Advice on strengthening hull

Discussion in 'Fiberglass and Composite Boat Building' started by IHTFP, Jul 16, 2008.

  1. IHTFP
    Joined: Sep 2004
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    IHTFP Junior Member

    Hi,

    I have a 22 Ft. fiberglass speedboat that had rotten stringers and transom and is currently undergoing restoration. The boat is completely stripped down to bare glass, all coreing and stringers removed. My question is: Is it a good idea from a strength perspective to add a fiberglass fillet to the interior of the chine and a good idea to fill the lifting strakes with glass before I install the stringers/bulkheads? I have attached an illustration for clarity. Thanks
     

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  2. kengrome
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    kengrome Senior Member

    I don't know what you mean by "coring removed", but ...

    If the boat didn't have the chines and strakes "filled in" before then why would you bother doing it now? Does the boat actually need more longitudinal strength in these areas? If not, just replace the stringers and transom -- hopefully after treating them with copper naphthenate to prevent future rot -- and that's all you should need to do.
     
  3. PAR
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    PAR Yacht Designer/Builder

    No, you don't "need" to fill these areas. The strength of the hull comes from the dramatic change in direction these elements make, much like the corner of a cardboard box.

    If you filled the strakes and filleted the chines, you'd add some stiffness, but not much really and unwanted additional weight would be the outcome. You actually want some flex in the hull, not a stone rigid thing.

    If you really think things would be better with some additional reinforcement, then fill the strakes with high density foam and tab this into the hull shell. This will apply minimal weight, additional stiffness and permit the hull to flex if it needs to. The laminate along the chine us usually pretty thick, but a fillet here wouldn't hurt things much. Again, a better choice would be to cut a wedge of foam and tab this into the hull, bridging the chine.
     
  4. IHTFP
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    IHTFP Junior Member

    Thanks for the reply's.

    Kenneth: By coring removed, I am referring to the wood coring in the transom. To respond to your question: If they weren't filled before, why fill them now? Well, the builder used some shabby construction techniques to manufacture this boat (which could have led to the rot problem). When I put the boat back together, I want to do it right, not cheaply or efficiently.
     
  5. kengrome
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    kengrome Senior Member

    You didn't say how old the boat is but I was guessing 2-3-4 decades. In those days lots of builders didn't have a clue that their stringers and plywood would rot when they encapsulated them in polyester and glass, or they didn't care so they went ahead and did it anyways without pre-treating the wood against rot. But now that you know this you can pre-treat the wood first (copper naphthenate is probably best) and then even when moisture gets to the wood it won't be able to rot.

    I thought you had completely removed the transom and were going to install a new one, but now that you say you only removed the plywood 'core' this brings up another important issue I didn't even think to mention earlier since you sound like you know what you're doing already -- so please forgive me if you already know this but ...

    If you use only epoxy on the new wood (after the rot pre-treatment and sufficient drying time) you'll have a much better result than if you take the cheap way out and use polyester or vinylester again. Those 'ester resins are not adhesives nor do they resist water as effectively as epoxy, so I'm hoping you already had it in your head to use epoxy exclusively this time around.

    Paul had a good suggestion to install some closed cell foam in the chines and cover it with epoxy and glass. You can go even further and use lumber instead of foam if you really want to add that much more strength. I don't know how poorly the boat was constructed so it's up to you to decide how much added strength the chines actually need. The foam itself doesn't add enough strength to measure of course, it's the glass on top of the foam and bonded to the hull that creates the structural stringer effect and stiffens the hull. But given the fact that the hull apparently has not broken apart already even though you say the builder didn't to the best job of it, adding a wood stringer at the chines is probably more hassle than it's worth.

    Usually when I rebuild a boat like this I make my main stringers taller or wider than the originals anyways, and if you're doing this your hull will become much stiffer than it was previously even without the foam/glass chine work. But you can easily use the same size stringers as before and still make them substantially stronger and stiffer by sheathing them with several layers of epoxy and +45/-45 biaxial cloth. Tolman skiffs are built this way from the beginning and that biax adds a ton of strength and stiffness.

    Sounds like you're on your way to doing a very good job on this rebuild. Good luck and enjoy yourself! :)
     
  6. IHTFP
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    IHTFP Junior Member

    Kenneth,

    Thanks for the response. The boat is 12 years old, but the wood in engine stringers and transom were like mulch, I literally sucked them out with a shop-vac. I definitely plan on using epoxy for all lamination. I also plan on using Greenwood Products XL plywood. What is your opinion of this brand of marine plywood? I had planned on using COOSA, but have been told that COOSA does not perform well in transoms because it can't resist crush like wood. My other issue is how do I recore a curved transom. My plan is to use 3/8 Greenwood and then vacuum bag the wood down to conform to the curvature of the transom. I plan to do this in several layers, probably 4 layers of 3/8 plywood with glass in between.

    Thanks
     
  7. kengrome
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    kengrome Senior Member

    I'm not familiar with that brand of plywood so maybe someone else can comment on it.

    I would have to see pictures of the transom before I could make any suggestions. When I've done this kind of task before I've completely removed the transom and started over from scratch.
     
  8. marshmat
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    marshmat Senior Member

    Ken and PAR have had some good points here, IHTFP. (what does that stand for, anyway?) The boys know their stuff....

    Vac-bagging the new transom in, a layer at a time, is not an approach I would have thought of, but it sounds remarkably logical. Photos would be a great help in assessing whether it's right for your situation.

    I like Ken's suggestion of +-45 biaxial for the stringers, if you're comfortable working with this stuff. (If you can't get it, a couple of layers of 0/90 on opposite diagonals has the same structural effect.)

    It sounds like you're wanting to start from scratch with the structural grid, and are not confident that the original builder got it right. (Odds are he didn't.) If you're not averse to a bit of light reading and some grade 10 level math, the $32 for Dave Gerr's Elements of Boat Strength would be well worth it in your case as it includes pretty much all the scantling guidelines and formulae you need to redesign the structure to exceed its original requirements.
     
  9. IHTFP
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    IHTFP Junior Member

    Kenneth,

    From what I have read, Greenwood Products XL Panels are supposed to be the Holy Grail of rot resistant marine plywood. I have very little knowledge in this arena so I am only repeating what I have read and heard others say.

    http://www.greenwoodproducts.com/xlpanels.htm

    XL® panels have been used in North America since 1985 and have proven themselves in virtually millions of boats, both large and small. The product has since expanded into applications such as bus floors, manure spreaders and permanent wood foundations- and not just in North America.


    XL® Panels essentially deprive wood destroying organisms of one of the four needed components for them to survive. XL® treated wood robs the organisms of the plywood panels as a food source.

    The CCA treating process used in manufacturing XL® Panels results in the chemicals becoming chemically altered within the wood, becoming insoluble, or fixed.

    XL® Panels are easy to work with, with the material delivered dry without residual wetness associated with other preservative treatment methods. Contact Greenwood Products for specific information or technical support.

    Preservative: Chromated Copper Arsenate - Oxide formulation or equal.

    Sizes: Standard thickness 1/4" through 1 1/2" in full 4' x 8' sheets. Also available in most widths and lengths up to and including 8' wide by 50' long.

    Glue Bond: Marine-type high solid content exterior glue.

    Surface: Tight-knotted high grade C face and back.

    Moisture Content: Kiln dried after treating in a controlled environment to 13 percent or less to assure uniform stability and flatness.

    Estimated Shipping Weight: 4'x 8' sheets.
    1/4" 24 lbs.. per sheet
    3/8" 31 lbs.. per sheet
    1/2" 43 lbs.. per sheet
    5/8" 54 lbs.. per sheet
    3/4" 64 lbs.. per sheet
    1" 82 lbs.. per sheet
    1-1/2" 131 lbs.. per sheet

    Identification: Each panel will be stamped with the XL® seal of quality and year of manufacture.

    Warranty
    The XL® preservative treating process allows designers and other industrial specifiers to take advantage of the benefits of structural plywood without the fear of decay.

    Decay in wood (often referred to as "dry rot") occurs when fungi encounter the following conditions. Remove any one and decay cannot take place.

    The presence of moisture or high humidity
    Typically wood that is 20% moisture content or less is not susceptible to decay. Components in boats often exceed 20% due to rain or direct contact with water, such as in bilge areas and carpeted decking. In many regions of the world, high atmospheric humidity has a tendency to accelerate decay in wood, if not properly addressed. Where water from washing a bus floor is trapped between the rubber or vinyl flooring and the plywood, rot is a factor, and a given in areas of high humidity.

    Food for the fungi (wood).

    Oxygen (wood, in deep water for instance, will not rot due to the lack of oxygen).

    A supportive temperature. Fungi, for instance, become dormant or non-existent in sub-zero temperatures.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Treated wood is lovely, but not particularly useful in this application. I know this sounds nuts, but hear me out. Unless the wood is embalmed in plastic, completely, then it will swell and contract with moisture content. This eventually sheers the contact patch it has with neighboring structural elements and tabbing to the hull shell, rendering it useless structurally. With the tabbing or light sheathing sheered away, moisture collects between the plastic and wood, then rot has it's way.

    The problem most of these production boats have is the lack of weep holes in the substructure, below the sole and a drain point aft so this moisture can get out of the boat. I can't tell you how many times I've drained hundreds of gallons of trapped water, living under a sole, with no way out, but a few sole breaches above to let more in.

    The bottom line, encapsulate the wooden elements. Every screw hole, notch, end grain, etc., then tab the sucker in good. Don't use any form of Douglas fur for these elements. This stuff checks and will self breach the encapsulation (even if lightly sheathed), making all your work moot in a few years.

    [​IMG]

    When doing a transom and you desire a little more strength, orient the plywood as shown (3 layers of 1/2" indicated). This places the cross grain in a better arrangement, which transmits loading better. Vacuum bagging sounds interesting, though I've never bothered. I use a slightly thickened epoxy mix and temporary screws to clamp them together. On a curved transom, I'll use a few jigs (2x6's with the radius cut into them) to permit the shaping.

    CCA is no longer available in the USA, being replaced with higher copper content chemicals, but the jury is still out on the ability of epoxy to remain stuck to these new treatments. I suspect it will, but with quality 1088 plywood and good encapsulation techniques, not especially necessary.
     
  11. IHTFP
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    IHTFP Junior Member

    PAR: Thanks for the response, very informative. Can you elaborate what you mean by "I'll use a few jigs (2x6's with the radius cut into them) to permit the shaping." I'm having problems visualizing what you are describing.

    Thanks
     
  12. bntii
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    bntii Senior Member

  13. IHTFP
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    IHTFP Junior Member

    BNTII,

    I have looked at COOSA and think it is a great product. But, I have been warned by a few people who work primarily on high performance powerboats that it does not work well in transoms due to the clamping pressure needed by the outdrive. They said it works everywhere else and works for transoms of inboard biats but it just can't handle the crushing force of an outdrive. I am interested in your opinion on this and if you have had different results.

    Thanks
     
  14. bntii
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    bntii Senior Member

    Hello IHTFP,

    I regularly use the Coosa products as a direct plywood replacement in high load/compression applications and have had no problems. I will however defer to the findings of the performance builders as I believe that their hands on experience should be heeded.

    Here is a decent table of the relative strengths of plywood products:
    http://www.worldpanel.com/plywoodcomparison.htm

    Good luck finding the same for Coosa

    Boy- someone is testy :)
     

  15. IHTFP
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    IHTFP Junior Member

    BNTII,

    I had planned on using COOSA all along until I was told about the compression problems. I have a few samples of Bluewater 26. I will try to contact COOSA and ask them for compression data on their product. Where in Maryland are you located, as I am in Maryland also? Also, I test fitted a sample strip of 6mm Okoume in my transom, and it curved around perfectly, so I believe the technique of laminating several layers of ply or coosa around the curve will work.

    Thanks
     
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