Glassing inside radius stiffeners - getting the glass to lay flat

MiddayGun

Super Anarchist
1,178
442
Yorkshire
Does anyone have any tips for getting bi-axial cloth to lay flat when glassing inside hull top hat stiffeners? 

The initial layer of CSM lays down nicely as the binder dissolves, but I find that because of the curvature of the hull, the cloth tends to bunch up at the top of the stiffener and give it a mushroom like top. 

Is the only way around this a load of relief cuts? Because i'll be essentially cutting right through the strands of glass it seems to defeat the point a bit.

Picture attached of what I mean, its not super clear in the image, but you can make out where the material is bunching up.

20220523_171111.jpg

 

cyclone

Super Anarchist
1,438
676
Maine
You could try using two strips that overlap atop the stiffener plus the fillet  smj  recommended.

 

MiddayGun

Super Anarchist
1,178
442
Yorkshire
Its not clear in the pic, but it has a small fillet & the sides are angled so its not a full 90 degrees to the hull. 
I'll try the next one with smaller strips and see if that helps. 

 

Startracker

Member
407
107
Van Isl.
Its not clear in the pic, but it has a small fillet & the sides are angled so its not a full 90 degrees to the hull. 
I'll try the next one with smaller strips and see if that helps. 
Bigger fillets never hurt anyone.  If you're trying to go hull - perpendicular edge -parallel edge - hull again _|-|_ you'll have problems.  1708 is great for ONE compound curve at a time.  I'm unclear about the csm, do you mean you're going 1.5oz mat - 1708?  How many layers of each total?  Epoxy or poly? 

Relief cuts are usually annoying.   I've done them in detailed compound curves but ONLY when I knew I was borderline for strength given how much area I had of good hull left.  Always a PITA.  

There are really 3 ways to deal:

1. Vac bag it.   This is great but with compound curves you sometimes get weird buckling.  

2.  Do it in strips(go as wide as you can, try  the full length of hull up over and down again in one piece, start with 3" strips and work down in width until it lays cleanly). The hull-stiffener-hull will then be in every layer.   Not quittte as good as one piece but still decent.  Is this an engineered activity where you are told how many layers or not?  If not I'd just taper my additional layers and add say 2 more of 1708.  That should more than compensate for the reduction vs all one piece.  and Work keel to rub rail keel to rub rail every layer. Think shingles on a house.  This is often factory choice.  Faster and easier. 

3.  Do it in full sections. This is more common with stringers in repair jobs or better quality new build.  So go hull-vertical face and either cut your pieces so they'll stay below the top edge, or lay up a couple layers at a time, then scissor trim wet to the top edge.  Two layers at a time both forward and aft face.  Then cap in the opposite way, the horizontal face and down both sides but NOT onto the hull.   FISKARS spring loaded or milwaukee no spring or worst case medical shears will mostly prevent the glass from bunching and sliding as you cut it wet.  micro serrated blades are key. 

 
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MiddayGun

Super Anarchist
1,178
442
Yorkshire
CSM as the first layer because I used Poly. 
The tabbing is 450gsm biaxial. And there are 2 layer of 600gsm uni along the tops of the stiffeners. 
Not sure what any of that is in American, metric just makes more sense to me. 

It mostly laid nice & flat, the CSM bulks out the radius' a little anyway, but just a couple of areas where I had material bunching at the top giving it an ever so slight mushroom shape around the edges.

I just had to replace some floors in the bilge that were cracked & split, since I've made them stronger, I'm just continuing them out into the hull. Previously there was no other reinforcement that the glassed in base of a moulded locker.

 

2airishuman

The Loyal Opposition
Bevel the sides of the ribs more, make bigger fillets, and round the tops of the ribs a little.  I use at least 1/2" radius fillets for ordinary 1-ply tape.  For biaxial you would want a larger radius.  Cut two pieces of scrap to the radius you choose (one inside, one outside) and use that as a gauge.

You could also try peel ply with battens and weights to hold the top in place.  Sort of like vacuum bagging, but only half way.

 

Startracker

Member
407
107
Van Isl.
CSM as the first layer because I used Poly. 
The tabbing is 450gsm biaxial. And there are 2 layer of 600gsm uni along the tops of the stiffeners. 
Not sure what any of that is in American, metric just makes more sense to me. 

It mostly laid nice & flat, the CSM bulks out the radius' a little anyway, but just a couple of areas where I had material bunching at the top giving it an ever so slight mushroom shape around the edges.

I just had to replace some floors in the bilge that were cracked & split, since I've made them stronger, I'm just continuing them out into the hull. Previously there was no other reinforcement that the glassed in base of a moulded locker.
Got it, in that case I'd use the strip method(like shingles) it's fast and easy and not fussy, strength will be fine, your issue will be more around the poly mechanical bond failing before the glass ever does.  Cheap and cheerful radius too: go to the pawn shop, get a 1/2" extension and few 1/2 sockets from the socket bin for cheap.  They make lovely a lovely inner radius.  Bigger the better, for something like this a socket 1/4" less than the the thickness of the stiffener will make for an elegant result.   +1 to what Sloop said about radiusing the upper edges with a big outer radius.  

 

MiddayGun

Super Anarchist
1,178
442
Yorkshire
I am using 45/45 biax, it's pretty much the only biax I seem to use on the boat. 

I'll try the bigger radius a the top & see how that works out. 

 your issue will be more around the poly mechanical bond failing before the glass ever does.  
Neither seems likely, the boat is full of secondary poly bonds that have lasted 35 years +. 
These stiffeners are probably overkill & unnecessary as well, but as I think the locker base I had to rip out was also used for stiffening I wanted to be belt & braces, hopefully never having to touch this area again.

 

teamvmg

Super Anarchist
1,972
104
I am using 45/45 biax, it's pretty much the only biax I seem to use on the boat. 
 
Looks like 0-90 in the picture.

could try 2 layers of thinner fabric 

if you do fillets along the edge, put the glass straight on before the filler cures, saves rubbing down/prep and it will stick like s..t

 

longy

Overlord of Anarchy
6,753
1,124
San Diego
You are working with two curves - hull and top of hat. Top of hat has smaller radius, so length of cloth against hull is longer than length of glass on top of hat. So that extra length has to go somewhere - in your case, it's bulging out. I believe the cloth you're using has the layers well stitched to each other, so the strands cannot move/splay. You need a cloth that is looser, so that strands can separate (make cloth "longer") as it goes 'down' hat & across hull. OR: since the main strength of a 'hat' section is the vertical component, glass hat in two parts. One piece hull up vertical, 2nd piece up vertical & across top then down. The vertical edges will still need to splay at the edges.

 

Zonker

Super Anarchist
9,663
5,621
Canada
and as a minimum use 10mm radius for fillets and outside corners - if you're good with glassing. Beginners should use 12mm (1/2") radius which is more forgiving.

You can also cheat a bit and use a bit of thickened epoxy on the top of the stiffeners to help hold the glass down.

+/- 45 biax you can shift it around and distort the weave to get it to lie flat but maybe some shorter sections would have helped as well. ~300mm long?

 

MiddayGun

Super Anarchist
1,178
442
Yorkshire
Half tempted to chop these out and start again. 
Most likely I'll just grind out the areas that are mushroomed and do some local re-glassing, and hopefully do a better job on the  other side. 

 

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