Stringer And Deck repair

[oceansswk]

Hello,
I have a 1988 Sport Craft and while I was working on the boat I noticed the fiberglass mat on the stringers was black, I began probing around and I found the stringers where like peat moss. I could not remove the deck without cutting it out, which I did today. Water was seeping out of the stringers once the foam was removed. I am not sure what material is the best, wood, Coosa etc. I read someone said pressure treated plywood is good for the stringers yes or no? Once that decision is made what material do I use to cover the material to encase. I will use epoxy not sure on the proper fiberglass. Do I cut the top of the stringer off and clean out the center or do I cut one side off to replace the core material ? I appreciate any help you can give me. Once the stringers are repaired I need to make a new deck.

[missinginaction]

Hopefully some of the more senior people will post as well and help you out. I'm not sure how large your boat is or what the engine mounting arrangement is, but I'll show you how I solved this problem on my boat. I'll attach some pictures showing the stringer fabrication on my 1973 Silverton. I used epoxy and 1708 biaxle stitchmat. This worked out OK, but PAR has since weighed in and told me that I could have saved some epoxy by using cloth and skipping the mat. I may have overbuilt the stringers, but what the heck, you live and learn.

PAR instructed me to remove the old stringers by using a long sawsall blade and bending the blade slightly so that it can be moved along the inside bottom of the hull. This technique works very well and you'll have the old stringers out in no time. Be sure that you've thought through the whole job though. I made a jig so that I could install the engine mounts exactly where they were originally and of course the stringers as well.

I made my stringers out of a product called Formular made by owens corning. It's just foam insulation and it doesn't absorb water. It isn't strong at all, but that's not the point. Once you coat the stringer with a number of layers of fiberglass and resin you will have a very solid assembly. Books such as Dave Gerrs "The elements of boat strength" will provide formulas that will determine how thick your fiberglass should be. Perhaps some of the more senior members here can give you a quick approximation based on the size of your boat.

Photo 1 shows the stringer being made up on the garage. Photo 2 shows the stringers being installed in the boat. At that point there was only one thin layer of glass on the stringer. This was applied in the workshop. The wooden blocks are installed where the engine beds bolt in, they are there since the foam is not strong in compression and I needed a strong material to clamp to. Photo 3 shows the stringers installed prior to painting. Photo 4 shows the original stringers and the jig I made so that I could install the new engine beds and stringers exactly where the old ones came out. If you study the photo you will see how the jig straddles the engine bay and the vertical pieces of plywood rest on the engine beds. Once the new stringers were ready to go all I had to do was install the jig and I had a guide to aid me in getting the new stringers in the correct location.

MIA

[PAR]

First off, welcome aboard Oceansswk. Can I assume you're from south Jersey?

As you've found out, your stringers are shot. This isn't uncommon and there are many, many previous threads on this site covering the different aspects of replacement. The key question now is, (picture your wife asking this) "will the boat be worth all this trouble when you're done". This can be a difficult question to answer, mostly because you don't know how bad a job this is yet. If the engine, electronics, trailer and hull are in good shape, the boat looks good and has some resale value, then go for it. On the other hand, if it needs an engine, the electrics and plumbing are shot and the whole boat looks like the Russian 3rd Army spent their last payday partying in it, then maybe she's spent and you should look for a different project.

Your choice of epoxy is a good one and all I recommend for the back yard builder. It's stronger, more water tight and easier to work with then the other choices, even though it does cost a little more.

When using epoxy, you don't need nor is it desirable to use mat. Use cloth or knitted fabrics instead.

The trick I told MIA (reciprocating saw) is a good one if you like to save time, but heed the warnings. You can just as easily cut through the hull with a tool like this, as easily as you can the stringers, so watch what you're doing.

Generally, you want to remove the damaged areas, clean, grind and other wise get back to good material, then bond and laminate new stuff in its place. Don't use pressure treated plywood for your stringers. Epoxy doesn't like to stick to the new PT chemicals they're using now. Besides, if you do it right, then you'll be dead the next time this issue comes up on your boat, which is pretty good durability if you ask me.

The first thing you should do is get the boat supported firmly. The trailer works well, especially if the keel area has several supports. You want to be able to walk around inside the boat with many of it's support elements removed, without the hull becoming distorted or warped. This often requires some additional bracing and blocking.

This will get you started.

[oceansswk]

I appreciate the information you both have given me. The boat is a 20 CC motor runs good. I asked the same question about value to a friend of mine who works in the marine industry an he said you could spend 10000.00 or more on another boat and the stringers could also be bad. Now that I have removed the floor and foam it was very poorly build and quality was not job one. There were no drain hole between compartments, matting was not installed on all exposed wood areas, etc. I think the sawsall is a great idea. The lumber industry has pressure microlambs, lvl and all are structural. Then I shopped XL panels from greenwood products very expensive 125.00/sheet. I do not know what to use at this point and would appreciate both inputs. Yes I am from So Jersey. Thank You both

[PAR]

Use dimensional lumber from you local Lowe's/Depot. Clear and straight grained 1 by stock is best. Laminate together if you need bulk.

You've discovered the reason most of these era boats die an early death. Manufactures were attempting to save on everything, resin, mat, labor, etc. The results are clear. You should check the transom also, which also suffers from similar issues, particularly is an outboard.

Use dimensional lumber on the longitudinal stringers. Use exterior plywood for the athwartship stiffeners and of course the sole (floor). Make sure the various compartments that are formed as the stringers and cross pieces go in, have drain holes to let out accumulated moisture at the transom.

See if you can find a Kamco building supply in your area (Trenton?), they'll have marine grades of plywood, which is the best thing for the sole.

Can you post pictures?

[oceansswk]

Hello Par, Thanks for your input. I will take Pictures tomorrow and post. I will see if Kamco is in the area

[AroMarine]

If you are near the shore Nacote Creek Marina in Port Republic and South Jersey Lumbermans in Mays Landing have marine plywood in stock

[oceansswk]

Good Evening Everyone,
I attempted to upload 4 pictures and is was rejected due to size. How do I upload pictures of the stringers. Yes I can go to Lumberman' s for the plywood. I am waiting on prices for epoxy etc. I did remove the stringers on the starboard side to start prepping that.
Thanks

[gonzo]

Mat and epoxy don't work well together. It has a binder that disolves with the styrene on polyester or vinylester resins. Epoxy won't bind properly to the fibers.

[PAR]

As a rule you don't need, nor do you want to use mat in an epoxy matrix.

[Fanie]

I attempted to upload 4 pictures and is was rejected due to size. How do I upload pictures of the stringers.
Download a little program called FastStone Capture, you capture a reasonable viewable picture off the viewing screen. Freeware.

Par has answered the stringer questions so many times he has a standard response he just copy and paste... :D

[PAR]

This assumes I'm willing to look up one of my previous replies Fanie . . .

[Fanie]

That wasn't what I meant :D One thing, we are lucky to have you on the forum, your advice is always sound and spot on. Doesn't leave much for the rest of us to comment on though ;)

[PAR]

Now you're just kissing up . . .

[missinginaction]

oceansswk.....

I used to have the same problem. When you go to upload photos note that file size restrictions apply. For instance a jpeg file can be no larger than 2mb.
So you may want to check the size of the photo file you're trying to transfer.

Now, I have a comment regarding the stitchmat. I certainly respect PAR's expertise. That said I also have confidence in Joe Merton at Mertons Fiberglass Supply. Joe recommended that I use 1708 biaxle stitchmat (knytex) to fabricate my stringers. I followed Joes advise and was very pleased with the results.

I 'm confused as I don't understand why this stitchmat (knytex) is considered to be a poor product to use with epoxy. Joe Merton made no mention of compatability issues with epoxy resin systems. It's in the boat and made a very substantial stringer matrix I did some research and found a book called Composites Design Manual by James A. Quinn. In the book it states (with regard to kyntek fabrics) that:

"All fiberglass reinforcement fabric used in composite laminates has a chemical finish.......compatable with the resin matrix with which they are used. This sizing is generally an organo-silane chemistry and is compatable with polyester, vinylester and epoxy resin systems."

I have no idea what an "organo-silane chemistry" is.

Help me out here..I'm just trying to understand opposing views from two respected sources.

Thanks

MIA

[PAR]

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.

[missinginaction]

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

[oceansswk]

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?

[PAR]

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).

[missinginaction]

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.

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

[tdvjensen]

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.

[PAR]

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".

[oceansswk]

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

[Fanie]

Now you're just kissing up . . .

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

[PAR]

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.

[tdvjensen]

.....you can use a fairly wide fillet....

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

[PAR]

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.

[missinginaction]

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

[Fanie]

Look, you can make all 'glass stringers.
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.

[oceansswk]

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.

[missinginaction]

OK, so you want to make one big trapazoidal beam that takes the place of the two stringers and fills in the space between the two stringers? More to the point, two fore and aft beams, one port and one starboard.

Well, that's certainly thinking outside the box (actually, I guess that you're proposing making a box of sorts, right?).

I have no idea on that. What I would recommend is waiting for a reply from PAR and the more experienced guys here. It just seems to me sitting here thinking about it, that you will use a lot of materials (foam and fiberglass/resin) that you don't really need to use. As far as the engineering aspects are concerned I'm not qualified to make a judgement. My gut tells me that I'd do a lot of research here though before I'd go forward.

MIA

[Fanie]

I think what is causing the confusion is the stringer should be 2" wide, 12" high and 16' long. A 12" WIDE stringer will not work (unless it's that high also).

Wood is quite strong, you have to make a proper stringer out of glass to replace the wooden one. I don't like wood because it is difficult to seal it properly.

[Fanie]

You have to fill the cavity between the stringers with a flotation foam with a CLOSED CELL STRUCTURE. If you slip up on the foam it will absorb water, become extremely heavy and you will have to do it over again.

[PAR]

Plexus is a specialty adhesive and really isn't the best way to go here (besides it'll cost 10 times as much and has a real short working time). In fact, it's not even as strong as the polyester resins used in your hull. It is good for difficult plastic bonds, but you don't have that here.

By the time you make the stringer stiff enough with just resin and fabric, you'll rival any wood cored version in weight. The only way around this is to control resin to 'glass content with vacuum bagging or infusion, neither of which seem likely on this job.

A big hollow box will likely buckle under load unless the box has an internal web structure or substantial core. It's size will require a lot of material.

With epoxy, you can seal wood up tight. I've worked on boats that I epoxied 30 years ago and the wood looks like the day it was entombed in goo.

I would strongly recommend not attempting to re-engineer the stringers system. Again, if you elect to go all laminate, you'll use a lot more goo and fabric. You will not save any weight, okay maybe an ounce or two if you're lucky, but you'd have to be careful about wetout and saturation.

Foam is fine, as long as the laminate stays attached to it or the laminate is thick enough so it doesn't matter if it stays attached.

Those are the usual options. Wood uses the least amount of goo and fabric and is probably the lightest. Laminate only will use the most goo and fabric and probably will be the heaviest, though it will not have wood in it. Foam cored will use some what less goo and fabric, but relies on staying attached to the core to be an effective load bearing member. It's best if these structures are slightly over built (read more goo and fabric) unless you can effectively control 'glass/resin ratios, at which point is will be the lightest of the bunch.

So, if working with goo and fabric is your thing, skip the wood and use a cored or solid laminate for the stringers. If working with goo isn't high up on you good times meter, then epoxy sealed wood and tabbing is the way to go, which will also be cheaper and faster then the other two.

[oceansswk]

I am going to bed the stinger in epoxy paste as recommended. I have a question regarding the transom. I was going to put a layer of 1708 on the transom shell the 1 -3/4", then 1708 then 1-3/4" then 2 layers of 1708 bonded to hull. Should I coat the plywood first with 2 coats of epoxy let dry then apply the layers as described above. I am using MAS Epoxy. thanks for all the information. Epoxy and fiberglass can become confusing.

[PAR]

Again, you don't need nor want the mat in you laminate. It adds tremendously to the weight and resin requirements. Biax or regular cloth will be all you need. I'm not sure what you're referring to with the double 1 -3/4" something or others, but I hope this isn't the thickness of the transom. Your transom is likely around 1 1/2" thick.

Epoxy is a type of resin. Fiberglass (what I call 'glass) is material, which can be a number of different products, such as 1708 combo mat or biax, etc.

You don't need fabric (1708 or other wise) between layers of plywood in the transom, just thickened resin. Good contact between the two layers of plywood is what you need and drywall screws can insure this occurs.

[oceansswk]

Par
The transom is 1 1/2" thick. The 1 is one piece of 3/4" . Can I put the 2 sheets of 3/4" together on the bench? or laminate wet at one time?
Thanks

[PAR]

Either will work. On the bench offers more control.

[SCORPIO26]

I know this is an old thread but I have been reading extensively here and have a question. First, I'm a DIY boat owner so forgive me if some of my questions are basic.
In reading this thread, I see references to the importance or lack of importance of core materials used in stringer construction. My question, if you have glassed over wood core stringers that are wet/rotting but have not changed shape, could you simply grind them and lay up more glass with epoxy resin to reinforce them and forget about the wood inside? Toatally removing old stingers and core and then rebuilding from a bare hull seems like reinventing the wheel so to speak. If the shape is there and you can build the thickness of glass with repeating layers of biaxial fabric and epoxy would that yield the strength need for proper support? I know I asked that question twice, just trying to clarify what I mean.

If this idea is not feasible, what would be better for engine bed stringes, dimensional lumber such as white/red oak or a laminate of marine plywood? BTW the stringers I'm talking about are about 2" thick and about 10" tall. I have no idea what was used originally.
I'm not currently doing a project, but I may have to in a year or two, so I'm trying to do my homework ahead of time. If I do have to rip and tear, I certainly don't want to have to do it twice.
Thanks

[tdvjensen]

Leaving a soaking wet/rotten core in the boat has 2 major problems that I can see right away.

One, the added rotten/soaked core is going to add lots of weight compared to a dry wooden core, nevermind the extra fiberglass and resin to make the grid strong enough to support the loads in the boat. As much work as it is, grinding out the core and replacing it is not that much more work than preping and adding more fiberglass to your grid. I also prefer to make sure I have done things right the first time. You wouldnt want to attempt a fix, and end up just replacing it all over again.

Two, stringers/bulkheads offer more than just hull support and a level surface to mount the sole. Depending on where and what accesories you have, the wooden core could be relied on for holding the fasteners of whatever is mounted to it. For example an engine mount. Rotten cores do not hold fasteners, and from my expirience, straight laminate doesnt hold very much better.

As for the lumber to replace it with, that all depends on what you have available and your budget. I like plywood for its uniformity, and you can make just about any thickness required by laminating pieces together. There are several types of marine plywood, some more exspensive than others. I was told that douglass fir is not the best choice for the sole because it requires a minimum of 6 oz cloth to properly seal, where as the okume mahogany (which is more exspensive) is lighter and requires only several coats of epoxy to be watertight. I used fir for my stringers/bulkheads because they are completely laminated in 4 layers of 17 oz biaxial and generously coated in epoxy. The sole I used okume mahogany because it wasnt feasible to fiberglass the underside of the sole.

[PAR]

What wood does as a stringer or core element is withstand compression as the stringer (laminate and all) takes on a load. If the wood core is mush, then the laminate attached to it will likely buckle or crush. You can increase laminate thickness to compensate for a mushy core, but there's a fairly large weight penalty, not to mention lots of working with goo and itchy materials. If the wood has a fairly high moisture content (probable) then cold weather can freeze it, possibly busting open the laminate.

So, as you can see it's usually best to remove and replace a spent core. Wood is used for several reasons: weight, strength, fastener holding, durability, etc. You can eliminate wood if you like, but you'll pay dearly for these products and they will not have all the benefits of using a wood core. For example you could use foam, but this doesn't hold a fastener worth a damn. You could use a honeycomb product and have the same problem, with both of these materials costing 10 times as much as plain old wood. Why do you think we still use it?

Douglas fir can make a fine sole, but as mentioned it does need a fairly heavy 'glass sheathing, but you'd want one anyway to prevent damage, improve waterproofing and for abrasion resistance. Okoume isn't a mahogany, though is a very nice, light weight hardwood. I much prefer it in many application, because of color and weight. It's not especially rot resistant so it need consider protection and it's not as strong as other species. Meranti is another common plywood species (often called Philippine mahogany, but it's not a mahogany either). It's a dark hardwood, that is fairly coarse, but finishes well, is about 20% heavier the Okoume and also stronger then Okoume. Sapele is a very nice hardwood plywood and is a real mahogany (African). It's strong durable and pretty, but also getting fairly rare and costly as a result.

For most repairs, Douglas fir will do, mostly because it's going to be coated and covered in goo and fabrics anyway, so who cares what's under all the plastic and paint.