Stringer repair and improvement

[K.C. Walker]

I need to repair one of the stringers on my DS1. The stringers appear to be a hollow U- channels of glass. So, grinding away and putting some glass tape and epoxy seems like an easy enough repair.

So, what I'm wondering about is improving the stiffness of the stringer by adding carbon fiber tape to the top of it. I have experience with using carbon fiber tape on both sides of a reinforcement and it does an amazing amount of stiffening and strengthening. So my question here is, do any of you have experience with doing just one side, i.e. the top of the stringer with carbon fiber? Or, do you have any thoughts regarding this?

It would be an easy enough experiment, because I'm doing the repair regardless.

KC

[GreenLake]

If you think of the stringer as a beam, then, when the water (or trailer) pushes up on the hull, the top of the stringer should be under tension and the hull below the stringer should be under compression.

The vertical members of the inverted U serve primarily to separate the tensioned part from the compressed part of the stringer/hull combo.

Adding carbon fiber to the top of the stringer would increase its tensile strength, which is what you want. Whether you glue your carbon to the outside or inside of the stringer-top should make much less difference, because the distance between these two positions is small.

[Roger]

And do tell us how you work with carboan fibre tape. Your skill in this area would add to the brain trust here, and likely benefit a number of us. Where does one typically acquire this tape, what shapes sizes thiicknesses does it come in, and how do you prep and apply it to regulare fiberglass (polyester), and how does it work with epoxy.

Looking forward to hearing about cft.

[K.C. Walker]

GreenLake, thanks for your thoughts and I agree.

Roger, thanks for your vote of confidence for me adding to the brain trust, BUT you may have the wrong candidate! lol

To take advantage of the strength of carbon fiber you need to use epoxy. It's really the only adhesive that is strong enough to bond with carbon fiber and hold up over the long run. West System is the most available carbon fiber tape and I buy it from Jamestown Distributors.

So I've been doing a bit more research and analyzing my particular stringer problem. First, I have found further evidence of problems with my stringers. I see sheer cracks where the fiberglass of the stringers attaches to the inside of the hull. This led me to think more about the core material's importance in the stringers. My investigations found that the original balsa core used in the stringers is actually a very good core material as far as sheer strength goes. Of course, if there are any cracks or ways for the water to get in the balsa wood rots out quickly. The stringers then are seriously compromised for stiffness and strength.

Adding carbon fiber without adding a core would have very little benefit it seems. So, my plan is to remove what I have for stringers and redo them using foam core from Jamestown Distributors. The only question in my mind is whether I am going to try to improve on what would have been original by adding more area to what I repair. I'm contemplating coring the hull below the water line or at least from the front of the centerboard trunk to the transom. I'll see how I feel about that after I removed the stringers. After all, I do want to go sailing sooner rather than later.

KC

[GreenLake]

K.C.,

I'm not sure I follow your conclusion that a core is necessarily required for a stringer. Hollow stringers are not an uncommon design and can be quite strong. For example, you can build them by laminating over (half) a cardboard tube.

If you see cracking where the stringer attaches to the hull that would argue that the stringer is strong enough to resist flexing -- otherwise it would bend with the hull and there'd be no cracks.

Therefore, it might be that the amount of material where it is attached to the hull is insufficient to translate the bending forces from the hull to the stringer, or that the geometry of that joint is not ideal.

It seems to me that sandwich constructions with cores, are most useful if you want to uniformly stiffen a larger, flat area, while keeping the weight down. However, using cores in places that have (potential) contact with standing water sounds like a really bad idea. Especially for material as sensitive to rot, like balsa. You're never going to be able to eliminate all chances of small cracks.

If you decide that cores are your thing, I'd suggest you use something that's waterproof and doesn't lose its strength if it does get wet.

[Roger]

like closed cell foam perhaps. The stuff that cheap camping pads are made from. Keep us informed of your project.

[GreenLake]

The point about balsa as a core material is that it's stiff (and light), and can be laminated well. (If it was waterproof it would be ideal). There are foam materials that fit the bill, but "camping pad" sounds like it would be too flexible/springy.

[persephone]

Balsa (or other light woods) are fine for core materials. As pointed out there can be problems from long term exposure to water, however this is the case with any core material. The only question is time (as in how long the core has to be exposed to it before problems develop). Remember that as you lay up any sandwich structure you will be applying epoxy resin to all sides of your core. This works to help seal moisture out (especially if you add a thickening agent as this reduces the porosity of the base epoxy resin). There are several thickening agents, some specifically designed to exclude water.
Places you can go to get ideas for core materials,
Aircraft spruce (prices will be high because most products have to carry FAA certification), they have a large selection of foam and honeycomb core materials.
Dragon Plate (carbon fiber products of all types) has a lot of information about their products, as well as selling pre-made sheets, laminate plates, straps, tubes etc...
If you can come up with a way to apply pressure to your stringers as they are curing so much the better. I have done vacuum bagging in the past. It can be difficult and you need access to the equipment. Another thought I had was to apply the release and barrier plies as if you were bagging, then cover the layup with sand. This will add strength to your stringers by compressing them to the hull and keeping air out of the layup.
Has anyone experimented with diagonal stringers (as much as is possible). I know in large (wooden) warship construction it was a popular technique for distributing the load of the guns, as well as stiffening the hull in general. Certainly diagonals could be used aft of the centerboard trunk, and forward of the seats (a diagonal on either side of the mast step).
Just my wooden nickels worth.

[K.C. Walker]

Thank you for your continued thoughts!

I cut out the stringers Saturday and what a soggy mess! They were balsa wood sponges and compost from the bow to about the aft end of the centerboard trunk. From there back they were marine grade plywood, which was wet but not decomposed. Also, the adhesion to the plywood was okay in some places and not in others. I was trying to decide if the plywood was a repair. I'm still not sure. The plywood was half-inch and the balsa was 3/8 inch. There was a stringers down the keel of half-inch plywood from the drain to the transom.

After cutting out the stringers I ground the inside of the hull between the seats from the transom forward to the end of the stringers. I ground out to the tabbing also, that is, the edges of the stringer that were attached to the inside of the hull. It appears that the stringer fiberglass was mat not cloth. Also, there was a lot of unreinforced resin. I guess used as faring. I also found a fair amount of cracking at the aft end of the centerboard trunk. That seemed to be filled with a lot of resin and I didn't see any reinforcing in it.

As a side note, I still itch from the fiberglass dust which made its way inside my Tyvek suit. I used a pretty good respirator, I hope it was good enough.

So, the hull is more flexible now but not as much as one might think and more so in some places than others. It was fairly flexible before removing the stringers. So I don't think they were contributing a great deal to the stiffness.

Now, what to put back in there? Go back with the original balsa? I don't think so! The structural foam that I've been considering is a marine grade structural foam called Core-Cell. It's lighter than balsa wood but is not as strong or crush resistant. It is however stronger than most structural foam and impervious to water. If you're interested in seeing a comparison chart of costs and strengths this one is pretty good http://www.corecomposites.com/media/Cor ... nChart.pdf . I found this place that sells offcuts of Core-Cell at discounts http://www.noahsboatbuilding.com/noahus ... =0&Tp=&Bc= . It wouldn't cost too much in foam this way.

Another option I've been considering is just using marine grade plywood as the core for the composite, using the West System epoxy encapsulation method. Being that I'm not covering that much area the weight difference between foam and plywood might not be that significant. And, plywood certainly would have more impact and crush resistance without having to put a lot of glass or carbon fiber on it.

I'm not considering hollow stringers because the points stress I think is too high as opposed to distributed load of a cored composite. I also want to cover more area. In removing and grinding out the stringers I found stress cracks and de-laminations where the stringers were attached to the hull and I think that is due to points stress load on a high stress area.

Thanks again for your thoughts guys!

KC

[GreenLake]

K.C.,

first a comment on stringer placement. I assume yours went longitudinal. They would then help the fore/aft distribution of loads, but doing little to stiffen the hull for flexing in the 90 degree direction. If your trailer applied loads not centered on the stringer, those loads would not be well distributed. A full sandwich floor section might not be as strong fore/aft as the stringers were, but I can see how it might fit your load profile. (I won't speculate further without drawings.)

Second, consider plywood dinghies. For a concrete example take the Mirror Dinghies. These are built from 5 mm plywood, with a single layer forming the hull. It seems to me, that laminating even a 1/4" (6mm) sheet of plywood onto your floors, with glass above, would add more strength there than you need. However, the added weight of that approach is not negligible. (You could reduce the top layer of glass fabric to its minimum, by the way, it's needed primarily for abrasion resistance.)

Third, you could try to figure out (from observed cracks and other evidence) where the bulk of your stress loads occur. That would allow you to cut a piece of plywood in some other shape, say and H or cross or a shallow T or E. The different prongs would stabilize against bending in different directions, but you would cover less area (with less added weight).

Finally, if you taper the plywood (in all three dimensions) the effect should be to get a more gradual transfer of forces, which would counteract the tendency for the stresses to concentrate at the point where the reinforcement starts. (For the same reason, there's always a fillet at the edge of a stringer, allowing the tab to meet with the hull in a gentle curve. The thickened epoxy used for that is not load-bearing - it just supports the curve. I suspect that is what you found).

Materials. There are low viscosity formulations of epoxies, such as SystemThree's RotFix, that one can use to seal wood - they penetrate much better than regular expoxy. They are used in the building trade to seal wood against recurring moisture. (The same company makes a marine laminating resin as well as a mixture that's optimized for filleting.)

[GreenLake]

A PS: found these interesting links on the performance of core materials.
http://www.yachtsurvey.com/HiTech.htm
http://www.zimmermanmarine.com/docs/Core%20part%20II.pdf

[K.C. Walker]

GreenLake,

Thank you for your last couple of posts. Your thoughts are much appreciated.

It's interesting to find all of the conflicting opinions about core material. While investigating the possibilities of core material the material that seems the most interesting to me is CoreCell. Even the the writer of the 1st article you linked to, who is anti-core, wrote this article about CoreCell where he seems favorable about it. Http://www.yachtsurvey.com/ATC_Core-Cell.htm I found this article that makes it sound pretty super. http://www.compositesworld.com/articles ... -boat.aspx

One of the reasons that was thinking about some kind of a core is that I think the fiberglass in the bilge area of my boat has broken down. It's either fatigued or was waterlogged or a little of each. It's much less rigid than above the water line. I've recently also discovered blisters in the gelcoat.

I've sanded down the inside enough so that I can start out by laminating some new glass in the bilge area. I was figuring on doing this regardless. I'm hoping that this will increases the stiffness enough so that I can just go with the quarter inch plywood stringers, and I'll use your idea of strategic placement. It will be easier to be sure that I have complete bonding to the hull rather than trying to cover too much area. And, tapering out the edges is a great idea! I'm thinking that I'll probably end up having to sand off the gelcoat below the water line and maybe do one layer of glass on the outside, as well.

Can of worms? I probably just should have gone sailing!

Thanks again for your thoughts!

KC

[GreenLake]

K.C.,

Remember, for stiffness (of a beam), it's the outer skins that should count most. If you add glass on the outside and a stringer/ reinforcement on the inside, then the existing hull should act like the middle of the sandwhich, where the main task is to separate the load-bearing parts.

Based on that principle, I would suggest that you consider carefully how much glass (weight) you want to introduce in the middle of the sandwich (i.e. between the hull and the support you are adding on the inside). Unless there are some contrary reasons I haven't considered, my hunch would be to keep that layer to a minimum.

Anyway, good luck, and let us know how it turned out.

[Baysailer]

I saw in an old post on this site where someone cut PVC pipe lengthwise for the form and put glass tape over that to form the channel. No core but there a form and it sounds simple and effective to me.

Fred

[GreenLake]

Something like that would likely be my first design if I was to add a stringer to my DS, but K.C. had ruled out any type of hollow stringer for his project. Some people use paper towel tubes (saturated in epoxy the paper would not cause a problem later, esp. if the hollow stringer is vented).

I seem to have a weak spot on my deck where I might add such a thing from below.