Stringer And Deck repair

MIA, other then the sizing issue, which I'll get back to, you just don't need or want mat in an epoxy layup. Mat is used with the polyesters and vinylesters, because it helps bulk it up and "key" it to the next layer. This isn't necessary with epoxy because of the molecule structure of the cured (epoxy) matrix.

In your case you used a "double bias stitch mat", which is a biaxial, non-woven (knitted) 45/45 (or 90/90) fabric that's lightly sewn to a mat backer. This is a material that's used in polyester building, because it bonds well with previous layers of material (regardless of what fabrics it might be) and has a high elongation strength (non-woven fibers). What this means is the fibers will be in tension and still stretching when the resin begins to fail, but the mat will likely have sheered and begun delaminating before this point (so what's the point). A better product for you would have been straight biax (double bias fabric, without the mat). The reason is simple, you'll use a fraction of the resin and your resin to 'glass ratio is better controlled. This makes laminates cheaper and more importantly stronger, because they don't have the mat (mat has next to zero strength) in them and you've a better resin/'glass ratio.

Mat and some fabrics have their fibers lightly bonded to each other so they don't fall apart when handled. Mat is commonly sized and pressed together. Some of the chemicals used, don't work well with some resin systems. In MIA's case he used a stitchmat product, which means that two 8 ounce uni directional fabrics were knitted together (at an angle) and the mat was stitched (lightly sewn) on. The mat also was volan treated (if it was knynex) so it is compatible with epoxy (even though you still don't want to use it with epoxy). Not all mats are treated this way, but since most experienced users will only use mat in poly layups, it's not an issue.

The bottom line, epoxy laminates don't need the physical properties of mat. This is darn handy as it sucks up resin like a 10 year old does ice cream. With an all cloth laminate, you'll have much better resin control and much higher strength. The only time I'll use a mat like material in an epoxy laminate, is to prevent "print through" and then I'll use a polyester fabric (usually a "veil"), which dramatically improves abrasion resistance as well as prevents print through.

 

Thanks for the thoughtful reply PAR!

I was down at the boat the past few days gluing and screwing. While taking a break I sat down and took a good look at a scrap of the 45/45 biaxle mat that I used for the stringers. I understand what you wrote in your last post. That said I still wonder......

When I was planning the stringer job I used the scantling rules found in Dave Gerr's book The Elements of Boat Strength. Now, I'm not crossing oceans in my little Silverton but I wanted to do the job right. I don't have my notebook in front of me, but working from memory (which isn't as good as it used to be) I remember that for a 25-26' boat with a 10 1/2' beam the schedule came up as something in the area of 1/4 to 3/8" for the laminate over the stringers. Looking at the biaxle mat I can see that the cloth is relatively thin. The mat is providing bulk and allowing me to build up a thicker laminate more quickly. I used 5 layers of 1708 biaxle mat. I'd agree that the mat alone would be weak. It's sandwiched between layers of biaxle cloth though, similar to a core. Wouldn't the biaxle cloth tend to stabilize the resin/mat that's sandwiched in between? Another thought I had is if I'd used cloth alone I'd have had to lay up many more layers. Or would heavier cloth have been advised. Then it occurs to me that if I'd used heavier cloth would I have had more difficulty doing the layup? The biaaxle mat was (for me as a novice) fairly easy to work with. It tucked and went around a radius fairly easily considering it's thickness. Sure did soak up the resin though. Reminded me of a girl I knew in college, but that was vodka.

I'm just trying to understand how this works, hope I'm not beating it to death.

Please keep in mind that my application is supporting a small block ford/velvet drive and a walters v-drive. I'd estimate that the whole rig weighs in around 700 -750 lbs. Not a lot of mass in the world of boats.

Thanks for the insight PAR,

Regards,

MIA

 

stringers

I have been reading the post on the 1708 so I called vectorply. He told me the 1708 would be good to use with the stringer and transom repairs. He said their glass is made to use with epoxy. I just emailed par a question regarding the stringers missinginaction made out of foam. My largest stringer is 2"X12" H can I use foam instead of plywood? this would eliminate the potential hard spot. The transom was black and wet so I have been removing the 2 layers at 3/4" thick. what a job! If i use plywood i was going to install 1/16" rubber strip on the bottom where it meets the hull for the hard spots, what do you think?

 

A small block with a velvet drive and a V drive will be over 1,000 pounds.

Dave Geer's scantlings are intentionally heavy, by his own admission and designed for polyester or vinylester resins systems, not epoxy, which is the whole point.

Without getting real technical, the poly resins are 2 dimensional on a molecular level, making them fairly weak, but epoxy is 3 dimensional, which offers very high molecular bonds. The result is epoxy sticks to itself quite well, but poly needs help and why mat is used in these resin systems. Epoxy laminates don't have to be as thick and more importantly can use the non-woven fabrics to full advantage (which the polys can't).

Ideally, you want the resin to begin to break down it's molecular bonds at the same time as the 'glass fibers begin to break. This occurs with biax and epoxy, because the resin and fabric have similar elongation properties. With poly laminates, the resin is the weak link and fails long before the fabric reaches it's max elongation. The results can be catastrophic laminate failure, where the bond just rips along the stronger fabric seams. Mat helps prevent this and why it's needed in poly laminates.

In small craft like your boat MIA, we run into laminate thickness minimums. When you compare poly with epoxy laminates, there can be a "multiplier" that could be used to reduce the average over all laminate thickness. Depending on load, use, etc. this is true, but there are some things you just don't want any smaller. End beds, mast steps, chain plate landings, etc. all need a certain minimum thickness for most of us to feel comfortable. Even though from a technical stand point and using epoxy you might be able to get away with a thinner laminate, you error on the too big side, just because it's "small enough".

John, you can use mashed potatoes as a core for your stringers if you want, but the laminate has to be thick enough to absorb all the loads. Generally, the core offers some compressions strength to the laminate so you can reduce the amount of material in the laminate. This means mashed potatoes are out (unless you cook them like my ex-wife). This is why you often see wood as the core for stringers and other reinforcement elements. Wood has good compressive and longitudinal strength for it's weight, bonds to epoxy exceptionally well and is easy to work with. Don't worry about hard spots, it's not an issue you need to concern yourself about. Bond the core directly to the hull shell, with fillets along the edges to make the tabbing lay down nice (and mitigate the hard spots).

 

Well that, and your point about scantling minimums for small craft clears this up in my mind. Maybe over the winter I'll have time to do a little reading regarding the chemistry. Interesting..

Thank you very much for the reply.

Best Regards,

MIA

 

Par,

I am in the middle of rebuilding my boat. I am curious as to your reasoning behind not worrying about hardspots under the stringers? This is a particular topic that I have been trying to get a decent answer on and is kinda slowing up my project, as I am at the point where the next step is bonding the damn things into the hull. The boat in question is an 18' open bow runabout, small block 350 I/O. If I dont have to worry about hard spots, with proper filleting etc, then I am all for that. It would definitely speed up the process.

I am also using epoxy and 17 oz biaxial fabric (no mat backing). Just curious about some more of the science behind it I guess, to make myself feel better.

 

Hard spots are when a surface has a dramatic rise in stiffness or density, such as occurs when something with a crisp edge, is glued to the back of a panel. If you push on this panel, it gives a certain amount, until you reach the place where there's a 2x4 (or something) glued on the other side. At this point the panel don't give nearly as much (dramatically so) to your push and this "hard point" will concentrate loads in the general area (actually on either side of the area). The concentrated loads can cause the panel to fail, just before or just after this location on the panel. This is the basic idea behind point loading or "hard points" (at least the ones we're concerned about).

To mitigate these stress risers in most boats, except those that are built exceptional light, you can use a fairly wide fillet, where the piece is bonded on the inside of the hull shell. What this does is gradually increase the density of the panel (as the fillet gets thicker) so there's less possibility of a "stress riser" under load and it gradually decrease in density as you move away from the hard point.

The other option is to change the way you attach things within the hull shell. You can use foam strips, beads of polyurethane, whatever and couple (really important) this treatment with healthy tabbing to the hull shell. This is the technique most production manufactures use, because it's easier and cheaper with it's lower labor requirements.

On light weight hulls, you can experience other issues, if you have hard points. On these types of hulls, weight is the concern, not the durability of the hull shell. If you have one of these hulls and it's "oil canned" or showing stress cracks at every bulkhead, then you may want to consider foam pads, but honestly, you'll never be satisfied with this type of hull, because it'll always be lightly built and causing "trouble" or developing "issues".

 

Hello Par,
I have a question with the stringers. I have a 2x12 and a 2x8 on each side of the center line approx 8"apart. I was talking to a locale fiberglass shop and he said it would be ok to make fiberglass stringer approx.12" wide and fill with high density foam eliminating the wood stringers and potential hard spots. I think it would be easier to make the glass stringer. The wood stringer are made so bad the do not fit the contours of the hull. I am not sure those angles can be made with a saw. Please advise

 

That was a compliment. Don't go read things between the lines now... I'm not that kind of gay I mean guy

 

A 12" wide stringer! Please do not take whatever this person is telling you to heart.

Look, you can make all 'glass stringers. The key is to use enough laminate (goo and 'glass) to make up for the lack of wood. It depends on what you want - a big thick, itchy, heavy, all 'glass laminate with lots of layers of material or a wooden core with a few layers of fabric over it to seal and tab it into the hull shell.

The reason wood fails in these boats, is because the manufactures want them to fail. They don't care what happens 10 years down the road. If they did, they'd insure the wood had three or more good coats of resin and sufficient material to make the wood waterproof. Of course this costs more in materials and labor and since they're cheap asses, they skip this part.

The wooden stringer doesn't have to fit the hull perfectly, when using epoxy (the only real choice for a novice). Just get it close, the epoxy is gap filling and can make up huge differences (like 1/2" gaps). The only important part is the top edge, which often has to be lined up with where the sole will live. If you can get the lumber close, it can sit in a bed of thickened epoxy, which will conform to the hull shape.

 

How wide would a fairly wide fillet be? Say like 1/2", or more like 3/4"-1"?

 

Fillets are sized to their use, the material it's on and what it has to do. In regard to the quote, a generous fillet will decrease the stress riser by slowly increasing the substrate thickness. In this application there wouldn't be a set rule, like is often used in taped seam construction, but a judgment call, depending on where the anticipated loads might be. For example inner bilge stringers, well aft on an outboard powered warped bottom hull, may need substantial fillet widths, say a few inches, to absorb the bottom loading. While forepeak locker bulkheads that land directly on the hull may just need something like 3 times the thickness of the bulkhead to mitigate the hard point. So, the answer is "it depends" on where this type of fillet is going to be employed.

 

PAR, I was looking at a previous post by oceansswk. I believe that he meant to say that his stringer was 12" high but somehow wrote 12" wide.

Oceansswk...I'm the kind of person who will do my research first. Sometimes, based on the research and my intuition I'll try something slightly unconventional. That's how I ended up with my foam stringers. I'll post a couple of more photos for clarity here. Rather than make an epoxy fillet for the stringer/hull joint I decided to cut a triangular piece of foam and epoxy it to the hull and stringer where they meet. PAR mentions this technique as an option in his previous post here. I'm not writing that to "kiss up", but just about everything I've done I've read about somewhere on this site.

Anyway.... The deadrise (which is, as I understand it the angle that the bottom of the boat "rises" from horizontal in a v-bottomed boat) was in my case 20°. So, based on that information I could easily use my little table saw and cut a piece of foam that would lay on the inside of the hull and on the vertical stringer. Because of the deadrise I didn't have right triangles but was able to transition from the vertical stringer to the inside of the hull with angles of 35° on the inside of the stringer and 55° on the outside. I said to myself "close enough".

Now just as PAR said about the stringers you're looking to build, the inside of my hull was not perfectly flat. So I mixed up some epoxy and silica thickener and built up a little "bed" where I wanted to place the stringer. Once the stringer was ready to go I plopped it onto the epoxy bed using my "jig" to get the stringer in the proper position and left it for a couple of days to cure.

What I had trouble comprehending at first was the concept that the material that one uses to "form" the stringer has little bearing on the stringers ultimate strength. The wood or foam or mashed potatoes as PAR has said is just a material for the fiberglass/epoxy laminate to drape over. It's the laminate that makes it strong. In my case I needed some material that I could clamp my engine beds to so I decided to make 4 inserts that I built into the foam stringer cores where I would eventually bolt the engine beds in. Note that I took the time to make a radius corner on the wooden inserts. I'm no engineer, but everything I've read on this forum advises one to spread out loads and eliminate "hard spots". I figured that a sharp 90° corner on the bottom of my wooden stringer inserts was looking for trouble down the road so I smoothed out that potential problem spot and hopefully spread the loads out.

As I hope you can see in the photos there is a nice, smooth radius from the stringer to the hull. The foam stringer was cut from 2" stock. If you actually do this you will find that after you cut a piece of foam to fit the deadrise of the hull you will have a piece of scrap that you can use to make a fillet. The angle on the scrap piece will fit since it is what is called a complimentary angle. It's difficult to describe but if you play around with it you will find it's true.

So that's my story for what it's worth. Hope that this helps. BTW, just using plywood would have been easier and quicker and just as strong if done properly. Sometimes I just like to try something for the heck of it, but always I do my research as best I can. I'm a scared boater.....I don't want any failures out there.

best regards,
MIA

 

True. In two of my boats I made mine all glass to gain some weight and because I don't like wood in my boats.

Don't laugh, but this worked quite well. I collected cigarette boxes from all the smokers I knew which I taped together. Then I'd glass them all round to the thickness I think was going to be strong enough and then glassed them in place. It worked pretty well. Gained quite a bit of weight on wood as well.

Off late I use closed cell foam to do the same. The added advantage is it adds a bit of flotation while providing a former for the glass. If you are carefull, you'll be surprised how much flotation (or water displacement) you can achieve, all for the better.

 

MIA,
I have 2 stringers on each side of the center line. The stringer closest to the center is 2"x12"x16' and the next stringer on the outboard side is 2"x8"x15'.
The outside dimensions of the 2 stringers is approx. 12". I was thinking I could make a fiberglass stringer approx. 12" wide and eliminate the 2 wood stringers and fill the center with foam. I believe Par said no way, use the wood. I called today to purchase Plexus adhesive for the stringers, $40.00/tube might have to purchase a case of 12, need 6, and special gun @ 275.00. I am calling one of the local boat builders tomorrow and see if I can purchase a gallon of each from them and mix my own batch.