to sink or not to sink!

Discussion in 'Boat Design' started by neil m, Dec 20, 2013.

  1. neil m
    Joined: Dec 2013
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    Location: manchester

    neil m New Member

    hello, I am just about to start a renovation of a wooden 12ft clinker dinghy I have come to my first predicament. the boat has not been in the water for ten years although the planks look in good condition and I cant see any gaps, my initial plan was to put her into the water to see how she takes up and to see if there where any real problem areas, which cant be seen at the moment. Then I was going to get to work on the small repairs and then varnish and paint. my predicament is that there are around 6 Ribs broken, so I am not sure if I should put her in now? should I just get on with the repairs and varnish and then when she is finished see how she takes up then?
    any advice would be of great help.
    thank you and merry christmas
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Broken ribs are a common sign in lapstrake hulls. It could be as simple as a bottom strike, but it's usually not. What I generally see are called "tension breaks", which are typically seen at plank to rib fastener locations, often perpendicular to the rib, though sometimes following the grain. What happens is, as the planking swells and contracts from moisture content changes, the fasteners pull at their holes and eventually one of two things happen - the fastener or it's hole distorts or the fastener has a good grip (as it should) and the rib is stretched until it cracks or breaks. Riveted and bolts boats are more prone to this, compared to clenched or screwed builds, but all lapstrakes suffer this indignity, with age, negligence and rough use.

    The most common locations for these types of cracks and breaks are at the more severe curves in the aft end of the boat. This will be at the turn of the bilge, where the bottom transitions to the topside planks. On a 12' boat, this turn will be pretty quick (tight radius).

    You might get lucky and she'll suck up fine, but expect some leaks, that will only get worse as you use her. The only cure it to find out what's really going on, repair or replace the ribs, restore the fastener holes and refasten this old gal. Lastly, no magic goo in a can or tube is going to fix her, in spite of what the advertising might suggest. I've seen lots of lapstrakes ruined with this treatment.

    Pictures of the ribs and the general condition of the hull would help.
     
  3. neil m
    Joined: Dec 2013
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    neil m New Member

    Thank you Par, that does help a lot, I will post some photos soon. Just one other thing regarding fastening her, would you give her a total refastening? not just the repairs? Oh yes I am not going to use any goo. I am going to try to be as true to her build as possible, I have some grown crooks coming for her thwart knees too.

    thanks again
    thanks again
     
  4. LCrosby
    Joined: Dec 2013
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    Location: Plymouth, MA

    LCrosby Junior Member

    Throw her in the water and see what happens.
    My grandfather used to say " If a lap strake boat
    has broken ribs get a dead viking and a match"
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I repair lapstrakes frequently and ribs can be repaired (several methods) or replaced. The approach you take is often dependent on the access and location. Replacement is usually preferred, as the unbroken potions of the rib may be just a little behind the curve in regard to breaking elsewhere. I've scabbed and sistered with success, though usually if not replacing I'll scarf a replacement portion (shown below), into the damaged area.

    Many shy away from lapstrake repairs, mostly because the work can be difficult to the uninitiated. The repairs use different techniques, which you need to get your head around, but once you do, the work isn't hard or any more demanding then other repairs.

    Once fasteners start to pull and their holes start to "egg out", you can use the hunt an peck method, replacing and fixing as necessary, but this can become a career, so I usually recommend just doing them all. Sometimes, depending on the boat's general condition, maybe just the bottom gets refastened. Usually once you notice "issues" the damage is much more than meets the eye, so wholesale faster replacement and hole restoration is necessary, to firm up the structure again. This is all a judgment call, based on condition and level of "movement" that can be found. You see, as the fasteners lose their grip on structural elements, the whole structure wracks and twists underway, making matters worse, including the leaks.

    By all means, splash the boat and see if she'll take up (don't flood the boat, just launch it), but don't get your hopes up. Much depends on the design, the life she's lead, neglect, abuse, type of construction, etc., so maybe you'll get lucky.
     

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  6. LCrosby
    Joined: Dec 2013
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    LCrosby Junior Member

    If you have cracked frames, they have to be replaced. It has to be one piece
    from the keel to sheer. A repair or scarf is bad. Especially if its in an area in that's flat like the bilge. Ill assume they are oak. Moving joints plus bilge plus white oak equals rot.

    You could sister the broken ribs. viz. put new ones alongside the broken.
    Might not look nice, but will do the job.

    Despite what anyone tells you, a lap strake hull must be threw fastened.
    Usually with rivets, but bronze nuts and bolts could be used.( just remember to saw the threads flush to the nut and key the end to lock it.)

    These planks pull outboard when they swell or are under stress. That is why you wood screws are not used in lap strake. Copper rivets.

    The whole point of the lap strake was to make it light. That is why traditionally they have small frames. No threads in the frames or the planks.
    Just smooth rivets. That way everything was free to move. Start throwing
    screws into smalls frames and............. I think you get the point.
     
  7. neil m
    Joined: Dec 2013
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    Location: manchester

    neil m New Member

    Thanks guys.
    I think I am not going to put her in yet. I think my plan going to scrape down old varnish and paint, replace broken ribs and refasten, there is some rot to the transom and the thwart Knees which I am going to grave in some pieces. I suppose I will need to re-rivet the whole hull which is a lot of work but eh ho, then give her a few coats of varnish and paint and then lastly pray to Neptune and hope she is kind to me and take up ok :p

    thanks again for all the advise
     
  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Cracked frames can be sistered and scarfed successful, though replacement is preferred. Steamed oak ribs rarely rot, unless quite neglected, which will rot any species. Sistering is the most common repair, as it's the cheapest and easiest to fit. Scarfing has the least impact on the boat, as under paint, you can't see them and there's no weight gain after installation. Rib replacement is difficult on a lot of boats and can be destructive. Both sistering and scarfing can be done on site, without having to pull any planks, which isn't the case for replacement, where at least 2 planks, usually more need to come off, to insert the replacement.

    It is best to through fasten at the ribs, but many successful designs have been screwed. Small craft are usually clenched or riveted, with larger boats seeing machine screws or bolts. Chris Craft used a pinched tip stove bolt nut, that digs into the frame as it's drawn up and screwed laps. In most cases, just the draw up will bury the nut sufficiently enough to prevent it backing off. I usually don't saw off the protruding bolt or machine screw, just clip them with a bolt cutter, which distorts the threads enough to prevent them backing off, without having to "stake" the threads. I do use a thread locker just to be sure.

    Frame dimensions and spacing are scaled to planking thickness and structural idiosyncrasies. They are small, because it's lighter to build this way, compared to beefier, more widely spaced frame arrangement. There are a few ways to handle the planking, structure interface. You can float the planking, much like what LCrosby is describing, but it's still attached, typically longitudinally. Conversely, you can hang the planks on the ribs themselves, with fasteners at each lap intersection. Both techniques work fine. Lap fasteners are also common in traditional lapstrake builds.

    The best thing to happen to lapstrakes builds, to improve their longevity and water tightness, is first in the late 1950's was the advent of employing polysulfide in the seams, greatly improving the seal before take up. And lastly has been the use of epoxy in glued lap structures, which if scaled properly, can eliminate most if not all of the ribs.

    Of all the traditional build methods, lapstrake is the lightest.
     

  9. LCrosby
    Joined: Dec 2013
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    Location: Plymouth, MA

    LCrosby Junior Member

    Well said PAR.......
    Wish I could put things into words like that!

    You might also think about sistering with a sandwich method.
    Rip enough (lets say oak) at around 1/8", same width and length as
    existing ribs. Get out enough to stack up to same thickness as existing.

    Tape off the area next to the ribs to sistered or replaced( it will get messy)
    Wet each side of the new rib strips with west system and stack them up
    like a lamination.

    Press it into place next to the old rib and clamp the top to the sheer plank.
    (by the way, make sure you cut the strips at least 12" longer than the existing ribs)

    You can then brace it off athwart or wherever until it cures.
    Next day Bobs your uncle, new laminated rib and epoxy everywhere!
    You can then decide if you want to sister or replace the old one.
    Don't forget to bevel the hard edges before you fasten it.
     
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