Preloading Stringers and Ribs

Discussion in 'Sailboats' started by Jamie Kennedy, Mar 18, 2016.

  1. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Context: I am looking to strengthen an Yngling for extended offshore sailing, along with adding some floatation, watertighnesss, reducing cockpit volume, all without adding too much weight. Aft of a major bulkhead where the chainplates are there are side tanks back to the aft floatation compartment. Forward of the chainplates it is more open, until you get to a forward bulkhead for a floation compartment. The mast is deck stepped, on the cuddy cabin actually, but there is a removeable support that extends to the forward part of the 700 pound iron keel. The hull weight is 700 pounds. It is normally raced with a crew of three. I am looking to sail solo, possibly with moveable water ballast and gear in the cockpit area. In the area forward of the chainplates I am interested in making it habitable so I need as much space as I can get there, but I also want to stiffening the hull, from oil canning, floating objects, and rig and keel stresses. For aesthetic reasons I like the idea of adding some wood.

    So I am thinking of adding wood stringers, running on hull diagonals, and then adding wood ribs, much like you might see in a skin on frame canoe or kayak. In this case the skin is already there, and I am adding some stringers and ribs (frames).

    Question: When adding stringers and ribs, is there an advantage to doing so in such a way as to preload the hull in tension, and to place the stringers and frames in compression? Also, is there an advantage to the stringers and frames being preloaded in bending, by bending them into place, either as single thicknesses, or minimal laminates, to maximize the bending preload? I suppose when superimposed on the compression, this would make them more in compression on the inside of their curve, and move the neutral axis towards the hull, but the idea would be to make them less prone to failure. Or would this make them more prone to failure.

    Clarifications: I would grind the hull and then epoxy and fillet the stringers to the hull. The ribs would be epoxied where they contact the stringers, but they would mostly hold to them because they are bent into them by compression. They might also be pinned somehow where they cross the stringers, perhaps with screws at the time of the gluing, but the function would be to pin the stringers and frames from moving out of position.

    Summary: Are there advantages and disadvantages to preloading stringers and frames in compression and bending and thereby preloading a hull in tension?

    [​IMG][​IMG]
     
  2. TANSL
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    TANSL Senior Member

    I do not see how you can change the position of the neutral axis if not change the shape of the cross section.
    Note that in each reinforcement, for example frames, the outer strips (or layers) are working tension and compression inner strips.
    I would let reinforcements as they are, without overloading them initially. But perhaps there is an advantage that I am not able to see.
    Just one comment: Shame place such strong, floating frames, because they lose much effectiveness.
     
  3. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Thanks TANSL.

    If you bend a bow by with a bending moment couple on each end it will have a bending moment throughout, with tension on the outside and compression on the inside. If you bend a bow by compressing it on the ends and allowing it to buckle some, while maintaining that compression, it will have more of the cross section in compression, and less in tension.

    Similarly, if you made a hoop by bending a member into a circle and welding it at the joint, the entire hoop would be in bending, with the inside in compression and the outside in tension. If you then wrapped a belt around that hoop, and tighted the belt enough, the belt would be in tension, and the entire hoop could be in compression, although the inside of the hoop would still have more compression that the outside. So as you are tightening the belt, you are moving the neutral axis out towards the belt, and eventually to the outer surface between the belt and the hoop.

    In the case of an existing somewhat thin skinned monocogue Yngling hull, the hull at rest has no residual stresses, at least not until you start to apply rig loads, and I suppose bouyancy loads as well. By adding the stringers and frames after the fact, I have the opportunity to do so in such a way as to load up the hull and deck. So I am wondering if it might be worth doing so.

    I think it might be worth doing at least a little to strengthen the joint between the hull and stringers and frames. Thinking about it some more, it might not be such a good idea to put the stringers in too much tension as I cannot go from one end of the boat to the other, but only from bulkhead to bulkhead. Plus I don't think I want to stretch the hull in length anyway, since the backstay and mast and forestay will be doing that.

    So I think it is just the ribs that I might want to put in compression, although I would have to be careful around the hull-deck joint. I would need the frame to take a decent radius there, and wrap right around to push up on the deck as well as out on the hull. I was thinking about this for a major bulkhead in the area of the cuddy cabin mounted mast, possibly to avoid having to have a strut from mast to keel. I got the inspiration from the Moore 24 with it's circular opening, although that hoop is not preloaded as far as I know.

    The third variable is whether the rib frames should be laminated in place, so as to reduce residual bending loads, or whether these bending loads might also be beneficial in some way. In a perfect circle that eventuall forms a hoop it doesn't matter, but if the ribs are end loaded, (which I want to avoid because of the hull-deck joint unless it is reinforce with a knee), then I think the residual bending is counterproductive to the idea of pushing out against the hull. If anything, I would want the ribs laminated in such a way that they are over-curved, so that they would have to be straightened as they are bent onto the inside of the hull.

    "Shame place such strong, floating frames, because they lose much effectiveness."
    - I see your point. I think they do this for fabric skinned kayaks and canoes for streamlining. In my case where I already have a more rigid skin it would make sense to have both stringers and frames against the hull, which is what I think you are saying. In my case I think there still might be an advantage to having only the stringers contact the hull, so as to spread the preloading forces against the hull more evenly.

    I'm thinking what I plan to do is to put the full and deck into a little bit of tension transversely, but not longitudinally, but still only a relatively small percentage of working loads. Also, as I am only doing this in the forward part of the boat, I think I am further limited to what I can get away with.
     
  4. TANSL
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    TANSL Senior Member

    I see that you know well of what you are speaking. I will indicate only one thing: the neutral axis is a property of the cross section. His position is the same regardless of the load that is under the beam. Even if the beam has a circular shape, the neutral axis of the cross section is the same, although the curvature of the circle varies.
    If you think you will have problems with the longitudinal strength of the hull, what I would do is put more longitudinal stiffeners and place smaller and more separate frames.
     
    Last edited: Mar 18, 2016
  5. gggGuest
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    gggGuest ...

    I'll tell you what starts to worry me here: the Yngling is a very lightly equipped raceboat intended for a pretty light crew. By the time you add all this extra structure, gear, habitation etc, is she going to be floating anywhere near her designed waterline, even with one of you rather than three light youths or women? And its not that small a rig,and she may be rather more tender with the majority of the new equipment above the waterline and less weight hanging out of the windward side.

    Are you sure you're not taking the shell a little too far away from what it was designed to do, and it might be worth sitting back and thinking about whether another starting point might be give you a better result?
     
  6. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I may have used the term 'neutral axis' incorrectly. I know that in curved beams the 'neutral axis' moves away from the 'centroidal axis'. Where I was mistaken is that for a given cross section and curved beam, the neutral axis still doesn't change even if an overall compression load is superimposed upon it. Thus both are determine by geometry, independent of stresses. I am not sure of the correct term for this axis of zero stress or zero strain when additional loads are applied. Thanks.

    Thanks. I think that makes good sense. Taken to the extreme I suppose it would mean adding another layup and keeping it monocoque rather than semi-monocoque. Both probably work well where it the hull shape is reasonable compound curves. The Yngling is like a small Soling. It is long and skiny, but not extremely so, and their aren't any really flat runs especially in the forward section.

    I am not entirely sure how much I need to strengthen it, but I am toying with the idea of modifying it for ocean crossings, like Webb Chiles is doing with his Moore 24. I did see some hull cracks last year when I sanded it down before applying a fresh coat of VC17 for the river. It was a crack in the gel coat that ran vertically, in the area just forward chainplates, and around the mast, but I am thinking that it was most likely caused by the trailer pads, and not while sailing. It did make me think about how thin the skin is though. I will look at the class rules again, but it is 700 pounds of fibreglass spread out over a 21 foot length and 6 foot beam. No core. I am sure it is very sound, like the soling, but maybe not for dropping off 40 foot waves and so forth. I want it too keep the water out, and have enough buoyancy to stay afloat if water does get in, but I don't want any major hull failures to happen either. Not sure what the modes of failure are I should be most worried about. I think if I hit a container I would ride up over it and hit it with the keel, but it could do some damage along the way. I think if a whale bumped me I would bounce off reasonable ok. I think the only real reason to make the hull stronger might be if I increased the displacement significantly, which I don't really intend to do. Nonetheless, I think some strengthening might be warranted and I think semi-monocogue adding some stringers and frames, particularly in the region between keel and mast, is something to look at. Just another layup in that area might be simpler and provide the most space, but I kind of like the aesthetic of adding some wooden stringers and frames. Plus the idea of preloading the hull and deck in that area interested me, and I can't do that with a layup. Cheers.
     
  7. TANSL
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    TANSL Senior Member

    Yes, you are right, if a beam is under bending, the zero stress (strain may be <>0) occurs at the neutral axis.
    About everything else you said, I can not comment without knowing more details of the boat. I think, but I can not be sure, that the vertical crack you found may occur due to a lack of longitudinal members. Although it is strange because these small boats, with full deck, do not usually have longitudinal strength problems.
     
  8. Jamie Kennedy
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    Jamie Kennedy Senior Member

    That's a good point. I am 200 pounds. Normal crew can be 600, although I know the rule limit is more like 500 pounds for women events. The idea would be to go solo, like Webb Chiles, but he weighs less than me and is giving up 3-4 crew in a Moore 24 whereas I am only giving up 2 in an Yngling. Plus I am looking at some moveable ballast, perhaps in the form of 20 gallons of potable water jugs. So I am really only giving up 200 pounds of crew weight. That would be taken up by the extra food and clothing and gear for even a minimalist ocean voyage. Drogue. Anchors. Rode. Some minimal batteries and solar panels or wind turbines. Electronics. Hull repair equipment. Weight added to reduce cockpit volume and add storage and reserve buoyancy.

    Design displacement is probably 700 pound hull, 700 pound keel, say 600 pound payload, for 2000 pounds. Water plane is probably 16 feet by 5 feet, if that, by say 0.7, for 56 square feet. Weight to immerse by one inch works out to about 300 pounds. I think the boat will sit and sail ok 2400 pounds, up from the design 2000 pounds. That assumes I keep the water out, which is especially important when solo, with limited battery power. So of the 400 pounds, above and beyond basic gear and fresh water, I think at most 100 pounds can go towards adding some buoyancy and keeping the water out, and I think at most 100 pounds can go towards strengthening the hull.

    So I am only talking about roughly 100 pounds of stringers and frames and strengthening. That could be used up easily with a minimal layup covering the mid section on the hull and deck, including the cockpit area after whatever I have done to reduce cockpit volume ( likely raised double bottom and some storage lockers above the raised double bottom.

    I think what would make sense, if I were to do this, would be to do as Roger Taylor does on the outside, which is to at least get down to the gelcoat to take a look, then get down below the gelcoat where there are any cracks, then reseal the outside. On the inside I can't see behind the side tanks in the cockpit area, and I think they go anyway when you put a raised double bottom in. The forward and aft bulkheads are plywood, and are still reasonably sound, but also reasonably deteriorated, so they might as well go. So on the inside I would remove everything to take a good look at the hull and deck and hull deck joint from the inside. From there I would repair what is there.

    From there, I would put back some fore and aft bulkheads and a raised double bottom and some buoyancy. Before I did that is where I think I would reinforce the hull deck joint, add some stringer, and then add some major ribs or frames in the area of cockpit, keel, chain plates, and mast. This is where I think it might be worth looking at doing it in such a way as to put the hull and deck in tension, somewhat, at least transversely. 100 pounds wouldn't add much when split between 5 hoops and say 5 or 7 stringers. Maybe 200 pounds would do it. The role of the stringers would be mostly to distribute the outward pressure onto the hull from the hoop frames, or ribs. The stringers and ribs might all be roughly square in cross section, say 1 inch square. 10 x 20 feet x 1/12 feet x 1/12 feet x say 30 pounds per square foot = 6000/144 = 40 pounds, plus that again to make it stick. So maybe I could double up all that and keep it under 200 pounds, except I also need some panels, to sleep on, for buoyancy compartments, for raised double bottom, and to keep the water out.

    So yes I can't add much weight, but I think if I add some frames in compression, and put the hull into tension, I might get more strengthening pound for pound.

    Anyhow, that is the context, but in general, purely theoretically, is there anything to be said for adding ribs preloaded with compression, to put a fibreglass skin into tension? Or does it always make more sense to keep residual stresses to a minimum?
     
  9. TANSL
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    TANSL Senior Member

    That's correct, imo.
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Might have been the trailer though. There are just 4 small adjustable pad, plus the keel support. When I worked on the boat I moved the keel support around, but I also played with the adjustable pads quite a but. I don't think I did it last spring though. I might have done it on the ride back from Cape Breton to New Brunswick. Or it could have been long ago. The boat and trailer are 1980 if I recall. Stuff happens in 36 years. All the more reason to take a good look before going offshore right.

    I don't want to do too much before I sail this summer, but it's worth looking at what might be feasible in the long run. The VC17 on their now is pretty thin except where it always builds up, which is in the flat section on the bottom between rudder and keel, which is the hardest to sand off. Probably not the area that requires the most attention either, but I might get all the VC17 off before putting it back on again. One coat. It will all have to come off eventually if I want to go offshore some day with something else, and one coat will come off a lot easier than several. Wouldn't hurt to take a look at all the gelcoat and fix it all at once.

    Iron keel is in good shape. Some pitting. There was some patchy fairing. Last spring I ground off most of the rust and gave it all a coat of epoxy, but I didn't do any refairing, because to do it right I understand you really need to get all the rust off right down to shiny white iron, because even under several coats of whatever iron oxide remains will keep moving. So that too this summer I think.

    Then I think I will get it in the water and go sailing. I can't do too much on the inside while afloat. I have my laser to sail though, so if I decide to I could take the boat out and sail my laser while removing all the insides to take a good look, and then put it all back in some form or another. In the very minimum, back into racing form in accordance with class rules, but with the raised double bottom they now allow. The forward bulkhead can also be brought back some. It has a range of 880mm to 1480mm forward of the cockpit, a range of +- 2 feet, which is 2 feet to 6 feet forward of the mast. I think 2 feet makes more sense than 6 feet, where it is currently.

    Adding stringers and frames and I think even reinforcing the hull deck joint is outside of class rules, but I might be able to do some things that are more easily reversible than others. The fleets in North America - there is a good one in Rhode Island - are pretty friendly and they would easily overlook anything that added weight and didn't provide any material advantage. As long as I didn't use the changes to increase rig loads I know there wouldn't be a problem. Still, it would nice if any illegal stringers and frames were reversible.

    Anyhow, what I think I have come to from this discussion is that basic hull maintenance would be the first step in any case. Thanks again.
     
  11. Jamie Kennedy
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    Jamie Kennedy Senior Member

    re: generally better to keep residual stresses minimum
    Thinking the same thing now. Just enough to get a good bond and keep things snug, especially considering cycling loading, but no more beyond that. Thanks again.

    Perhaps on the most practical side, if you do get some hull damage, you don't really want the crack you need to repair at sea opening up too much extra, or worse, tearing further along the hull, because the damn ribs are wanting to rip the hull from stem to stern. Recipe for catastrophic failure, however remote, but which is the very thing I am trying to prevent.

    Over a 10 foot frame though, say 1/10" or compressive strain, just enough to have it sit snuggly in place and help clamp the adhesive as it hardens. Over a 1 square inch cross section it would be a compressive strain of roughly 100 pounds, I think. I don't think that would cause any great harm, even if repeated every 2 feet or so. It wouldn't be much different than if you had a team of people lift the boat up by the gunnels, if it had gunnels. I think that would be ok. The important thing would be not to put anything in that would really load up the fibreglass too much, or some frame material that can take more compressive strain than wood, so that it might open up a 1 inch gash if it had the opportunity. I think wood at 1% strain is ok. 1/10" over a 10 foot girth. If it got moist it would want to expand some, even longitudinally, but it would also lose compressive strength. I think I would have them epoxied and varnished regardless. Still, a 1% compressive strain when installed dry, might become a 2% compressive strain once it absorbs 10% moisture, which it still might over time, even under epoxy, if it was installed at say 6% moisture. I think I'm still safe, but worth mentioning.

    Correction, longitudinal expansion with moisture would be much less than 1% for a 10% gain, so not an issue.
     
  12. SukiSolo
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    SukiSolo Senior Member

    I would be tempted to keep the stress to a minimum. Laminating the stringers in situ BUT with a polythene sheet underneath, so you can get them out and dress them later then refit. There's almost no 'hardness' on a fair hull in terms of curvature other than the bend radius the timber will take. Fairly light weighting/clamping will hold it in place when the positioning is correct. TBH, I've put in extra floor battens and stringers often just tilting the hull sideways until a vertical clamp ie weight will hold it. Light taping stops it sliding around, so a cradle is useful and stops squeeze out running everywhere. When you fit, tape the sides (on floor) and remove when part cured, leaving a perfect face for filleting.

    Before doing anything, check out the oldest hard used Ynglings you can find, to see IF there are any constructional weaknesses that show up over time. As some of these things were only meant to last a year or so, for an Olympic campaign they might not be quite so 'solid' as may be desired for your purposes.....;) They were/are not a popular boat in the UK, so almost all over here were Olympic hopefuls campaign boats.
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Thanks SukiSolo. They are very solid boats though, built like an old Finn dinghy, not a disposable boat like a 470. The hardest looking one I've seen are some old O'Days that have been left in the sun and rain. I think some of these have even been fixed up though. The fibreglass is all solid. There is some wooden knees and bulkheads and floorboards that need replacement, but not a lot of material in those. Some of the iron keels weren't cover in glass, and become very pitted and rusty, but the hull keel joint is a very solid deign. Eight 1/2" stainless bolts, six of these staggered. Reasonably wide hull keel joint. Not a terrifically deep keel. 3'5" draft, so about a 3 foot keel.

    I think the biggest challenge would be keeping the water out, and keeping the gear and fresh water weight in the cockpit area, while still being able to use the very small cuddy area for a refuge at times. Getting to the forestay is an issue, say if I needed to take a jib down in a storm. I think I would want to move some weight even further after before doing so. There are no lifelines or pulpit. I think I would need to add some toe rail grab rails, in addition to safety line of course. I was thinking of working from a forward hatch. This would pretty much guarantee a flooded compartment under storm conditions, but if the compartment is small enough it might be a good option. I would simply bail it dry. Maybe I could have some sort of paddling jack and skirt when working from this hatch, as kayakers use, but I am not sure how I would get into it before opening the hatch. So it would still need to be a very small compartment, forward just far enough to reach to the forestay, kneeled down.

    The Yngling is still very popular in Norway, Sweden, Finland, Denmark, Holland, and Australia.

    Here are some lads getting a boat ready for 2015 Worlds in Norway. You can see how high the raised double bottom is, but it can still hold a ton of water. I think I would reduce this volume and keep weight aft by storing my clothing and food and water in waterproof duffel bags that stow under the deck, and some in the middle of the cockpit. I could keep some bags light and some bags (water) heavy, and move them around when needed. You can see the hatch to the after buoyancy compartment. You can also image there is not much room forward. There would be even less if the raised double bottom was extended forward, but their might still be a cramped sleeping space. I would still need access to under the raised floor, through watertight hatches, and it would need to be compartmentalized. That might be the way to go rather than an open space forward with ribs and stringers.
    [​IMG]
     
  14. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Here is a great look at the inside of an old boat with floorboards removed.
    You can see partial frames for low floorboards, and the forward buoyancy compartment.

    [​IMG]
    Bigger: http://www.sailboatlistings.com/view/43361#
     

  15. gonzo
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    gonzo Senior Member

    Floating frames is a system used with good success. However, a Yingling is just fine as it is. Adding a lot of frames is not going to make it a better ocean boat; just heavier. A vee berth forward and maybe extending the cuddy is all you may need. Those boats are sailed hard, specially in sailing schools, and don't show any structural defects. I think you are fixing a problem that doesn't exist.
     
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