Hatch Gas Spring/Strut Calculator

[tony.latham]

I can compensate by attaching the struts to the galley counter...

That's the way my current teardrop is. And it works fine.



But of course, it depends on how you've attached your counter to the walls.

On this new build, I'll attach the lower brackets to the walls. But I've been dithering over how to attach the counter to the walls since it adds a great amount of stability to the walls where there is no spar. How's your countertop attached?

This morning (I think) I've decided to add a piece of aluminum angle underneath the counter ends and do it in that fashion. I was going to glue and screw a piece of 1" hardwood block in but that would get in the way of my stove.



Tony

And I concur with that sentence:

[Tom&Shelly]

I can compensate by attaching the struts to the galley counter...

That's the way my current teardrop is. And it works fine.



But of course, it depends on how you've attached your counter to the walls.

On this new build, I'll attach the lower brackets to the walls. But I've been dithering over how to attach the counter to the walls since it adds a great amount of stability to the walls where there is no spar. How's your countertop attached?

This morning (I think) I've decided to add a piece of aluminum angle underneath the counter ends and do it in that fashion. I was going to glue and screw a piece of 1" hardwood block in but that would get in the way of my stove.



Tony

And I concur with that sentence:

Well, no time like the present to plan, since I missed the opportunity before starting the build!

I haven't attached the counter yet, but did provide the hard point in the galley wall skeletons. A little low as it turns out, but I can compensate for that. The baseline plan is to use pine cleats, glued and screwed. I believe I can build something that way I could sit on, so a mere 100 lb gas strut wouldn't be a problem, but as we look at possible galley designs, I agree they get in the way. Like your idea of aluminum angle.

A strong alternate, for me with gas struts attached to the counter, would be to use steel angle brackets: https://www.mcmaster.com/angle-brackets, with bolts through the counter to the gas struts. That transfers the force to the skeleton wall, and so wood screws there would be the weak link. A solution to that would be bolts all the way through the wall to a plate on the outside, which then introduces issues with possible water intrusion and ascetics. Think both of those could be solved though. (Water intrusion is no worse than where I run wires to the side lights--could seal with something. Seems like a good place for a reflector--hiding the plate.)

Short answer is I'm still considering options, but now with new insight concerning the gas struts.

Tom

[Tom&Shelly]

I didn't put enough skeleton in the upper galley portion of the skeletonized walls. I can compensate by attaching the struts to the galley counter, but, in that case, it would be better to make the struts' "extended length" longer than the calculator recommends by an inch or two. Does anyone see a problem with that?

Tom

Think I answered my own question here, as I see McMaster-Carr's discussion of gas struts: https://www.mcmaster.com/gas-struts (click on "more"). Dan's calculator must assume the hatch is of approximately uniform density (weight per area). (Dan please correct me if I'm wrong.) Therefore, if I want to use a longer strut, I could add weight to the bottom of the hatch, if necessary. As it happens I wanted to add an extra strut and blocking down there to support a dust seal anyway, so I may be okay.

Tom

[danlott]

I didn't put enough skeleton in the upper galley portion of the skeletonized walls. I can compensate by attaching the struts to the galley counter, but, in that case, it would be better to make the struts' "extended length" longer than the calculator recommends by an inch or two. Does anyone see a problem with that?

Tom

You can use struts that are longer than the calculator recommends, but it will affect your upper and lower strut attachment points and the required force rating.

There are several variables that go into calculating the proper gas strut force and placement. A change in anyone of the variables affects all the other variables.

Once you know your finished dead weight of your hatch and the distance from your galley hinge and lower strut attachment point PM me and I can work with you to determine the correct strut length, force rating and upper attachment point.

Dan

[Tom&Shelly]

I didn't put enough skeleton in the upper galley portion of the skeletonized walls. I can compensate by attaching the struts to the galley counter, but, in that case, it would be better to make the struts' "extended length" longer than the calculator recommends by an inch or two. Does anyone see a problem with that?

Tom

You can use struts that are longer than the calculator recommends, but it will affect your upper and lower strut attachment points and the required force rating.

There are several variables that go into calculating the proper gas strut force and placement. A change in anyone of the variables affects all the other variables.

Once you know your finished dead weight of your hatch and the distance from your galley hinge and lower strut attachment point PM me and I can work with you to determine the correct strut length, force rating and upper attachment point.

Dan

Thank you Dan. I appreciate that!

Tom

[danlott]

Just a reminder. The spreadsheet calculator is designed to be utilized when your gas struts are installed in a more vertical position similar to how they are illustrated in the picture. A more horizontal position will require more force and the calculator is not designed for that. Also the more force required will place increased forces on your hatch, galley and hinge area, which can lead to failure.



Dan

[MickinOz]

Just starting to think about struts. Was going to fabricate some sort of timber supports for the hatch - hoping to make it look a little traditional.
But gas struts are so cheap, and you just have to mount them, not fabricate them....
The calculator looks really good. It helped me determine the dimensions of the struts with minimal pain.

But I do have a few questions.
Overall length of my hatch per the diagram at the start of this thread is about 47".
Weight measured per diagram is about 13 kg /29 lbs which I think is a reasonable weight for a 62" wide hatch?

Max length of strut is 24 inches. It would foul my galley shelf if I used a longer strut with a lower mount.
85% of stroke is 9 inches on an ebay 24" strut.
So the hatch end of the strut ends up 9 inches from the hurricane hinge.
This appears typical from reading the thread.

So you end up with about 4/5 of the length of the hatch outboard of the hatch end of the strut.
Thinking about good old Archimedes and his lever, this must put a fair bit of upward leverage on the hinge.
Has anyone found this stresses, stretches or tears loose the hinge?
My hinge is secured with a ten gauge screw every 6 inches, and glued down with sikaflex 292.
My expectation is it will stay exactly where I put it, but thought I'd ask, anyway.

[tony.latham]

Has anyone found this stresses, stretches or tears loose the hinge?

I've certainly seen photos of this. It's one (screw-holding) reason I use oak for the hatch spar.

Weight measured per diagram is about 13 kg /29 lbs

I'd say that your hatch is remarkably light. Which is good.

I think it's important to think about how the strut brackets attach structurally to your hatch. When the hatch is closed, they exert a lot of force.





Tony

[MickinOz]

I'd say that your hatch is remarkably light. Which is good.

Thanks. I was aiming for light as possible, so it is only 4 vertical frames, no horizontal spars, and I soaked the single ply skin so it curved around the frame easily when I put it on.
Worked OK. There is about 10-12mm of spring back.
More concerning is the twisting flex I can get in it when I lift it by one corner instead of in the middle.
I may yet retro fit an internal skin to stiffen it.

I have been looking at the Mechanix Illustrated 1947 article. I may explore that simple 3/8 rod setup for struts. Looks like low structural stresses would ensue.

[tony.latham]

I may yet retro fit an internal skin to stiffen it.

That'd turn it into a torsion box and stiffen the heck out of it. I add foam board inside my hatch to support the two sides of the 1/8" ply.

If you go with the rods for support, make sure they lock the hatch in place in case a gust of wind grabs it while open.

And yes... I fixed my above post.

Tony

[kevin13]

When you get the spring force you need with the calculator is that per strut or for both struts

[tony.latham]

When you get the spring force you need with the calculator is that per strut or for both struts

That’s for each strut.

T

[ELM]

The gas strut calculator worked perfect for me. I can lift the hatch with one finger and it just raises slowly.

[danlott]

The gas strut calculator worked perfect for me. I can lift the hatch with one finger and it just raises slowly.

Glad to hear that it worked well for you.

Dan

[saltydawg]

I was going to do all the math for my rear hatch, and just went with linear actuators, no math except length and even thats not important. The other nice thing is I did not need to figure out a latch as the actuators hold it closed. They where even about the same price, I found outdoor rated ones with 225 lbs of force each on amazon for 59 or 69 bucks.