Re-tabbing Bulkheads on a Production Catamaran

Ready Thy Archers!

Hello everyone and welcome to the weekend. As a new member I thought I might ask for some collective wisdom. Even though I have already conducted personal research on this site and others, I still would appreciate some feedback for my particular situation.

Background: 38 foot (11.55 m) European production catamaran that was built in 2003. Solid polyester/vinylester hull below the waterline with core on the sides above the waterline and a balsa cored deck (note, I still need to open up a section and positively identify the core material used in the side hulls, but let’s say balsa for now). Dry ship is listed at 7,260 kg in the documents and when we last had her weighed during haul out in Grenada the lift operator said just over 10,000 kg (so she’s running heavy). I also know that when dealing with some thru hulls back in 2013 that the solid hull thickness is just shy of 25 mm along the bottom. I am the second owner, after buying her out of charter in 2007 – so she did a little less than 3 years in the BVI charter trade.

Problem statement: After numerous years of full-time cruising in the Caribbean, we have noticed that many of the smaller bulkheads and furniture paneling has separated from the hull. This specific manufacturer has a history of poor varnish application and service life. Finally, after one too many rough passages, we began to notice that some of the glue joints (it is believed to be Plexus, but if anyone knows what was being used in France back in 2003 please let me know) have failed at the wood interface. It is only the wood interface that the adhesive has failed. During some destructive testing, I did observe that the bulkheads were varnished prior to tabbing and that the varnish under the fillet has lifted from the wood, thus creating a tabbing failure (through no fault of the actual adhesive). See picture #1 for an example.

At this time, in light of my inspections, all of my issues are with what I would characterize as non-structural bulkheads. But any chance I get to stiffen (and lighten) her up is envisioned and desirable. There are four bulkheads that I would say are structural and they are tabbed with glass with an air gap of “x” mm (as I still need to measure that).

My repairs will be using West System epoxy, because that’s what I’m familiar with.

Although this cat is far from perfect, I am attached to her. And I have plans on making more memories in the future onboard… even if it does mean slipping down more 20’ waves during a gale because we got a bad weather forecast

So here are my initial questions in reference to the cartoon shown in picture #2:

1. I am not sure if I will re-use the marine ply or fabricate composite panels (I’m really leaning to composite right now). Either way should I provide a cove profile on the edges of the bulkheads as shown? And is there a recommended radius for this feature? Is there an industry ratio of bulkhead thickness to cove radius?

2. What type of bedding compound? I see everything from thickened epoxy to commercial offerings. Knowing that we see some flex (if only because of my luck for rough weather), so advice on which is appropriate for a semi-flexible environment? I see Arjay 4001 has many claims about how flexible they are…

3. Use the bedding compound only between bulkhead & hull or use a foam strip between to prevent stress risers? And if foam is recommended for this situations what dimensions are we talking about here and what type of foam and density?

4. An older thread on this site seemed to recommend a 4 mm standoff between bulkheads and the hulls. Many authors echoed that that ‘sounded right’ but what is the rationale behind 4 mm (as opposed to say 3 or 5 mm)? And is it appropriate to have a standoff at all in my situation if large fillets are used? Note in picture #1 there is no gap in this construction (and thankfully no detectable cracks either).

5. If a standoff of “x” mm is desirable, would it be acceptable (to a future marine surveyor) to use the bedding compound between the bulkhead edge & hull and create the fillets all at once? Therefore eliminating the air gap and use of foam strip?

6. Should I glass in a continuous length of tape on the hull under the bulkhead edge prior to installation (for both solid hull and cored hull portions) for these smaller bulkheads? I notice 150 mm wide tape is often referred to in this context, but what is the rationale? Is it an industry standard or experience says it’s vastly better than say 100 mm wide tape?

7. Tape used in tabbing over the fillets – use how many layers for a non-structural bulkhead and with what thicknesses and which order? I have noticed that in some web discussions it is/was permissible to start with the widest tape first and use smaller tapes over the top when using epoxy resins (but apparently not polyester). Although no real reason was given as to ‘why’.

8. Thoughts on removing the existing adhesive? Chisel or oscillating multi-tool? Or…?

9. Any professional references for me? I’m not above paying for an experienced engineer to provide assistance (I’m in Northern Florida). I am however, a bit hesitant as most I’ve talked with personally don’t seem to know multihulls in general (and that can get expensive fast). I did get one response from the designer of record about this issue and the response was:

“Bulkhead attachment to the hull: You could have fill the gap between the bulkhead and the hull with charged resin, but it is not the main thing. You have to do again the bonding with 2*400gr/m² biaxial cloth taping between hull to the bulkhead.”

Which I really appreciated. But I had many questions about the response and unfortunately did not get any follow-up. I can appreciate the delicate nature of the business as I’m sure they are always very cautious about litigation when giving email communications of this nature.



OK, that’s it for now. Let those ‘arrows’ fly, I’ve got my ribs exposed!



Thanks – Coastal Ogre

 

Coastal Orge. I am still thinking about this, but my initial reaction is how much work do you want to do and how much lighter will the result be. Yes a foam glass panel can be substituted and it will be half the weight of the ply bulkhead and probable save you 5 kgs of weight if the bulkhead is a square meter (10.7 square feet) in size. But the work to cut out the original, create and install the new panel with bog and tapes, cleanup and fair the surface of the tapes and make it make it look pretty is not trivial. On the Multihull Structure Thoughts thread are two C106.pdf's have bulkheads/ribs for a 40 foot cat (base design for original Catana 40). It gives a guide to suitable bulkhead materials and glass tapes. My focus on weight is in my experience before a racing series we would unload our boat and only put back the really required items, this pulled up to 600 kilo's out of the boat. Do you need that generator, inflatable dingy, portable fridge, unread books, spare blankets, extra rope etc. I will think more about how you may be able to repair, upgrade the existing structure and get back to you.

 

Without knowing the excact structural design of your boat I'd tab all the bulkheads and paneling on the hull without gaps to get a solid monoqoque construction. There's online "Gougeon brother on boatbuilding", read it first and follow the instructions. PVC foam as Divinycell instead of ply to save weight for most places..

 

If the solid hull skin is already 25mm on the centreline the weight race already hard to catch up. Every little bit helps though. Personally I'd just accept that it's on the heavy side & just put back as built- with some multis just need to believe that they're better motor sailers than sailing machines and enjoy for the space & view. Generally the gap you mention is to avoid print to topsides, if epoxy and tabbed over a cove less issue.

Jeff

 

Oldmulti: Thanks for the reply. My driver to replace the ply with a composite panel (if acted upon) would be part of a broader effort to remove weight from the forward two cabins (and arrange them differently - as we don't have much use for those bunks except storage). Should I go this route that bulkhead would be replaced with a 360 degree rib, somewhat similar to the rib used by the manufacturer where the leading edge of the bridge deck meets the hulls (in that same cabin). Regardless of the approach, the primary concern is to do it right the first time and (perhaps) make it a better arrangement.

I had already downloaded the C106.pdf's and have been reviewing them in earnest. I felt the timing couldn't have been better. The tabbing and rib information has been insightful to say the least.

Regards - Coastal Ogre

 

Teddy Diver: Thanks for this information. I have reviewed much of the Gougeon and West Systems' literature. One of my concerns with this approach would be creating a linear hard spot/ stress riser against the somewhat thin production hull. There are considerable 'web tales' about brand "X" monohulls that had their bulkheads re-tabbed (by owners) directly to the inner hulls and suffered extensive hull cracking.

However, I do have to admit that this hull has held up well, in spite of me driving her over 10k blue water miles. And if the manufacturer did it that way, then it might be just fine. But if I am conducting a repair, which is more effective in terms of protecting the integrity of the boat and longevity?

Regards - Coastal Ogre

 

Waikikin: Aloha! And thanks for your insight. Yes, she won't win any regattas, that's for sure (we usually jump on someone else's boat for that). But she eats up the miles at 6-7 knots getting me to and from the Eastern Caribbean. In regards to the second part of your last sentence; are you saying use epoxy for bedding, or no bedding, or...?

Regards - Coastal Ogre

 

1) I don't see any reason at all for doing that. Extra work with nothing to gain but more weight and risk creating air gaps which can at some later time collect water, bad for ply bulkheads.

2 & 3) If you use a foam strip between bulkhead and hull surface, it will provide all the flex you want, and the pudding can be hard like thickened epoxy and with some short fibers if possible. You could use ready made commercial product if wished. I used a foam strip in my own smaller and much lighter trimaran, with dimension perpendicular to the bulkhead 5 times the thickness of the bulkhead, and 12mm thick at center (same width as bulkhead), and gradually reducing to zero towards both edges. The core of the hull is also 12 mm for my boat, and the idea was to double the core thickness at the bulkhead location and gradually reducing to the original to reduce stress risers. On your boat the core thickness is most likely between 15...25 mm. Same density and type of core as used on the hull is ideal for the strip, to optimize distribution of the shear load for the core.
Most likely core density if a foam core is used is 80...100 kg/m^3 and at the time of construction crosslinked PVC is most likely. SAN foam will be fine too at this time, better toughness, and shear properties not too far away.

4) The softer gap made of foam is only effective if those large rounded fillets are also mostly made of foam, with hard pudding only at the surface. That's what the 5 times bulkhead thickness wide foam strip is for. Put thin hard pudding on that and then first layer of taping on that. If you require thicker rounding radius, put that on next. And then the last layers of taping. The taping will then effectively become locally sandwich construction with hard core at the rounding area, but single skin both at bulkhead contact and hull contact, which is critically important, as the taping need to be able to flex with the panels there. Otherwise loading between existing panels and the taping will have stress risers at the critical bonding area.

The idea of the gap is to allow hull panels flex at the bulkhead position, to reduce bending stress of hull surfaces at that location. See from any textbook covering structural engineering how clamped edge panels have that maximum bending stress at that location, while simply supported panels have it at mid span. The optimum condition for panel supports is between simply supported and clamped, and that is what the gap between bulkhead and hull surface is all about.

5) It would reduce the bending strength of existing hull panels there, but for such a heavy boat, it's likely the inner and outer skins are plenty strong enough for that. After all, that is how it was initially made. It's just more unnecessary weight.
6) only if you make it as in 5), not needed or useful if not.

7) Since the original was 800 g/m^2 overall, I wouldn't use anything less for that size of boat. I would prefer to use +-45 orientation though. Easier to form to required shapes, and still far stronger than needed for any load perpendicular to the hull surface. In that direction the taping will take much more load in tension or compression than the hull panels can. But for the shear load that is not the case. +-45 orientation is stronger than 0/90 orientation, but still most likely weaker than the hull surface. Think taping transmitting sideways load between bottom of the hull or deck and the bulkhead. Or vertical load between hull sides and the bulkhead. Would prefer 3 layers if possible to find thin enough with +-45 orientation. And one of them over the foam strip and below core made of pudding at the rounding location. You can and should reduce width of layers and use widest first, but use minimum 50 mm wide contact area between panels and taping at all locations. Therefore minimum width is = (100 mm + distance of the pudding + foam strip.) Stagger edges of every layer at least 5 mm to avoid thick edges of taping overall.

8) trial & error method works fine in this case.

Ps, it's not likely the hull is 25 mm thick at all single skin areas below waterline. 25 mm is more than thickness of single skin hull at ballast keel location on some similar size monohulls. The bottom of your boat will not have anywhere near that magnitude of such localized loads to require such thickness. It's quite possible there was overlap of reinforcement at the area you measured the thickness of hull of your boat. Some manufacturers don't spread them out. Or minikeels or some other loaded area were nearby perhaps?

Pps, 800 g/m^2 is not adequate for any structural bulkhead transmitting rigging loading or localized loads from the bridge deck. For epoxy lamination it could be enough for structural bulkheads just supporting hull shape and limiting panel size for hull in shear and bending.

 

fastsailing: thanks for this insight, it is much appreciated.
1.,2., and 3. - all good!
4. - I got everything except for that part in bold. Are you saying 'use only one layer of cloth to tab the non-structural bulkheads, so they can flex?' Because the wording in 7. seems to contradict that point (would prefer three layers).
5.,6.,7., and 8. - all good!
p.s. - agreed, and I should have been more clear in my wording. I knew the thickness along the bottom area (close to the fixed keels), but I have no direct evidence elsewhere (at this time).
p.p.s. - all good!

Regards - Coastal Ogre

 

4. I was writing about structural bulkheads only, because it does not matter how you tape non-structural bulkheads.
Everything will flex under load, it's just a matter of how much flex. Putty made of epoxy & microballoons or such will be brittle (compared to foam) , and break with far less flex than anything else with or without a proper core, when subject to bending. So if you use putty between layers of taping, it can only be done in areas where there isn't much flexing. In such places it increases bending strength of the taping substantially and allows taping to carry more load without any risk of delamination in high curvature areas. The hull panels are the ones that flex the most under load and thus require that taping laminated into hull panels must be able to bend with the hull panel, otherwise you will risk having gradually spreading debonding between taping and the hull panels. That means taping must be single skin where-ever it is in contact with and transmitting loads to hull panels further away from the bulkhead area, and if you use foam strip between bulkhead and hull panel, the latter will also be able to flex a little at that location, in other words, the hull panel is closer to simply supported at the bulkhead, allowing the hull panel to withstand more bending load than with fully fixed support.

There are a lot of boats made that don't follow these instructions. They may very well be strong enough regardless, but for the same strength they must use thicker and wider taping overlaps to the panels and thus be heavier. In other words, I'm just describing how to optimize weight for a given strength requirements and type of materials used without excessive amount of work needed or expensive materials or working methods. And strength here means all things affecting the end result, including but not limited to: bonding strength, avoiding unequal distribution of stress as much as reasonable both in taping itself and bonding, strength of taping in all directions that matter, etc.

 

fastsailing: thanks for your help and explanation!

Regards - Coastal Ogre