Brand Name of Foam

[The Froon]

Folks, can I trouble you for some suggestions on which brand of foam to use as a spacer between bulkheads and the fiberglass hull - so as to avoid hard spots, cracking, etc.?

Thanks,

Brian

[Hirilondë]

I don't think it makes the slightest difference what kind of foam you use, never mind brand. All it does is keep the plywood away from the hull while you do the tabbing. It serves no purpose in the end, it is just buried stuff. Sticky faced foam weather stripping works well.

[The Froon]

much obliged

[mitiempo]

Try and pick a foam that is closed cell though otherwise it will absorb resin from your tabbing or fillets. Ideally you can use a hard enough foam that you can cut it in a trapezoidal shape and save i=on epoxy when filleting as shown below.

[Tallystick]

Would a sheet of packing foam be an acceptable choice? A sharp knife can cut angles into it, or possibly a table saw.

[mitiempo]

Yes, that should work well.

[Hirilondë]

The trapezoid shape makes for a nice detail, but I doubt it does anything to improve the bond. Bond is adhesion to stuff. Your prep work on the hull and plywood, and your choice of resin are what effect that. If anything, your cloth will like a rounded inside corner most of all for bending around. So the trapezoid will still need a little thickened epoxy. And even thought the angles are better, it has 2 corners. All the foam needs to accomplish is to keep the plywood off the hull before and during tabbing to avoid hard spots. If you want it to add a cosmetic detail that you like then go for it. Everyone has there own way of doing things. But many techniques, though they look cool and sound good don't really make for a stronger or better anything.

Your packing foam sounds like it comes with a great price tag, I would surely consider using it.

[Quetzalsailor]

Hmmm. I have my doubts about the use of foam between the hull and the edge of a bulkhead actually working to 'reduce hard spots'. I think what you're really trying to do is to keep water from wicking into the end grain of the plywood. In the good old days, when our old boats were built, the tabbing was laid up in polyester resin and the end grain was seldom coated out. Polyester resin does not really stick well to wood, especially given the wood's coming and going with moisture content. Now, most of us probably do our repair work with epoxy resins and we coat out the end grain. Thus the debonding problem and the rotting due to trapped moisture in wood is much reduced. An adequate thickness of tabbing, intended to transfer the strength of the plywood (in tension, compression merely requires the plywood to stay put) will be more than stiff enough to cause any hard spot we'd fear. None of us intend that the tabbing is elastic. None of us are engineering the bottom panels to 'float' free of the frames as is sometimes done in ocean-capable power boats. A nice, stiff assembly of tabbing actually spreads the 'hard spot' out into a hard area, much wider. Additionally, tabbing fails in shear, as debonding between the wood and the tabbing, or it fails in 'peeling' the cold bond off of the hull. While adding a nice tapered bit of foam allows for easier, gentler layup of tabbing, it does poorly for the peeling since the distance and flexure in the tabbing is longer. I think a better solution would be to coat out the plywood in epoxy, fit it as neatly or haphazardly as you might, then use the epoxy to fillet between the plywood and the hull - taking care to fill the errant gap. Then the fillets do something good structurally as well as to form an easier layup.

[mitiempo]

I disagree. The hull will move and any part of the bulkhead edge that touches the hull will be a hard spot you don't want. By using a spacer of foam the tabbing takes the entire load spread over a much larger area evenly. The edge of the plywood bulkhead does not and cannot touch the hull.

If the bulkhead was a perfect fit to the hull there would be a lesser chance of a localized hard spot, but one the entire area of the bulkhead edge would exist. And that is next to impossible to do anyway as the hull has high spots and low spots because of the roving used in the original layup.

It doesn't have to be foam. Morris uses spacers and after one side is tabbed the spacers are removed before tabbing the other side. Same result when done - the hard bulkhead edge is kept away from the hull.

[Tallystick]

It would also be advantageous for the strength of the fillet to add chopped glass fibers so that the epoxy performs better under tensile and shear loads. Epoxy thickened with cabosil or similar silica particle reinforcement doesn't add much for tensile and shear strength. You need a reinforcement that acts on longer length scales to transfer the load. You can buy fiberglass whiskers or cut your own from cloth using sharp scissors.

[mitiempo]

The fillet doesn't add strength nor does it really have to - it is just there so the biax or whatever you are tabbing with doesn't have to make a 90 degree turn.
See how Tim attaches bulkheads on Northern Yacht for examples

[Rachel]

Interesting thread. I tend to agree on it being a good idea to space the edge of the bulkhead away from the hull and then let the wide tabbing transfer the force/load to the hull and not the edge of the bulkhead.

I, too, don't see where the filleting needs to have particular strength. I thought it was to make for a gradual transition, which the cloth/glass likes. I have wondered if it matters if the cloth adheres to that section (i.e., if it did not adhere as well to tapered foam as to an epoxy fillet would that matter)?

[mitiempo]

Probably not Rachel. The Morris method of using spacers only until the tabbing is in place leaves the bulkhead/hull gap empty.

[Tallystick]

Yes, I agree with that for the most part. My theory is that reinforcing the fillet would help resist delamination, but I don't have data to back that up, so merely a suggestion that is easy enough to do, and couldn't hurt.

[mitiempo]

The goal is for the tabbing to stay connected to the bulkhead and the hull.

With epoxy and better fabrics like biaxial we end up with a better finished product than they ever could have. We care more than the generally unskilled hourly workers that did the glass work on out boats, while smelling polyester 40 hours a week. I think the goal is to fix/rebuild to the standard that we can achieve with today's products with reasonable effort. We have a lot more information available than the builders did and the decades since have shown us what works and what doesn't.

[Tallystick]

True, and good points. Delamination would start at the edge of the wood/fiberglass interface, which is why I propose reinforcing the fillet to reduce flex in that local area.

As far as preparing the wood for epoxying the tabs, what would you recommend? I'm planning on sanding it with 36 grit to increase the surface area for the epoxy to grab onto.

I wish the previous owner of my boat had frequented this site. There were numerous half-ass repairs that seemed okay on initial inspection, but I've had to totally redo. One area on the deck had been "repaired" using rigid foam board with only paint and no glass over it. That was discovered while tapping the deck with a screw driver looking for rotted spots in the core, and the screwdriver went 1/2" into the deck.

[mitiempo]

When I built the interiors of boats (Columbia 26 and 34) we used all teak ply for the bulkheads and the edges that were to receive the glass were shaved to remove the top teak veneer. That gave a better bond as teak is oily. But that is all that was done. They lasted a while as that was 40 years ago.

In my experience with epoxy you get a first rate bond with a smooth surface. Anything I have changed or had to remove in the past has been incredibly difficult and all my mating surfaces were smooth or close to it. In my memory West in their manual and in the Gougeon's book the surfaces were always smooth - even when cold molding a hull. I think the area bonded and the thickness of the tabbing is most important. You want the tabbing, at least on a structural bulkhead, to be thick enough that it cannot flex if the bulkhead tries to move. For a main bulkhead I would use 3 layers of 1708 biax, staggered so the first is mostly on the bulkhead, the second mostly on the hull, and the third in the middle possibly.

I don't think the fillet does anything except make a curve for the tabbing to follow. In the end it doesn't matter if it is there or not. The hull/bulkhead gap doesn't have to be large - 1/4" or less, as long as the bulkhead is not touching the hull. When I put partial bulkheads in my boat I used 1/2" Ethafoam because it was what I had on hand. When the bulkhead was put in place the foam was compressed to a lot less than that, 1/4" or less. That is a small gap for the tabbing to span.

[Rachel]

Yes, I agree with that for the most part. My theory is that reinforcing the fillet would help resist delamination, but I don't have data to back that up, so merely a suggestion that is easy enough to do, and couldn't hurt.

I can't see in my mind how reinforcing the fillet would help at all. I haven't had any reason to reinforce thickened epoxy with fibers lately, so I can't remember - does it make it any less "creamy dreamy" for spreading?

If not, then I guess the only potential "hurt" would be one of time*, the cost of the fibers, the slight additional mess/hazard, and the risk that - before you know it - it's a "known fact" that it's a benefit, or that you need it (a variation on the theme in Hirilonde's signature).

Rachel

*both in general, and as applied to a potential smoke bomb.

[Tallystick]

Hmm, okay you're talking about using multiple layers. I misunderstood and thought one layer was being used. Only a single layer was used on the bulkheads on my boat, and most all of them have delaminated. Using multiple layers as you describe accomplishes the same thing that I had in mind, stiffening and strengthening the glass around the curvature where the wood interfaces with the fiberglass. Also my suggestion of reinforcing the fillet was meant to refer to thickened epoxy applied on the exterior of the tab, to strengthen the glass over the radius. In the case of using multiple layers, and that is what I'll plan on doing, I'll likely apply my thickened mixture over the layers to smooth it out, both for strengthening and a nicely paintable surface.

Rachel, yeah I have plenty of small glass cloth scraps, so may as well use them for something. I know for a fact from composite materials classes that using multiple length scales of reinforcements increases strength, because it provide higher interfacial surface area that has to be ripped apart before the material fails, and they transfer the load along the axis of the fibers. They will increase the tensile strength of the epoxy mix. Chopping the fibers shouldn't be hazardous as long as they don't get into your lungs.

[Quetzalsailor]

Hey yawl! Don't dismiss the fillet. If you're using West's filleting additive/filler, or similar, you're creating a structural adhesive. You need not add fiber to it. Plenty of small boats are bonded solely with fillets. Tabbing can be made with single or multiple plies of 'glass; the determinant is how much stress the tabbing is intended to handle. Bulkheads, engine beds and serious stuff require more 'glass than, say, a shelf or a battery box.

Back to bulkheads and hardspots. I'd opine that the hardspots that we see on some hulls are more a function of extended cure over time. The same thing is what causes print through of fabric pattern. I read in Professional BoatBuilder that an old white hull that lived in northern climes and then was moved to the south and painted black would show print through - years later! Other causes of hardspots include the hull simply being structurally too light. My dad observed another boat sailing a parallel course and saw the hull pucker in with every gust. No doubt, his '72 Morgan 27's deck was being lifted with those same gusts! That same Morgan's tabbing at the bulkheads near the mast were peeling off the hull, and the whole hull was bending to open the famous 'Morgan Smile' between hull and keel.

[Hirilondë]

Hmmm. I have my doubts about the use of foam between the hull and the edge of a bulkhead actually working to 'reduce hard spots'.

I have always questioned whether the foam accomplishes anything in reducing hard spots. But that is the theory behind it. I think a well done filet will fill most of the voids and make the entire mating surface equally hard. Then the tabbing becomes an extension of this.

I think what you're really trying to do is to keep water from wicking into the end grain of the plywood.

I never heard that before, but then I am often amazed at some of the stuff and reasoning people come up with. If you question the integrity of your tabbing to keep water out then seal the end of the plywood first.

In general there is a lot of over-engineering of boat repairs. Some one finds something rotten or broken and is convinced it is the fault of the original process. While this is some times true it is often not. And then the process begins to build the perfect repair job, often based on guessing what will be better. Then these guesses are talked about, or written about and pretty soon the totally unproven technique is an expert's method. A long time and very accomplished fiberglass repair associate of mine has done numerous structural repairs of boats run up on the rocks. He has worked with engineers from the manufacturing company to come up with the methods and glassing schedules for these repairs. He has never used foam nor has any engineer suggested he do so. I am not going to tell anyone not to use foam, I simply don't have any proof it doesn't help. But I am not going to tell anyone they should use it either, for the same reason.

[mitiempo]

They left a pretty big gap though!

[The Froon]

Appreciate the robust thread on this subject...it's why I love this forum so much. I am inclined, from the reading I have done here and elsewhere, to see benefit in creating a "foam" spacer between the unforgiving edge of plywood and a constantly flexing fiberglass hull. Cutting a trapezoidal piece of foam will enable me to more easily form that fillet, and I bet require less of an expensive material (thickened West System epoxy).

Going for 1/2" closed-cell foam....unless I read further on the matter that causes a heading change. :)

thanks all,

Brian

[mishnish]

Ok, time to weigh in here! :D

A bulkhead or any transverse division forms a "shear/compressive" connection to the hull. That is to say, the majority of loadings placed on it by the hull are trying to "rotate" it or "squish" it. These loads come from the hydrostatic pressure of the hull acting as a distributed load over the bottom portions, and point loads such as a chain plate pulling up on one side trying to "rotate" it, and the compressive point load of the mast foot, about which this moment is generated.

So a good connection of bulkhead to hull is working in shear mainly. Compressive forces are less important as the materials involved are good in compression. We typically use Biaxial tape oriented +45 degrees -45 degrees on bulkhead tabbing as this is exactly right to transfer these shear loads to the hull - the fibres are being stressed along their length, which is their strongest direction. In the case of using 0 degrees 90 degrees cloth, the 0 degree fibres would be loaded in shear - very bad as this is the fibres weakest orientation, and you would be relying on the shear strength of the "matrix" - epoxy or polyester resin.

Also, before I go on, if there is any movement at all in this joint, it has failed and is not working as designed!!! A bit of foam will not stop a hard point developing on the hull, indeed its the least of your worries!!!

In any joint, distributing load out over a wide area is most always a good idea, and this is what you are trying to achieve with tabbing and fillets. You are in effect, making the plywood edge several times wider. Also the bonding benefits from this increased area, as this means less stress placed on the joint for the epoxy bond to deal with.

Now this is where I start to disagree with what has become accepted technique. The addition of the foam, in my opinion is causing a reduction of area where the bulkhead is fully bonded, and thus transfers any hard spots out to where the bonders contact the hull anyway. If anything, the point load contact area is less than with just a ply bulkhead thickness as you have just removed this from the joint. Loadings in the joint are now fully transferred to the bonders, and there is far less shear area for the joint to work with.

To be fair, this can all be designed. The use of a packer can be acceptable based on calculation and knowledge of the shear loadings in the joint, then you develop your laminate schedule to cope with the loadings. The use of a high density foam packer or trapezoid can be particularly important when dealing with high spec cored laminates where the core thickness is particularly large compared to the skin thickness.

Now in our little world of old plastic classics, I find the technique unnecessary and pointless. We typically are dealing with thick monolithic laminates, especially on the bottom shell. The hull is usually of the order of thickness or greater than the plywood bulkhead, and so is locally very stiff. If you ever see a bulkhead through the hull of these boats you have a thin hull, and perhaps it needs some more engineered bulkhead connections.

So on the Contradanza project, I know I typically have about 10mm laminate thickness over the bottom shell, reducing on the side shell to about 8mm. Using a 12mm ply for bulkheads, and then buttering up the edge of the ply for full contact, filleting for the same width either side of the bulkhead and then bonding with tabbing gives me nearly 40mm effective contact area. This is the situation you are trying to achieve and is called "Aspect Ratio". Having your bulkhead contact area several times larger than the hull thickness is moving things in a very favourable direction every time, and is much more effective at reducing "hard spots" than a bit of foam.

Hope I have not caused any death by boredom!
Best Regards
Chris

[triton318]

Yeah...what he said!

[mishnish]

Yeah, I will be punished for that diatribe... no doubt my bulkheads will show through now! lol!