External Chine Logs

Discussion in 'Hydrodynamics and Aerodynamics' started by fredrosse, Feb 16, 2012.

  1. fredrosse
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    fredrosse USACE Steam

    Building a simple plywood hull, one site indicates external chine logs make for better hull performance than ordinary interior chine logs. This seems unrealistic to me, and probably promoted just to make building easier, without the need to notch the frames for the ordinary chine logs.

    Have there been any tests or other confirmation of the statement that exterior chine logs somehow help the performance of the hull?
     
  2. PAR
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    PAR Yacht Designer/Builder

    A lot depends on several factors; chine shape in plan and profile, log shape, Cp, target speed, hull shape, etc., etc., etc.

    The external log is typical of cheap, flat bottom craft, but really isn't big enough to offer much in regard to preformance enhancements. In fact, if you manage to get going fast enough to plane off, then the external log would be a hindrance, not a benefit. At slower speeds and if the chine has some bury, it can act as a runner, offer some directional and lateral stability, though not much if of typical chine log proportions.

    Simply put, if you want efficiency, then the bottom and sides should be as clean as possible to promote reasonably flow. This means no protrusions unless they are absolutely necessary (like a centerboard or rudder). If traveling under S/L 1.4 then you'll want well rounded chines and underwater appendages, but if capable of getting on plane, then you'll want a crisp chine edge and probably less draggy appendages too.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Performance ? Ease of build ? What type of boat is this ?

    External Chine logs are hard to build and on the bottom dont increase speed, they provide chafe protection. I wouldn't consider external chine logs on a skiff strong, nor fast , nor desirable.

    When time comes for service you will have to tear them off and make a big mess.

    If you are building with epoxy and ply stick with the proven biax tape chines then add spray or chafe gear in such a way that you must not violate the integrity of the joint when servicing.
     
  4. PAR
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    PAR Yacht Designer/Builder

    External chine logs are much easier then internal logs, unless of course you've never built a boat like, this and are just speculating about their usefulness. The presence of a chine log suggests, it's not a taped seam build and again, unless you are just speculating about a build method, with which you haven't any expertise, maybe it's best just to post what you actually know.
     
  5. DCockey
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    DCockey Senior Member

    Phil Bolger speculated that external chine logs migh "reduce eddys" or something similar though his primary reason for using external chine logs was ease of building. As far as I'm aware the "reduce eddys" part was pure speculation without anything to back it up.

    I would expect external chine logs to increase the "eddys" and slightly increase resistance compared to a simple, sharp corner at the chine, though the increase might not be noticeable. That's based on wind tunnel work including flow visualization on rear of "fastback" cars.
     
  6. DCockey
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    DCockey Senior Member

    I expect the external chine logs fredrosse asked about are on a simple plywood flat bottom or v-bottom boat. During construction the sides are bent around the frames and/or mold without the chine logs in place. Next the chine logs are bent around the outside of the sides and fastened in place. No need to notch the frames nor to carefully fit the ends of the chine logs. Then the chine logs are cut to length and beveled.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Chine logs as a rule aren't big enough to really affect the preformance envelop much. It depends on hull shape, type, preformance envelop Cp, etc. A boat with deeply immersed chines could benefit from runners, which are a fair bit different then the usual external chine log, seen on small craft. Many ships have used runners for generations with great success, as have small craft with specifically shaped and located pieces. A runner and an external chine log are quite different in application, shape, location etc., so lets not confuse the issue by thinking one can act as the other.

    Fred it would be helpful if we knew the model of boat you're building. This will quickly determine if the external logs, will be anything more then an easier way to make, the bottom/topside panel joint.
     

  8. fredrosse
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    fredrosse USACE Steam

    I was looking at a build site for a Mayfly16, and came across the external chine logs. The author had the following statement about the external chine logs... :

    ""Don't the external chine logs slow the boat?" Bolger didn't think so. In fact he thought that on a flat iron skiff the external chine acted as a dam to slow the flow of water around the chine, reducing the eddy at the chine and making the boat faster! I don't know about that. I've had boats with both internal and external chines and they go the same as far as I can tell. "

    This is what prompted my question on the forum, and thanks to all who provided useful information.

    The sidewheel steambaot "Margaret S." (19"-3") has internal chine logs, and so does the 55% size replica hull I am currently building. The small hull will fit on the sidewheeler when trailering, and will serve as a tender when the Margaret S. is anchored. Docking a sidewheeler has its issues, so I will anchor often. I also plan to use the small hull explore with my wife (in her own kayak), but then I will have a small electric trolling motor to help me out.
     

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    Last edited: Feb 19, 2012
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