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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
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  #21  
Old 03-20-2010
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Quote:
Originally Posted by Mechsmith View Post
I would never join a hull to a cockpit with anything rigid. It will break sometime.
I completely agree; and I will add that if you put in a rigid pipe you will also have a big problem steming the flow if the pipe breaks or if the boat is swamped and taking on water via those drains.
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  #22  
Old 03-21-2010
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Interesting thread.
A boater at my marina nearly sank last year when his boat flooded. He was close enough to shore to be towed in and hauled out before the boat actually sank. Turned out it was a solid drain tube that cracked. This was on an older boat, maybe 30 yrs old or so. Hunter 27 I think.

I would tend to agree that some allowance for flex has to favor hoses.
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  #23  
Old 10-04-2012
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Re: Cockpit drains

I know this topic has been cooled for 2 years or so, but there are still new guys like me who are coming fresh to the problem. I share a Nic 38 centre cockpit motorsailer with a friend and we are seriously worried about going blue-water without doing something about the cockpit drainage. We have (as designed) 2 x 0.5" dia skin fittings (one port, one starboard, linked to 4 x 0.5" pipes at floor level, not crossed. The floor is just about 3" above the waterline. When sailing with the drains open, we get a little water in the cockpit coming up the drains. I dread to think how long it would take to drain a cockpitful if we got a big wave on board...... though actually it would trickle/pour down into the engine space and thence to the bilge, where we could pump it out. But if we got swamped repeatedly, then the water going to bilge would win against he pumps, the waterline would go down and the drains would sink us.
So I want to put in some 2" dia drains maybe 2 each side. We cannot go to transom as there is not enough freeboard to do so. We are also thinking about raising the cockpit floor by about 4" and bringing the drains up to this height. Comments please! My buddy wants to sail to the Canary Isles from Portugal, where we are wintering, but I object until this problem is solved.
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  #24  
Old 10-04-2012
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Re: Cockpit drains

Nere,

Properly sized cockpit drains should be able to empty a swamped cockpit within one minute or less when the cockpit is full. Almost all production boats fail this, with the exception of open transom boats.

Having 2 drains feed four pipes is really pretty useless, the amount of water that can be moved is determined by the size of the opening, and the restrictions. In this case, the four pipes really just act to ensure the choke point is at the scuppers not the pipes, but so long as the scuppers are flow restricted, the system is as well.

And any reliance on draining the cockpit by way of the bilge pumps is really poor planning. In bad weather there is likely enough water coming into the hull from other things, that cockpit overflow very well may swamp the boat.


The real question is how large do the drains need to be. Without knowing the size of the cockpit, I can't begin to guess, but assume that a 1/2" scupper can move a max of around 6 gallons/minute assuming it is 20" underwater.

Keeping the same depth (20 inches under water the following are ideals)

1" = 25 gallons per minute
1.5"= 56 gallons per minute
2" = 100 gallons per minute

From this, just figure out the volume of the cockpit in cubic feet. One gallon is equal to .133 cubic foot, and figure out how many, and of what size drains you need to match that size.

Realistically most boats don't meet this requirement, but if you are doing the work, you might as well do it right. And normally there isn't a major problem increasing size by a good bit, once the drain ways have been identified.
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  #25  
Old 10-07-2012
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Re: Cockpit drains

Many thanks for your reply. It seems we both feel the designer (back in 1964 or thereabouts) did not fit adequate drains. The first point I would like you to consider is why you quoted the flow through a drain point 20" underwater. In fact, ours are about 20" underwater, but I've always thought that was just because there is not a convenient point on the hull nearer the waterline to exit them. My point is that the distance underwater should not make any difference to the flow rate - it's the depth of water above the waterline which matters... and we have only 3" + depth of water in cockpit. Would you agree?

If so, and we are only concerned with shifting the top 2000 gallons of water in the cockpit, my feeling is that draining this to just ABOVE the waterline would be safer than installing large ports 20" down, so that there are no huge holes below the waterline, which would DEFINITELY sink the yacht fast if anything went wrong with the pipe connections at any time (yacht on mooring, owner absent for example).

So I am thinking that large horizontal pipes from just above the cockpit floor (currently 3" AWL, but 7" AWL if we raise the floor) would be the safest option for draining the "swamp-water", allowing the existing tiny drains to finish the job. I guess I would put non-return valves in the big drains, so water doesn't get pushed back in when sailing heeled, but these would have to open with only 1-2" head of water or else they would defeat their object. Or else I would fit them with on/off cocks that were in easy reach without having to open a locker, for example.
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Old 05-22-2013
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Re: Cockpit drains

Can someone explain why crossed drains are still used.
Als long als the thruhull is under sealevel, it makes in mine opinion
no difference crossed or not crossed.
Not crossed has the advantage more room under the cockpitfloor
and shorter run so faster waterflow.
So what do you think?
HermanD
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  #27  
Old 05-22-2013
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Re: Cockpit drains

Quote:
Originally Posted by HermanD View Post
Can someone explain why crossed drains are still used.
Als long als the thruhull is under sealevel, it makes in mine opinion
no difference crossed or not crossed.
Not crossed has the advantage more room under the cockpitfloor
and shorter run so faster waterflow.
So what do you think?
HermanD
Because on some designs if not crossed water will enter the cockpit from the low side drain as the boat heels.
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  #28  
Old 05-22-2013
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Re: Cockpit drains

As the boat heels and the outlet off the low side drain is under sealevel then the water
can flow back. Only ,if under heel, the outlet comes above sealevel the water cannot flow back. But on what boat comes the outlet above sealevel under heel??
If the outlet stays under sealevel, crossing is (in mine opinion) compleet useless.
HermanD
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  #29  
Old 05-22-2013
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Re: Cockpit drains

Mine are not crossed. I have one two inch drain in each forward corner of the cockpit. They lead through a "T" to one through hull that is just below the waterline. (about four inches below the cockpit sole when level).

Your depth to drain will always be at the top of the water that you are floating in. This is where you have to trust the designer a bit. Your boat should float on its side. When the boat is on its side any water in the cockpit will go over the gunwales. (By this time it will probably have a little yellow in it (:-) .

When it's getting rough you should have the companionway doors shut to keep water out of the cabin. A full cockpit shouldn't sink you. A full cabin could. (Basic design again). You need to draw a little picture of things so that you can visualize it and you will probably be able to relax a little about it. Perhaps a photo of the stern and a level would help.
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  #30  
Old 07-24-2013
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Re: Cockpit drains

anyone reading... make sure to click on the link at the top. It is a beautiful boat
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