broaching essentials

Discussion in 'Boat Design' started by coopscraft, Dec 25, 2013.

  1. coopscraft
    Joined: Dec 2013
    Posts: 48
    Likes: 1, Points: 8, Legacy Rep: 10
    Location: oregon

    coopscraft Junior Member

    Working on a surf boat design from scratch(see my other thread). Under an ash breaze, what design characteristics lend better to broach prevention? My thoughts so far are as follows, 1, design for controllable surfing. This would make following seas a source of power and provide a solution to the broaching issue. However surfing is a hull design I don't know anything about. Or2 a long rockerless bottom would resist being pushed to yaw, but require more effort on the oars to compansate when the sea tries to steer. Or 3 use lots of rocker sacrificing speed for manuverability. What say u all? Assume I understand learning to avoid broaching conditions is the best first line of defense. This is about the boat not the boater.
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,390
    Likes: 1,023, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You could use devices that can be temporarily deployed when 'surfing', to shift the CLR rearward, and retracted at other times.
     
  3. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    I think that if you are going to design a proper hull to prevent broaching in surfing conditions you need to study what is happening in the water first. While I have some experience in following waves, I don't know all the underlying principles.

    As the wave front moves forward, how is the water under the surface moving to allow the wave to move forward? This is the movement, along with gravity, that determines the forces on a boat.

    One thing I do know is that a long deep rockerless bow is what you do not want. I'd think the Aussie surf boats driven by ash breeze must be close to ideal given their pedigree. Of course they must be compromised somewhat by the need to both go out through the waves as well as return down wave. A boat needing to go only one way will probably have a different ideal shape.
     
  4. Baltic Bandit

    Baltic Bandit Previous Member

    I'd suggest you are correct on this approach. And part of what is happening in a wave is that the water at the top of the wave is moving "forward" (direction of the wave train) relative to the water on the front face of the wave. So part of what is causing the broach is that the back end of the boat is being accelerated more than the front end (ie being pushed from behind).
     
  5. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Since I live near coastal water, observing waves is very common. It's clear that water is moving forward in the upper part of a wave as well as up and down as the wave passes. On a beach or shore, water crashes forward on the shore and then clearly recedes between waves. That makes it clear that there must be a circular motion to the water in waves. Forward in the upper part, down in front of the wave and aft on the under side. The amount of circular motion must be directly proportional to the height or energy of the wave. No complex science involved, just observation although this reasoning is surely a simplification.

    The form of a boat to negotiate on waves should be able to handle these motions and maintain balance and steering control. The task is then to imagine a boat that works with motions of the water rather than fighting them.
     
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,390
    Likes: 1,023, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The "sweep" crewman is critical in keeping the thing running straight when on a "run", and not always successfully even with an experienced and very strong man in charge. I doubt these boats are optimized for broach resistance, they are, after all, racing craft, with a finish line at the shoreline. If they were designed to be broach-proof (to the extent anything is broach-proof), they'd probably surf poorly and only ever win if the rest of the field came unstuck. "winning" for the OP is getting home upright. Another thing to be aware of is a shore "dump" where the bow may hit bottom, that will almost certainly cause the boat to go side-on.
     
  7. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    CLR aft helps but there are other important factors.

    Water on the front face of a wave is 'compressing' ; the water in the trough and the water at crest are moving towards each other - on the face side.
    On the back of the wave it's the opposite; the water particles move away from each other. It's a necessary consequence of the changing height of the fluid surface.
    What this means is that the bow of the boat can be moving forward through the water while at the same time the stern is going backwards through the water. This would be in the extreme case of the boat measures one half wavelength and is moving a bit slower than the wave celerity.
    This alone will already tend to always turn any floating object transversal to the wave on the face, and straighten it back again on the back of the wave.
    It also means that the rudder when the stern is near the crest and its forward moving water loses much of its effectiveness.
    Thus moving the rudder far behind the stern into a differently moving mass of water can be very advantageous. Also it allows pushing the stern laterally even when there is no relative water motion at that moment.

    Another extremely important element is how the boat trims wih heel.
    You absolutely must avoid a shape that trims by the bow with heel, as this will cause the CLR to move forward.

    Thus advantageous shape for reducing (eliminating is impossible) broaching tendencies is a 'bluffer' forebody than aftbody as this makes it tend to lift the bow (at the expense of the stern though) when heeled.

    Broaching is a combination of complex dynamical motions that feed into each other to the point of loss of control , so the idea is to reduce the positive (but undesirable) feedbacks from one motion to the others.

    You'll notice that the australian surf boats have a large amount of reserve floatation in the 'shoulders'

    Incidentally a lot of modern wedge shaped sailboats are about the worst possible combination of design features for broaching, which is why they are forced to use twin rudders.
     
  8. gonzo
    Joined: Aug 2002
    Posts: 15,498
    Likes: 1,039, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

  9. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Well, that probably shows my ignorance of what the surf boats are really about but I think the rest of my arguments are correct.

    Tcubed, are you supposing that water is compressible?
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,390
    Likes: 1,023, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Standard practice in those surf boats is for the 4 rowers to abandon their oars and move aft to keep the nose up, then resume rowing when the wave flattens out, not an option for the OP. I doubt there is a safe, reliable formula to get what he is after.
     
  11. KJL38
    Joined: Jun 2008
    Posts: 97
    Likes: 13, Points: 8, Legacy Rep: 75
    Location: Tasmania

    KJL38 Junior Member

  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,390
    Likes: 1,023, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It is a useful article, of course it applies to boats that can't keep pace with the wave train, it is much easier to run the bar or surf if you can hold the back of waves. Rowing, of course, doesn't offer that advantage. For braver souls than moi ! :D
     
  13. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    No of course not! (although it is compressible in the strict sense, but that has no bearing on this)
    What i mean is that water particles moving towards each other have to go somewhere.
    In other words; the water columns on the wave face are getting compressed horizontally so thus elongates vertically. The reverse happens on the back of the wave.
    Interestingly, it means that waves that are two boat lengths long are the most effective at yawing it, or the worst for broaching.
     
  14. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    I assumed that you did not really mean that water is compressible but it might be better to avoid the term and use pressure instead. I look at a wave train like it is harmonic motion?
     

  15. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    Yes poor choice of terms on my part.

    The pdf report linked to above is good and worth reading from pp 15 onwards.
    However they do not talk about what i'm really trying to underline here; the importance of looking at how a boat trims with heel.
    Also important to consider when designing a boat to have low broach tendency are the inclined waterlines, and especially heel induced yawing moments .
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.