designing a boat that doesnt broach

Discussion in 'Boat Design' started by nzboy, Apr 16, 2013.

  1. nzboy
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    nzboy Senior Member

    Another sad event all to common in this neck of the woods. What kind of design changes could make a trawler surf in rather than broach, because a wave has gripped it under the stern? 8558795.jpg
     
  2. Petros
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    Petros Senior Member

    It seems to me what ever you might do in changing the hull shape would make it unmanageable in normal operation. I am no expert in hull design, but I have worked many years in aerodynamics and understand dynamic stability. I have also experienced a broach in much smaller boats, when the water under the hull is moving forward it appears the boat wants to swap ends. IOW, when moving forward in the water the hulls maintains directional stability because of the forces on the hull, keel/skag and rudder, but when the water is moving from rear to front (like when the wave passes under it), the boat becomes directionally unstable and wants to turn around. The same effect would happen I suspect if you were to push the boat backwards rapidly in calm waters. Also, when the stern is lifted out of the water, and the bow is buried, the center of lateral resistance moves way forward, ahead of the CG, making the hull more directionally unstable.

    There is no way to make the boat directionally stable with water flowing over it in both directions. A really large rudder and a deep fin keel might help, but that would cost a lot of extra drag in normal operation.

    Best bet might be a computer controlled rudder and trim tab arrangement, where when it becomes directionally unstable the computer reacts faster than the helmsman does, and keeps the boat in a straight line. Of course this assumes the rudder stays effective when partially submerged and in the foaming sea. Perhaps if you use two rudders that are slight angled outward, so at least one will have control power at all times even when the hull is rolled over. This is how unstable fighter aircraft operate (and why many have twin tails).

    The question is would the cost and extra maintenance be worth the cost and frequency of a broach.

    Maybe someone more knowledgeable about boat design can contribute.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    You can't. All monohulls broach, under the right conditions.

    It is more about education of seamanship than design.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously this enquiry relates to barred estuarine entrances and displacement boats that are unable to keep up with the speed of the wave train. The only approach that would seem prospective to me is something that can be lowered into the water akin to a surfboat sweep oar (with plenty of surface area), well behind the boat, but needing to be able to be gotten out of the way when not in use. Just an idea.
     
  5. Milehog
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    Milehog Clever Quip

    Nicholas Brett Eklund, 36, of Greymouth, died after the 15-metre wooden trawler Lady Anna, which he was skippering for the first time, capsized as it made its way into the West Coast town's port.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Commercial offshore fishing is close to the top of the list of most dangerous occupations.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Imo, it should have a form of a sphere. A ball-like trawler. :)

    Otherwise, it will broach because of the physics of wave motion, which is a pretty interesting phenomenon though also a counter-intuitive one. The mass of water is not moving forward as the wave advances, but is performing a circular motion. It can be better comprehended by taking a look at this nice illustration: http://hyperphysics.phy-astr.gsu.edu/hbase/waves/watwav2.html
    or by carefully observing this animation (Check the "water waves" section): http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

    So, what happens is that water particles near the wave crest are moving forward, while the particles near the wave through are actually moving backwards. This particular characteristics of ocean waves is the main origin of broaching, which has the following dynamics:

    Imagine a boat placed somewhere between the crest and the bottom of a wave, in front of the wave. The stern (closer to the wave crest) will be pushed forward by the water particles (which are moving forward near the crest), while the bow (closer to the wave bottom) will be pushed backwards by the backwards-moving water particles.
    It is an unavoidable fact, due to this particular nature of ocean wave motion.

    If the boat's centreline is not perfectly perpendicular to the wave direction, this wave motion will generate a force couple which will tend to rotate the boat's centerline in a position perpendicular to the wave direction. In other words, the boat will broach. Once in this position, the boat is at the mercy of the incoming lateral waves.

    Cheers
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Water is certainly moving forward in wholesale fashion when the hollow wave breaks, and in the above pics it was the boat going broadside to that that finished it.
     
  9. MikeJohns
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    MikeJohns Senior Member


    The Grey river runs out through the mouth and the SW swell runs in peaks on the bar and breaks with a strong undertow.

    That makes it a very treacherous entrance.
     
  10. daiquiri
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    daiquiri Engineering and Design

    The breaking waves are an extremization of the above mechanism, which is a fact.
    What the above physical explanation tells you is that a boat will want to broach even in non-breaking waves. It is a natural phenomenon which stems from the principle of minimum energy (of the closed system "water-boat").
    Cheers
     
  11. tom28571
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    tom28571 Senior Member

    Boat and ships can have a tendency to broach in following waves whether the water is moving relative to the hull or not, whether moving forward or aft relative to the hull. A broach can be the summation of several factors, not just one.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member


    Needless to say, this is correct, fast planing boats can broach violently when their forward speed is much greater than any differential in speed between water at the crest and the trough.
     
  13. daiquiri
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    daiquiri Engineering and Design

    So can sailboats too - when the rudder force gets too small (due to aeration, excessive heel, too small rudder area, or whatever) to counteract the forces and moments generated by the sails.

    All correct indeed. But, the boat in the OP was neither a sailboat, nor a planing powerboat. It shows, imo, a boat which has broached due to wave action.
     
  14. Gib Etheridge
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    Gib Etheridge Junior Member

    I've often wondered if keeping tension on a large drogue would keep the hull lined up. Even if the prop came out of the water the boat's momentum would maintain at least some tension. A drogue can be quite useful in a following sea, it seems it could be useful for landing rowing boats through surf if it works. Anyone tried it?
     

  15. sean9c
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    sean9c Senior Member

    Not a good idea. It'd really give the boat something to trip over.

     
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