what's the logic of a cockpit drain ending under the waterline???

My cockpit drains on a Mirage 28 were fitted with large seacocks. It turned out that they were plain brass ball valves and the operating spindles had rotted such that the handle turned but the balls didn't. In my case they were open just a fraction. I replaced them with 316 stainless ones (because I was working on a construction site and had access FOC). As far as I know since I've sold the boat, they are still in place.
 
Tranona said:
Afraid your thinking is not logical. If properly installed the seacock is not vulnerable. Some advocate that every outlet below the waterline should have a seacock on it, but cockpit drains because of their function might be an exception.

I have (and others) have already answered your question. The drain is below the waterline so that it always drains irrespective of the load in the boat (therefor where the waterline is) and when the boat heels. There may also be a constraint that prevents running the drain to the transom.

Suggest you draw out the profile of your boat, marking the waterline and the height of the cockpit floor and drain outlets in relation to the waterline.Then draw the same lines in section when heeled at 30 degrees. You should then appreciate why the drains are sited where they are.
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so here is my logic :
I didn't say exactly a seacock was vulnerable, I said that it was "an added vulnerability". It is an added link in the chain of "things which can break".
I did "the drawing"...as i've already written, the cockpit will drain no more than the level of the sea level...when the boat heels, the sea level gets closer to the cockpit sole, and the cockpit will have its lowest side full of water (till sea level) no matter what (concept of the communicating levels). So the lowest on the bilge is the drain, the more water will flodd the lowest side of the cockpit of the heeling boat ....
Am I wrong?

I agree with "the constraint" preventing to get the drain outlet at the transom (have to check that next time I get on the boat, but from here I guess it's a matter of not enough slope for the pipe)
 
Seacocks do not break if they are made of the correct material. Their only "vulnerability" is that if they are not operated regularly they may seize but then you are no worse off than if you don't have one at all.

If your drains are on the transom the cockpit may backfill in a following sea. My boat is the same style as that in pvbs photo and water comes back through the drains which are the openings around the drop down transom. Not a big issue as the drainage area is far greater than normal pipes.

However this like so many things in boat design and build, there are pros and cons of different solutions and what you have is standard practice. If the drains are big enough then they will meet the requirements of the RCD. So difficult to argue why one should want to change the layout.
 
Tranona said:
Nothing unusual about what you have' usually because going through the transom is either inconvenient or does not allow enough drop to drain the cockpit. Why would you want to close the seacocks when you leave the boat? Many boats do not have seacocks at all but straight drain tubes under water. The seacocks are there just in case the hose or clips fail which is highly unlikely, or you want to change the hose with the boat in the water - again highly unlikely.

Just make sure the hoses are sound and well clipped to the seacocks, leave them open and stop worrying about it.
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Good practice is to have two hose clips on underwater drains.
 
guernseyman said:
Good practice is to have two hose clips on underwater drains.
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With no real evidence as to why. Have you ever had a clip fail or tried to get a hose off a spigot? Does your car have double hose clips on all its hose connections, even those that have fluid under pressure?

Seacocks are not under any real pressure and hoses don't come off on their own. Double clipping stems from the early days when through hull pipes were probably lead and there was mistrust of this new fangled flexible hose plus stainless worm clamps were not invented. But hoses are now reliable and proper clips do not fail - but the mantra is still there!
 
Tranona said:
With no real evidence as to why. Have you ever had a clip fail or tried to get a hose off a spigot? Does your car have double hose clips on all its hose connections, even those that have fluid under pressure?

Seacocks are not under any real pressure and hoses don't come off on their own. Double clipping stems from the early days when through hull pipes were probably lead and there was mistrust of this new fangled flexible hose plus stainless worm clamps were not invented. But hoses are now reliable and proper clips do not fail - but the mantra is still there!
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Spot on, another of those folklore "truths" which don't stand up to scrutiny.
 
What is more likely is a small split in the pipe. It means that to use a wooden plug one has to remove the pipe or cut it altogether; assuming it cannot be bound & sealed. With a stopcock one can just turn the handle & deal with the leaking pipe in a sensible manner. With no stopcock it could be a different matter. the amount of water that comes through quite a small split can be surprisingly large, as I once found out. If one leaves stopcocks open one has to be sure that the rest of the system is sound-- is one always sure???? Well at least it is a reason to nip back to the boat mid week I suppose.
 
There are several considerations. Here's a few more....

A following sea can - and does - lop over the stern into the cockpit. You don't have to be below 50°S to experience this, for it has happened a few times to me, around 50°N.........

In a truly small boat ( how small is small? ) that weight of water can depress the stern rather a lot - and the next following sea can lop more briny into the cockpit.... and so on. That could spoil your day.

Having 'more than sufficient' cockpit drains matters, and it is certainly worthwhile checking that the whole run(s) are sufficiently clear. I'm aware of one quite popular 24' design where the TWO 1" cockpit drains were 'siamesed' into just ONE low inaccessible outlet..... and one of those upper branches was choked with rotted leaves, when probed.
 
My early Westerly design had two cockpit drains at the stern allowing the rain or other water to drain out itself when boat was left untended or even sailing. The cockpit sole was hardly above the waterline when stationary. The cockpit drains had a simple pull up/ push down facility so was easy to shut them off if ever required . No interconnecting linking hoses at at all, just a fitted plastic unit that was fitted on original assembly. Worked a treat, oh craft never sank on me
 
Our fairly modern and high spec Motor Sailer has 4 cockpit drains, all 2 inch I/D pipe, no seacocks.

Two for the rear cockpit, quite short and well protected from stuff moving around, plus two for the front cockpit, a fairly unique feature on a sailboat. This has grills in the two front corners, about 3 metres of reinforced top quality hose going down to the through hulls, no seacocks.

This front cockpit is known as the Jaccuzi. It is rumored-but not yet substantiated-that some US owners have blocked the drains, filled the area with water and relaxed.

It does look very much like a hot tub in truth.

The hoses and connections are checked every season, last time they were as new.

I cant see the point of seacocks on drains-it will end in disaster if they are shut and the vessel is in the moisture laden UK.
 
rotrax said:
Our fairly modern and high spec Motor Sailer has 4 cockpit drains, all 2 inch I/D pipe, no seacocks.

Two for the rear cockpit, quite short and well protected from stuff moving around, plus two for the front cockpit, a fairly unique feature on a sailboat. This has grills in the two front corners, about 3 metres of reinforced top quality hose going down to the through hulls, no seacocks.
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On or 27 ft yacht we have 2 x rigid 4 inch cockpit drains though from cockpit to stern via lazerette. They took a wave over the stern mid Atlantic that filled the cockpit, it took 30 minutes to drain via standard 2 x1 1/2 inch flexible pipes.

Brian
 
alexlago said:
so here is my logic :
I didn't say exactly a seacock was vulnerable, I said that it was "an added vulnerability". It is an added link in the chain of "things which can break".
I did "the drawing"...as i've already written, the cockpit will drain no more than the level of the sea level...when the boat heels, the sea level gets closer to the cockpit sole, and the cockpit will have its lowest side full of water (till sea level) no matter what (concept of the communicating levels). So the lowest on the bilge is the drain, the more water will flodd the lowest side of the cockpit of the heeling boat ....
Am I wrong?

I agree with "the constraint" preventing to get the drain outlet at the transom (have to check that next time I get on the boat, but from here I guess it's a matter of not enough slope for the pipe)
Click to expand...

I may have completely misunderstood all this, but isn't this all about the Bernouilli principle ? Having the drain under the water generates low pressure on the drain side and so draws water out (like a self bailer in a dinghy). If it were above the waterline there would be no Bernouilli effect, no ?
 
There is a recommended time to empty I've seen quoted but unfortunately can't remember what it is.
halcyon said:
On or 27 ft yacht we have 2 x rigid 4 inch cockpit drains though from cockpit to stern via lazerette. They took a wave over the stern mid Atlantic that filled the cockpit, it took 30 minutes to drain via standard 2 x1 1/2 inch flexible pipes.

Brian
Click to expand...
 
Benny257 said:
I may have completely misunderstood all this, but isn't this all about the Bernouilli principle ? Having the drain under the water generates low pressure on the drain side and so draws water out (like a self bailer in a dinghy). If it were above the waterline there would be no Bernouilli effect, no ?
Click to expand...

Yes, you've completely misunderstood it.
 
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