Hatch with flange or flat base?

[dahlke]

Happy holidays

I would like to replace the old ventilation hole-thingy in the cabin (not the head) with a hatch. I seem to remember Bob has done the exact same thing - although I can't seem to find the pictures I remember seeing I'm looking at a Lewmar size 10 ocean hatch (and trimkit). Size 10 is 32.4 cm x 32.4 cm. I'm worried that if I choose a bigger hatch the curvature of the deck will result in a big gap (>1 mm) between the deck and the base of the hatch. Lewmar has two versions of their ocean hatches - one with a flange and one with a flat base.

Hatch with flange: http://www.lewmar.com/products.asp?id=8018&lid=25098
Hatch with flat base: http://www.lewmar.com/products.asp?id=8016&lid=25097
Trimkit: http://www.lewmar.com/products.asp?id=7964&lid=24920

With one should I choose (flange or flat base)? Lewmar doesn't seem to provide much information.

[Bob McGovern]

Hi Mads. Whatever hatch you choose, the barrel-shaped deck of the Ballad may require creating a flat surface to get a good seal. Hatches really don't stay waterproof unless the mounting surface is very flat. Or at least as flat as the hatch frame, which isn't always. I prefer a slightly raised curb (3cm or so) to keep water away from the sealant and provide a suitable mating surface. There are lots of ways to make a curb, from belt-sanding a plywood ring to making a composite ring (my usual method) to casting a raised curb in place using thickened epoxy. It is quite simple work & will result in a strong opening that prevents leaks.

Our new hatch is 20x20" (50x50 cm); we hope it doesn't interfere with the boom or vang! The cutout is about 40x40cm. It has a flange. This area of the deck has a wire running thru it on a diagonal from port aft to stbd forward; it's almost impossible to miss cutting it if putting a hatch in this location. The core on our boat is about 1" thick and the two skins are joined by evil little loops of roving. The foam core really needs to be cut back from the opening and filled with putty/resin/epoxy filler; some say 1cm is enough, but I like going a full 2cm+, so there is no danger of the bolts leaking later.

I made a glass-reinforced ring in a mold, ground the bottom to fit the curve of the deck, and epoxied it to the deck (ground down to bare GRP). The ring was spot-sanded until the hatch sat flat. Then I wrapped one layer of fabric from the cabin inside, up the opening, then over the collar & onto the deck; then the whole thing was filleted smooth on the outside, blended to the deck, & temporarily painted. The hatch is through-bolted using binding posts ('sex bolts') & bedded in sticky butyl tape. It will need to come out when we paint the deck.



This approach is far stronger than it needs to be, but that's just how I tend to work. What's important is a close-fitted mating surface between hatch and deck, so the sealant is not asked to bridge any gap. The installed hatch:

[prjacobs]

That looks good, Bob ... I think I see a future project when I'm out of things to do!

[dahlke]

I must agree with Peter - that looks very nice indeed Great job, Bob! I'm now considering a bigger hatch

How did you make the mold?

I'm going to replace the main cabin light (the one in the ceiling) with two or three build in LEDs. I'm 6'3" and got tired real fast of bumping my head against the big original ceiling light. So no matter what I'm going to need to do a bit of rewiring. I already have the new LEDs ready now it's just a matter of waiting for spring

[Sanlen]

Looks really nice Bob I would like to do that mod on our boat but am a little concerned since we have installed a rodkick and also sometimes store a liferaft (in valise) on the roof. Looking at this picture (my youngest son on foredeck ), I think only a small hatch will fit without interfering with the rodkick.

[Bob McGovern]

Vang clearance may become an issue; the Ballad's boom is small, so a Boomkicker may be one answer; or just a shorter rigid vang with steep angle.

http://www.boomkicker.com/

We did locate the hatch as far aft as possible; the cut-out barely encompassed the old mushroom vent location. A smaller hatch might be needed for low-angle vangs.

Casting a ring is easy, either in a mold or on deck. For remote casting in a mold, I used a scrap piece of melamine for the base; any flat bit of panel product would work & could be covered in plastic sheeting. Then I screwed another (2-part) piece of particle board on top, with a groove between the parts: the inner perimeter of the groove matches the hatch flange/deck cutout, plus some wiggle room. The outer perimeter represents the outside dimensions of the hatch base. Smooth and wax the pieces well, reassemble the mold, and pour/lay your resin mix into it. I used 2cm strips of fiberglass tape and slightly thickened epoxy (silica filler). You can also use polyester resin, chopped glass fibers, glass beads, or (for insane strength) lengths of wetted-out loose roving laid around the circle. You want to go slowly so the casting doesn't overheat & warp. Poking it to get the bubbles out saves filling them later.

When it is fully cured, you can grind the bottom side carefully to fit the approximate deck curve, epoxy it to the deck, and then fair it in. I wrapped it with a layer of glass fabric, but that's really not needed. The epoxy bedding & filleting plus the thru bolts will hold it in place forever. I'll grab a couple more photos today.

Like you, Mads, we are switching over to LED lighting everywhere, so clipping that little wire (which serves the old dome lights) was no great trauma. The added light and ventilation from the hatch is a great improvement!

[Bob McGovern]

Here's a couple more photos of the deck hatch on Fionn. The cutout, with foam core gouged out (ready for epoxy filler) and top skin ground down to raw glass:



We braced a piece of waxed plastic laminate underneath to prevent drips. A better photo of the finished curb:



The paint is temporary, since the whole deck will need to be prepped & repainted anyway. And here is a 30cm straitedge to show the deck curvature roughly where the mushroom vent is/was:



That's a lot of gap for a sealant to bridge. A simple way to approximate a curb might be to grind away the inch or so inside the hatch dimension (maybe before making the cutout?), build a little dam with modelling clay, fill inside it with thickened epoxy, place the hatch itself inside a plastic bag, and press the hatch right down into the epoxy. When cured, remove the hatch and there is crude curb you can shape into the finished item. Could also used a piece of waxed melamine (shelving material) to create a flat surface by pressing in. Then make your cutout. We did something similar for the chainplates: