windows and hatch rails

[JeffM]

I want to put simple windows in the cabin I am building, so I am considering Lexan.  But it's pricey stuff, so I don't want to experiment too much.  Has anybody here worked with it?  How much of a thickness do I need for sturdy windows that could be as much as 10x18"?  How is it cut?  How fastened?  How sealed?

While I'm trolling for advice, here's another question: what should I make my hatch slides out of?  My main and forward hatches will both slide, and will overlap, with the main sliding over the forward.  My best idea so far is to have one set of double-edged rails, with the forward hatch using the insides of the rails and the main hatch the outsides, so they can be moved independently without interference.  I'm thinking the durable, slippery plastic (UM-something-or-other) that I've heard of used for bottom protection and whatnot.  Ideas?

[Charlie Jones]

on the Lexan- it's extremely easily worked- you can cut it with a skill saw, saber saw, band saw or table saw- it can be cut ( shaped) with a sharp wood chisel, or planed with a hand plane.

It's also easily scratched so leave the paper on until you are done, or tape over the bare Lexan with duct tape or packing tape before working. Drill holes slightly oversized and use pan head screws NOT countersunk ones, Counter sunk screws will eventually break out the edges due to expansion/contraction of the plastic.

As for thickness I'd say that depends on how you intend to sail the boat. If you are gonna be inshore most of the time, 1/4 inch should be fine. If you are going offshore a good bit, then for ports that size I'd go with 3/8. I just put new ports into a Pearson Vanguard for a customer who will be sailing offshore and used 3/8. He was able to purchase a 4 x 4 sheet by the way.

I can't see how you'll have one hatch slide over the other, and seal the one that goes over the top- got any pictures or sketches?

But your cabin sides may determine how thick you can go- if the :exan is bolted through to the cabin sides, then whatever you need will work- if it has to fit pre-existing frames, you are limited to what they will handle.

[Hirilonde]

Since you want to discuss material and its properties it would be best to start with the proper name for it.  Lexan is a brand name, not a material.  The material I believe you are referring to is polycarbonate.  It is made by several companies now.  For dead lights, aka fixed ports, like you are describing, it is the right choice. I agree with Charlie about thickness and working characteristics.  I find it is more dimensionally stable than most give it credit for though.  I find expansion and contraction isn't really an issue.  McMaster-Carr has a great write up about the stuff in the catalog. 

It is however easy to crack by fasteners.  I think the break out Charlie refers to is really caused by uneven pressure caused by the fasteners or damage caused by the countersinks themselves that manifest themselves as a crack later.  It is easier and safer to use pan head screws, but carefully countersunk screws, only lightly fastened, with a flexible bedding can work.  Most manufacturers recommend using a special long taper bit for drilling holes.  These however only work properly in a drill press where you can resist the tendency of the bit from grabbing and drawing itself into the hole too quickly.  Slow easy drilling with a really sharp slightly over-sized bit is the key.

For a project like yours, unless you have trim hardware it can simply be a piece cut to the shape of your opening except larger.  The edges can be rounded over with a router, and the piece then bedded and screwed or bolted over the opening from the outside.  Simple, neat, and very functional. 

Your hatch description has me confused too.  Off-hand I think you are looking to achieve more than you can.  If I read you right you want most of the middle of the cabin top to open up, and for it to be covered by 2 slides that overlap when open?  I see 2 issues here.  That the opening runs from the aft bulkhead to  significantly forward weakens the cabin top considerably.  As Charlie has pointed out it will be near if not impossible to make it weather proof. 

I understand you want the opening to provide head room when on the head?  I suggest you think this through more and try to achieve that goal while simplifying your design and minimizing the size of you opening in the top.  A really large opening compromises a lot of the strength your frame and cambered ply are trying to achieve.  Make and post a sketch of your top.  It will be a lot easier to comment and critique if we can see where you are going with it.  This design is going to be a challenge, and the result will require a lot of compromise.  Balancing cabin height, looks, function, strength, etc. is a challenge on a 25'-27'  boat, never mind a 20'.  Naval architects have been struggling with this for years.  No design will meet every criteria you want.

[Charlie Jones]

[quote author=Hirilond

[PAR]

I've always attached these plastics with rubber washers and the fasteners went into over size holes. Expansion can be absorbed if there is room, which the rubber fastener gaskets provide.

You can buy aluminum extruded tracks for sliding windows. twin 1/4" tracks are common. If not you could use single U shaped extrusions, found at most big box stores.

[JeffM]

Thanks very much for the info and recommendations on the lexan--oops: polycarbonate! 

    My wood-and-cloth top (now defunct, since I canibalized it for some of the parts for the new roof) had a full-length slot top.  Needless to say, it wasn't strong enough to put significant weight on, though I could safely lean on it for balance when coming off the foredeck or walking round the sidedecks.  I closed this opening with two overlapping hatches--aft over forward so they would shed water properly--that fastened to the slot edges with tabs that rode under the edges.  Because the aft upper cover needed to almost fully overlap the lower forward cover, it had only two such tabs rather than the more secure four on the lower.  (I had a provision that could secure both hatches firmly when closed, though I never used it.) 

    In the new roof, these tabs wouldn't work, since the slots are no longer than the covers.  My best idea so far is to run a single rail on either side of the hatch openings the whole length of the roof.  The rail would be a "T" in cross-section.  The lower forward hatch cover would have a ridge that would catch under the inside of the "T", while the aft upper cover would have a ridge that would catch in the outside of the "T."  The fit would necessarily entail a certain amount of slop in the cover overlap, and between hatch edges and coamings, but the old version did. too, and it kept rain and spray out pretty effectively.  If I can figure out how to post drawings, I will.

[Hirilonde]

If I can figure out how to post drawings, I will.

You can simply photograph a sketch or use something simple like MS Paint, then post as you would any picture.  Not hi-tech, but it works.

example of MS Paint:

[JeffM]

Okay: camera is down, don't have paint.  Spent a loong time fussing with the "draw" functions of Word, and here is the best I can do.  Top is cross section through cabin top at main hatch looking forward, so you can see how hatch cover hugs outside of hatch coamings (which are ash).  (I made an attempt to show one version of ledges meant to hold hatch cover down, best visible on the right side of the hatch cover.)  Next image is profile of roof showing how main hatch cover is higher, and can ride over fore hatch cover.  Last image is plan view of roof without hatch covers, so you can see that main cover will ride outside narrower fore cover.  (Dotted lines show beams supporting roof; these are centered about a foot apart.)   All this is rough and eye-ball proportioned, but near enough to get concepts across.  To help get bearings, main hatch is 24" across and about 22" fore-and-aft, while fore hatch is about 17" fore-and-aft.  Fore hatch cover slides back until it bumps into coaming of main hatch (I only realized the necessity of this after my last post).  Main cover slides forward to overlap fore hatch cover.

The question is this: what holds hatch covers down?  Best I can come up with off the (hardware store) shelf is some sort of ledge sticking out from edge of hatch cover and riding under a ledge screwed to roof.  This would be easy (though ugly) in aluminum, but I guess would corrode and begin sticking very quickly.  The ledges could also be of wood, but I don't know how they'd last and be dimensionally stable with all the wear and tear.

Jeff

hatch.doc

[PAR]

This is a common home made method. Pictured are a hatch and cabintop at the corner, where the slide lives. The slide is a strip of aluminum, brass, bronze or I've seen HDPE used too. It's screwed to the under side of the hatch frame.

This slide often is fitted in a grove, but the groove can clog with sand and salt, so I've made them open at the bottom to drain out what ever might get in.

The cabin top has a set of tapered slides (tapered fore and aft to assist sliding open) and usually a covering piece to hide the end grain of the cabintop. The top of these slides very often is equipped with another strip of HDPE (good idea) which helps the hatch slide easily and doesn't wear out the wood.

Provide lots of slop in this arrangement for expansion. The benefits are it's easy to build, doesn't have anything sticking out to snag lines or fingers, easy to clean and is "captured" so it can't fly off trailering down the road.

To install it, the end piece of the hatch frame has to be cut shallow or made removable. The hatch is slid into it's location and the end piece reinstalled.

I've seen this same technique used on really big, heavy hatches, with rollers much like drawer slides to move the heavy hatch easily.

This is harder to describe then build.[attachment=1]

[JeffM]

Thanks, Paul!  Just what I was looking for.  Let me see if I've got this straight: in the attached image, you would put an HDPE strip on the cabin top beneath the slot for the metal slide to ride on?  Also, in what sense are these grooves in the coaming tapered?  Are they deeper forward to make more slop when the hatch is open?  I can't figure out why you wouldn't want them parallel (with adequate slop built in) their whole length.  Aluminum for the slides screwed to the hatch edge would certainly be easy and cheap, but I picture the aluminum corroding, getting crusty, and eventually jamming.  I suppose they could be replaced easily, though.

[PAR]

The attached image above has been modified to include labels and a basic side view, showing the hatch in the closed position and open (dotted line).

The hatch generally looks better if mounted at an angle (shown) which also helps it slide forward when opening it, but more importantly does not suddenly slam closed if the boat pitches.

On top of the cabintop mounted rails (which the hatch rides on) I usually screw down a thin strip of HDPE, which helps tremendously with heavy or sticky companionway hatch covers. It also will not eat into the wood of the hatch (a way cool thing).

The problem with most home made sliding hatches is they are made with too tight of tolerances. The nature of the sliding hatch requires it literally wobble back and worth as it's shoved each way. The gaps should be huge, say 1/8" or better. The slop around the aluminum strip, the fit of the hatch frame over the rails, etc. all need to be fairly sloppy. You'll find that drawers in cabinets also need this level of slop to work reliably. Anyone who has tried to open an old plastic sliding hatch on a well worn production boat, can testify about how hard a close tolerance hatch is to open or close.

Yes Jeff, in plan view, the rails are parallel. With the slop built in, HDPE and dry lubricants on the aluminum slide, corrosion shouldn't be an issue. In fact, corrosion is a maintenance issue anyway.

[JeffM]

Paul, thanks enormously for taking such pains!  It's all crystal clear, now.  As it happens, the slope of my cabin top will make it unlikely the hatch would close unexpectedly.  It also happens I'm much better at slop than at close tolerances.  (I knew there was a good reason for my indifferent woodworking skills!)  I'll immediately go looking for the right sort of HDPE.

[PAR]

Go to Wal-Mart and pick up a cheap cutting board. The vast majority will be made from HDPE. Cut to length, butt edges to make long enough, etc. It's supper easy to cut and machine. It doesn't glue or hold fastener threads worth a damn, but counter sunk through fasteners work fine in this application. Using HDPE strip on the hatch (as well) will insure nothing wears out except the strips.

[Hirilonde]

PAR's sketch is typical of many boat's companionway hatch.  It is very similar to my Renegade's.  This is why I mentioned earlier that a slider was not going to make your cabin even close to water tight.  They can be made water repellent however.  I think you will find it is what you are looking for, but not water tight.  Most cruising boats use sea hoods and/or dodgers as well to keep these hatches even drier.  I can't see a dodger fitting under your main sail, and adding a sea hood can be done later if it proves necessary.  In the mean time, it seems you are headed towards some final plans for your hard top. 

[JeffM]

I want the hatch reasonably weather-tight.  That seems doable--at least in the weather I'm likeliest to see.  Keeping the sea out, on the other hand, is a job for hatch placement, which is why I'm a bit nervous about my off-centerline companionway.  My hope is that the port edge will be above the water in a knockdown, and that a closed hatch will at least slow water sloshing in via waves until I can get back upright.  This is guess-work, since I have only my one capsize test to go on, and that in sheltered water with the boat empty and configured quite differently.