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07 March 2014

Flush Hatches in DIY Ply

Looks like this when put together.

Most real sailors advise not to go barefoot. HA. Muh dawgs cain't breath! If it's warm and sunny, the clothes come off; and that means gumboots, too.

But I do hate stubbin' my toe, or worse, turning an ankle. And Anke and I love to sleep on deck, of a starry night. Hatches in the cockpit, mounted proud, are rain on our parade.

So, one day I was envying fiberglass and metal boat flush hatches out loud when it came to me! A plywood sub-deck is the key. Or at least a key.

Considerations include the following:
  • The cantilevered trough/coaming structure is inherently weak... it needs considerable beef.
  • The trough has to be wide enough for easy (and frequent) cleaning.
  • The trough has to be deep enough to keep water from spilling over the coamings.
  • A gap under the hatch lips allows debris to drain without binding.
  • The hatch has to have enough room to be raised - angling up, over and leaned back along one edge.

In SLACKTIDE, we built a set, and - despite beginners' mistakes - they work very well, despite needing work on the first four points.

Downside is that these are relatively complex, especially with ply/foam/ply decks. They add considerable building time and material. But, given that we dislike footwells and want a big hatch for access to the main hold,  the results are worth it to us.

Shows the two main components;
Deck and Transom with on-edge framing, and the SubDeck Assembly

The larger opening is about 3ft6in square.

The on-edge framing provides most of the stiffness. The flat, SubDeck framing should be strongly joined (glued and fastened) into it, and it, in turn should be strongly joined to any structure available. A large hatch will need a bulkhead, mid-span, to add further support. We'll put ours under the gap between hatches.

Joinery is a matter of preference... hatch lips and coamings could be ship-lapped or dove-tailed, for example. We tend to just use simple overlaps, edge gluing and nailing. Fast, has never leaked, and if something bad happens to it, it's easy to repair. We'll fillet inside corners, in some cases, but not often.

Main thing to remember is that when two pieces meet, the uphill one should overlap the downhill one. Water is thus encouraged to flow by the joint. If ship-lapping, the lowest laps follow this rule.

To tell the truth, that 1/4in ply SubDeck is mainly a contiguous membrane to cover our lazy joinery at the bottom of the trough. If you're a real boatwright, you can approach the trough/coamings in a number of more elegant ways.

One bit not shown... the end of the trough tends to dribble down the transom, meaning we have to 'wipe our beehinds' often. We've been meaning to fit some copper (flashing) to form a small spout out clear of the transom.

This aft draining hatch isn't totally necessary - the usual thing is to use a drain hose. But they are a pain when they clog. If you do use one, consider a straight shot of tubing that can be poked clear with a rod (1/4in all-thread works as well as anything's going to... the threads help grab obstructions).

If built right, you probably won't need a gasket for inshore sailing. But it doesn't hurt. We like to mount them on the underside of the Hatch itself, so that it will press down on the coamings. Neoprene works best, but closed-cell weather stripping is cheap and easily replaced. If you can find an old wetsuit, they work great.

So there ya have it... if you end up sleeping out on one of these... sweet dreams!


  1. Saved this by screenshots for future use.... much appreciated. I saw these hatches on your current boat and immediately saw their value. This is one virtue of a catamaran that most monhullers never enjoy: a nice flat open space for any number of delightful uses. Just a few nights a year of profound meteor showers alone justify such added complication in building but these diagrams really ease DIY effort. Right on.

    1. Hi BonRobi,

      Glad these help out!

      We normally don't do anything like full plans, and what there is is usually scribbled on paper. But doing a lumber count in my head was beyond me, and doodles weren't up to it. Hence the SketchUp drawings.

      It's been very helpful to look at them from every angle... they're by far the most complicated structure on the boat, but you're right about the payout!

      Dave Z

  2. Posting on behalf of JOHN:

    Hello Dave,
    Your flush hatches are a great idea. Are they hinged at all, or just set in place? Are there any latches, or are you just relying on gravity to hold them in place? How do you grab them to lift them up?

    My understanding is that the deck hatches access the holds at each end of the boat. Will those holds also be accessible from inside the boat? If not, it may not be necessary to insulate the hold-portion of the deck, saving time and material. Even if the holds are accessible from the interior it may be worthwhile insulating the bulkhead between the hold and the interior, install a well-sealed door, and not bother with insulating the hold.


    1. Hi John,

      They could be hinged, but we don't... this lets them be moved around as portable work surfaces.

      For a handle, we've fixed a strip of webbing at the forward edge (due to the 'gap' located aft, the hatch can only open from forward... other wise its lips are pried hard by the narrow troughs). This flops, a bit, but doesn't catch on anything, and mostly lays dociley flat... if you step on it, it's easy on bare feet.

      We haven't installed dogs (latches), aft. Still mulling that one over, though there's never been the shadow of a need (waves have NEVER walked aboard). Would definitely do something if headed off-shore, though. Trick is something that won't get underfoot.

      The strongest contender I've come up with is to hinge the aft edge, and lead a tie-down line, fixed to the underside, via fairleads through the bulkhead into the interior, where it can be secured at a cleat. It would need to be long enough to stay threaded with the hatch fully open.

      Forward - hatches proud - we use a tie-down line. It's fixed at an eyebolt to one side, run across through a second, back through a mid-line loop (trucker's hitch style) and secured with a rolling hitch. Maybe ten seconds slower to set up, relative to dogs, but cheap and reliable. No handles required.

      In this boat, there will be a quirk, forward. The Fwd Hold and Anchor Well Hatches are close, in line and near equal in size. Plan is to extend the deck of the Anchor Well Hatch aft to the Hold hatch, and hinge between them. To open the Anchor Well, we'll flip it back onto the Hold Hatch, and sit on its underside while working anchor gear. Since the Anchor Well is self-draining, the lips needn't be close fit, allowing the play necessary for this to work. I think.

      Lots of room for experimentation and step-wise refinement. Hinges and dogs are never as simple as it seems they should be. I've probably spent more time thinking about them, with less succes/satisfaction, than most anything aboard!

      Dave Z

    2. Hi again,

      RE Holds: Our holds are separate and water-tight (for back-up positive buoyancy).

      We've decided to insulate, despite apparent lack of need. We didn't on SLACKTIDE on that same logic, and have regretted it due to lots of condensation.

      Chances are this is due to living in a rainforest. Even with ventilation, there are lots of days we're just exchanging damp air. Meanwhile hold temperature is influenced by the woodstove (albeit slowed by insulated bulkheads), passive solar and sea-water. On damp days (especially going to cool night) when the holds are warmer than ambient, moisture condenses onto the uninsulated walls.

      So, as Bugs Bunny would say, "DISS means WO-AH!" This time, we're going all out!

      Dave Z

  3. Here is a possible solution, in case the link does not wook, Google "Lee Valley Magnetic Secret Latch". Not sure about working vertically, demo was on a horizontal opening door.The nice part is no parts sticking out to stub a toe on, but you have to make sure not to loose the magnet, or place it near your compass,41399,41403&ap=1

    1. Hi Dennis,

      That's a smashin' thang! And good price, too. I could even see a DIY version (nothing more than a sliding rod).

      The one thing I'd be concerned about (a general worry for sliding mechanisms) is when things freeze up... I could sure see myself getting locked out!

      One of our mentors used a simple nail lock for his cabin security. He'd drill a horizontal, slightly oversize hole and slide in a nail (to its head), then give it a tap with a hammer to set flush. Paint its head the same color as the surroundings. Insert the nail to lock. To open, get under the head with the tip of a pocket knife and slide it out (or a magnet!). Worked like a charm.

      Looked just like any ordinary nail unless you knew its secret identity.

      Dave Z

    2. If the top panel was attached with countersunk screws, on the chance that the mechanism did freeze up, you could undo the screws to access the hold & mechanism. It would not be convienient, but nothing would be damaged. Hopefully it would be a very rare occasion that you would need the alternate access.

    3. Good idea... could always leave a screwdriver stowed under a deck seat as back-up.

      Given the kind of stuff we keep in the holds, it would seldom be urgent that we get the lid up. For use on the companionway entrance, moreso, but far less likelyhood of freeze-up.


      Dave Z

  4. This is what we did for the cockpit in Kairos. If there is an error in our approach (and there probably is except for the cockpit cap or cover we've decided to put on, it's that we likely didn't make the depth of the drain troughs deep enough, nor probably our stern scuppers large enough for very, very fast drainage. That could be a problem if water comes in big and fast. We may try a fix, later, once the boat is otherwise completed. We could add another layer of decking (fastened down with epoxy glue and bronze) on both hatch covers and deck, raising both, and adding a corresponding amount of depth to the drains. With 3/4 plywood this would add a bit. The scuppers can always be enlarged, too. We'll see.

    1. Hi Peter,

      Re Scuppers: We've never shipped a drop of water in the cockpit in all our years of sailing (inshore). Good drainage is still cheap insurance, but hopefully never to be used!

      Re Depth of Trough: We underestimated ours the first round, too. Especially, grass and other debris tracked aboard can jam and dam. To help that situation, we cut down our hatch lips from 1 1/2in to 1in, allowing a little more throughput.

      We just completed WAYWARD's aft deck (post soon), and may have swung a bit far in the other direction... They'll drain like weasels, but will interfere somewhat with below-decks storage.

      Livin' and learning!

      Dave Z

  5. As to insulating the holds... From your experience it looks like we may have to retrofit hold insulation in the Kairos. Already we've been plagued by condensation. Ah, Cool Temperate Rainforest!

    1. Hi Peter,

      You might consider starting with good ventilation, overhead insulation and full insulation in that order, as necessary.

      We've always been derelict installing hod ventilation, an it may well be that's all it takes.

      Condensation was worst in LUNA, which had access from the galley. Crew respiration and cooking are major sources of ambient moisture. With separate holds, the problem was lesser, and only seemed pronounced on the under deck overheads.

      One Q&D 'solution' is water tight, plastic bins along the deep ends, where puddles collect.

      Dave Z

  6. I've still got a question as how exactly you made your anchor well self-bailing. Some sort of interior floating flap-valve? (Probably not. That seems overly complicated by KISS DIY principles.) A simple hole let into the bottom of the hull forward of the bulkhead separating the anchor well from the forward hold? That would have to be right at the bulkhead, I'd think, on the lowest part of the forward hull bottom curve in order for the space to drain. (Higher than at the very bottom, it would always be holding water.)

    1. Hi Peter,

      We bored drain holes P&S as you describe, low and against the bulkhead. The hole and any fairing can be epoxy coated, or a 'fancy' lining of copper tubing fit and glued in.

      In my opinion, self-bailing beats free-flooding. This implies drainage above the waterline, at least with the boat upright and at rest.

      If the low end of the anchor well, say, is below waterline, a deck or filling (tar+cement?) can raise it for good drainage. Ventilation/acess is a problem under such a deck, so consider excluding air with expansive foam? Or possibly shaped foam, bonded in and glassed over?

      In WAYWARD, we avoided those complications by keeping the anchor well high on the bow curve, angling its bulkhead to bring its base higher while giving good access from above.

      Dave Z


About Me

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Anke and I live aboard WAYWARD, our T32x8 ketch. We sail by wind, tide and muscle in the waters of mid- to northern Southeast Alaska. We try to maximize the joys of life, and minimize the chores. ........ We live between the communities of SE Alaska, but drop in to visit with friends. Lately, we've worked, every other winter, care-taking Baranof Wilderness Lodge in Warmsprings Bay. This has given us a window on Web. ........ We're working toward a subsistence lifestyle, somewhat impeded by addictions to coffee, chocolate and cheese. ........ We think TEOTWAWKI is looming, and while we won't be ready, we'd at least like comfortable seats.