Yet another stringer repair post...

Hello all. I've been reading the forums for a while now but this is my first post and I'm sorry, but it's probably going to be a long one.

I recently acquired an old 19' boat with rotten stringers but is otherwise in good condition. I have wood and fiberglass experience but have never replaced boat stringers before although I've read everything I could find online and feel confident.

I'm planning to use woven roving matterial and epoxy for the stringers because that's what appears to be there now. The existing stringers are made from wood (2"x8" it looks like) and I'm wondering if I can use a foam core to build the stringers instead of wood to reduce the weight and perhaps double-up on the w/r to achieve strength. Can I do the same with the floor?? There are 3 stringers and I intend to install bulkheads to give more support to the floor. This is actually my first boat so I want it to last me a while (2 or more seasons) and don't want to have to do this work again.

The floor is partially torn out and there appears to be a bulkhead just in front of the bilge area. If this is the case and the stringers are solid to the rear of the bulkhead, am I okay to glass the new stringers to the bulkhead or am I better to remove everyhing and replace it all??

While at the rear of the boat, I have a gas tank question. The boat has an outboard and a removeable fuel tank. I want to increase fuel capacity and keep the weight balanced so I will be installing a battery on each side and was considering a second fuel tank to keep the weight even and connecting them. The reason for this is to avoid a single large tank that would cover up my access to the bilge pump. Is this reasonable or should I just install a single large tank and plan to remove (or move) it when I need to access the bilge??

For the flooring, I have heard mixed information about whether or not to leave some of the old flooring. Some say to leave 2" and I've seen where others have removed it completely. I would like to remove it completely but if this is not the best way, I can leave some of the old behind. Keeping in mind that I want to use glass over foam to keep weight down, is it possible to use this type of construction epoxied directly to the hull, stringers, and bulkheads and have enough strength in the floor??

The boat is a closed bow and I've always preferred bow riders or even center consoles. Does the top deck provide support to the hull or is it only there as an attachment point for lights and the railing?? I obviously don't want to hack it apart if it's structural. If I don't modify the deck over the bow, I would like to utilize the space under the deck for storage. If anyone has any dieas for me on how to build storage into this area, please pass them on so I can build them into my plan.

My last question is regarding seats. There are currently the back-to-back convertible seats that fold flat installed on both sides. Since I intend to use the boat mostly for fishing and want to maximize walking space I will be building battery boxes / seats into the rear of the boat (with a removeable cover for access to the bilge and fuel tank(s)) plus a helm and passenger seat and not utilizing the existing back-to-back seats leaving the floor relatively open. In case I want to take more people along, I'd like a way to make the back-to-back seats somehow removeable yet still firmly connected to the floor when installed. I'd like some type of latch that won't get in the way too much when the seats are not installed. If such a thing exists, please provide a link or manufacturer so that I can do further research.

I will be starting this project in the next week or so beginning by cleaning out the leaves and debris followed by demolition of the floor. Any ideas, tips, suggestions, answers, or criticism are greatly appreciated. If I'm planning to do something that sounds stupid, please don't hesitate to tell me because as I said, I don't want to have to do this work agian.

Thank you in advance.

 

You can use foam core for the stringers and deck. They are plenty strong and light, but you just need to be sure of the lamination schedule - it is different with foam core vs. wood. Since these are all flat pieces, you can vacuum bag them on a table for more strength.

Joel
Boatbuildercentral.com

 

Joel,

Thank you for your reply. Is there a specific foam that should be used or can any closed cell foam that will not deteriorate over time be sufficient?? The assumption I made is that the foam is only to give a structure while the fiberglass cures and since it would not be load bearing would be relatively insignificant other than that it should not hold water.

 

The type of foam does matter greatly. Specific foam cores are designed for specific applications. If you do not use the correct type, you could have problems. I would use different foam for the stringers than I would for the deck. Peel strength, compression strength, shear strength, tensile, and density are a few of the things that need to be right. Also, the weight and type of directional glass to be used with the foam core is important.

Joel

 

Foam

I would get some 3/4" divinycell foam. In the stringer case it is being used strictly as a former for the glass. It is very easy to work with and does not have to be straight like plywood. The stringers will be more effective if they taper inboard at the bow rather than being straight in plan view. Use some putty to fillet the outboard intersection of the stringer and hull to avoid a sharp bend in your fiberglass overlaps. As long as you are using the good stuff for the stringers, skip the woven roving and use a +-45 knitted fabric like 1708 over the foam. Make sure to grind the hull where the glass wants to stick and wipe with acetone. Stagger the ends of the overlaps........you've heard all this before?

The rest of your questions are hard to picture without seeing the boat but I can tell you this: the stringers are the backbone of the boat and should be continious. If the bulkhead you mention is in pretty good condition, I would trim it away so it does not interfere with the stringer installation. After installing the stringers, use some of your foam to fill the gap and glass the stringer bulkhead joint.

Why do you need epoxy? Although it won't hurt to use it, I'm not sure the cost is justified. Any opinions?

 

Use foam pads cut into trapazoid strips made of the same material for the stringer to rest on, this is the prefered meathod in construction.

There are many reasons to use epoxy over polyester. For one, the epoxy will bond better to the fiberglass hull. If you going to go to the expense of foam core and directional glass why not get the most out of it with better resin. Its not much more on a project this size. Its like putting old tires on a race car.

If you use polyester resin, I would just use wood as the core.

 

Now I'm starting to reconsider the foam. Obviously weight is a major concern but the cost of marine specific foam may be a bit more than I'm willing to spend on this boat right now. At almost $100/4'x8'x.5" sheet, it's going to be quite costly to use so, for the stringers I think I'm going to double-up on 3/4" plywood. That should accomplish saving weight (over the heavy lumber in there now) and keep the cost reasonable. There are insulating foams available (weather resistant but not marine specific) that are comparable in price to plywood that I was considering if it would be safe to use. For the floor, since I am adding bulkheads to help support the floor, I am thinking that 1/2" plywood should be sufficient if it's glassed well.

Derek, you say that in the stringers that the foam is just used as a former for the glass and if this is the case then I should be able to use a less costly foam for the stringers and perhaps the divinycell for the floor. This would acomplish both weight savings and keeping the costs reasonable. Again, I only want to do this if it's safe. As far as the fabric and resin, I really don't want to mess with polyester resin because I am confident that epoxy will make a secure bond with the hull and wood or foam that will be used. For load bearing structures like the floor and stringers, I really don't want to take a chance with polyester since I have never used it before. I chose woven roving for the fabric because that's what appears to be holding the current stringers in place. Is there an advantage to using 1708 over woven roving?? Perhaps easier to use, less costly, stronger?? I'm definitely open to suggestion on this one.

I took a bunch of pictures of the boat last weekend and haven't even cleaned it out yet as you can clearly see from the pics. I'm planning to start cleaning and removing the floor after this weekend. The pictures can be found at http://www.tombarcia.net//Boat/080604.

Thanks again for your help!!

 

1/2" is overkill if you glassing both sides. I would use 3/8 or even 1/4 depending on the support. If you get 3/8, you can double it up for the stringers. This is exactly what is done on most of our boat designs of this size. Use biaxial tape to secure the stringers:

http://www.boatbuildercentral.com/categories.php?id=1

 

Fabric

TBarCYa,
Think about what you are trying to accomplish. Wood stringers providing strength with fiberglass holding the wood in place, or fiberglass stringers providing strength with wood or foam holding the glass in place while it kicks?
In either case the forces on the stringers are the same during use. The tops are in compression when you hit a big wave, which is the largest force the boat will see. The best orientation for the fabric on the top is fore and aft. Some builders add a layer of unidirectional fabric on the tops of stringers. Hi performance builders use uni-carbon for its great strength in compression. The forces on the sides want to twist or buckle them. The best orientation of fabric to counteract this would be + - 45 degrees.
Now lets compare woven roving to knitted fabrics for this purpose.
The 1708 would be perfect for the sides and not so perfect for the tops, although not bad.
The half the fabric in woven roving oriented athwart ship would be going along for the ride and adding little strength. The half running longitudinally would be helping greatly on the top but not much on the sides. In addition, due to the nature of woven fabrics, the orientation of the fibers changes direction repeatedly in the weave, further compromising strength.
As concerns foam, the main consideration is that the resin you use does not react with it. Price out low density urethane. It is cheaper and also easy to shape, although it makes me itch.
After looking at your photos I think wood may be a good cost effective choice. I would recommend that unlike the original builder of this boat, that you fully encapsulate the stringers in glass to protect them from degradation by sea water.
By the way JR is correct in his statement about the trapezoid strips, although a putty fillet works too and may be easier near the bow where the deadrise gets steep.
I think that's a fine looking boat and as I often say to the chagrin of my wife, "Somebody could fix that".
Good luck
djs

 

I think I'm confused by your message. Should I be looking for a strong wood stringer using fiberglass to hold it in or would I be better off with a foam core stringer covered with the right glass?? Which is the better way to go?? I want to avoid wood because I don't want to have to deal with rot again. If I can get the low density urethane then I don't mind spending a bit more time and money on the right glass to do a good job and end up with a strong boat.

If I understand you, then if I stick with wood I can use the woven roving like the manufacturer did but I would be better off with bi-axial. If I go composite, then I need uni-axial for the top (running with the fibers parallel to the stringer) and bi-axial for the sides?? In either case, when using bi-axial I need to align the tape parallel with the stringer so that the fibers in the tape align at a 45* angle to the long side of the stringer. Am I in the ballpark??

As far is the prep work, my plan is to cut the stringers out and replace them one at a time. Once I get a stringer out, I'll grind the old woven roving off the hull and use the stringer as a template to make the new stringer. I plan to use a 1/4" radius bit in my router to round the tops of the matterial so there's no sharp corners for the fabric to navigate. For installation, I'll put down some epoxy and wait for that to kick before mixing up a batch of filler which I'll use to make a radius where the stringer meets the hull. I'll make sure the tape spans at least 4" from the stringer along the hull so I know it'll be strong.

Hopefully I will get out to look for foam tomorrow so I can decide whether to use foam or wood for the stringers.

 

Uni

Unidirectional on the stringer tops would be nice but not manditory. If you wrap the foam with couple of layers of 1708 you should be good to go.

 

Excellent. So 1708 to wrap the foam and should I also use the 1708 to attach it to the hull??

I was doing some investigation and found that polystyrene is often used in surfboards and was wondering if this was an acceptable foam to use for the stringers?? Epoxy is mandatory when using polystyrene but I was planning to use that anyway.

What is a good fabric to use for the floor?? I'm not sure if the floor is going to be wood or foam but if it's wood, then I'm planning .25" glassed on both sides and if foam then .75" glassed on both sides. Would I use a different fabric depending on which material is used for the core??

 

Hat section

Just to be clear.

Prep the hull.
Lay foam stingers in place on hull. Do what you have to to keep them in position.
Cover foam with the fabric and overlap onto hull.
Do another layer and stagger the edges on the hull.

The glass forms a hat section.

Don't forget the trapezoid or putty fillet.

 

10-4

-Tom

 

Keep it up...

Tbar,

I couldn't help but notice that the pics of your boat are strikingly similar to what I am looking at on mine. I have a 19' runabout that I am currently cutting the deck off of now. I have been glued to this forum reading every post possible. I would be greatly appreciative if you could post your progress on here so I have a reference to check against. I am unsure of what material to use on my stringer repair, but plan on decking with marine plywood, fiberglass, and paint (I don't want to recover w/ carpet). I will be interested to hear about what material/process you use on repainring your stringers. I was unaware of the many different grades of mats, resins, plywood’s, etc.. Thankx