Avoiding yaw / broaching in following seas

Discussion in 'Powerboats' started by Mat-C, Feb 18, 2010.

  1. Mat-C
    Joined: May 2007
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    Mat-C Senior Member

    I was wondering what design elements would normally be included to prevent a displacement hull from suffering from yaw / broaching in a following sea.
    There seems to be an increasing number of boats that are only designed to go into the wind / waves. Many have wide, flat transoms (which I know to be two contributors) though admittedely many of these are intended to allow the vessel to travel at higher speeds....
  2. Guest62110524

    Guest62110524 Previous Member

    well offshore power boats are more of a compromise than anything else
    yes you have to keep the sections aft less full to help prevent the broach, but then you need them full to stop squatting, you need the forefoot deep vee, but not so deep that it will dig in Deep vee(shape) to stop pounding
    perhaps the most important and oft overlooked , is the rudder Properly designed rudder is a real MUST, to help stop the broach before it starts How often do you see a boat with a piece of plate as a rudder, or a piece of wood with no foil or balance about it? often
    I like the Dashew approach very similar to Don Sheads. steep entry, as in knife sharp, fairly full sections mid and flat to 9 degrees deadrise aft
    What is on your mind, what are you building?
  3. mark775

    mark775 Guest

    For the information of those with, perhaps, heresay or intuitive but not practical knowledge, speed is a defence against broaching and with speed under power (not surfing), small rudder adjustments make for sufficient steerage to stay on course. This falls apart when safety dictates that the boat slows down to bare steerageway. Then, the wave is underwhelming the boat, no significant water is flowing over the rudder, it is often in dirty water, ventilated (butt up on top of the wave), and the vessel design becomes paramount. In other words, the rudder has darn little to do with stopping a broach most of the time.
    Enough power to ride the back top of a wave as long as possible but not the front, which might induce surfing, is a key. If one can actually outrun the seas, it works well...up to the point when it becomes unsafe to do so, and that is highly subjective.
    All designs are a compromise. If seaworthyness is the over-riding criteria, full displacement boats are the best way to go. Underpowered semi-displacement yacht/trawlers, the worst. It is very easy to be under-powered, BTW. Ballast aft, short of inviting a poop. Ballast a bit on the weather shoulder if not running directly downhill - this can be as simple as "hey - you sit there (be specific)!. Anticipate a broach and steer and power to avoid - once you chop the throttle to avoid surfing, it is your hull and how it is ballasted.
  4. daiquiri
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    daiquiri Engineering and Design

    I guess you are talking about powerboat broaching here... So in addition to what has been said above, I would like to add one more fact about broaching, as follows.

    The wave motion is of a pretty interesting nature, and is counter-intuitive at first sight. The mass of water is not moving forward as the wave advances, but is rather performing a circular motion.
    It can be better comprehended through a nice illustration like this one:
    or with this animation (requires the installation of Shockwave Player):

    So, what happens is that water particles near the wave crest are moving in the same direction of the wave, while the particles near the wave trough are moving backwards. This particular caracteristics of ocean waves generate another (we could call it "natural") source of broaching, which can be explained as follows.

    If you imagine a boat hull placed somewhere between the crest and the bottom of a wave, at the front side of it, you can then understand that the stern (closer to the wave crest) will be pushed forward by the water particles in circular motion, while the bow (closer to the wave bottom) will be pushed backwards.
    It is an unavoidable fact (because it is due to the nature of ocean wave motion) and the forces thus generated will be superimposed to other hydrodynamic forces.
    If the boat's centreline is not perfectly perpendicular to the wave direction, these opposing forces will form a moment which will tend to rotate the boat's centerline in a position perpendicular to the wave direction. In other words, the boat will broach. The only hull which wouldn't be subject to this broaching component is a perfectly spherical one.

    The cure for this is a suficient forward boatspeed and a well-designed rudder.
  5. Easy Rider
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    Easy Rider Senior Member

    Once your'e a bit sideways on the face of a wave one word describes your best chances of survival and that is control. Helmsman experience, low CG, lack of high windage, slack bilges (chines) for gracefully sliding sideways, balanced lateral resistance below the WL, double ended boat ect ect all help avoid the broach but control is probably most important. If you can motor along w your boat at 3-4 knots and turn very quickly you can probably avoid the broach. I can turn my 30' Willard 180 degrees in a 90' fairway and as long as I have the strength to pump the helm back and forth .. all seems well. Power steering and a bit faster than my 2.75 turns lock to lock w 45 degrees rudder deflection would be close to bullet proof. With 6 or 7 turns L to L I think control would be lacking. My rudder is large and of flat plate design but seems adequate. OB and IOB boats have a big advantage with control as long as their propellers stay in the water. Daiquiri makes it abundandly clear why the following sea is such a problem but w enough control it should be manageable.

  6. gonzo
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    gonzo Senior Member

    I think that one of the main causes of broaching in modern racing type boats is rudder ventilation.
  7. Mat-C
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    Mat-C Senior Member

    Thanks guys...
    Whoosh - I'm not actually building anything, just an interested amateur...:p
  8. souljour2000
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    souljour2000 Senior Member

    A good subject... riding a following sea ...this is one of the skill sets that I need much more practice with...I just finished making a dolly for my new toy..a 7' 8" sailing pram...no sails or mast for her yet..but I have beefed up her transom with glass/epoxy to hold my 5hp tohatsu...I think I need to get her in the water and practice running from NWesterly waves in the bay in some moderate chop..maybe tomorrow afternoon before the cold gets here again.....
  9. TollyWally
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    TollyWally Senior Member

    From a pragmatic viewpoint, when it really gets bad, a good way to keep from broaching in a following sea is to turn around and jog into it until it mellows out. Technically that's called getting the **** beat out of you.
  10. tunnels

    tunnels Previous Member

    A good easy way to get a handle on this is take a rowing dinghy out in the waves ,row into the waves first and see the effects the boat has going into the waves the bow always wants to lift and even hang in mid air because of the balance fore and aft ,Then at the right time turn and go with the waves at some point the boat wants to slide backwards and as the wave catchs up with the boat and it tilts nose down it wants pick up speed ,At that point when it accelerates it will begin to surf and can over take the speed of the wave and then you need to either slow down or be able to steer properly , the bow want to bury its self in the back of the wave its just caught up with and thats dangerous . If there is not enough Bouyancy and the balance is nose heavy it can nose dive , go under and fill with water rather quickly . It can also begin to turn one side or the other and bury at the same time and could throw you out as the hull begins to roll over and even more water coming in to the point that it sinks .
    Learn from play , When i was a kid we played in the surf in small boats without oars and 99% of the time filled the boat with water , but it was fun and learned a lot .
    Inflatable surf rescue boats will some times sustain broken floor boards when they reach the bottom of a surf trough ,the motor at the back wants to keep pushing down and the bow is trying to force the front up and out so momentarily there is a suction under the centre of the hull , it would bend quite badly if the tubes are inderinflated and soft-ish !!
  11. Ad Hoc
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    Ad Hoc Naval Architect

    It is called good seamanship.

    All boats under the right conditions will broach.
  12. MikeJohns
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    MikeJohns Senior Member

    Just about any reasonable boat can be rendered reasonably seaworthy with a fit and able and knowledgeable crew.

    But for smaller sailing vessel specific characteristics, read Marchaj's book "Seaworthiness the forgotten factor" Then look for papers and articles particularly by Martin Renilson(AMC) and Andrew Claughton(Southampton) . There have been some good Japanese wave tank studies as well on sailboats.
  13. tunnels

    tunnels Previous Member

    I totally agree if you cant feel what is happening to the boat and how it is behaving you should not be there . Trust your in-stinks they know before you do !! :D
  14. MikeJohns
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    MikeJohns Senior Member

    Exhaustion and motion sickness related incapacity are the bugbear of small vessels in extended heavy weather. Then its nice to have boat that you don't have to fight.

    Although I'd add that size really matters and there are plenty of small boat's that cannot be made reasonably seaworthy by anyone and they definitely shouldn't be out there.

  15. Bglad
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    Bglad Senior Member

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