putting in a hatch/skylight

[Crip]

Evening all.
I want to put a hatch/skylight into the roof of the cabin on my boat. The summer's coming and it's stuffy enough down there already.
I've obtained a 12" square, Lewmar hatch, the frame (aluminium) of which is designed to be mounted on a flat deck. The cabin roof has a camber.
My question. Is it going to be easier to cut out the hole in the cabin roof first, so as to scribe the camber onto the wooden top and underside frames that I will have to mount the hatch on using the cut-out piece, or, is there a method whereby I can build the frame first, then cut out and assemble the parts in situ? If you follow my drift...

Cheers,

Crip (http://crip.moorey.net/toplevel/boats.html)

[paladin]

forget the hatch..unless you just want a lot of light........obtain (beg, borrow...steal...or dogonnit just go out and buy) about 4-6 Nicro 4 inch solar vents. install half forward and half aft...and set the blades for one set to exhaust and the other to intake and you will get a lot more ventilation.....especially if you have a wooden boat....and they work all dfay and all night on the internal battery and charge during the day.......prevents mold and mildew....

[Bob Cleek]

Paladin's right. Even a few old fashioned cowl ventilators would probably be better. You don't want to go cutting into your cabin top before you know what effect that's going to have on the structure, anyhow.

[Rosebud]

Crip,

The aluminum hatch needs a flat frame to sit on - called a coaming. Since your boat has a curved cabin roof, you can build a coaming - that is a wooden frame that is curved on the bottom to fit flush on top of your cabin roof, but flat on top to let your hatch sit tight. Make the coaming 2 inches tall or so at the lowest point of the curve. It will be 1 inch or so on the high side - depending on the curve. Make the coaming 1 to 1.5 inches thick and bolt it through the roof.

You don't need to cut into your roof first - this coaming will maintain the original strength.

Good Luck!
Good Day,
Jim and Kim

[ 05-05-2002, 07:42 PM: Message edited by: Rosebud ]

[Crip]

Thanks Paladin. Thanks Bob.
If ventilation were the only consideration, the Nicros would do the job. The trouble is going to be passing beers... umm a spin through a fan. It might put a bit of a head on 'em or shred it I mean.
Rosebud's on my wave length. I figured that building the 'coaming' might be the way to go but am hung up on getting the camber of the cabin roof right... any ideas on that one?

Crip (http://crip.moorey.net/toplevel/boats.html)

[Charlie J]

Take a strip of thin plywood about 2 inches wide and slightly longer than what the hatch will be. Mark a centerline on it (vertically). Mark a center on the cabin roof. Align the two centers, block up the ply so it's level athwartships. Scribe a line using a block and a pencil, sliding it along the cabin roof. If you have a changing camber, do the same thing for the other side of the hatch. Cut to the lines, then use that as a pattern to scribe the coaming and work it down to the line. Keep fitting it to the cabin roof till it fits exactly. Then install.

[Rosebud]

Crip,

Another variation on Charlie's method would be to build the coaming first out of full dimensioned sides - like 1x4 or whatever you're going to use. Fasten it together temporarily. Set it on the cabin roof and shim it level. You can mark the curve right on the fore and aft pieces using Charlie's method. The ends of the curve give you the bevel needed for the bottom of the starboard and port pieces. Take the coaming apart, cut the curves and bevels, put it back together permanently. Then it should be pretty easy to fit and you'll get a good flat top for the hatch. You can use the cutout curved pieces for and inside frame if you want.

Good Day!

[JimD]

Charlie's method (or variations of it) is a great way to scribe many curves, about the only trick to it is the obvious need to keep the block with the pencil perpendicular to the curve you're copying while you're moving the block along

[Matt Middleton]

Not trying to be contrary, but it seems to me you'd want to keep the spacer or block vertical rather than perpendicular to the curve- that would yield a mismatched radius between cabin top and hatch frame. It probably won't be much of an issue in practice anyhow, since the difference in radius would only be a few inches compared to the presumably large (several feet)radius of the cabin top. You'd just have to sand/plane to fit.
Keeping the block vertical will give the same radius, while keeping it perpendicular to the curve will use the same center and give a larger radius.
Good luck with the install!
Matt

[Charlie J]

Absolutely true and a very good point. Especially on tighter radiuses (radii?) I doubt it would have a significant bearing on something like this though since the camber of a normal cabin top is so large. But for smaller radius curves you can sure get some lousy fits if you don't keep the block or compass aligned parallel- and ruin some good wood in the bargain.

[Alan D. Hyde]

It seems that the geometry of what you're trying to do means that to obtain a precise copy of the curve, your scribing tool must be in a constant perpendicular relationship to the chord of the curve being copied.

Alan

[ 05-08-2002, 10:44 AM: Message edited by: Alan D. Hyde ]

[Crip]

Ah ha!
Now we're getting there. Many thanks to Charlie, Jim, Matt and Alan.
So, having tried this scribing method before and almost always ended up with results as described by Charlie -
can sure get some lousy fits if you don't keep the block or compass aligned parallel - on anything like a tight curve, my question now is this: How do you keep the pencil/scribe perpendicular to the cord of the curve. 'By eye' would seem a likely answer, but lets assume I'm a boss-eyed old codger with the shakes and my boat's in the water with a bit of a chop up... exagerating of course but interested to hear if there's a fail-safe method for this or whether it's just down to practice and 'eye'.

Cheers for all your help so far.

Crip (http://crip.moorey.net/toplevel/boats.html)

[Ron Williamson]

Crip
Log housebuilders do this all the time using scribers with adjustable leveling bubbles.This method is pretty failsafe,but the scribers aren't cheap.
R