how big can the hatches be in the third bulkhead?

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max
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how big can the hatches be in the third bulkhead?

Post by max »

Hey there,

I'm starting in on a new build and have decided that I'm going to make custom hatches in bulkhead three following the same basic premise as the aftermarket ones - the backing plates will overlap on the back of the bulkhead by 1.5" around the opening, and extend into the opening 0.75" for a lip for the hatch itself to rest on.

I'd like to make these bigger then those that are marked on the kit bulkhead (which are roughly 21"x11" on mine) - I'm a pretty big guy myself, and I figure being able to get my head and shoulders through the opening to reach all the way forward will be useful. I'm thinking of extending the hatches to be roughly 24" high, and making them wider at the base of the opening, going as wide as I can and still have enough material all the way around the hatch opening for the backing plates.

The question I have is for anyone who has experience with this - is there a "maximum size" of hatches that can be used in bulkhead 3? Is there anything to take into consideration around structural integrity of the bulkhead with extra large hatches? Is having larger hatches a bad idea for some reason?

Any advice or input is appreciated!

Thanks!
Max
Rob Hazard
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Re: how big can the hatches be in the third bulkhead?

Post by Rob Hazard »

Hello Max,
I had the same thought as you, so I did some experiments by progressively enlarging a hole in a large piece of scrap ply. My hatches are oval with one flat side along the mast box, narrower at the top, wider at the bottom, and just large enough that I can put one arm over my head and slide into the bow far enough to reach anywhere in there. They are not hinged, just gasketed all the way around. Sorry I don't have a photo, since I am not near the boat now.
I began with a straight vertical line parallel to the mast box, about 2" out, and stayed about 4" above the cockpit sole. The other controlling point was where the cuddy side met the side deck. I think I left about 3" there. The top and bottom curves are both arcs,The top arc with a radius of ~4 1/2", the bottom arc radius ~7", and I connected them by simply springing a batten.
I trusted the egg-crate structure of the boat to spread the mast loads. I made the doubling flange, where the gasket lives, extra thick both to further stiffen the bulkhead and to make it more comfortable against my ribs while I'm working in there.



I posted a question on the board asking if anyone had made hatches big enough to crawl through, and the answer back then was no, but I went ahead anyway, and I'm happy I did.

Cheers,
Rob (#197 Puffin)
max
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Re: how big can the hatches be in the third bulkhead?

Post by max »

Thanks Rob.

Based off your experience, and with a single picture that Keith Nasman posted of his hatches somewhere (that I've since lost the picture - d'oh!) I decided to go about expanding the bottom edge of the hatch. So the hatch overall is roughly 11" along the top, about 21" tall, is about 1.5" away from the mast box and top edge, and the outboard edge angles out roughly matching the angle of the bulkhead itself, so it goes from 11" wide at the top to roughly 15" wide at the bottom, keeping at least 1.5" of bulkhead all the way around the opening.

I used a ~ 3" radius to round each corner, and it looks good and I can pretty easily fit through the hatch hole now. I've just finished making templates for the door (both a negative and positive blank) and the backing that will be glued to the back of bulkhead 3 and extend into the open space somewhat to provide the lip that the hatch will rest against. I made the templates out of some leftover cheap plywood I had lying around, once it stops raining up here in Seattle I'll clamp these up and use a router to cut the real pieces out of marine ply. Hopefully I don't have to wait too long for the rain to stop ;)

Fair winds,
Max
Rob Hazard
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Re: how big can the hatches be in the third bulkhead?

Post by Rob Hazard »

Max,
It sounds like you've pushed your hatch size a bit closer to the limit than I did. I think you'll be fine, though, because I think the major rig loads are borne by the cuddy top, sides, and the interior egg crate structure that supports them.
Maybe John Welsford would render a more authoritative opinion...

Do you plan to hinge your hatches? I opted to just tether them so I can leave them open while I'm lying down in the boat. I drop them inside the compartment and they're out of my way.

Cheers,
Rob
max
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Re: how big can the hatches be in the third bulkhead?

Post by max »

I don't think I want to hinge them for the same reason you mention Rob - I like the idea of being able to completely remove them and get them out of the way from time to time. I did buy some of the plastic turn dogs so I could attach them with that, but I haven't really decided yet. I'm also considering using the bungee cord trick on the back of the panels similar to how some kayak hatches are held flush. Either way I figure I don't have to decide on this for a while yet 😉
max
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Re: how big can the hatches be in the third bulkhead?

Post by max »

Gee Rob, now you've got me thinking about whether the extra material being gone will affect the structural integrity... 🤔

Well I've cut the router templates but haven't actually cut into bullhead three yet, so now I'm pondering a few options:
1. Just go for it. By adding a layer of fiberglass around each hatch hole and then 9mm plywood on top of that extending into the hatch hole to form a shelf for the hatches to sit on, it should be good.
2. Stick with the same plan as above, but add a layer of fiberglass to the entire front side of bullhead three
3. Look at reducing the size of the hatches a bit, perhaps shaving an inch or so off the vertical (I'd have to modify/make new router cutout templates).

Hard to decide when I don't know what forces are on the bulkhead. What do you (or others reading along) think?
Rob Hazard
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Re: how big can the hatches be in the third bulkhead?

Post by Rob Hazard »

Sorry about that, Max! :D
But if you're going to rethink a feature like that, NOW is the time!
I thought long and hard before cutting out my hatch openings, and I made them as small as I could comfortably squeeze through with one arm over my head, and I kept them as close to round as I could, to avoid any corners that might cause local stress points.
I even considered putting a large hatch on one side and a small one on the other side, leaving more of the bulkhead intact, available for installing a compass, reading light, and whatever else might prove handy. In the end I went with the 2 matching ovals. Each one is closed with 7 of Russell Brown's turn-buttons.

All in all, I think the mast in a Scamp is well supported, especially compared to the usual thwart-with-a-hole arrangement you find in most boats of Scamp's size. But, I am NOT an engineer, let alone a Naval Architect!
GregWads
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Re: how big can the hatches be in the third bulkhead?

Post by GregWads »

I made mine somewhat the size of what Rob described. I stayed 2 inches away from the Mast Box. The backing plate ring for each hatch is cut from one piece of 1/2 plywood. The ring is 3 inches wide all the way around, giving me a 1 inch inside lip. The outside edge of the rings are right up against the mast box and are epoxied directly to the mast box & the bulkhead, with a layer of fiberglass over the box and down across the rings for good measure. I think this should be plenty strong as the bulkhead & backing ring is nearly 7/8 thick. The floors are only 18 mm or just a bit less than 3/4 of an inch.
Rob Hazard
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Re: how big can the hatches be in the third bulkhead?

Post by Rob Hazard »

A correction:
If you look at the topic "SCAMP Hatch Kits" in the Announcements section above, you'll see the Keith Nasman designed his own custom hatches for B-3 before I did. He even included a photo.
Rob Hazard
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Re: how big can the hatches be in the third bulkhead?

Post by Rob Hazard »

/Users/roberthazard/Desktop/IMG_0939.JPG

B-3 hatches, *just* big enough to crawl through.
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