OK Hull from RAFNAR

Discussion in 'Boat Design' started by Manfred.pech, Mar 27, 2017.

  1. Manfred.pech
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    Manfred.pech Senior Member

    http://rafnar.com/leiftur/

    "Established in 2005, Rafnar has developed a new hull design called the OK Hull. It promises to be more efficient and provide superior performance when compared to competitors, especially in bad weather. SuperyachtNews.com flew to the shipard’s headquarters in Reykjavík to learn more about the platform and its new fibreglass boats.
    Inspired by Fredrik Ljungström’s circular arc hull design, the OK Hull was created by Rafnar's founder Össur Kristinsson, who dedicated six years to the development of the scalable platform. The research process involved tank testing over 100 different one-metre models before building a six-metre prototype." http://www.superyachtnews.com/design/rafnar-unveils-new-models and http://www.mby.com/gear/rafnar-ok-hull-iceland-coastguard-test-45596/2

    https://www.google.com/patents/US8726823

    https://vimeo.com/user43267810

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  2. kerosene
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    kerosene Senior Member

    I think I can see some inspiration in the image. Somehow makes me think that it might be a wet ride.


    Sorry - liitle sleep.
     
  3. Manfred.pech
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    Manfred.pech Senior Member

    Thank you kerosene for your comment,

    yes, there can be a wet ride and a lot of spray: https://vimeo.com/191772109 But there are images which might be helpful to understand more about the flow around the hull and the "inspiration".

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    The inspiration might be two percent but the following research and development have been the biggest part. A hundred 1m models have been tested and then a 6m prototype.
    The result is not a displacement hull to plough the water like a displacement vessel and not a planing deep vee and it does not jump or slam in the waves like the vid shows from the planing rib (vimeo above).
    The hull remains in the water and moves with up to 40kn without problematic and dangerous motions.
    The underwater shape reminds me of a conjunction of a bigger deep vee planing hull with a smaller displacement circular arc hull with a short keel (Ljungström). Such a marriage of hulls is known from Atkins (NA) "Rescue Minor" and some Bolger designs.

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    But the RAFNAR conjunction of the two hulls is technologycally advanced to a unique unit. The patented RAFNAR hulls ( https://www.google.com/patents/US8726823 ) are very stable in choppy seas. They do hardly slam, give a safe, comfortable and fast ride over long distances and are very suitable for the rough waters around Iceland. That is the result of all news and reports I was able to find about Rafnar OK Hulls.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Numbers, numbers, gimme numbers first!

    (and my dear Kerosene; guess you were disappointed when you woke up from your filthy wet dream.........)
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Lots of washing off of speed when the bows dipped in the following sea, even with the motors trimmed well out. Don't like that much. And not much indication of what happens when hammering into a head sea. Videos don't often show that pesky angle to the waves !

    Edit: those comments refer to the 1100, I just noticed the video of the "850" in a head sea, which seemed to ride pretty well.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member


    A Rorschach test for boaters ?
     
    1 person likes this.
  7. rasorinc
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    rasorinc Senior Member

    Mr Efficiency, good answer you get points........
    What do you see? I see a large floating refrigerator under power with the doors down.



    i
     
  8. kerosene
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    kerosene Senior Member

    Obviously a butterfly.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    What I see of the orange boat in a following sea causes me to want to know more about the camera boat, which does not lose forward momentum noticeably, but the orange one does. Seems to be its weakness.
     
  10. Manfred.pech
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    Manfred.pech Senior Member

    Sorry, it is not so easy. Aside from the simple dimensions there is not much to be found. Length Overall, Hull Length etc. are here: http://rafnar.com/kerfi/wp-content/uploads/2015/08/Flengur-Brochure-1608.pdf . But I would like more as for instance the lift to drag ratio of the OK-Hull.
    There has been tanktesting in Maloy/Norway, in Potsdam/Germany and in Vienna/Austria but RAFNAR seems not to be interested to publish the results. A hundred 1m models have been investigated, modified and tested again - but nothing was published -- or I was not able to find it.

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    Together with scientists of the Iceland University Reykjavik motion responses (MR) such as vibration and slamming during navigation were investigated. MR cause discomfort to crews and passengers right from the beginning of every sea journey, and often lead to injuries, especially in rough seas.
    It was found, that the RAFNAR OK-Hull design reduced hazardous levels of slamming by up to 95 percent (six instances, versus 112 on a standard rescue boat). These results were published in a short article http://rafnar.com/kerfi/wp-content/uploads/2015/08/Rafnar-Keeping-comfort-at-sea.pdf and this seem to be the most important arguement for RAfNAR to sell their Ribs.

    "Basing his thinking upon a scalable series of ‘circle-segments’ making up the eventual formula for the unique OK keel and hull shape, Ossur Kristinsson determined after endless model testing in the towing tanks of Potsdam, Vienna, and Maloy, Norway that he had perfected his concept. With his final prototypes of open RIBs being extensively tested by the Icelandic Coast Guard and the local Search and Rescue teams in the extreme North Atlantic conditions around Iceland – and sea-trialing for some 3000 nm in the process – Ossur has pronounced a sweet-spot in what he refers to as his ‘base model’, scalable from the smallest prototype of 6 m up to 40 m and beyond. This concept formula has been patented as an invention in the United States.
    An engineer originally trained in prosthetics and orthotics, and owner of a variety of boats and yachts over the years – up to 40 m in length – Ossur Kristinsson had become convinced that the traditional concepts surrounding hull-form and water-flow dynamics were nowhere near fully explored or developed. The uncomfortable slamming and instability of a planing or semi- planing hull and the inefficiency of a conventional displacement hull, drove him to re-think the shapes and radii making up the ideal underwater keel and hull shape. Understanding clearly that having as much of a vessel’s hull remaining in the water for as much of the time underway as possible promoted stability, comfort and good sea-keeping performance, Ossur’s goal was to perfect the hull and keel shape so as to pass the displacement form through the water as efficiently and effortlessly as physics would allow, thereby allowing bodies to transgress accepted limits passing through water as ‘displacement’ boats.
    In his sights was the target of dramatically beating the displacement water- line limit speed rule, while at the same time sustaining comfort, stability, and in-water performance. Ossur believes that two important physical aspects of water are to be taken into account when considering hull-shape: firstly, water is incompressible, hence the violent slamming experienced when a planing or semi-planing boat bounces along atop it, and secondly, the surface tension of water is tremendous with regard to penetrating forces, again augmenting slamming effect and the counter-forces experienced when a vessel tries to punch through a moderate or large sea and waves. In Ossur’s words, ‘the water fights back’ zone whas when hitting waves at an approximately 45° angle.
    The results show that Rafnar’s craft Stefnir is significantly more stable than the comparison craft. Rafnar’s innovative hull and keel design is therefore an important feature when it comes to reducing the risk crew members are exposed to on board, not least when it comes to the negative long-term effects of prolongued exposure to whole body vibration.
    This means that crew safety is greatly improved, the risk of physical damage due to drasitcally fewer slams is significantly reduced, and the crew is therefore able to focus on the task at hand in a more efficient and safe manner.
    The patented hull and keel design used in all Rafnar models is a key factor that contributes to better on-board safety. In fact, a trial conducted by the University of Iceland in Icelandic conditions in January shows that Whole Body Vibration (WBV), which can negatively impact the crew’s health, especially if endured long-term, is significantly reduced in Rafnar’s boats.
    The trial, that compared Stefnir and a common coastguard model of a similar size and weight, factored in different angles when hitting waves, wave height, cruising speed and wind factors, and showed that Stefnir performed significantly better than the comparison craft.
    The ISO 2631-1:1997 standard defines two different WBV health guidance caution zones, as 2.8m/s2
    and 5.6 m/s2. Moreover, according to the standard, the VDV health guidance caution zone is defined by vibration dose values (VDV) between 8.5 m /s 1.75 and 17 m/s 1.75.
    When measuring instances of 1gz z zz, the hazardous condition limit, at the bow of each craft, Stefnir only showed six instances compared to 112 instances in the comparison craft. THAT IS 95% When looking exclusively at instances at the cockpit that surpass the 0.57g health guidance caution zone, the Rafnar craft again performed much better than the comparison craft, with only 6 registered instances compared to 33 instances at the cockpit of the comparison craft." http://www.affairaction.com/Articles/RIBs.pdf
     

  11. JosephT
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    JosephT Senior Member

    It appears Ossur Kristinsson has realized some of the benefits of today's wave runner/jetski hulls. The protruding lower hull + chines allow the boat to bounce around in rough water without all the shock of a flatter hull. Once on plane it will also be more efficient as well (notice how the boat rises up out of the water onto the lower hull chines...less surface drag).

    It's quite obvious the OK hull must be properly loaded to perform in an optimal way. Too much weight overall or too much on the bow or stern will surely affect how it performs. Provided crews are properly trained on load & balance it should do a nice job. I can see the design catching on in due time.
     
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