Racks cook book - layup suggestions welcome

[roland_trim]

BMG needs to start getting the Wings together. Wondered if anyone out there has any comments on the proposed reciepe (except the spelling of this word):

1 layer 200g glass +/-45
1 layer 300g uni (0)
2 layer 200g glass (0/90)
1 layer 300g uni (0)
1 layer 200g carbon weave (0/90)

There have been no sums done on the above!

[joe_moore]

What's the thinking in the glass sandwiched between the uni-d?

My understanding is that there's little point in using carbon and glass in the same direction as the glass will stretch before it takes any great load so the majority of the force is transferred to the carbon anyway?

I'd hazard a guess that the thinking about the ratios of hoop, longitudinal and torsional stiffness make sense. How much torsional load is transferred into the racks assuming the hull is "stiff enough" in the first place?

That turned out to be a lot of questions as I'm thinking out loud.. i guess the short answer would have been "I don't know Roland, but it's spelt 'recipe'..."

[ross_burkin]

What diameter tube are you using? Is it going to be the same all round?

I could be talking rubbish, but that seems like a lot of material. I only know what I've worked out for myself.  If I was building tube for racks I would go with.

Weave 0/90
bi-ax 45/45
uni 0
weave 0-90

With a fairly large diameter and maybe a weave in the middle just to bulk it up a bit and get some more hoop fibres in.

Joe brings up an interesting point about using both glass and carbon. In a high load area the different stiffness
and stretch properties might become more of a concern? I've never heard anyone mention it before. Windsurf masts seem to get on fine with a mixture in layup but are they designed with these characteristics in mind?  The load on a windsurf mast and the space frame for racks must be miles different though.

[Phil Alderson]

For Aqua's wing that did not break I used
0-90 200g woven glass
2 layers of 400g glass Uni
0-90 200g glass

For Primal's wings I used
+- 45 200g carbon Weave
2 layers 300g Carbon Uni
0-90 200g Carbon Weave

[Tim Noyce]

The diameter of the wing was quite a lot larger on Aqua than Primal though if I remember rightly? That must make a difference to the overall strength of the tube.

Trapezing on Aquas racks was really comfy!

[roland_trim]

Many thanks for the suggestions, as always I'm open to as many more as the readers are happy to supply.

What's the thinking in the glass sandwiched between the uni-d?

Increasing local crushing/buckling strength and stiffness by separating the two layers of Carbon Uni.

The glass will provide next ot nothing in the axis, but will transfer shear between the uni layers. With the weave on the ouside the tube, hoops of glass also do next to nothing for hoop stress, but as the carbon and glass hoops are spaced apart by the uni you get another local bending stiffness increase from composite action between the sandwiched glass and outer hoop of carbon.

In short the glass between the Unis is there as a spacer which can transfer shear. 

For Aqua's wing that did not break I used...

Many thanks Phil, I'm tempted to simply copy the primal layup (and forget the packing layer) can you confirm the diameters?

bi-ax 45/45

Ross, if you are around in when we make these you are welcome to come and have a gander (I'll provide beers for spectators). Hayley has perfected the strip method for laying up weave +/- 45 that saves a fair bit of cash in biax.

[Phil Alderson]

Aqua's racks were made on a 68mm drain pipe
Primal's racks were made on a 50mm diamater ali tube.

Primal's racks were scary light and needed some extra reinforcement around the supports to help spread the loads.

[Stuberry]

When i was thinking about this the other day I reckoned:

200g carbon weave
30cm wide 300g UD wrapped round (about 2 layers)
200g weave

I'm not sure I understand the benefit of the +/- 45 layer, I cannot imagine where twisting loads will come from.

IIRC Primal broke the very front of her racks on a couple of occasions when tipping the boat over to rig. I'm not convinced I'll put my racks that far forward because it will need to be strong/heavy and when it's that light I'll trap off the gunwhale.

I've seen you laminate tubes before but can I come for the beer?

[joe_moore]

Windsurf masts seem to get on fine with a mixture in layup but are they designed with these characteristics in mind?  The load on a windsurf mast and the space frame for racks must be miles different though.

The thing you need to bear in mind is that nearly all of these are mass manufactured by a few large Chinese factories. The carbon percentage is purely a trade-up sales trick. The lower percentage carbon masts really are there in the range because "carbon sells".

I personally use a 90% carbon mast as it's what was available cheap 2nd hand. At that kind of percentage it's substantially lighter than the equivalent glass mast (as it's a narrower taper for the same stiffness) and the 10% glass content is used in the inner layer for hoop stiffness. Glass does quite well at this because it's nice and springy and can take a knock. The rest is longitudinal uni-d with a loose 45/45 weave over the outside. I've heard of people having trouble with 100% carbon masts breaking at the wishbone when used in waves, I guess because of the point loads.

[joe_moore]

I'm not sure I understand the benefit of the +/- 45 layer, I cannot imagine where twisting loads will come from.

If you have a torsionally weak hull it will twist when the rudder is working. In theory that would put some twist into the racks. I'd go out on a limb and say you probably don't want/need to rely on the racks to stiffen the hull up though 


For reference, Taking Liberties racks are made from some old carbon I14 mast which is 3" in diameter and tough as old boots. Very comfy.

[Phil Alderson]

I'm not sure I understand the benefit of the +/- 45 layer, I cannot imagine where twisting loads will come from.

IIRC Primal broke the very front of her racks on a couple of occasions when tipping the boat over to rig. I'm not convinced I'll put my racks that far forward because it will need to be strong/heavy and when it's that light I'll trap off the gunwhale.

We first had the wings stopping at the forward support, then after sailing in light winds decided they would be useful extended fwd for when Carol was crewing infront of the mast. It was the re-attachement that failed, not helped by the very heavy trolly.

I think the 45's help to stop crack propagation, if you do not have any then all your fibers are 0 or 90 so cracks can run along the length just breaking the 90's with the 45's in there you get a much more robust structure.



 

[Stuberry]

Thanks for the explanations, makes sense now.

I'm also thinking it will be a good idea to laminate the outside layer of weave in squashed wasp (carbon/kevlar hybrid) for robustness when rigging the boat on it's side on concrete.

[Will_Lee]

Do you have any squashed wasp? I'd like to blag some if you do as we may be making a new stock quite soon and I like the look of it.

[Stuberry]

I don't, but I have a good source that I will PM you the details of. (Or any other members)

[phil_kirk]

Knowing that a good connection  (built in condition) can reduce bending deflections at mid span it might be worth adding short additional lengths of uni and or more +-45 where the racks tubes meet the deck beams if the racks are going to extend forward or aft of these connections by any great margin.