Laminating a Rudder's Trailing Edge...WTF?!?

Perfectly logical Catbuilder, if you apply the chart at a 1:1 mix of the components 406-407 you get product 406.5 that rates 3.5 out of a possible 4 on the chart, basic interpolation. there's another way to do the trailing edge too with uni roving inside the very shallow v of the trailing edge, its done in three stages often using some melamine faced sheet to hold the "line" of the foil sides. Regards from Jeff.

 

Whew! Glad to hear some of the experts here giving me the "not crazy" (when it comes to mixing up bog) prognosis.

When you are putting all money you have, plus a significant chunk of unearned income into something, you get a little touchy when someone says it will fall apart.

 

When have you been told that, and why?

 

Both times were on this forum by people saying that my materials are incorrect and will not stand up. In this thread, by Tunnels. In a previous thread, by PAR.

Both suggested that a colloidal silica and microballoons mixture bog was weak and not durable... that I was doing something wrong using these materials as epoxy fillers in the applications stated above.

In the case of my heated discussion with PAR, it was regarding using this type of bog to make a "Payson Butt Joint" in the following bulkheads:



The Payson Butt Joint was made just as the Payson diagram shows, using multiple layers of glass and bog. I was told in the other thread that the colloidal silica and microballoons mixture was not going to hold up on these bulkheads.

Here is what the joint looked like in real life:



Now, in this rudder thread, I was told it would not hold up on the trailing edge of a rudder.

Hearing this twice makes a guy nervous, since I would imagine there to be a fairly well defined use pattern for any and all of these fillers.

 

Catbuilder; what you have been doing is fit for purpose and typical for foam construction. It would take more than 100mm flex in your rudders for the silica/resin trailing edge to crack.
PAR was also correct by saying that by adding fibers like milled glass will add strength especially in bending situations.
In my build the only coves that contained milled glass were in the connecting beams.

 

And besides, as large as your boat is, you have room for a spare rudder, do you not?

 

I think you have had an incomprehension with PAR back in the thread about bulkheads. PAR knows his stuff when it comes to working with wood and resins, and shares his knowledge whenever asked for an advice. So I really believe you two have had just a moment of misunderstanding there.

You say that you have used epoxy thickened with microbaloon + colloidal silica as filler or bog (as you call it). Now, these are the facts:

1. colloidal silica is used as an adhesive filler
2. microbaloons are used as a fairing filler and to make the epoxy mixture lighter and easier to sand.
3. the mixture of the two can be anything in between, in terms of mechanical and "cosmetic" properties.

I believe that the fact n.3 is what has alarmed PAR and others, since you have not specified the mixing proportions of the two types of additives.

First, about the Corecell strips. They have the primary function to transmit compression and shear loads between the glass laminates. These are not very high loads - in fact the Corecell, though being a resistant material, is a far cry from the mechanical resistance of the glass laminate, and even of the epoxy alone. So, imho, you can use your mixture for joining the Corecell strips together with no problems and no worries.

Thing change when it comes to direct glueing bulkheads to the hull, via a T-joint. That's where you want a good adhesive filler. Which means low content of microbaloons. The ideal is - no microbaloons but just silica or microfibers as thickening additives.

However, no need to panic. Consider this:
What microbaloons do is to replace resin with gas-filled micro-pockets. In this way the filler becomes less dense (lighter for the same volume), but it also leaves smaller cross-area paths for the stress loads to flow between the bonded structures across the bond (mechanical stress cannot be transmitted through gas pockets but only through solid resin).
You can then understand that when there's a big area across which a load can be transmitted through the glue bond, the local stress will be lower ( Stress = Load / Area ). This area is biggest (100%) when there are no gas pockets (microbaloons). By introducing spheres filled with gas, the percentage of the local cross-area of the bond occupied by resin becomes smaller, so the local stress will be higher. The important thing here is that it doesn't become higher than what glue can carry.

So what can you do to make sure that the local stress doesn't get too high across the bond? Simple: give it enough section area across which loads can be spread. Make the fillet along the perimeter of the bulkhead with a generously large radius. You can find a good illustration of that concept here: http://www.efunda.com/designstandards/plastic_design/radius.cfm

See this video, from the time-point 1:20 : http://www.youtube.com/watch?v=44GrmoXDabA&feature=player_detailpage#t=80s . Take note of the large-radius fillet they have used for those T-joints. It serves to allow the loads to pass between the hull and the bulkheads across bigger cross-sectional area. Even when you reinforce it with a biax tape, a large-radius fillet will allow for a better glass adhesion and less probability of delamination under cyclic bending and shear loads.

Now, if you want to be reassured about the quality of the job done, make a specimen of a T-joint identical (epoxy filler recipe, fillet radius, curing parameters) to the ones you have used for your bulkhead works. Once cured, test it with bending loads and see how it behaves. Ask your designer to help you out with establishing a bond-failure criteria. He will know better than we do the loads acting on that bulkhead and the scantling requirements. You need to feel confident with the results of the tests. If the bond fails, then increase the fillet radius, until the it doesn't fail any more.

It's 1.30 a.m., I better stop this thing if I want to be productive tomorrow.
And don't worry, if you have followed the instructions from your designer your boat will not fall apart.

Cheers!

 

As I mentioned previously, spherical filler materials (Q-cells, balloons, etc.) haven't the surface area, to make an effective or noble element, in a structural fillet or bond. These materials are bulking agents and as such need to be reinforced in the matrix with something else, such as silica, milled fibers, etc. This is in keeping with the list above from West System, which plainly shows that 407 and 410 are fairing compounds. 407 has some "stickiness" but very little strength, as it's mostly phenol balloons, though stiffer then 410 which is pure micro balloons.

In Cat's situation, the balloons serve as a bulking filler for interlaminate bonds, which some folks do, though I feel is a much weaker technique. A much better bulking agent for this type of bond is milled fibers or cotton flock, but both are tougher to sand if you have to.

Again and as I attempted to explain previously, the only time I use these round filler materials, compared to the fibrous choices, is to fair and smooth surfaces. The only time I would add these round bulking agents in a structural joint, would be on the occasions I need to sand them after cure. Of course the fillet would be stiffened with fibrous materials first, then the balloons added to make the mixture smoother and easier to sand. Often this is a two step process, where the structural aspects of the joint are addressed initially, then top coated with a fairing compound to "finish it off".

In all these thickened mixtures, silica is assumed also added to the mixture to control viscosity.

 

Thanks for the posts, everyone. This is very reassuring to hear.

My coves on those bulkheads are about 50%/50% colloidal silica and phenolic microballoons. They are also a 2" (50mm) radius. These structurally less significant bulkheads (pictured above) are held in with just those materials, as per the design. Only the beams and beam bulkheads require the chopped glass or microfibers.

My designer specifies several bog recipes, but doesn't specify what a "structural bog" is. I have been assuming this means to add chopped glass or microfibers to the mix.


My American designer calls this stuff bog and so do many Australians that I have had conversations with - they build a lot of catamarans over there. In fact, my bulkhead specifications are exactly as AndrewK mentions above (thanks, Andrew).

Well, this is all a relief. Hearing that you're doing something so common in a build incorrectly has the potential to make someone a bit jumpy.

PAR: I think the reason my designer uses large, fluffy coves rather than dense, smaller ones is weight. He's very adamant about keeping weight off his boats so they will perform. Obviously, he's done the structural analysis on these boats and I know he's got them on a diet. Because they are on a diet, they have lower overall stresses via momentum, at least. Also, I think he's figured out that a fluffy, large cove weighs less than a denser, yet slightly smaller one. The cove dimensions are laid out in his plans.

Your method makes good sense using the 2 step process to put just a finish sanding coat on after the initial fibrous cove. I will probably do that on my structural (beam) bulkheads and beam to bridgedeck attachments. Will save a lot of sanding time.

Again, thanks, everyone. I'm glad to have all these great posts and descriptions here. Daquiri's definitions and logical layout helped tremendously as well.

 

This situation has changed a bit, thankfully. I have eliminated the problem with filling in the weave on my interlaminate bonds.

I have moved over to infusing everything.

Now, the heavy uni side of the triax is placed face down against the foam, leaving the nicer, smoother biax side up with peel ply attached to it.

When I pull the peel ply after an infusion, I barely even have to sand anything. It's just amazing.

 

Cat, you, I and your designer aren't very far away from each other in the way we look at these things. I too attempt to save weight in preformance designs. I have to admit never having so lightly loaded a bulkhead that his mixture would be acceptable (to me), but this isn't that unusual. In other words if the bulkhead was found to be this lightly loaded, I'd probably use something else, like a ring frame. That's just me.

I also know a few that use bulking materials as an interlaminate bond promoter. I'd be hard pressed to do this, preferring to use 'glass, but again this is just me, wanting to use a similar material within the laminate, particularly if it promotes a better bond.

In the end of this huge learning experience, you're going to have a sweet cat and lots of places you're very proud of. Of course as with the rest of us, there will be other places where you'll say "if I had to do it again I'd . . ." Keep up the good work.

 

As I ramp up my plans for the build next year, I have been studying this as well as several other threads. One answer I did not find; Having ample sawdust and router shavings from pressure-treated yellow pine, I am considering using it for fillet material for just the sort of coves you have been discussing here. Do you consider it to be an appropriate filler for these joints?

 

Question: What is the difference between colloidal silica (West 406) and "Cab-O-Sil?"

They seem to be the same in descriptions, but one costs much less than the other. I have been using off brand colloidal silica, but the place I'm currently ordering from only has West System colloidal silica. They also carry "Cab-O-Sil", but I'm not sure if it's the same thing. I was considering ordering some.

Hoyt: As a data point, I'll tell you what my designer says about wood shavings. He says they're strong, but absorb lots of resin and make a heavy joint. Again, he is very obsessed with weight, which is why my boat has nearly the same numbers as a Gunboat 48 before rigs and such are put on. For a monohull, I wouldn't think twice about the weight.

 

Fumed silica, colloidal silica, Cabosil, Aerosil, same stuff, the latter are brands. Regards from Jeff.

 

Yellow pine is a softwood and it's average mechanical properties are (ISO 12215-5):

• flextural strength: 60-70 MPa
• elastic modulus: 8.5 GPa

On the other hand, epoxy has typically these properties:

• tensile strength: 90-110 MPa
• elastic modulus: 10.5 GPa

So, since epoxy resin alone performs better than this softwood, the inclusion of wood sawdust and shawings will decrease mechanical properties of the epoxy-wood thickened resin.
Another issue is related to grinding. If after curing there remain wood shawings not completely soaked with epoxy, after grinding some of them might be left with wood exposed to air and humidity, and would become subject to rotting and a path for water intrusion into the bond, thus weakening it.
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So, considering all of the above, I would suggest you to use wood shavings and dust as a fairing filler only and not for bonding.

Cheers