cockpit drains

Discussion in 'Fiberglass and Composite Boat Building' started by robwilk37, Oct 3, 2013.

  1. robwilk37
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    robwilk37 Senior Member

    ok. so ive read all the threads and links i can find and still dont have my head around the theory. heres what id like to do...

    40' cutter, large-ish aft cockpit. id like to just run 4 big (4"+) solid FG pipe straight down and exit below the water line. glassed in and no seacocks. always open and ready to work. pipe of thick section so no worries about getting crushed and sinking the boat, and even if they where to be breached the area of the boat they are in is of small volume and has a gasketed entry.

    ive never seen this done but then i havent been around as many boats as most on this forum. i like it because its simple, i have the material, and i can put in as many as i want. so some questions...

    1 - given equal drain capacity, will gravity fed water move faster if draining to air (say out the transom above the water line) than it will going straight down and exiting below the waterline? in my head i think not but then i seem to remember something about 'differential head' affecting flow from my school days.

    2 - do drains have seacocks because they are usually connected to rubber hose that can slip off or puncture?

    3 - is solid glass pipe not typically used just because it is inflexible and expensive? some other reason?

    4 - ive done the math, even in a knockdown the leeward drain would be above the waterline, so is there another reason to cross the drains rather than go straight down?

    5 - ive got about 14" from the cockpit sole to the waterline. how likely is it that in a seaway heeled ill be constantly shipping water in and out as the boat plunges?

    need to get this sorted before i return to the boat. thanks to all.
     
  2. michael pierzga
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    michael pierzga Senior Member

    Below the waterline holes collect marine growth. ID is reduced. Thru transom are superior if your cockpit alows them

    Ive never seen a below the waterline thru hull without a seacock.

    On some boats with deep cockpits a non return line is needed to prevent flooding and blowback when heeled . The farthr from centreline the greater the chance of blowback....try to keep your hull piercing near or on centre.

    Cockpit drains can never be too big. Big holes eat personal belongings and ropes so they need to be grated. A rope vacumed thru a cockpit drain and fouled on prop or rudder will ruin your day


    A solid fiberglass tube ... No flex link..must be correctly engineered and bonded to the hull because it is now structural.

    Even with thru transom drains a pair of drain holes should be set into the front of the cocpit so that the cockpit will drain if the boat is off trim...hauled at the shipyard bow down

    rail down drainage
     

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  3. PAR
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    PAR Yacht Designer/Builder

    Large diameter pipes straight down wouldn't be the logical route. In an aft cockpit sailer, the cockpit corner scuppers would crisscross, to prevent down flooding when heeled. If properly arranged, you'll still have sufficient slope to the leeward scuppers to drain off the cockpit at normal angles of heel.

    The quick way to figure this out is a scale sketch of the cockpit sole and hull's midship section (body plan) at various angles of heel.

    Scuppers aren't typically considered structure, even if 'glass tubes, tabbed to the hull, unless it's an all out racer, needing every ounce of laminate counted for something.

    There's nothing wrong with 'glass tubes, directly bonded and it's often done this way, to prevent leaks at junctions. Hoses are used, so you can route around things, as a direct path to the through hull may not be practical on some designs.

    If you want simple, direct them aft, while still crisscrossing them, from all four corners. This way pitch and yawl are handled as well as roll.
     
  4. alex_sailor
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    alex_sailor Junior Member

    HI PAR

    Thanks for that note, as I am restoring an old 70's boat where the drains went to the same side thru-hull as the seacock, and wondered about that, so I would be recommended to criss-cross as you suggest, maybe even fore and aft.

    Alex
     
  5. PAR
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    PAR Yacht Designer/Builder

    A self draining cockpit is dependent on only one thing, sole height above the LWL. If you have the height, yep, install scuppers. Sailboats typically need to have crossing scuppers, because they flop over underway, but a powerboat can live with straight out the sides or an aft slope sloe (bad idea in a sailboat) to transom scuppers.

    Again a simple scale(ish) drawing will quickly show if you have the room. An hour with a tape measure and a sketch pad will be all it takes.
     
  6. DCockey
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    DCockey Senior Member

    And then there are the sailboats and powerboats with a self draining cockpit and extremely large aft transom scuppers - an open stern.
     
  7. michael pierzga
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    michael pierzga Senior Member

    Open cockpits only work when the steering station is on top of or aft of the rudder post. For boats with steering forward, or deep cockpits, over the transom drains are impractical.
     
  8. DCockey
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    DCockey Senior Member

    Why don't open cockpits work with the steering station forward as long as the cockpit sole is sufficiently high and has sufficient slope aft?
     
  9. michael pierzga
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    michael pierzga Senior Member

    The reason you moved the steering cockpit forward was to gain interior volume. A typical forward steering cockpits sole would be the engine rooms ceiling.

    Unless its a race boat, open transomes waste much interior and deck storage space
     
  10. michael pierzga
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    michael pierzga Senior Member

    Remember to install grates over the drains. Drains eat snickers bars, flying fish, squid, jellyfish, weed and become clogged.

    I prefer ss countersunk washers with two ss bars welded between
     

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  11. PAR
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    PAR Yacht Designer/Builder

    An open transom is just a weight saver, not a cockpit drain and typically used on a racer. Forward helms are like Michael suggests, for interior volume though typically, you have more then just a forward helm, it's also raised considerably, often for an aft cabin, which wouldn't be possible with an aft cockpit.

    Cockpit drains do two things and the first is obvious, drain boarding water. This can be accomplished in a few different ways, but isn't as big a consideration, as even bilge pumps can handle a few inches of boarding spray, on a windward slosh. This isn't the big one. It's how to handle the weight of a boarding, wave that can fill the footwell or cockpit. This is when you can get your butt in a sling quick. If the cockpit sole slopes aft, the bulk of the weight will also be aft, which can make the boat all but helpless as it drains. On the other hand, if the cockpit sole slopes forward (and drains from that end) the boat will still wallow around a bit particularly initially, but as the weight is concentrated much closer to midship, she'll recover faster. This is a serious consideration on offshore yachts and one of the little design details, not much discussed or appreciated by non-designers.
     
  12. michael pierzga
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    michael pierzga Senior Member

    Ive never met an open transom boat that i could live with.

    Poor Access to steering gear, autopilot and aft storage make them clumsy. Add a waterproof bulkhead forward of the rudder post and it becomes even worse

    When an open transom boat takes water over the deck...the water certainly drains fast..along with every sheet tail and your trusty crew

    Its possible to have effective over thru transom drainage without an open transom.
     
  13. PAR
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    PAR Yacht Designer/Builder

    I have an open transom boat and have no difficultly with it. I have installed a safety rail that can swing over it, but rarely use it. It's mostly for those uncomfortable with the hole in the boat's butt.
     
  14. michael pierzga
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    michael pierzga Senior Member

    On the sea bed are hundreds of flashlights that ive lost overboard via open transoms. Perhaps thousands. These flashlights are expensive and drive up the cost of boat ownership
     

  15. PAR
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    PAR Yacht Designer/Builder

    Lanyards are a handy thing . . .
     
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