21' Bowrider Re-design

Discussion in 'Boat Design' started by CaramoanReef, Jul 6, 2016.

  1. CaramoanReef
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    CaramoanReef Junior Member

    I purchased a 21' bowrider with a 150 four stroke Mercury. So far every piece of wood I have touched on this boat is completely rotten, so I am cutting everything out, with the exception of the front seats which seem too difficult to remove.

    There were 5 half inch plywood stringers going the whole length of the boat (covered with just 1 layer of CSM!), and 1 bulkhead towards the middle. There are cracks in the hull that leak and need repair, probably due to the stress caused by having broken, rotten stringers.

    I have the chance to re-design the layout of the stringers and bulkheads - the question is: should I? Should I replace what was there with the same design, or should I try to make it better? Opinions and suggestions welcome!

    I am planning to make foam stringers with adequate amount of CSM and WR. Is 1708 overkill for 1/2 inch stringers?

    I'll post some photos. Thanks!
     
  2. CaramoanReef
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    CaramoanReef Junior Member

    Bowrider Photos

    Here are some photos of the tearing out process!
     

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  3. CaramoanReef
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    CaramoanReef Junior Member

    Update - everything cleaned out

    Here is the latest: all the rotten wood has been taken out, and we are now grinding everything down to the original hull fiberglass.

    Tomorrow I will prop it up on a wood structure since the trailer it is on seems to be distorting the hull. I don't want to install new stringers when the hull is tweaked like that! This is a big job, but it is going well so far.

    I am thinking of using 1/2" foam covered in fiberglass as the new stringers, and a hexagonal honeycomb type of fiberglass sheets as the new deck.

    Any advice is welcome!
     

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  4. mickyryan
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    mickyryan Junior Member

    I would go back to it exactly the way they had it , I mean you could make them taller if you wanted but if you make them shorter to lower center of gravity id say add more stringers to compensate for strength.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The issue of having the thing go out of shape bedevils these projects, a proper cradle to support the boat would be a big job in itself, but really what is needed. Otherwise undulations and twisting may appear.
     
  6. CaramoanReef
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    CaramoanReef Junior Member

    Stringer Design

    Thanks for the advice! I am definitely keeping them the same height, and I will replace all 5 stringers. There was only 1 bulkhead, and I was thinking of adding another - if for nothing else, just to help support the deck. I was also thinking of doubling up the width of the stringers - from 1/2" plywood before to 1" foam core. Does anyone see a problem with ADDING strength to a hull? Will I make it too rigid?

    Thanks for your help.
     
  7. CaramoanReef
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    CaramoanReef Junior Member

    Thanks for the advice on building a proper cradle. Do you have any advice/links/books that can help me build one that will give the boat adequate support? I read somewhere that just putting 2 supports on the back corners and one on the front will support it properly.

    Any advice welcome!
     
  8. CaramoanReef
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    CaramoanReef Junior Member

    Composite Stringers

    I decided to purchase some polyurethane foam board to fabricate the new stringers. Since the board comes in 8'x4' pieces, we used 3 equal size pieces to construct the full 16 foot length. Lots of measuring and cutting to get them just right.

    One challenge was that the 2 stringers on either side of the center one are curved inward at the bow. We will make a wooden form to hold the right shape when we lay them up.

    Planning to put one layer of CSM around them before putting them in. Then we will lay them up with a layer of "combo-mat" and tab them onto the hull.

    Anyone want to give their vote on whether to put foam spacers under the stringers before tabbing, or glue them directly onto the hull with epoxy? I have seen both opinions on the boards. I am leaning towards just gluing them to the hull.

    You can see a photo of a piece of the old stringer. 1/2 inch plywood - rotted, delammed, and cracked... what a mess that was! No more plywood for me!
     

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  9. CaramoanReef
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    CaramoanReef Junior Member

    Oh, BTW, I glued 2 half inch pieces of foam to get a 1 inch stringer. I am hoping that will give some additional strength over the glass around the 1/2 inch plywood!
     
  10. mickyryan
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    mickyryan Junior Member

    well foam wont be as strong as plywood ... ever however if you double the layup I am guessing that will compensate for the wood , as far as tabbing it with spacers you don't need to because foam isn't going to cause hard spots anywhere and the foam isn't the strength its the glass, that said if you glass them first then you will have to because that could cause hard and soft spots in the stringer.
     
  11. PAR
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    PAR Yacht Designer/Builder

    Really, two layers of CSM will be strong enough? You've done a lot of this before?

    The foam isn't a structural element, just a mold for the laminate to land on, until it cures. No need for foam spacers under foam. Minimal tabbing would be 3 layers of 1708, boths side of the stringer, but erroring on too much isn't a bad idea. The stringer sheath should be at least the same schedule.
     
  12. CaramoanReef
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    CaramoanReef Junior Member

    From what I have read, the strength is in the glass on the outside. I have read discussions of hollow stringers being just as strong as plywood ones, but not advisable due to the chance that they may fill with water...
    This foam is really soft and easily formed into a shape. After applying a coat of resin onto this foam, it soaked it all up and when it dried, it now feels like rock.
    I hear about creating "hard spots", but I haven't seen anyone post an example of this actually happening. If the stringers and bulkheads are spaced properly, isn't this unlikely to happen? I am thinking of using some standard marine epoxy to glue the stringers to the hull and then tab them in with poly and 3 layers of combo-mat.
     
  13. CaramoanReef
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    CaramoanReef Junior Member

    Thanks, PAR. 1708 is not available out here. Will 3 layers of combo-mat (450 CSM + 16 WR) give adequate strength? I am hoping that increasing the stringer width from 1/2 inch to 1 inch will provide some additional strength as well.
     
  14. PAR
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    PAR Yacht Designer/Builder

    First, pick one, as you can't use polyester over epoxy. Either use all polyester or epoxy over polyester, but never poly over epoxy.

    A combo-mat is what you're using, though I've never heard of the combination you're using, I suspect it's actually 2412 or similar, which is a 24 ounce rove, stitched to an ounce of mat. The stuff you're using is probably a stitch mat. Mat isn't necessary with epoxy and just sucks up costly resin.

    Hard spots are stress risers and there's lots of previous threads about this issue. They do occur and need to be avoided. If the foam is glued to the hull, the problem goes away, though a fillet will be necessary, under the tabbing so you can have a reasonable transition from the vertical plane of the stringer, to the hull shell (this prevents hard spots). This will let fabric will lay down neatly, making full contact, without puckers, wrinkles and voids. You can use roving if you like and it's necessary with polyester, but you'll use a lot of resin with it. Cloth or knitted fabrics are much better, but if using polyester, you'll need a combo fabric (cloth with mat stitched to it), to insure interlaminate bonds.

    Download the free user's guides, from westsystem.com and the epoxy book from systemthree.com. These will nurse you through some of the products and techniques. The sheathing over the stringers will bear all the loads the boat might impose on them, so don't skimp.

    Lastly, it would be a sin to do all this work on a distorted hull. Put it in a cradle and insure it's straight, especially along the aft bottom sections. You might have to jack, strap and cuss it into position, until the missing structure is replaced and cured for a few weeks.
     

  15. CaramoanReef
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    CaramoanReef Junior Member

    PAR, thanks for taking the time to give advice. One thing that struck me that you said, "Either use all polyester or epoxy over polyester, but never poly over epoxy.". So here is what I have done: I glued down the stringers today with "Cord Marine Epoxy". Some of it got onto the sides of the stringers as I was smoothing it out. Should I sand off the epoxy that got on the sides of the stringers before glassing them in with polyester resin and combo-mat?

    I did some calculations with scantling numbers from Dave Gerr's book "The Elements of Boat Strength". 3 layers of the combo-mat will get me well over the .14 inches I got from calculating the laminate thickness for stringers. My issue is that the numbers also suggest a 2.3 inch stringer width. The original center stringer was 1/2 plywood and 14 inches tall. I am replacing it with 1 inch poly urethane foam and 14 inches tall. Do I need to adjust the laminate thickness because of the height and lack of width?

    I will be using "Poly Patch" to make some nice fillets so the glass lays down nicely, like you said. I am planning to tab them in at 5 inches on either side to make sure it gets a good strong bond, then 2 more layers on top of that. I will get some photos of the stringers in place tomorrow and post them tomorrow night.

    I will also take a photo of the combo-mat I am using. I am in the Philippines, so there may be different stuff out here.

    I really appreciate the help, as I am on my own out here with no one to give me advice (everyone builds their boats out here with wood and no fiberglass). This is my first attempt at any kind of boat repair, and I don't want to mess it up as my family will be riding in this thing.
     
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