Floating Chine Logs

Discussion in 'Boat Design' started by Matt M., Dec 3, 2019.

  1. Matt M.
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    Matt M. New Member

    I am building a McKenzie River drift boat and the directions call for a floating chine log that is not connected to the ribs. No explanation for the reason to do this is given, and from a strength perspective it seems counterintuitive. Does anyone know why the designs would recommend a floating chine log and whether it is advisable? Would it be a stronger hull if I go ahead and attached the chine log to the ribs. If I do it, my plan is to attach the hull ply/chine log/rib with a single screw. Thanks for the info/advice.
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Contact the supplier of the plans with your query, seems a reasonable start.
  3. bajansailor
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    bajansailor Marine Surveyor

    Maybe this is a way of allowing for any unfairness in the lines?
    Maybe they are considering the chine log to act like a spline used for lofting - if it is 'floating' then it will find it's natural curve, rather than the curve it would be forced to adopt if it was attached rigidly to every frame?
    I would think though that once it has found itself, it would be a good idea to secure it in place.
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  4. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Ya, I read that the same way.
    It's the order of attachment that's important.

    Partially installing each screw into the ply/chine
    before coming back and driving them all home into the ribs.

    Could that be correct Matt?
    Does it make sense?
    bajansailor likes this.
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Heard story about how in WW2 the UK govt contracted with some house builders to build boats. Problem was the house builders didn't understand the need for wooden boats to flex, and built them "too strong" in all the wrong places, and as soon as the boats got in some waves, or took on some cargo, they started busting themselves apart.
    Could be something like that? Sort of like how a throwing tomahawk's handle is supposed to be bit loose.
  6. sharpii2
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    sharpii2 Senior Member

    My guess is this is to protect the chine log from rupturing should the boat encounter a rock.

    my reasoning goes as follows:

    If the chine should hit a rock, the blow will be distributed along its length instead of being localized between two frames. This will allow a greater amount of flex to absorb the blow. If it were attached to the frame ends, there would be far less length between secured ends, and this would require a much shorter span, which is much thicker for its length, to take the same bending loads. If it is strong enough to handle them, no problem. But if it is not, it is very likely to break into as many as three pieces within the frame bay. As the surrounding plywood is very likely to have cracked all the way through, a rather large chunk may be ripped from the hull which would be almost impossible to patch or repair on site.

    But if the chine log does not rupture, there will only be through-hull cracks in the damaged area which can be either patched or plugged with available material.

    What cannot be handled with brute strength can often be handled by resiliency.
  7. fallguy
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    fallguy Senior Member

    Lotsa reasons I am guessing..

    If you get hung up on a rock in whitewater; the chine log acts as both a chafing board and since it moves; you would avoid getting wedged as bad, or you could possibly unscrew the chine log in process and save the boat, but in order to unscrew in whitewater; you'd need the job to be simple.

    Have you ever gotten sideways in a canoe up against a real log? The canoe can quickly wash under the real log with pressures so high it is almost unmoveable.

    If your boat were to wedge against an odd shaped rock or between two rocks in the current, the chine logs ability to move or be removed may help. Or it may simply be there to double as a sponson and a chafing board in which case once it is all chafed to hell; you can remove it more easily for refinishing.

    But the real insurance of a simple disconnect is the likely reason. The boat might be able to be rescued from being wedged and becoming part of the river. And for some this will seem laughable, but it is truly not. For this reason, the screws would be bedded in seam compound and not caulk as well and you could even put wingnuts on the inside for a quick removal.

    If you never intend whitewater use, you could probably add a third connection, but I believe it would be below the waterline, which is still another reason to keep it simpler and avoid connections that may result in leaks.
  8. DCockey
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    DCockey Senior Member

    The plans probably originated from Roger Fletcher. His building method has the chine log inside the planking and captured in place by notches in the frames, but not fastened to the frames. The chine logs cannot be removed without disassembling the boat.

    My guess is the same as several others. The chine logs are floating to allow them to move relative to the frames in the event of a collision.
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  9. philSweet
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    philSweet Senior Member

    Cutting the frames clear of the logs and stringers prevents rot pockets from forming in the end grain of the frames, and it can also lower stress concentrations due to swelling and contraction of wood in different grain orientations, and lessens damage from freeze-thaw cycles. So it is considered good practice in general.
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  10. DCockey
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    DCockey Senior Member

    Roger Fletcher's construction has the notches in the frames sized to fit the chine logs - no clearance. He shows "sealant" but not fasteners between the frames and chine log. Roger's plans are based on his study of old boats from several builders. My guess is the floating chine logs was in the old boats. Either it developed because it was found to be the best solution - or - one builder did it and others copied that builder.
    fallguy likes this.
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