inches of water or more you are in all sorts of trouble long before the water reaches the gunnels.
 
I know /forums/images/graemlins/smile.gif The boat in question is on a marina finger berth, is dead level and the water is flat calm.

I have been trying to ascertain what would be a reasonable length of time before anyone spotted anything amiss. Neither the liveaboards next door nor the marina patrols had noticed, so the thinking is that the flooding must have been relatively rapid. If the boot topping had only gone below the water say 6 hours before then it is unlikely that a cursory glance would have seen anything. The boat had been visited 2 days previously and all was OK then.
 
Apologies, I had just come from the Jester Forum and though this was just theory. Did not realise you were sinking.

Is it rain water getting in, are you okay now.
 
And there was me on my bike half to Bristol with a bucket.

Interesting thread though. Last year (I won't say summer) I woke up to find water widdling in from the grease nipple on the stern gland. The clear plastic pipe had come off. There was a dodgy little fitting that had broken. Managed to replace it with an olive.

I used rags covered in grease and junbille clipped on. The rubber bandage sounds a good idea.

For other leaks I can recommend waterproof epoxy putty. A friend says he has used it for underwater repairs in swimming pools.
 
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