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Strengthening Cabin Top


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Hi all – I’m finally planning to replace the headliner in our 37ft classic plastic this winter and, naturally, I’m thinking of ways to make the project more expensive and miserable. Since I’ll have open access to the bottom of the cabin top, I was hoping to strengthen the aft section with a number of glassed or epoxied transverse ribs or stringers. I suppose these would be wood flexible enough to contour to the top’s curvature. Any tips? Waste of time?

The forward cabin top is noticeably stiffer, supported by two full length transverse bulkheads separating the head and forward sleeping cabin. Decks are approx. 7/8” fiberglass with balsa core.

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Make a template of the curve of the inside cabintop. Use cardboard or plywood doorskin strips hot melt glued to each other in short segments. Have a helper.

From the template, make a curved mold (bent piece of formica with thin plywood backing (1/4" or less depending on curvature))

Wax and polish the mold. Several coats. Then hair spray with the cheapest nastiest thickest hairspray you can get. Hairspray is PVA, an alcohol based mold release.

Lay up your beams (thin strips of wood or foam covered with glass) on the mold.

When cured, pop them off the mold. Then glue them to underside of deck with thickened epoxy. Use several little dabs of hot melt glue at the very end and a few along the length of the beams between the epoxy. The hot melt glue will hold nicely until the epoxy cures.

Trust me that this is MUCH easier than gluing up beams overhead.

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You could also make laminated beams by laminating them on top of the cabintop, covered with masking or packaging tape. Use weights to hold the timber while the glue goes off. For more stiffness add 5-6 layers of uni glass to the bottom of the beam, so you get glass, timber, glass, balsa, glass. 

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If you’re going the template route, you could also use that to simply trace it and cut out beams from a plank. (2x8?)  No need to laminate: just install.  It should be easy to find a plank with grain that follows the curve you'd need at Home Depot since so little of their stock is straight grained. ;)  

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If the balsa core is the reason why it's no longer strong why not just scrape out the core material and vacuum bag up some 1/2" or 3/4" Divinycel followed by 1708?  That surely would check both your boxes. 

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5 hours ago, RImike said:

If the balsa core is the reason why it's no longer strong why not just scrape out the core material and vacuum bag up some 1/2" or 3/4" Divinycel followed by 1708?  That surely would check both your boxes. 

Core is in fine shape, totally dry. 

On 8/27/2019 at 4:25 PM, Zonker said:

From the template, make a curved mold (bent piece of formica with thin plywood backing (1/4" or less depending on curvature))

Thanks Zonker, really helpful. Stupid question...just heat up the formica and gently bend it to the template? 

12 hours ago, Alcatraz5768 said:

You could also make laminated beams by laminating them on top of the cabintop, covered with masking or packaging tape. Use weights to hold the timber while the glue goes off. For more stiffness add 5-6 layers of uni glass to the bottom of the beam, so you get glass, timber, glass, balsa, glass. 

Another great idea, and another totally dumb question... the beams will retain the curved shape after the epoxy dries? I would have assumed that it would just pop back straight again after the weight is taken off  

 

Thanks guys - as you can tell woodworking is not my strong suit. 

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No, the formica will bend easily. Just don't glue it to a piece of plywood and then try to bend it.  Make a support to the correct curve shape, then put formica on top. Small spans won't even need support.

Yes, the beams will keep the shape. The epoxy holds them in place if they are laminated beams that are clamped to a curved mold shape.

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59 minutes ago, ghost37 said:

Another great idea, and another totally dumb question... the beams will retain the curved shape after the epoxy dries? I would have assumed that it would just pop back straight again after the weight is taken off  

Sometimes you can get a minor mount of spring back.  A minor amount would help when you glue them in as you could just prop the centre of the beam in order to clamp it in while the glue cures. Having said that the tighter the radius the more spring back you are likely to get. I suspect your radius will be such that the spring back will be hardly noticeable but could be enough to help with the final gluing in place.  

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When laminating several pieces in a curve, the fraction of the bend that springs back depends on the number of laminations.  

If 2 laminations, 1/4 of the bend, if 3 lams, 1/9, if 4, 1/16.  If n laminations, 1/n^2 of the bend goes away.. 

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It's always good to remember Newton and hope your NA did too.  You may want to take a step back or two and look at the project in relation to where any noodling or stiffness is going.  If it is a genuine fault of design that needs to be addressed then you want to make sure the stresses have somewhere to go and you are not shifting a problem from one place to another.  Some of the classic plastic boats were a little wiggly but you can chase your tail and the initial design by trying to make it something it's not. This being said after doing the aforementioned for several years.

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1 hour ago, Diamond Jim said:

When laminating several pieces in a curve, the fraction of the bend that springs back depends on the number of laminations.  

If 2 laminations, 1/4 of the bend, if 3 lams, 1/9, if 4, 1/16.  If n laminations, 1/n^2 of the bend goes away.. 

I had a shipright guy tell me you had to do min 3 or more odd layeres for no distortion.  Any truth to that?

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10 hours ago, SASSAFRASS said:

It's always good to remember Newton and hope your NA did too.  You may want to take a step back or two and look at the project in relation to where any noodling or stiffness is going.  If it is a genuine fault of design that needs to be addressed then you want to make sure the stresses have somewhere to go and you are not shifting a problem from one place to another.  Some of the classic plastic boats were a little wiggly but you can chase your tail and the initial design by trying to make it something it's not. This being said after doing the aforementioned for several years.

Definitely a valid point. I don't think it's an actual design fault, just figured that while the headliner was down I would try to improve the structure and learn something new. It's lasted 42 years and one circumnavigation as is! 

 

12 hours ago, Diamond Jim said:

When laminating several pieces in a curve, the fraction of the bend that springs back depends on the number of laminations.  

If 2 laminations, 1/4 of the bend, if 3 lams, 1/9, if 4, 1/16.  If n laminations, 1/n^2 of the bend goes away.. 

Really helpful, would have never thought it was a specific relationship like that. 

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She's a Gulfstar 37, 13 years older than me. Probably not the first boat the comes to mind for a lap, but she handled it really well. We've done a constant number of systems, rig, mechanical, and structural improvements over the years. More to go! Hoping to take off again in a few years when the fiance is done with grad school. 

Here's how she sits now...

 ghost2.thumb.jpg.1a07d2593f68fac31ae0eaae71ce464d.jpg

and here's her a few years ago...

GHOST.thumb.jpg.7e79ff486af7588f919ed2b9d843b956.jpg

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On 8/28/2019 at 4:44 PM, ghost37 said:

Core is in fine shape, totally dry. 

Thanks Zonker, really helpful. Stupid question...just heat up the formica and gently bend it to the template? 

Another great idea, and another totally dumb question... the beams will retain the curved shape after the epoxy dries? I would have assumed that it would just pop back straight again after the weight is taken off  

 

Thanks guys - as you can tell woodworking is not my strong suit. 

IS the cabin top the same thickness as the deck (With a 7/8" core). it has to be close to 1 1/4 inches thick, no? How much more strength there do you need unless the core has failed or no longer bonded to the layers of fiberglass. 

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23 minutes ago, RImike said:

IS the cabin top the same thickness as the deck (With a 7/8" core). it has to be close to 1 1/4 inches thick, no? How much more strength there do you need unless the core has failed or no longer bonded to the layers of fiberglass. 

Oops, I misspoke above. Decks are actually closer to 1 1/4 (more in places, less in others) and cabin top is 7/8 (total thickness, core included). There's no structural issue, no soft core, no delam... this is merely a project I wanted to undertake to learn something new and add a little more, likely unnecessary, strength. I am a glutton for punishment when it comes to boat work!

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13 minutes ago, ghost37 said:

Oops, I misspoke above. Decks are actually closer to 1 1/4 (more in places, less in others) and cabin top is 7/8 (total thickness, core included). There's no structural issue, no soft core, no delam... this is merely a project I wanted to undertake to learn something new and add a little more, likely unnecessary, strength. I am a glutton for punishment when it comes to boat work!

If you want the wood work, you could go that route. To "curve" wood to the contours of the cabin top, you can steam it, and then vaccum bag it up to force it to the contours, I've done this before with 4 layers of 1/4" laminate to give me a total 1" thick material. After the wood dried out I removed the vacuum, coated it in thickened epoxy and put it back up there and let of bond for 24 hours. I spent more time sanding/routering and varnishing than I did everything else.

For purely structural applications, you can laminate fiberglass cloth around a foam stringer. Foam is there mostly to give the cloth shape and is not structural. After it's there for the ride. I've done this for stringers in boats before however I see no reason why it couldn't be applied to the cabin top as well. 

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  • 4 months later...

Back from the dead... this project is the definition of "scope creep". I ended up taking down the existing gross vinyl headliner, removing virtually all deck hardware to rebed - including hatches, making new bedded G10 backing plates, and repairing some very localized wet core spots. Now I am finally planning to do this part of the project. Bought a 1/4" Coosa Bluewater board that I will use to make five 3" wide beams with either 2 or 3 layers of Coosa (so end thickness will be 1/2" or 3/4"). I'll epoxy these in place and then glass, which is my next question...

Was planning to use 3 layers of this: https://www.jamestowndistributors.com/userportal/show_product.do?pid=2076&familyName=12in+Fiberglass+Tape

Easier to bend and wet out than biax right? Or do I just use 1708 biax since it's ultimately stronger?

 

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Use a straight biaxial +/-45 degree layers (double bias for our antipodean friends) without mat. The mat is just to bulk it up for hulls etc. and doesn't make much sense for a beam. The 45 degree layers will bend much easier over a slight radius on the corner of the beam than 0/90 tape. The tape has 1/2 the fibers running sideways to the beam so much weaker on the flange. 

If you have room use 3/4" deep depth beams (beam stiffness goes up as depth CUBED).

So 3 layers x 1200 (12 oz) layer of biaxial tape. 12 oz x 6" wide roll x 50 yd = $45/roll 

https://www.boatbuildercentral.com/proddetail.php?prod=Biaxial-Tape

or 2 layers of 17 oz x 6" x 45 yard = $84

https://www.jamestowndistributors.com/userportal/show_product.do?pid=1442&familyName=Fiberglass+Biaxial+Cloth+Tape+-+6+inches+Wide

If your beam is 3" wide x 3/4" deep then you can overlap the 6" tape on the bottom flange of the beam where you need the thickness.

Like this:

image.thumb.png.9fbf8259bf33e6441c5aa596c5eb61ab.png

 

 

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What Zonk said.

There is little I can think of that is limper than wetted glass cloth. Trying to get it laid overhead would need at least 6 hands.

Use biax, stronger, stiffer (thicker) and drapes nicely around radii.

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Wow Zonk. I owe you a stiff drink if our paths ever cross, very helpful. Will try to see if I can get away with the 3/4", should be fine. Will radius the edges of the beams and build up a half way decent fillet on either side. 

Jon -  Planning to lay up overlapping 2-3 ft strips as opposed to a full 8ft strip. I'll see what I can get away with. 

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If you are laying it up overhead, the simple solution is to first wet it out on a flat plastic covered table (4 mil drop cloth, taped tight to table). Then sprinkle a moderate dusting of cabosil onto the wetted out cloth. Like dusting a cookie with icing sugar or putting cinnamon onto a coffee drink.

 Squeegee it into the wetted out cloth. This will make the resin thixotropic (non runny) and stickier.

Roll up your length of cloth and starting at one end, unroll and squeegee or using your hands, place into position as you unroll it. I did about a 15' deck beam this way and it worked fine. Wear a baseball hat or similar in case it droops down into your hair. You can start with a 3' length for the first piece, but after you do 1 piece, you'll find it sticks very well this way.

Peel ply optional but always recommended!

Make sure the bottom beam core radius is about 3/8" and fillet the inside corner where beam core joins the overhead with thickened epoxy putty too. Didn't show it on the drawing.

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An alternative to the wet layup with cabosil is to do a dry layup with a light misting of spray adhesive on the fibreglass.

Spray adhesives are used in resin infusion to keep the dry stack in place.

I have used AirTac which seems like an aerosol version of contact cement.  3M 77 is a similar product.

Actually, it looks like 3M makes a Hi Tack version designed specifically for resin infusion: https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Hi-Tack-Composite-Spray-Adhesive-71/?N=5002385+3290539321&rt=rud

Only works with epoxy.  I have tried it on polyester, but it appears that the styrene or some other chemical dissolves the adhesive because the layup fell off just as the cloth was becoming saturated.

Just make sure to wear a hat have a lot of drop cloth in place because there will be lots of drips.

 

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37 minutes ago, SloopJonB said:

Don't bother with the hat - just do the laminating right before you need a haircut. :D

The fact that my fiance is still living with me on our boat is a miracle. Every night it looks like a workshop. And she helps too! TG I had the boat before I had the fiance...

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20 minutes ago, weightless said:

I'm confused. Would you then release the part and glue it to the overhead with epoxy?

 I was just saying that as an alternative to doing a wet layup over the part, it is possible to do a dry layup, which is how I prefer to laminate when possible. 

I was only talking about the overhead lamination of glass fabrics .  Any part/beam was not part of the equation  - I believe it would have to be fastened in place first using either method.

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17 minutes ago, 12 metre said:

 I was just saying that as an alternative to doing a wet layup over the part, it is possible to do a dry layup, which is how I prefer to laminate when possible. 

I was only talking about the overhead lamination of glass fabrics .  Any part/beam was not part of the equation  - I believe it would have to be fastened in place first using either method.

Okay, but what happens to the spray adhesive? You'd need to use enough to hold the whole layup to the overhead. That seems like a different thing than tacking bits in place into tooling. No?

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41 minutes ago, weightless said:

Okay, but what happens to the spray adhesive? You'd need to use enough to hold the whole layup to the overhead. That seems like a different thing than tacking bits in place into tooling. No?

I'm not sure I truly understand your question

In any event - to answer what happens to the spray adhesive?  I assume the spray adhesive remains.  Potential for contamination?  Yes, but 3M says about their Hi Tack 71 "Formulated to have little to no intrusion in the resin infusion process to reduce delamination risk"  I assume AirTac and 3M 77 are much the same since these have been used in resin infusion.

On to your second point, I assume you mean how do you do multiple dry layups in one go?  I have done it with simultaneous laminations of 1708 and 1200.  Just used the spray adhesive to join those two, then a fine spritz on the stitched mat portion of the 1708 and offered it up, then wet it our with a foam roller followed by squeegeeing with a plastic spreader.  I don't know if this will work with more than two dry laminations at a time.  I doubt adhesion will be an issue, but I suspect full saturation of the fabric would probably be one.  So if you want to do more than two laminations, you may well have to wait until the initial laminations have cured.  Good idea in that case to use peel ply so you don't have to prep the cured epoxy/glass.

On the other hand, there are limits on the wet layup method as well.  I think you would have to wait until the innermost laminations have begun to gel before applying subsequent laminations.

 

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22 minutes ago, 12 metre said:

I'm not sure I truly understand your question

In any event - to answer what happens to the spray adhesive?  I assume the spray adhesive remains.  Potential for contamination?  Yes, but 3M says about their Hi Tack 71 "Formulated to have little to no intrusion in the resin infusion process to reduce delamination risk"  I assume AirTac and 3M 77 are much the same since these have been used in resin infusion.

 

Not a great question I'm afraid. I think you've answered what I was trying to get at though -- the epoxy bond across the spray glue line between the glass and overhead is okay in practice.

 

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