designing a boat that doesnt broach

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

  1. Eric Sponberg
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    Eric Sponberg Senior Member

    I agree. The best defense against broaching is to not get into a broaching situation in the first place. Stand off, keep your bow to the waves, wait for better weather, or choose another safe harbor with more managable surf if that is what you are trying to do. If you suspect broaching conditions on your return, don't go out.

    You can design different hulls with different broaching characteristics, but you cannot design a hull that won't broach at all.

    Eric
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    The MCA offer the following guidance for those running/operating HSC boats:

    broaching.jpg
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    All sounds good in theory, but in practice people who use such entrances cannot be sure what swell conditions will prevail on their return, and it may be the only port available to them. On more than one occasion I have gone offshore in the early morning through a bar that was in a "tame" mood, only to return in the afternoon and find the swell was up, from some distant influence un-related to local weather. The best insurance is not to traverse bars at all when the tide is running out strongly, which is maybe a third of the time. Ideally time your entrance/exit to the middle of the run-in tide.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Would be an option for slower craft, imo. It would need to not burst out of the front of waves though.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I suspect the problem with the bar that claimed the boat in this instance is the complication of strong outflows of floodwaters that are causing the swells to stand up dangerously.
     
  6. nzboy
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    nzboy Senior Member

    Im not sure the river was flooding at the time I think weather events in the tasman sea affect the swell combined with out going tide from river mouth. My observations are 40-50 ft boats with deep forefoots and flat sterns are the common denominator in broaching. Over 60ft and there is less problems Another picture is of a local boat departing greymouth that was designed with the bar in mind McBride%20Red%20Fishing%20Boat%20in%20swell.jpg
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You may well be right NZboy, the water in the original pics does not look dirty, but I've seem some frightening video of fishing boats trying to negotiate that bar, the water was brown and pushing out hard, in those circumstances even a rising tide is not going to help much. I suppose a boat with plenty of beam is going to be more survivable, but the running costs will blow out.
     
  8. daiquiri
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    daiquiri Engineering and Design

    That is a well-known fact (and also a very logical one) and your observations are correct.
    Cheers
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Can't be too categorical about that without reference to whatever keel(s) are present. Having gone into the broach, however, I'd rather be on a boat with the broad flat stern, rather than a double-ender, which will likely go over sooner.
     
  10. KJL38
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    KJL38 Junior Member

  11. tom28571
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    tom28571 Senior Member

    Quote:
    Originally Posted by nzboy
    My observations are 40-50 ft boats with deep forefoots and flat sterns are the common denominator in broaching.

    Can happen to much larger vessels too. I remember the helmsman working very hard to keep our 392' Gearing Class Destroyer running straight after weathering a typhoon in the Pacific in1952. The swells were high and extremely long and the ship wanted to slew around when going downhill. And yes, the Gearing does have a deep - sharp bow and a flat stern. On such vessels, there is no ability to modulate the prop thrust rapidly and so it is all about steering.

    On small boats, the ability to direct and pulse the thrust is the pilots main defense against broaching. Outboards and IOs' shine in this regard. Inboards rely on skegs and keels to help with directional stability but the small rudders on many are not up to the task when moving slow in difficult inlets.
     
  12. Petros
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    Petros Senior Member

    I was wondering if a double ender would not have as much tendency to broach, but I have seen sea kayaks broach when in a following sea when they tried to surf down the face of the swell.

    If what you say is true about deep bows and flat sterns, than would it reduce tendency to broach with a shallow bow and a sharp stern (double ender)?
     
  13. Willallison
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    Willallison Senior Member

    True, so long as the ability to apply power is there, for when there's no power, there's no appreciable steering. This is also the case with pod drives...something that I've yet to see mentioned in any of the press, which appears blind to anything but the positive attributes of pods. ( Call me a cynic, but the term advertising %'s comes to mind...)
    Anyway... I was talking to the delivery skipper of a 45ft production planing powerboat that was fitted with pods a while back. He described it as the worst handling boat he'd ever been on and swore that he would never again go to sea in a pod-driven boat. When faced with a potential broaching situation, the only way to get the boat to respond was to power up, which tended to drive the bow into the next wave. The result was that by the time it had made the trip from Sydney to Hobart, a number of the bulkheads had parted company with the hull and the (absurd) fishing tower had all but ripped itself off the deck.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Marchaj said that substantial long keels and a deep forefoot on models in the wave tank often could not be induced to broach at all. That's in some of his work on fishing vessel safety and I think in "Saeaworthiness" as well.

    And to follow on from this there are two issues here, one is the broach and the second is what happens afterwards. For vessels that operate out of barred ports and expect to return in all weather they should arguably be required to have a significantly higher stability requirement. But unfortunately there is a culture of complacency and disregard for safety that ultimately claims many commercial fishermans lives. Certainly in NZ and Australia.
     

  15. MLC
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    MLC Junior Member

    I have no personal experience with double-enders but my dad fished on one, (they were common in the old days), and he says that double-enders were very good in fallowing seas, and was the main reason that feature was common on trollers of the time. Of course other design considerations make double-enders impractical for most fisheries.

    Another thing my dad says is that fishing vessels with low L/B ratios tend to suffer form broaching more than narrower vessels. But when speaking alaska fishing vessels, many of which are length restricted, a low L/B ratio could be 2.5/1 or 2/1.

    Vessels with narrow entrance angles also tend to broach badly, as do vessels that sit bow heavy.

    I do not fully understand the dynamics involved but I think that the idea is to make the bow full at and above the waterline, minimize transom immersion, while at the same time, try to keep the center of buoyancy aft.

    Thanks for this thread, it's very interesting
     
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