I’ve just started to add flush hatches to my MC 13. After searching around the forum I’ve got more questions.
On my last build I used strong backs to hold the shape of the hatch’s, seemed to work well. I ordered the CLC kit and it calls for a doublers. Adding thickness to something seems a poor way of gaining stiffness. The instructions even warn about “spring back” when removing the hatch/doubler from the form. It would seem difficult to determine the amount of “spring back” for both of my hatches that have a fairly moderate camber. Wouldn’t strong backs hold the camber better?
Also, someone suggested glassing the deck before cutting out the hatches to hold the shape of the deck and hatches together. I was going to cut the hatches before glassing but now I’m thinking about after. I can see advantages and problems with both.
The drawing on the plan calls the sill a “rim”. But doesn’t show adding a rim. Is the current conventional wisdom requiring a rim? I can see the advantages but do I really need to?
Thanks in advance- Doug
Without a rim your hatch cover will fall into the boat, so yes, you do need a rim.
Glassing the deck to control spring back when cutting the hatch cover won’t do all that much. Fiberglass is very strong in tension, not compression. Spring back will be trying to compress the deck glass on the hatch cover and it will mostly succeed. The layer of epoxy saturating the glass will actually have a greater effect on reducing spring back because fully cured epoxy is very strong in compression.
The other thing to keep in mind is that cutting cured glass/epoxy is more difficult than cutting bare wood. It’s rougher on the saw blade and requires more force, which could lead to cutting accidents. You will also get a neater cut (important for flush hatches) without the glass.
The formers on the underside of the hatch in your picture would hold the hatch cover shape better. It’s just a matter of determining the correct radius to cut them to. That said, there may be other reasons why the cover thickness is doubled. It may need the extra thickness to fit properly and form a watertight seal. It may need the extra thickness to provide strength for wet re-entries. And it could be that if the cover thickness is doubled anyway for other reasons, it may provide enough stiffness that the formers are not required. It may be worth a check with CLC support to find out if there’s some other reason for the double thickness.
Laszlo
Thanks Laszlo! The question about the rim or sill is that I have read where people refer to what the drawing points to as the rim as the sill and the rim is a thin ring 1/2” wide (maybe) on top of the sill’s (rim) inside diameter. I think it would add to the stability of the gasket holding it between the deck and the rim.
I had the same thoughts about cutting out the hatches first. It would make cutting much harder the only benefit would possibly be to keep the wood at the cut from splintering but I plan on taping the cut line to help prevent that.
Thanks again, I will proceed with cutting out the hatches
OK, now I get it. You’re talking about something like this:
That’s the WD12 hatch, BTW. The raised piece of wood on the inner part of the opening is there for leak control. It compresses the foam to form a hopefully watertight seal. and helps contain any minor drips that get past the cover and foam. It probably also stiffens the entire assembly.
Laszlo
That’s it! I have the kit but I don’t think it came with one. I’ve got the material so now I have to try to make one of those little guys
Beautiful job! All of your work is immaculate.
Thanks, Doug
Hi Doug,
the use of a doubler (added to the back of a hatch) is pretty common for designs with flat hatches like the shearwater stitch and glue series as well as the wood ducks stich and glue. but not typically an approach you would see with a curved deck where a form is typically the most efficient approach to locking the shape of the hatch in and preventing ‘spring back’. So for the millcreek series with ‘tortured’ (aka curved) deck, the forms or strongback as shown in the picture IMG_6412 above is the typical approach.
with a little attention to shaping the form you can get a very very tight/close fit.
in my experience i have not used the technique of backing it up with glass before cutting for all the reasons that Laszlo cited. it simply won’t reliably keep the curve.
in the pictures below i demonstrate the use of a form to hold the shape (acknowledgement to laszlo on his consultations) that also has built into it the hooks for internal hold downs (no dogs holding the hatch in place - internal bungees.) The result is hatches that you have to come up close to find.
one other suggestion i have for a good fit is to not make your final form to hold the hatch in its final shape until you have permanently attached the hull to the deck. the reason for this is you want your curve that you will be locking in with the form to marry exactly to the final curve of the deck. the curve of the deck is not finalized until the hull and deck are pernanently mated. this last scrap of advice is perhaps more relevent to strip or hybrid builds where a lot of the hatch works is done prior to the hull and deck being mated…but can come into play in stitch and glue on occasion.
I did “invisible” hatch hold downs on my Chess 17 and Shearwater Sport. I didn’t cut out the hatches until the Chess 17 until the deck was virtually complete - glass and all. That worked fine. There was quite a bit of spring back, and I did have some difficulty getting the strongback curvatures correct to re-match the deck. I have one main strongback with hold down hooks incorporated as in Howard’s second photo above, plus smaller strongbacks located forward and aft of the main strongback. If I had it to do again I would have made templates that PERFECTLY matched the outer deck curvature PRIOR to cutting out the hatch, carefully noting their distance/placement on the hatch surface, then used those curves to set the curvatures for my strongbacks. (The inside and outside curvature being the same, by definition.) I think this would have prevented me from having to de-bond and re-do the strongbacks, even though I’d tried to use a plastic strip, etc. to get the curve estimate correct in my first attempt.
I think a doubler instead of a stongback is fine for flat hatches. My Shearwater Sport is simply doubled (as per plan), but I did glue on wood “hooks” for the invisible hold-down system.
In alll cases my hatches have the inner rim, with the gasket in the groove formed between the inner rim an the deck. That way you can put a large hold-down force on the hatch, thus compressing the gasket (of appropriate thickness), but only down as far as the inner rim allows, thus keeping the outer surface nice and flush.
Great feedback guys! Pretty much covers all of my questions and then some. I feel more confident now in the direction I’m headed. I’m comfortable using strong backs so I think I’ll go that way due to the considerable camber on my boat especially the hatch towards the bow. I really like the idea of making templates before cutting the hatches out plus it delays me having to cut the hatches out by another day, that task really gives me heartburn
The inside radius of curvature is shorter by the deck/hatch thickness compared to the outside radius of curvature so the inner and outer curves are not the same. Or are you saying something else and I’m misunderstanding you?
Laszlo
Laszlo is correct that if you want to make the ‘perfect’ match you would have to take your template from the inside of the hull as the outer curve is not equal to the inner curve due to the radius being smaller. as a practical matter, on the tortured deck boats, you typically do not have access to the inside…so it gets tricky.
that said, the outside would be very good approximation and in most cases folks would never notice…but if you take a couple extra shaves with a block plane towards the ends of the form to add some additional curvature to the template that you took from the outside, you can better approximate the inside arc and get a very good fit.
to the extent you want to be a total weenie on the matter, because your form does not go all the way out to the edge, (in order to clear the sill) even if you took an inside arc, the last 3/4 to 1 inch of the hatch would spring back slightly, so on a tortured deck with signficant spring back, its almost impossible to be perfect…but nobody will notice 
thank god for strip builds…where you can simply shape the curve without the torture…
Or, in a more genteel fashion:
“Let us not be too pedantic, for all love”
Alternatively, make the original template from the outside and then subtract the hatch cover thickness to get the exact curve of the uncut deck on the inside.
Laszlo
I was ready to start sawing away at the hull/ hatch when after reading everyone’s comments did a pivot and decided to go with the ideas from the forum. I found this contour gauge on Amazon which looked perfect for the job at hand. I will take the radii minus the deck thickness at the locations for the stongbacks/forms and transfer them to some heavy cardboard and cut out the forms from 1/2” plywood. Then start on sanding the forms to fit the hatch, of course taking extra off where needed. I will update on how things went.
Thanks Again
Doug
I do stand corrected in the way I used my words regarding the outside/inside template. I did draw an appropriate “inside” arc spaced one hatch thickness away from the arc of the template for the actual cut-line of the strong back. I should have made that more clear so as not to lead anyone into a mistake.
And again I should be more clear - that was when I was attampting to make templates “by eye” with cardboard and drawing along a flexible plastic strip I taped over the hole where I’d already cut out the hatch. The lesson learned was that it would have been much better to have made the template prior to the cut out, as is being shown and discussed above.
I understand what you meant, no problem. In fact your comments made me stop cutting my hatch out and take actual measurements where the stongbacks are located before hand!
I drew the stongbacks out on extra thick poster board then started to cut it out with a bandsaw but I stopped. Why make a copy of something when I should just draw it out on the plywood first, oh well I’ll save the templates for insurance. Today I plan on cutting the strong backs out of plywood at the outside diameter radius then draw another line the thickness of the deck and sand back to it. That should be a good place to start when I cut out the hatch. Then just sand a little here and there.until it looks “invisible”. Haha
Here’s what I’ve been doing….
While the deck is off the hull and has been fiberglassed inside and out I make a laminate of FG and CF that conforms to the underside of the deck and is larger than the hatch opening. This is formed on plastic sheeting and is not glued on yet. Then I cut the openings. Then I’ll trim the laminate and cut an opening in it that forms a lip for the hatch to rest on. The piece I remove is then bonded to the underside of the hatch cover. Then the lip is bonded to the underside of the deck. This makes a hatch cover that is very stiff and a lip that has some flex. I do NOT use a spacer or a raised lip. I then use a very thin gasket (1/8th “) and internal bungees to hold it down. I also drill 2 holes in the cover and use deck line that doubles as a grab loop and tether. I’ve done this on five or six boats and find it attractive and quite water tight.
I hope this makes sense…
I had to read it a couple of times but I understand completely. I’ve never done a flush hatch before so I don’t have any preconceived ideas about how do it. When I got my hatch kit I thought I made a mistake. The instructions were not great. After trying to think of different ways to do it I literally have been thinking in 3D for all types of iterations.
Your version is definitely thinking out of the box, I like it. Looks very light too. What was the gasket material that you used? Using a flexible composite sill is impressive. Do you think it would work on a hatch with a significant camber? What thickness did you make the sill?
Down the rabbit hole I go, thanks! Doug
The sill is less than an 1/8th”. That first pic is actually from a Shearwater 17 hybrid. The following ones are from a MicroBootlegger Sport which has a much more curved deck. I’m available to chat offline any time. I’m not the fastest typist…
Also, I, ehem, “borrowed” this idea from another builder.
