Is there a way to calculate tabbing requirements on bulkheads?

Discussion in 'Boatbuilding' started by leaky, Feb 12, 2018.

  1. leaky
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    leaky Senior Member

    I've been standing up these corecell panels - they are not intended to be structural bulkheads rather they are walls for a bathroom, however they kinda function that way. Hull is solid glass in the ballpark of 1/2 inch thick (not really sure haven't drilled a hole in it yet but edges are that thick, bottom is usually thicker on these, probably 5/8 anyway), round bilge, semi displacement full keeled downeast; top/roof is some sort of foam core with minimal glass on the bottom (I suspect 2 layers of 1.5 ounce mat)..

    These hulls do not need support forward of where the stringers stop, builder says it's good to have structure but all optional what is done up there - usually they have something like a v berth, bunks, etc.. but bulkheads are not required, I'm mostly using them for bathroom walls but obviously they are creating some structure.

    The panels are 5/8 thick 5 lb corecell, a layer of 1708 and a layer of 3/4 ounce mat as a veil. I do a fillet with hull & deck putty, mostly to make it so the glass lays in nice and to hold them in place. Originally I went at it based on some various things I read, recommendations from others, and the idea that the same schedule that goes into a light foam panel ought to be fine for tabbing - so I ran a layer of 1708 and a layer of 3/4 ounce mat/veil as tabbing (using VE), but reading various things I question this a little, would like to be sure to get it right the 1st time but also do not want to pile on weight or actually cause a problem by taking something that can flex a little and making it rock solid.

    Is there some formula that outlines such tabbing w/ light foam bulkheads like this? Or what do you think about that tabbing strategy?

    Thanks in advance!

    Jon

    2ndheadbulkheadflanged.jpeg
     
  2. philSweet
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    philSweet Senior Member

    You've come to the party a bit late. There are a few reasonable ways to have gone about this and this really isn't any of them, but not to worry. You've installed a relatively weak and stiff (compared to the hull) bulkhead hard against the hull. One way to get away with this is to not tab it at all and let it float in a channel built into the hull. The other is to make a seriously structural ring frame using the bulkhead as a mold. This would involve feathered/staggered tabbing that totals about 5/8 of the hull thickness (from memory) all on one side, and just regular tabbing as you described on the other. That full thickness needs to extend about 10 hull thicknesses, so 5 inches, molded on the bulkhead, and then taper out.

    Back of the envelope calculation suggests 7 layers of 1708 on one side. If 12" wide tabbing was used, carefully lapped 1/2 to 5/8 inch each, you would be in the ballpark. Now you have a bulkhead mounted to a ring frame.

    An alternative that might still be attractive is to carefully scribe the bulkhead one "hat section" thick - probably 1.5 inch hat would do - and saw it out and grind back the fillet to the hull and install hat section. You could do the bottom and top in separate jobs and not even remove the bulkhead. Hat sections are engineered for this job. Now the bulkhead interface is soft enough you don't need to go crazy with the tabbing. I'd give this option a hard look because it may still be as fast, and I doubt there is much cost difference either.

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  3. leaky
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    leaky Senior Member

    Is what you are saying to try and eliminate hard spots or is it in regard to the panel holding together?

    Am aware of concerns with hard spots but these boats they do not even use a fillet on the stringers or main bulkhead, is just coosa on the stringers and a very rigid foam core structure on the bulkhead, dropped hard onto the hull and tabbed in there with a 1608/1708 with no space or even ramp, hundreds running around like that. I figured putting a fillet on it was improving upon typical in the spirit of hard spot prevention but maybe that's not what are concerned with?

    I could fairly reasonably cut the hull side of it out and do whatever to make it right (hell could trim out the part that meets the hull, slip some corecell in there with a ramp and put it back together). Wouldn't a wider fillet have the same effect though? I guess I need more explanation of what you are getting at.

    Thanks,

    Jon
     
  4. leaky
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    leaky Senior Member

    Oh and the other thing I wonder about - the top is tabbed to a foam core structure, never read anything indicating there is an issue putting a bulkhead onto a foam core structure so that's very confusing to me too.

    Thanks,

    Jon
     
  5. leaky
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    leaky Senior Member

    Interested in more info on this for sure - especially what the main concern is; I am aware of the idea but like I said just not something I've ever seen on these boats and I thought a fillet was good enough on a light foam core panel like this. Just double checked some pics of one in progress of a build and someone who is fairly experienced has a plascore main bulkhead with the same layup tabbed hard into the hull with no fillet.

    You can't see it but there is a fairly identical bulkhead 26 inches in front of the one in my picture BTW.

    In the meantime I did some quick calculation - on the hull side, 4 bulkheads starting at 12 inch tabbing via 1708 (or biax anyway) adds ~ 25 lbs if it was done starting a 12 inch. That's a practical solution I could bang out in a day and I like the idea of adding structure to reduce movement versus welcoming it, which tends to lead to wear and tear over time.

    Jon
     
  6. rxcomposite
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    rxcomposite Senior Member

    Here from LR SSC. Tabbing start first at 50 mm each side of the angle to meet minimum shear requirement for resin. Tabbing thickness is around 1.5X the face thickness of the cored laminate. Compliant resin is important as it absorbs the shear load, fillet provides stress relief, and bonds the foam to the skin.

    Afraid the formula or calculation is quite complex as it requires calculation of it as a whole. That is starting from material properties up to the dimensions of the attached part to determine the neutral axis of the web and shear load on the junction and web of the attached panel. Core to single skin.png
     
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  7. leaky
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    leaky Senior Member

    I talked to a few people about this today and re-checked how others are doing it on these boats, another builder I know is going to give me a buzz later who usually has a good handle on taking old & new schools of thought into something that makes sense so his feedback might be the most valuable. Regardless still interested in info - was hoping PhilSweet might expand a bit on what he said related to my questions.

    In short though after looking at 2 ongoing builds and talking to the builder of mine and two others who are in the business of these boats, the feedback I got was that the concepts of the foam hat, ring frames, etc.. are not well applied to these boats and doing anything beyond a fillet you are trying to fix a non existent problem and possibly causing a new one.

    Checked 2 ongoing builds, one a very prestigious builder - they use hull and deck putty fillets about exactly how I'm doing it with panels of 5 lb foam and 1208 (nearly exactly what I've got)... another smaller builder who does rugged commercial boats with lots of bulkheads and right angle structures does no such fillet, cuts similar panels to as tight of a fit as he can get them, and tabs at a right angle.

    One explanation I got as to why the difference was basically, these hulls are of a heavier schedule and are generally built much more rugged than the lighter and more thoroughly engineered higher performance craft the concepts were designed around - a thicker hull doesn't mind a bulkhead adjoining it at a right angle and flexes less to begin with.

    Jon
     
  8. philSweet
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    philSweet Senior Member

    I'm not too concerned about hard spots, I'm concerned about the bulkhead getting frittered away at the interface. Not something that will happen quickly, but it is a heck of a lot easier to prevent this now when you can get at it easily than to try to fit in new parts later. I don't like the ring frame idea much either, but it will prevent this from being a problem at the cost of some unnecessary weight and expense. I haven't ever heard hat section/trapezoid section transitions were a bad idea anywhere. What I am seeing is that the recommended size of the sections has decreased over the years. I used 1 1/2 inch hat because that's what we stocked. The information so far all concerns structural bulkheads, and even if this isn't required by the designer for the benefit of the hull, I think you need to treat the transition zone to a depth of ten hull thicknesses as if it was one. Your panel of lightly skinned H80 or similar takes a good bit of finesse to get right compared to a 5/8 inch sheet of mahogany plywood.
     
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  9. leaky
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    leaky Senior Member

    Thanks you very much for that. Certainly would rather make corrections than have things come apart later, but then also I've got alot of surface area of panels and a weight budget particularly up in this area, as it can throw off balance, which gets skewed by plywood, so outside of the simple ideal of "no wood" there are some good reasons to use light foam core. The stringers for instance are X 4 made of 26 lb coosa, adding 100 lbs there is of no concern.

    Here's another question - I pulled out one of my books (which doesn't seem to object to any of what I've done so maybe is of no use) and one of the things they detailed some is in race boats they tend to support light bulkheads with small close spaced stringers to eliminate such problems. One thing I did not mention is within the structure here I've got a few things going on; there are 2 fairly identical bulkheads, one forward of this by 26 inches, secondly once I figure out if/what to do about these bulkheads I have 2 feet of stringers to run from the original that the builder installed up to the pictured bulkhead. Within those two bulkheads there will also be "stringers" connecting them together on the bottom as well, but they are to be spaced differently for different reasons (namely to support the shitter on one side with a platform likely also joining the bulkheads together, and on the other side a structure to set the sink pedestal atop which could be a similar platform like the shitter side.. Forward of the forward bulkhead there is then a bunk tabbed into it - effectively a L with the long part running forward along the hull, a typical v berth. Quite a bit of structure going on there. How does that feed into the same design considerations? Seems like as structure is added it begins to eliminate potential wear and tear too, lots of light structure versus a little heavy structure.

    Also worth mentioning - another option if the strength of the panels is the main concern is as you go adding tabbing to a degree of 30 lbs then actually strengthening the bulkheads may make more sense (or even part of them). For instance if the bulkheads were stronger, and one was to bring the existing fillet out to a full 1.5 inches per side, tab over that normally, then the interface between hull & bulkhead is 3.75 inches of solid material transitioned out, plus tabbing - that would be a half day project of minimal cost as would adding some 1708 to the existing bulkheads (ie maybe just along the interface if that's the main concern), likely all on the same weight budget but without the brute force of a ring frame.

    What I was given that was "bad" was basically when you use foam as the fillet on a wide angle, what can happen is you delaminate the fillet due to the inherent lack of peel strength with foam, ie attempting to peel the fabric from the face of the fillet since as you make that foam part wider the angle becomes closer to what you'd do to peel the tabbing away from the foam; the bulkhead has little control over the hull if the forces pull during twisting, bouncing, or whatnot. If that happens then you have a free floating thin piece of tabbing connecting your bulkhead to the hull which certainly fails. Lots of folks really negative on the foam fillet thing, I guess it comes up a lot, at least the crowd I spoke to quickly dismisses it, one person had a bone to pick over the idea of softly mounting stringers stating it undermined the structure they are supposed to provide.

    I don't know - what I try to do when questions arise is get lots of feedback and sort through it - when you talk to 10 people that all are certainly well experienced on a subject with vastly different views, I think somewhere in the middle usually lies the right idea.

    Jon
     
  10. leaky
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    leaky Senior Member

    The other thing that goes through my head too is man w/ VE and 1708 over this 5 lb corecell the stuff is actually a lot tougher than sometimes it's made out to be. I was breaking up a piece of what I considered a failed laminate the other day to fit it in a contractor bag, was ~3 ft by ~8 inches - I laid it on a stump on one end and stomped on it with my boot, it bent at an awful angle, the sprung out and manged to smash my shin. I picked it up and there was no apparent damage. I then set it differently and after a few tries managed to bend it a full 180 degrees and fold it over but even the outside skin of glass was fairly intact, I only managed to crush the core and somewhat buckle the inside layer. As I consider these things trying to imagine how you destroy it in a boat without destroying the whole boat :). I'm sure like you say wear, tear, and time can slowly beat things down but I wouldn't expect plywood to be much more ornery than that.

    Jon
     
  11. leaky
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    leaky Senior Member

    This is one of the things that kinda got me here, a set of bulkheads in a Wesmac 46 - 1208 over light foam core, hull & deck fillet which then is tabbed over. You can see the coring on the side of the hull which does not extend below the turn of the bilge to get an idea of positioning and this is a monster boat compared to mine. Not saying there is nothing here to consider just one of the examples I've seen on the subject.

    Jon

    westmacbulkhead.jpg
     
  12. rxcomposite
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    rxcomposite Senior Member

    Jon, you might be confusing the part. That is not a bulkhead. Looks like a storage area to me. Bulkheads/web frames are major structures to connect the bottom, sides and top of a boat. When it has a hole in it it is called a web frame with an adequate height of the web to support the load. Some boats are designed with a minimum of bulkheads designed for the panels to adequately handle the load thus must be tabbed properly.

    Sometimes, depending on bulkhead spacing, frames/ring frames, adequately braced, adequately capped, are added in between to reduce the stress on the panels. It ranges from 1 or more frames in between the bulkhead and remains a major structural element. In order to save weight, areas which requires beefing are sometimes built in with continuous cabinetry such as tabletops/bed tops/floors are built in (longitudinals) and divisions (transverse) it becomes part of the interior of the boat. The section/s becomes relatively stiff.

    Once the initial design has been proven by calculations to handle the load, additional cabinetry/storage maybe installed or designed. It is not a structural part of the boat, designed only for the load it will carry and nothing else. You can just stick it (nails, if wood) with thickened epoxy and minimal filleting or even Sikaflex it. It may appear to be redundant, or looks like it is stiffening the boat, but you can rip it out and still have the integrity of the boat intact.

    If the original design shows only two bulkhead/web frames then anything added is redundant and does not have to meet the criteria for major structure.
     
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  13. leaky
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    leaky Senior Member

    Thanks - that may be the point I kinda had in the back of my head but wasn't thinking about clearly. I believe in my case these are not required either, they are really just bathroom walls not intended to meet any requirement, just like that closet in the Wesmac? The hull, top, and structure required all basically get put in by the builder and as a blanket statement they tell you if you want to put an engine in it, and go racing around without a deck even, it's got all it needs.

    Typically in a commercial lobster boat style finish there would never be a bulkhead as nobody cares about such - there would just be a sort of large v berth, open in the center, likely without cushions, with hinged tops for storage - and those boats really get thrashed fishing year round in some cases. In a boat with creature comforts like mine you start having "bulkheads" for bathrooms, closets, etc..

    Similarly that would be the sound reason why the builder basically just states to attach them to the hull with fiberglass in whatever manner is easiest?

    I spoke to a 2nd boat builder this morning about it, designs and builds these sorts of boats, really wanted his input because this particular person tends to be savvy with more modern ideas/improvements (so if anyone I talked to was going to find an issue it's this guy usually). He said basically when he attaches similar panels he uses a 1.5 inch pvc pipe to make a fillet with hull and deck putty, and that such sizing is engineered and proven. His 2 cents was it was fine as-is but he recommends 2 layers of 1708 on the tabbing to prevent wear over time, and after talking where we got was it would make sense to put a fillet over the existing tabbing to make the footprint a bit wider, then cap that off with a layer of 1708 & veil of 3/4 ounce mat. Tonight I did the prep on the tabbing areas so I can well bond to it - as one way or the other sounds like I'm going to be adding there.

    Jon
     
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