jnye

Companionway Hatch

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Hoping for some help. I’m considering redoing the companionway hatch on my Olson 30 along the lines of what’s pictured. One difference being my piece will be curved to match the deck radius.  I’d like to make this a thin as possible but able to withstand a pretty good impact (250lb guy trips and falls on it plus). I’ve asked the supply houses but not getting much feedback. So here goes… What layup schedule should I use? I’m anticipating a 3 or 4mm core (corecell?) and some combination of 2x2 twill and biax. Willing to go with carbon to add structure/save weight.  Instead of over-building (over-paying), I’d rather have some expert guidance. Recommendations as to number of laminations, carbon vs. glass, fabric weight and orientation? This will likely be a hand layup.

Or am I going about this all wrong and should consider some alternative?

Thanks

Companionway 1.jpg

Companionway 2.jpg

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That slide setup looks like it was specifically designed to catch and route water inside the boat.

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How much thickness do you have available for the sliding hatch? That really dictates the layup. I can run some numbers for you, assuming a fat guy standing on it, and then falls on it.

Does corecell even make 3mm core?? Huh. They do make a 1/8" (3mm) thickness. But that's pretty close to useless and won't save you much over a solid laminate that is a bit thinner.

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6mm foam core with a medium weight triax on both sides. Have the unis run side to side. Glass is fine if it's not a gp boat. Brush over with a thin filler mix then sand. 

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6 hours ago, Zonker said:

How much thickness do you have available for the sliding hatch? That really dictates the layup. I can run some numbers for you, assuming a fat guy standing on it, and then falls on it.

Does corecell even make 3mm core?? Huh. They do make a 1/8" (3mm) thickness. But that's pretty close to useless and won't save you much over a solid laminate that is a bit thinner.

Thanks. I'll be fabbing the guides for the hatch so can work with whatever thickness is required. Yes, I've seen corecell that thin. If 1/4" would be better, I can do that. Or go solid. Any numbers you can provide would be appreciated.

6 hours ago, Alcatraz5768 said:

6mm foam core with a medium weight triax on both sides. Have the unis run side to side. Glass is fine if it's not a gp boat. Brush over with a thin filler mix then sand. 

Appreciated. How about something like this in carbon? Or e-glass?

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Why not do this in acrylic? I had one made a few years ago out of 1/2" tinted acrylic by a local plastics store.  Gave them my plans with the appropriate deck radius along with $300, and 2 days later had a new companionway hatch.   

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20 minutes ago, Stigaro said:

Why not do this in acrylic? I had one made a few years ago out of 1/2" tinted acrylic by a local plastics store.  Gave them my plans with the appropriate deck radius along with $300, and 2 days later had a new companionway hatch.   

Interesting. Didn't realize this was an option. Though I'm guessing the acrylic would be a lot heavier than the composite. Thicker than I was anticipating but could work. Have used Select Plastics in Norwalk CT. Will give them a call.

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It will be heavier but it lets a lot of light in, which is nice. 

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Plus 3 on the tinted acrylic. May be a bit heavier, but no strength issues, and as Stream mentions, additional light in the cabin is a bonus. 

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If the little bit of extra weight of acrylic is a problem, just skip lunch or leave a couple of cans of beer ashore.

I mean really, you are talking about something like 0.005% of the boats weight.

It's a sailboat, It ain't a Formula One car.

 

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Yeah I agree weight dif is minimal... only other thing is op said he wanted curved hatch slide... not sure why but that would complicate getting a straight 1/2 cut 

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Keep in mind this is an O30 - I assume for racing.

Assuming the hatch is roughly 2'x4', then a 1/2" acrylic one would weigh about 25 lb.  The problem isn't just the added weight, it is where it's added.  At cabin top, you end up with reduced RM, increased pitching moment.The only place worse would be at the masthead.  The thing about weight savings is it's a philosophy.  So while in an individual case like this you may only save 20 lb, if you can do this with 10 items on the boat, you've saved 200 lb .

Alcatraz is on the right track - a 1/2' foam cored one with 13 oz skins would weigh about  4 lb to 5 lb max.

Structurally, I like to look at things from a Limit States perspective (known as LRDF in the US) which looks at the probability of failure (everything has a probability of failure, the key is what is the acceptable probability) from the Ultimate Limit (where the structure fails by breaking apart) and Serviceability Limit  (where the structure fails in the sense that it no longer performs it's intended function from things like deflection and deformation)

Generally speaking, the Serviceability Limit dictates a stronger and/or stiffer structure.  For example, the size and spacing of floor joists in a house built to code are way over designed from the Ultimate Limit requirements since people don't feel comfortable walking along a floor that deflects more than L/360.

What I'm getting at is that 1/2" foam core with 13 oz skins would most likely meet the Ultimate Limit requirement but may deflect more than desirable under the load of a 250 lb person falling on it, or perhaps even stepping on it.  The question is, is that amount of deflection acceptable for the amount of times said 250 lb person falls or treads on it?  Personally,  i would say yes, but realize there may and likely will be  noticeable deflection when such an event occurs. 

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2 hours ago, SloopJonB said:

If the little bit of extra weight of acrylic is a problem, just skip lunch or leave a couple of cans of beer ashore.

I mean really, you are talking about something like 0.005% of the boats weight.

It's a sailboat, It ain't a Formula One car.

 

More like .35%. Still not much but a handful of similar decisions can start to add up!

2 hours ago, fufkin said:

Yeah I agree weight dif is minimal... only other thing is op said he wanted curved hatch slide... not sure why but that would complicate getting a straight 1/2 cut 

Think the curve would look good and slot in under the deck with minimum loss of headroom (which is already seriously restricted). Figured also the curve would add some structural integrity. Only talking a 3/8" rise over a 22" width.
 

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13 minutes ago, 12 metre said:

Keep in mind this is an O30 - I assume for racing.

Assuming the hatch is roughly 2'x4', then a 1/2" acrylic one would weigh about 25 lb.  The problem isn't just the added weight, it is where it's added.  At cabin top, you end up with reduced RM, increased pitching moment.The only place worse would be at the masthead.  The thing about weight savings is it's a philosophy.  So while in an individual case like this you may only save 20 lb, if you can do this with 10 items on the boat, you've saved 200 lb .

Alcatraz is on the right track - a 1/2' foam cored one with 13 oz skins would weigh about  4 lb to 5 lb max.

Structurally, I like to look at things from a Limit States perspective (known as LRDF in the US) which looks at the probability of failure (everything has a probability of failure, the key is what is the acceptable probability) from the Ultimate Limit (where the structure fails by breaking apart) and Serviceability Limit  (where the structure fails in the sense that it no longer performs it's intended function from things like deflection and deformation)

Generally speaking, the Serviceability Limit dictates a stronger and/or stiffer structure.  For example, the size and spacing of floor joists in a house built to code are way over designed from the Ultimate Limit requirements since people don't feel comfortable walking along a floor that deflects more than L/360.

What I'm getting at is that 1/2" foam core with 13 oz skins would most likely meet the Ultimate Limit requirement but may deflect more than desirable under the load of a 250 lb person falling on it, or perhaps even stepping on it.  The question is, is that amount of deflection acceptable for the amount of times said 250 lb person falls or treads on it?  Personally,  i would say yes, but realize there may and likely will be  noticeable deflection when such an event occurs. 

Agreed though the sliding hatch part is only 24x24 (approximately) so half the weight.

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So you're talking 6 or 7 Lbs difference.

The light and the looks are worth that.

Plus, trimming a piece of plexi to shape is way faster and easier than laminating and finishing a cored piece.

One of the first things I do with a new boat is to replace any solid hatches and drop boards with tinted plexi.

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1 hour ago, jnye said:

Agreed though the sliding hatch part is only 24x24 (approximately) so half the weight.

Thinking about it again, normally I'm hesitant to suggest using carbon on a non-structural item, but carbons primary advantage over e-glass is stiffness, so carbon skins would greatly reduce the tendency to deflect (if that is at all important) and you could probably get away with fairly light weight carbon - say 12 oz or maybe a bit less.

Here'a quick back of the napkin calc:

- 250 lb Static Load equates to roughly 500 lb Dynamic Load

- Assume bending predominates over shear

- 24'" width means a max bending moment of 6000 lb-in

- F = M/D = 6000 lb-in/.5in = 12,000 lb

- Tensile & Compressive Strength of standard carbon is approx 600 Mpa = 87,000 psi

- Assume SF of 2.5 = 35,000 psi

- Carbon area = 12,000 lb/35,000 psi = 0.34 in2

- Carbon L = 24 in

- Minimum Carbon Laminate thickness = .34/24 = .014 in

- Single skin 8 oz cloth thickness = .016 in > .014 in

So single skins of 8 oz carbon on 1/2" foam should suffice.

Total 8 oz carbon area is roughly 8ft2/9 = .9 sq yd

Total laminate wt is roughly .9 x 8 x 3 = 21.6 oz = 1.35 lb

Total wt = 1.35 lb + .17 ftfoam x 6 lb/ft3 = 2.4 lb

Estimated wt savings = 10 lb

OR...  do it in acrylic for much less effort.  :D

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I'd go with a dynamic load of 1000 lbs, assuming a big guy falling on it, so that would add another 1.35 lbs of carbon, which is tolerable. I think you may have also forgotten the weight of the epoxy, which puts you over 4 lbs, which is totally unacceptable. ;)

Also, don't forget that acrylic will have less photonic pressure due to its transparency.

 

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1 hour ago, IStream said:

I'd go with a dynamic load of 1000 lbs, assuming a big guy falling on it, so that would add another 1.35 lbs of carbon, which is tolerable. I think you may have also forgotten the weight of the epoxy, which puts you over 4 lbs, which is totally unacceptable. ;)

Also, don't forget that acrylic will have less photonic pressure due to its transparency.

 

Nope, that is what the "3" in the .9 x 8 x 3 is - assuming a less than optimal hand layup. :P

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Yeah I agree weight dif is minimal... only other thing is op said he wanted curved hatch slide... not sure why but that would complicate getting a straight 1/2 cut 

I had a curve to match the deck put on mine.  I measured about a 12 ft radius, but only over about 20" wide.  Place did a great job bending it.  Not a problem for most plastics manufacturers. Of course, flat is cheaper and DIY, just doesn't look as good.  The one I replaced was flat.

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Keep in mind this is an O30 - I assume for racing.

Assuming the hatch is roughly 2'x4', then a 1/2" acrylic one would weigh about 25 lb.  The problem isn't just the added weight, it is where it's added.  At cabin top, you end up with reduced RM, increased pitching moment.The only place worse would be at the masthead.  The thing about weight savings is it's a philosophy.  So while in an individual case like this you may only save 20 lb, if you can do this with 10 items on the boat, you've saved 200 lb .

Alcatraz is on the right track - a 1/2' foam cored one with 13 oz skins would weigh about  4 lb to 5 lb max.

Structurally, I like to look at things from a Limit States perspective (known as LRDF in the US) which looks at the probability of failure (everything has a probability of failure, the key is what is the acceptable probability) from the Ultimate Limit (where the structure fails by breaking apart) and Serviceability Limit  (where the structure fails in the sense that it no longer performs it's intended function from things like deflection and deformation)

Generally speaking, the Serviceability Limit dictates a stronger and/or stiffer structure.  For example, the size and spacing of floor joists in a house built to code are way over designed from the Ultimate Limit requirements since people don't feel comfortable walking along a floor that deflects more than L/360.

What I'm getting at is that 1/2" foam core with 13 oz skins would most likely meet the Ultimate Limit requirement but may deflect more than desirable under the load of a 250 lb person falling on it, or perhaps even stepping on it.  The question is, is that amount of deflection acceptable for the amount of times said 250 lb person falls or treads on it?  Personally,  i would say yes, but realize there may and likely will be  noticeable deflection when such an event occurs. 

1/2" acrylic weights 2.8 lbs/sqft. A 2'x2' hatch cover would weight 11.2 lbs.  If this is too heavy, use 3/8" and get down to 8.5 lbs. Even with carbon, I suspect you won't get it less than 4 lbs. So you're saving 4 lbs at best, likely a lot less.  Hardly worth the all the effort I should think.  

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"Single skin 8 oz cloth thickness = .016 in".    0.016 in = 0.356mm.  Well maybe for a decent hand layup.

A useful rule of thumb for vac bagged composite layer thickness is 100 gm/m2 weight = 0.1mm

So 9 oz (305 gm/m2) = 0.3mm         (That's why the 8 oz being 0.356 jumped out at me.)

My design suggestion:

Since you're not thickness limited, a 1/2" core makes the most sense to limit the deflection (not the strength). Otherwise it will be strong enough, just very bouncy.

And then a single 15 oz triaxial carbon skin, a bit resin rich because otherwise with a single skin like that you might get pinholes and water will get in. A single 8 oz skin is very thin for puncture / water resistance for a hatch. Maybe a thin extra strip of glass/carbon where it runs in the guides to account for wear resistance? If you're really confident in your glassing skills go with a lighter laminate. Make sure the 0 deg layers in the triax run port/stbd. across the width of the hatch.

Shear might be an issue if the guides are very narrow and all the load is transferred over a narrow ledge. Make them about 1" wide. Again the 1/2" core helps here.

 

I like acrylic drop boards for light but not as fond of them for companionway sliders because they are slippery as ice when wet.

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I would do the acrylic, especially if it's so easy.

I have to rebuild mine - a more standard box type - and will probably do acrylic on top of a wood frame.

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"Single skin 8 oz cloth thickness = .016 in".    0.016 in = 0.356mm.  Well maybe for a decent hand layup.

A useful rule of thumb for vac bagged composite layer thickness is 100 gm/m2 weight = 0.1mm

So 9 oz (305 gm/m2) = 0.3mm         (That's why the 8 oz being 0.356 jumped out at me.)

My design suggestion:

Since you're not thickness limited, a 1/2" core makes the most sense to limit the deflection (not the strength). Otherwise it will be strong enough, just very bouncy.

And then a single 15 oz triaxial carbon skin, a bit resin rich because otherwise with a single skin like that you might get pinholes and water will get in. A single 8 oz skin is very thin for puncture / water resistance for a hatch. Maybe a thin extra strip of glass/carbon where it runs in the guides to account for wear resistance? If you're really confident in your glassing skills go with a lighter laminate. Make sure the 0 deg layers in the triax run port/stbd. across the width of the hatch.

Shear might be an issue if the guides are very narrow and all the load is transferred over a narrow ledge. Make them about 1" wide. Again the 1/2" core helps here.

 

I like acrylic drop boards for light but not as fond of them for companionway sliders because they are slippery as ice when wet.

Thanks. This is pretty much what I've settled on. Going to run a few strips of progressively wider tapes along the side edges to beef up and help transfer the load at the guides. The hatch will also have lateral "ribs" at the front and back - the front piece to butt up against tabs to keep the hatch from falling out; the aft piece to aid in grip and butt up against the leeboard. Guessing these will add a reasonable amount of structure. Settled on doing this this way because (a) it'll be cheaper than acrylic assuming the $300 number is in the ballpark; (b) my time is free and I've got plenty of it; (c) should look pretty good; (d) it's a race boat with no meaningful interior so light below is of limited value; (e) weight; (f) might be tempted to tackle the cockpit re-build if I went with something easier to do.

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I would do the acrylic, especially if it's so easy.

I have to rebuild mine - a more standard box type - and will probably do acrylic on top of a wood frame.

The stock O30 hatch is essentially a box with angled sides. The sides mate with a molded flange on the interior and angled runners on the exterior. Once in place, the hatch can't escape because the fixed width of the hatch pinches up against the narrowing width of the flange/runner gap. The whole contraption sits proud of the deck by several inches.

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If the slot is straight, and the slider is curved, how is it going to slide out of the way?

I made a curved slider for my F-24, but it pops into place and does not slide.

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On 3/23/2018 at 1:46 PM, thinwater said:

If the slot is straight, and the slider is curved, how is it going to slide out of the way?

I made a curved slider for my F-24, but it pops into place and does not slide.

The curve is outward to the sides so the left and right sides are straight (and the plane the hatch sits on is flat).

The form being put together. Fore and aft view. Layup should start tomorrow.

IMG_3035.JPG

IMG_3036.JPG

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