Reinforcing frames with SS straps.

Discussion in 'Wooden Boat Building and Restoration' started by Hansen Aerosprt, Jan 14, 2010.

  1. Hansen Aerosprt
    Joined: Mar 2007
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    Location: SF Bay

    Hansen Aerosprt Junior Member

    Anyone have any thoughts on reinforcing frames with thru-bolted stainless steel straps?

    Info:
    Boat is 50ish year-old heavy-built 25' carvel planked sailboat. Frames are 1.25"x1.25" - planks are 1" thick. None of the frames are cracked at the moment but these boats (one design) typically suffer from it at this age and those used regularly in the fleet are sistered extensively as a result. Thinking of using 3/16" x 1" 304 bar stock formed over the inside of the frames and thru-bolted at the center of the planks as a low impact and time-efficient method of refastening and reinforcing this hull before it loses it's current integrity and shape. Clearly not 'traditional' but maybe a good alternative to sistering the whole boat in advance?

    Any thoughts from experienced repairmen is appreciated.
    Thanks!
     
  2. keith66
    Joined: Sep 2007
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    Location: Essex UK

    keith66 Senior Member

    I am currently working on a 16ft close seamed carvel dinghy built 80 odd years ago, she has close set ribs of square section rock elm. I have numerous breaks & cracks to repair and the breaks always start on the outside or side that faces the planking. I have retimbered many dinghies & a few yachts & never seen seen a break that starts on the inside.
    To do any good the strap would have to go on the outside of the boat hardly practical.
    In any case the practice of Doubling or Sistering frames is nothing more than a bodge & poor workmanship. Effectively you double the strength of the frame or rib except at the point of the original break. In other words the sistering creates a hard spot & the sister is guaranteed to break at the same point.
    One boat i saw years ago had been sistered completely from keel to sheer not once but twice, no less than three steamed timbers side by side & all of them were broken at the turn of the bilge All that held the top half of the hull to the bottom was the sternpost & stem.
    Back to your boat, i would keep an eye on it & repair properly as you go along
    If you dont want to replace a frame or rib completely scarf & glue a section in.
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Sistering on a break creates a futtock and is a good,time honored practice. Throughbolting will create entry points for water and weaken the overall structure.
     
  4. keith66
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    keith66 Senior Member

    Maybe sistering on a sawn frame boat creates a futtock in which case it might be acceptable but on a steamed timber boat it still creates a hard spot to be avoided. Its still an ugly, cheap & nasty repair.
     
  5. gonzo
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    gonzo Senior Member

    No, I am talking about sistering steamed frames. That is a good application for laminated in place frames sometimes. What makes you think it creates a hard spot? The load is spread on a very large area. Have you ever done a repair like this? It is standard practice and an accepted repair. Do you have information to the contrary or is it an opinion based on cosmetics?
     
  6. keith66
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    keith66 Senior Member

    By sistering a steamed frame or rib the frame width is effectively doubled except at the point where the break is in the original rib ergo you have a hard spot or concentrated stress point in the sister right next to the already broken rib. It is bound to fail again sooner than later. It may be aceptable in an old tore out boat that is being kept going for a few years before the inevitable bonfire. But in a good boat?
    I was taught that doubling was not good or acceptable practice, & would rather do the job properly.
     
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Lets dispel a few myths. The vast majority of steam bent frame breaks are tension cracks. This does not occur because of point loading or stress risers, but as a result of the unforgiving nature, of the fasteners during moisture content changes. This of course assumes no breaks as a result of impact or other damage.

    Sistering or doubling a frame, does not increase point, loading or stress risers, it decreases them, by offering a bigger contact patch in which to transmit loads. This isn't conjecture, it's simple physics.

    This said, sistering isn't the best way to make a broken rib well again, though it is an accepted and well used method (read hundreds of years).

    Ideally, the compromised frame should be replaced or at the very least have a scarf repair across the affected area. Alternatively, laminated repairs can also be quite acceptable.

    In the real world, it's costs a lot more to do the "better" repairs, so the majority of owners, looking at a neglected structure with many repairs necessary, will opt for the sisters.

    I did a 50 year old powerboat recently, a lovely thing 26' hardtop with 42 frame bays. 38 of them had cracks or clean breaks. With the exception of a handful, which may have been impact damage, all where perpendicular to the frame and right at a fastener. Classic examples of tension breaks and cracks. Many ribs where is several pieces.

    I could have sistered most and replaced a few that were just way past any hope of reuse. Fortunately the owner loved this boat and was having the bottom planking replaced too, so all the frames where removed and new installed. This repair easily cost over twice as much, then had I'd used a sistered rib repair, but the owner wanted her like new, which I was grateful for.

    In defense of sisters, the repair can last just as long as a full rib replacement, though it can add considerable weight to the boat. Insurance companies consider it acceptable, which often forces the hand of a repair shop.

    Personally, I'd rather see scarf or laminated repairs, but on a budget, sisters are perfectly acceptable.
     
  8. Hansen Aerosprt
    Joined: Mar 2007
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    Hansen Aerosprt Junior Member

    Thanks for the replies.
    My inquiry was directed at reinforcing a boat that has no broken frames (yet) but under the intended use (racing on SF Bay) probably will if something is not done. That is, I was looking to prevent likely problems in advance as well as get some new fasteners in the planks without overly weakening the existing frames. This is a relatively heavy-built boat with steam-bent frames but typically, they wind up with broken frames as time goes on. I'm all for traditional methods but at the same time not opposed to trying something different...
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Rather then try to second guess the future, develop a good preventive maintenance routine. This is the easiest and least costly method of handing issues. It also mitigates them to a fair degree because if a problem does arise, it's found and sorted out quickly, which is the key to wooden boat ownership, unless you've got more money then you know what to do with.

    In short, more fastener penetrations into the frames, even if a steel backer is installed, will weaken the wooden element considerably. You could compensate with thick steel straps, which would be best cut and fitted on the sides of the frames, not the back, but now you're adding a fair bit of weight too.

    Gluing the steel in place has some merit, though the dissimilar materials will eventually cause issues with each other and their bond.

    An analogy could be something like coating your car with foam to prevent the inevitable dents and dings in the body. I'm suggesting if you address each ding and dent as they occur, you can prevent them from causing rust or other issues with your car . . .
     
  10. Hansen Aerosprt
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    Hansen Aerosprt Junior Member

    These boats are iron fastened and removing the old fastenings is usually fruitless. Thus, re-fastening means more penetrations in any case. But, I'm with you on the maintenance side. What I was wanting was prevention instead of waiting for problems to develop. It is certainly easier to wait and see...
     

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

    Admittedly, removing fasteners is an art form that only comes with considerable practice and tool purchases and/or making. I have several home made stubborn fastener removal tools, some having a rather medieval flare to them, which I enjoy.

    If you want prevention, then get the iron fasteners out as soon as you can. Get the ones you can and cuss about the ones you can't until you have no choice. I spend a lot of time convincing owners to do this, as it saves a lot of money then if they bring it to me, with nothing but iron sick bilges and planking. I usually show them the difference, the damage that is done (or will be) and the things I have to resort to using to get the damn things out. Then I'll ballpark a price that's probably 30% more then the worst case scenario, just to help reel them in, on the do it now gig. It's your call, but "you can pay me now, or you can pay me latter . . ." is the typical reply.

    Don't get me wrong, I have no interest in moving to the bay area to work on your boat, but the iron will have to come out eventually. You can wait until the frames are ruined and planks are popping, or you can get started.
     
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