Does this hatch profile look manufacturable?

[gpr5027]

Hi all,

I'm Geoff, and this is my first post here. I'm in the process of designing a teardrop and have found a lot of good information on this forum. I've never built a teardrop and I have yet to get off the tube to begin prototyping of any sort - so before I get too excited and too far down any one path, I figured it's time to start asking for feedback. I couldn't find a build thread where anyone had attempted something like this, but if there's any out there I would definitely appreciate the link(s).

So for all of you hatch experts, how difficult do you feel it would be to construct a hatch for the profile below using "industry-standard" techniques (i.e. vertical wooden ribs that define the profile, horizontal wooden ribs for stiffness, plywood skin on exterior / interior)? It looks like sealing could be a major issue if there's even the slightest bit of warpage in the hatch relative to the walls due to the dual-radius nature of the profile. I'd like the hatch to extend all the way to the frame, not above it as I've seen some similar builds done. I'm highly driven to make this shape work if I can, and pursuing the wooden construction route is preferable for cost reasons as opposed to laser cut or waterjet aluminum parts (though this is an option). Has anyone ever done this? Thoughts, questions, criticism?

Thanks ahead of time.

Hatch Profile Iso.PNG (280.58 KiB) Viewed 3280 times

Hatch Profile XZ.PNG (41.8 KiB) Viewed 3280 times

[edgeau]

It is more extreme than I did but I did have two radii to make my shape. I only went to vertical not back under like your plan. What I failed to consider was the grain direction in my ribs. Once I put the ply on I had 1 or 2 cm spring back. Get the ply that is made to be flexible for the skin and use plenty of 'the mix' to seal it.

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[tony.latham]

for the profile below using "industry-standard" techniques (i.e. vertical wooden ribs that define the profile, horizontal wooden ribs for stiffness, plywood skin on exterior / interior)? It looks like sealing could be a major issue

How wide is your cabin? And how long is your hatch? I would make sure the hatch can be covered with a single (5'x5') sheet of Baltic birch assuming it isn't greater than 5' in width.

I build using a modified Fredrick's style hatch. Dust and waterproof. "Industry standard?" Perhaps not, but this style hatch is popular.



The hatch ends are removed from the walls while still on the bench. The spacers and lip pieces are also added to the galley walls while still on the bench. (The internal galley wall forms a rain gutter and mates with the seal.)

Obviously, I don't see the need for extra ribs in the hatch. (You don't need them in the front curve of the cabin, so why in the hatch?)

Here's the wall/hatch detail. (I build sandwiched walls.)



As far as building with that profile –-as long as it's less than 5' wide-- I don't see a problem with the exception of that little whale tail at the bottom. I'd have to think about it for a night or two.



Tony

[gpr5027]

How wide is your cabin? And how long is your hatch? I would make sure the hatch can be covered with a single (5'x5') sheet of Baltic birch assuming it isn't greater than 5' in width.

The cabin is something like 56" across, I'd have to check for the exact number but it's driven by the width of a full-size mattress + a inch or so of clearance on either side. I haven't measured the length of the hatch yet (I change things so often that it's still largely undefined) but working within the 5' length for BB is one of my constraints. So, let there be an assumption that the hatch can be covered with a single 5'x5' sheet.

This post was extremely helpful and has given me some food for thought. I like what you did and I think I could accomplish something similar to the cross section you posted of the hatch detail. The main difference is that my frame will utilize welded steel uprights and crossbeams to do most of the structural lifting - which is what the exterior and interior plywood walls will mount to - but I think I could get away with deleting the 1/8" spacer and the extra 1/4" galley wall, by leaving the interior wall proud relative to the exterior wall. So that the edge of the interior wall is what will interface with the seal on the hatch side. If that is hard to visualize, I will get some photos posted once I get around to making these changes in CAD.

Also, your "build #4" thread in your sig is fantastic. I'm pulling lots of ideas and inspiration now. Do you have any more information on this "Fredericks hatch"? I saw a book mentioned somewhere on the forum..

[aggie79]

I think the hatch is "do-able" except for perhaps the reverse curve portion at the bottom of the hatch. The ribs/spars won't cause issues, but getting the exterior ply or aluminum to reverse curve will be tough. As Tony stated, a consideration is available material sizes with regard to the length of the hatch. With a long hatch, you will hatch need to butt joint or splice your plywood and lap exterior aluminum. Another consideration of a long hatch is weight.

My hatch is almost 6' in length and has a compound curve. The last portion has a tight radius. I had to butt joint the plywood skin and overlap the aluminum exterior. The plywood is butted at a cross-spar.









The bottom of the hatch seals to a perpendicular face/end of the floor. (The sidewalls extend beyond the end of the floor.)



I'd recommend working out all of the details before you start building the hatch so you don't box yourself into a corner.

[tony.latham]

The cabin is something like 56" across...

A queen-sized foam mattress will fit in a 60" wide (mines 57.5 on the inside) cabin just fine. Depending on your chassis, you might want to bump it up 4".



Tony

[saywhatthat]

As much trouble some have with hatch spring back .Why not build the hatch first then build the rest of the pod to fit
la trappe?

[tony.latham]

As much trouble some have with hatch spring back .Why not build the hatch first then build the rest of the pod to fit
la trappe?

If you build the frame in situ, it'll fit. If you add 1/2" gussets on the ends, it doesn't spring back.





I find building a hatch a fun project.

Tony

[gpr5027]

Thanks for the feedback guys. Y'all have done nice work. My plan as of right now is to design every detail down to the last piece of hardware before I commit to building. This way, I'll also know if I can afford it

Tony, I like the idea of building the hatch in place. What kind of hurricane hinge did you use? And is that edge trim along the roof necessary? (I guess this would be underneath of the hinge once installed)

Tom, in your build thread I noticed you mentioned that you would've used the "frederick" style of sealing if you could do it over. Are you referring to the profile of the sides (as Tony's cross section diagram illustrates) or only to the hinge? How well do your sides seal?

[edgeau]

I also found trying out some ideas with scrap wood first helped me avoid expensive mistakes. With hindsight I woul have done more of that

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[tony.latham]

Tony, I like the idea of building the hatch in place. What kind of hurricane hinge did you use? And is that edge trim along the roof necessary?

That's Vintage Technologies' HT06 Flat Hurricane Hinge.

Edge trim? There is none.



Tony

[gpr5027]

Edge trim? There is none.

Err, sorry - Edge gasket? Weatherstrip? Edge seal? I'm still learning the lingo here. Where your roof skin ends and where your hatch begins, you notched the edge of the roof and installed some type of gasket. It looks like it lives underneath of the hurricane hinge once everything is buttoned up. Is this necessary, or is it extra insurance against water ingress? How did you know where to locate it relative to the hinge?

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Edit: Hatch detail coming along..

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[tony.latham]

Edge trim? There is none.

Err, sorry - Edge gasket? Weatherstrip? Edge seal? I'm still learning the lingo here. Where your roof skin ends and where your hatch begins, you notched the edge of the roof and installed some type of gasket. It looks like it lives underneath of the hurricane hinge once everything is buttoned up. Is this necessary, or is it extra insurance against water ingress? How did you know where to locate it relative to the hinge?

Capture.PNG

Capture1.PNG

Edit: Hatch detail coming along..

HatchDeatil1.PNG

What you are seeing is the 3/8” gap between the oak cabin spar and the first hatch spar. The gap is needed to allow the hurricane hinge legs to fit in.

Good eye.

Tony


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[pchast]

As to the cost of building....

I'd suggest that its a big project and so costs and material acquisition
can be staged as needed. For me that meant that I could finance things
out of future pay checks. It was more do-able.

[noseoil]

I don't see any problem with the hatch design. "Tumble home" would be the boat-building term for having the reverse curve on the bottom (actually the top decks above the sheer), but it does limit the overall amount of galley space at the bottom. I chose a simple Benroy design to utilize as much floor length as possible (a first build), since we needed all of it for the design & stuff which was going into the build. There just wasn't enough room, without the straight front & rear at the deck. Just keep looking at builds, to make sure you have it all designed & thought out prior to starting. The small stuff can kill you later in the build if something is left out in planning.