Stringer Replacement Materials

Discussion in 'Fiberglass and Composite Boat Building' started by Islander11, May 9, 2011.

  1. Islander11
    Joined: May 2011
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    Location: Lake Champlain, VT

    Islander11 Junior Member

    I ripped out the floor and gas tank of my '88 21' Seebold Eagle sport boat. Tank was foamed in with a plywood access panel and carpet over it. Stringers are both rotted and the bulkhead behind the tank also. Damage seems to be limited to areas where the foam contacted the stringers, and near the gas tank access port, which is near the middle of the back seat.

    The floor is solid except for this center section. I cut it back to solid wood just in front of the engine. Everything around the engine compartment seems solid.

    So my plan is to replace both stringers and the bulkhead, and replace plywood floor with needed blocking underneath. I plan to use rubber "coin" floor instead of carpet.

    I started removing the bad stringers, so far so good. They are badly rotted, so easy to remove. I'd like to leave the outboard side of the stringers fiberglass as is, and epoxy the new stringer in from the inboard side, then re-glass them around and aft of the tank compartment.

    A few questions:

    - Anyone know what type of cloth I'm dealing with (see pictures...last one is a section I cut from the side of the stringer)? It seems it have heavy strands that run front-to-back, and a thread side-to-side. The whole boat uses this, so I'd like to get the same unless there is a better choice.

    - Should I be using epoxy? I'm not sure what the original hull is. Can you tell from the pictures? It's a green layup. 3rd pic shows the hull under the section of stringer I removed. There is no chopped strand on the boat, and the hull and deck are glassed together. Assuming its an epoxy job, I plan to use West System, as I have some experience with it.

    - What's the best stringer wood to use? I've seen a few threads saying Douglas Fir. Is there a grade or flavor I should look for? I assume something better than lumber grade? Other woods to consider? Have a good hardwood dealer nearby.

    Many more questions to come...just getting started with ordering materials, etc. I think I have most tools I'll need...

    Thanks!
     

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  2. Islander11
    Joined: May 2011
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    Islander11 Junior Member

    Cambara for Stringers?

    I just spoke with our local hardwood store...they suggested Cambara as a possible stringer material. They said it is popular for decking, and is naturally rot resistant.

    They also have Douglas Fir in a woodworking grade (not lumber).

    Any other suggested woods I should consider?

    I planned to stick with wood because the rest of the boat is still original, but I could be talked into a foam or other man-made material. I need a finished size of about 1 1/2" wide, 3-4" deep (tapered), and a little under 8' long.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Cambara is fairly heavy and dense, with good rot resistance. The problem wasn't the wood selection, but the techniques from the manufacture. This is a very common issue and typically it boils down to cheap basterds and/or stupidity. The factory usually skimps on resin saturation of the wooden elements, not enough fabric and insufficient encapsulation. Couple this with moisture and/or fuel absorbing foam in direct contact with poorly coated wooden stuff, well . . .

    You don't have to suffer these indignities. You can use enough resin and you've wisely selected a much better one in epoxy, which is far superior to the polyester the factory used. You also can insure complete encapsulation and fabric coverage. All of this loving attention will insure your repair will long out live the factory's attempt.

    It's difficult to tell if the stringers were structural cores or just forms. The thickness of the fabric around them is the way to tell. If there's a lot of fabric, then the wooden core was just a form, a spacer to separate the fabric skins. If the fabric is relatively thin, then the wooden core is a structural element and needs to be replaced with the same.

    I wouldn't use fancy woods on this, I would just bulk up the fabric and make sure what ever you use is well encapsulated. 3 coats of resin, then the fabric goes on. Douglas fir is a common material, both solid wood and plywood. It's cheap and readily available. If you use foam, the fabric thickness has to compensate for the lose of the wooden element integrity. This just means more goo and fabric, which cost more and is messier then wood.

    The fabric you're looking at is a heavy 'glass roving. Use biax or a combo product like 1708 instead. For your stringers, use two pieces of 3/4" thick wood, epoxied together, rather then one 2x4. Epoxy has a difficult time with wood thicker then 1", so glue up thinner pieces.

    The key is make sure the fabric you put on is as thick, if not thicker then what you saw when you first started tearing things out. This will insure strength and bond. If you use solid wood, instead of plywood, make sure it's knot free and straight grained.
     
  4. LMB
    Joined: Nov 2008
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    LMB Junior Member

    In my opinion the DougFir would be the best all around choice. You could get by with construction grade Spruce or Pine from a lumber retailer, but if the Fir is not too much trouble to acquire I'd go that route. Two layers of 3/4" plywood will work as well. The most important part will be the workmanship. Good prep, fit, and glass work is very important. More so than the materials or resin choice. If it's a production built boat it almost certainly polyester. You could properly repair the boat using polyester, vinylester or epoxy. If you are familiar with West System and have the budget, I'd go that route. An experience glass professional could do a good repair with polyester. Fabric doesn't have to match the original, just needs to be adequete layers, including some heavy weave like 18oz double bias or woven roven. Leave the chop strand mat out if working with epoxy. It has a binder that holds it together that doesn't break down properly with epoxy. Plenty of info online, just keep searching and reading.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    LMB, it's helpful to read the thread before posting.

    Mat isn't used with epoxy because it's not necessary as a bulking agent as it is with polyester or vinylester. The "binder" you mention remains in suspension if used with epoxy and does no harm, contrary to the rumors.

    Lastly yes, fabric thickness does need to match or likely exceed the original laminate. This is likely why it failed in the first place. This plus not enough resin saturation on the wood.
     
  6. Islander11
    Joined: May 2011
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    Islander11 Junior Member

    Thanks for the helpful information. I appreciate your time to help me on this project.

    It's a fun sport boat with a stepped transom and pad keel, the engine (Merc Alpha 260hp) runs like new, everything else is structurally sound, the interior is still in pretty good shape, and it sits on a custom bunker trailer that is still in great shape, so this is a project worth some time and money to get it right.

    I was disappointed with what I found upon ripping out the plywood. The stringers were only attached with one layer of flimsy fabric. Seems like the resin separated from much of the wood. The corners between the stringers and bulkheads were not glassed, and had separated in a couple cases. And of course the stringers must have been uncoated because they are totally rotted.

    The aluminum gas tank was foamed in and was pressing directly on the belly of the hull...no separation. That combined with a weakened stringer system resulted in bending/twisting stress on the tank that caused a crack in the aluminum right at the heat-effected-zone next to where an internal baffle was welded. Now that I understand the cause, I will probably just get the tank welded to put a patch over the bad spot. I'll reinstall it properly with neoprene tape to create an air gap under the tank and stress isolation.

    Seebold is an experienced builder of outboard race boats (they dominate the Champ Boat class), so they knew better. Looks like they fell victim to the same cost pressures as everyone else. But there's still plenty of sloppiness in the work...it doesn't cost much to put some glass on corners of structural members!

    In a way, its nice to see because I am certain I will be able to do a much nicer job on the repair!

    Going back to my original questions, here's an updated plan:

    - Use a heavy cloth like 17oz fabric, or 1708. No need to duplicate original cloth.

    - Use epoxy for the repair. I like West System, and won't need too much, so the cost is OK with me.

    - For stringer and bulkhead wood, use straight-grained, clear douglas fir. Plan to laminate 2 1x4's by first coating with 3 coats of epoxy, laminating them together on the last coat. I'll plan to make then the same dimensions, which had them separated from the hull by about 3/8".

    I'll work on getting materials on hand, and also continue the ripout.

    More pictures, questions, and followups to come...Thanks again!
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Personally, I don't find a need for combo fabrics, like 1708. From a strength view point, the more layers of a lighter fabric, the better it will be. I prefer to use straight 12 ounce biax for tabbing (attaching things to a hull shell). I use it in tape form as well as rolls. 45/45 degree biax is the material of choice, as it places all the fibers across the joint or fillet, which makes sure they all are participating in load transmission duties. Naturally, I'll layer it up to suit laminate thicknesses. Make sure you stagger the edges and seams too.

    Yep, you've found what most find, lots of short cuts and stupidity in areas that are "out of sight - out of mind". The builders knew better, but . . .

    Log onto West System and System Three web sites and download their user's guides for an overview of techniques. Then look into Bateau.com for a price on 2:1 "Marinepoxy" that is far cheaper then West, for the same strength.
     
  8. LMB
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    LMB Junior Member

    Par, I did read the thread, as I always do. If you'll notice the post times you'll realize we were posting at the same time. Good to know we were giving similar advice.:)

    About the mat/epoxy issue - I agree. I researched this exhaustively a number of years ago, thinking I had made a mistake using mat with epoxy and finally decided the work was not compromised just made it unnecessarily hard on myself. Mat is difficult to wet out with epoxy but I still ocassionally use it on small repairs and some gap filling/bonding type situations.

    Also agree that the 12oz fabric will be easier to wet out and a good choice in this application.
     
  9. Islander11
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    Islander11 Junior Member

    Thanks for the advice...I usually buy supplies from Defender Marine, and they have a 12.5oz cloth that sounds like a good choice.

    Anyone care to guess how much cloth and epoxy resin I'll need for this job?

    Looking at the pictures above, I'll be glassing both stringers (mostly on one side), replacing the bulkhead and short stringer behind the tank, reinforcing the bulkhead in front of the tank, and I'd like to add a fresh layer of cloth on the sole after its replaced. Also need to allow for coating all the new wood. The size of this section is about 8' x 3' total, and the top of the sole will be about 8' x 5'.
     
  10. missinginaction
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    missinginaction Senior Member

    Just a comment on the 1708 biaxle mat. I used this on my restoration (to build stringers, just as you are going to do) on the recommendation of my fiberglass supplier.

    You can do this, but as PAR accurately points out it's probably not the best approach. My stringers (5 layers of 1708 over 2" closed cell foam former) are plenty strong, but they are also very expensive. That mat soaks up resin like a sponge and at what? $80 a gallon or so you will use a lot.

    After actually doing this I'd follow PAR's advise if I were you and use fiberglass cloth, avoiding the mat. You'll need additional layers so the layup might take you an extra day but you'll get a stronger assembly and use less resin, thus saving some money in the bargain.

    Have plenty of disposable gloves, old clothes, old shoes, and work methodically and slowly. I'd advise spending a few bucks on a tyvek suit. This is gooey, messy work, especially if you've never done it before. You might want to use a medium or slow hardener so that you'll have time to set the cloth before the resin kicks off. ANother tip, don't try to layer more than two or three layers of cloth at one time. The resin is exothermic and will generate heat as it cures. Too many layers will cause heat to build, not good for the strength of the structure.

    Regards,

    MIA
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Thanks Bart. Mat makes you efforts more expensive and difficult. You don't need it with an epoxy layup, which saves time and money. Mat uses 3 times as much resin to wet out the cloth, so you've been warned.

    As to how much fabric you need, well it's fairly simple math to add up the square footage and multiple by the number of layers.

    Sorry LMB, I didn't notice the posting times. I should be more observant.

    Back to tabbing the stringers and stuff. A good guess would be three layers of 12 ounce, 45/45 biax, with staggered over laps. This tabbing should extend onto the hull shell at least 6". Always grind a fresh spot when tabbing to the hull shell. In other words, in the areas where the tabbing will land on the hull shell, take a grinder with a 40 grit wheel and hit all around the area, so fresh, clean hull shell is ready to receive the goo.
     
  12. Islander11
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    Islander11 Junior Member

    Thanks everyone for your input. I especially appreciate the warning about how much epoxy gets soaked up in the mat. That sounds like a tough lesson to learn the hard way!

    I'll stick with a 12oz cloth for the stringer repair.

    How about re-glassing the sole? Should I use the same 12oz? It currently has the heavy roving (shown in the pictures above) on the section of the sole I am keeping. The center panel will be new.

    My initial thought is to put one layer of 12oz fabric on the new wood to bring it to the same height as the original, then sand the old roving enough to clean it up, and use a fresh layer of 12oz over the whole sole (probably 2 pieces unless I can find a 5' wide cloth). I'd like a nice smooth uniform surface that I will glue a rubber coin flooring material to (wet carpet is a lot of the cause of this rot, so I'll switch to a waterproof flooring and a snap in carpet).

    I have a great Milwaukee 5" orbital sander that will be perfect to freshen up the glass before laying up new cloth. It's basically an angle grinder with an orbital head. It's one mean sander, but not so aggressive as a plain grinder (doesn't catch and go wild on me). But if needed, I also have an angle grinder, which I think I'll need to grind down the old stringers.

    I'll be sure to get a Tyvek suit, and I'm prepared with an arsenal of old cloths. I'll take this opportunity to upgrade my respirator too.

    I've done some glass work before, so I think I'm beyond the "sloppy newbie" stage, but its been a couple decades, so I'm sure there will be a relearning curve. I plan to mask off everything around the repair area to avoid a dust mess, or mishap with spilled resin.
     
  13. PAR
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    PAR Yacht Designer/Builder

    The sole doesn't need biax, so use a conventional cloth instead of the knitted stuff. Again, thinner layers makes for better laminations, so a couple layers of 8 - 10 ounce would be preferred on the sole and more importantly will make fairing and smoothing much easier too.. Use some biax around the edges to "tab" the replacement sole back to the existing hull shell. You only need the respirator when sanding. Epoxy doesn't have the VOC's that the poly's have. Don't get me wrong, you don't want to hover over the open can and work on an early Friday night buzz, but generally the high molecular pressure causes the vapors to dissipate very quickly, after you open a container of epoxy (read a few seconds)
     
  14. Islander11
    Joined: May 2011
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    Islander11 Junior Member

    Update

    No progress to report on the boat, but I do have some materials coming in. Epoxy is on hand, and cloth should be any day. Also got some PPE as was suggested above...

    Hope to get some time to finish ripping out the stringer I started on this week. I plan to do one side at a time for the replacement stringers.

    More to come...
     

  15. missinginaction
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    missinginaction Senior Member

    Some spring up here isn't it Islander? I was hoping to get to work on my boat in March. Here it is almost June and I've done almost nothing. Mother Nature's just not co-operating this year.

    Keep us posted and be patient......

    MIA
     
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