Motion in a Seaway

Discussion in 'Stability' started by Tad, Jan 23, 2007.

  1. Tad
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    Tad Boat Designer

    Seaworthiness (or lack thereof) is a combination of a number of factors including design, crew preparedness, construction, and maintenance.

    In another thread there seems to be an endless circular discussion of heavy vs light displacement and the suitability of these factors, mostly boiled down to subjective opinion.

    One area where we can be somewhat objective is in the motion (accelerations) of a particular hull form in a given seaway. It would seem to me that the discussion below was figured out 30 years ago by Bruce Farr. Make the bow fine to minimize pitching up when meeting a wave, and make the stern flat and wide to quickly damp any pitching.

    Tad



    Axe Bow could form basis of fast superyacht design

    By IBI Magazine/David Foxwell

    An interesting new hull concept from Damen Shipyards in The Netherlands is said to offer a range of benefits for fast craft, including superyachts.

    The Axe Bow is a specially shaped bow that has an almost straight stem, thus maximising waterline length and prevents a bow from coming out of the water. Slamming is said to be reduced, sea-keeping dramatically enhanced, and downtime and fuel consumption are reduced.

    The origins of the Axe Bow date back to 1995, and to Damen's 'Enlarged Ship Concept', a project undertaken in close collaboration with the hydromechanics department at the Technical University of Delft, the aim of which was to improve the performance of fast monohulls by increasing length substantially, whilst maintaining speed and without increasing cost.

    As Jaap Gelling, Damen's senior architect explained, the Enlarged Ship Project was a great success. "What we were seeking to do was improve the performance of a fast monohull in a seaway without increasing cost. Increasing length really helped. The empty space in the foreship created the opportunity to optimise the hull shape — mainly the bow — for enhanced sea-keeping, and the extra length could be used to optimise the layout."

    In 1997, Damen and TU Delft embarked in the 'Optimised Enlarged Ship Concept', which led eventually to the construction of the first Enlarged Ship, a Stan Patrol 4207 coastguard vessel of some 42m (138ft). Many more hulls making use of this concept followed and were built for, among others, the UK Revenue & Customs, Dutch Coast Guard, Vinamarine in Vietnam, The Jamaica Defence Force and South Africa's Department of International Affairs.

    "The next step was the Axe Bow," Gelling explained. Starting in 2003, and working once again with TU Delft, also with Maritime Research Institute Netherlands (Marin), the Royal Schelde Group, and the Royal Netherlands Navy, Damen pushed the envelope further still and embarked on the creation of what was to be an even more revolutionary hull shape.

    "Operability is limited by motions, particularly accelerations," Gelling explained. "Most speed reductions are voluntary, and crews on board fast vessels tend to react to extremes, not average accelerations. We took our experience with the Enlarged Ship Concept, and went a stage further, creating a design with even less flare, an even deeper forefoot, even higher freeboard, and an even more slender foreship."

    In a subsequent project, the 'FAST Research project', intended to examine the performance of a range of advanced hull shapes, Damen and its partners compared the performance of the Axe Bow with that of the Optimised Enlarged Ship Concept and a wave piercer; and working closely with TU Delft and Marin, they contrasted the flat water resistance, accelerations in head waves, accelerations in stern waves, course stability in stern quartering waves and speeds of 25, 35, and 50 knots.

    "The work we did clearly demonstrated that the Axe Bow has superior sea-keeping characteristics, with 50 per cent lower peak vertical accelerations compared with the Enlarged Ship — which already provided vertical accelerations that were some 50 per cent less than those of a conventional monohull — and the Axe Bow had lower resistance in the speed range up to 35 knots," Gelling explained.

    He said the Axe Bow still manoeuvres well at low speed, and 'listens' very well to rudders. The draft is a little larger than a conventional hullform, but not that much greater; he agreed, the Axe Bow is a little more complicated — and a little more costly — to build, but not excessively so.

    The result of the 'know how' gained by Damen during the development of the Enlarged Ship and subsequently during the Axe Bow projects is a series of new 'Sea Axe' crewboat designs, a number of military and paramilitary vessels (the Stan Patrol 3307 and Stan Patrol 5009), and a fast superyacht design.

    The first craft with the Damen Axe Bow were delivered last year to Oceanteam Crewboats BV, based in Amsterdam, The Netherlands. Also on order or already under construction is a pair of 50m (164ft) Axe Bow crewboats due to be delivered in late 2007 and early 2008, respectively, and Damen believes that the hullform is equally applicable to other applications — such as fast ferries — and fast superyachts where limited volume in the foreship is not an issue.

    (23 January 2007)
     
  2. jehardiman
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    jehardiman Senior Member

    LOL :D God I wish it was that easy....

    "Axe Bow"?..."a project undertaken in close collaboration with the hydromechanics department at the Technical University of Delft, the aim of which was to improve the performance of fast monohulls by increasing length substantially, whilst maintaining speed and without increasing cost".

    Old hat...and will most likely be discarded for the same reasons that it was discarded 80 years ago....wetness and surge...which BTW, were not mentioned as factors in the above C&P.

    [​IMG]
    http://upload.wikimedia.org/wikiped...inia_At_Speed.jpg/300px-Turbinia_At_Speed.jpg
     
  3. Guillermo
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    Guillermo Ingeniero Naval

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  4. Trevlyns
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    Trevlyns Senior Citizen/Member

    I tend to follow jehardiman on this one and would like to add the following thought.

    Common sense and a little “thinking outside of the box” say that a craft does not necessarily always move with the seas. Despite being in Britain, NOTHING rules the waves!

    With these modern designs with fine bows and broad “sugar scoop” sterns, I always conjure up this picture of the yacht in a heavy following sea… The broad stern is easily picked up by the wave and the fine bow (love the term “axe”), offering little buoyancy or resistance, is unceremoniously dumped into the trough. Result – a pitch pole.

    Sure, we may be talking racing yachts in ideal conditions, but there is also a real world out there. Fashion and trend should never jeopardise safety.
     
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  5. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think the X bow is designed to be able to stay in one position with minimal movements or accelerations? Then it will always lay head to the wind.
     
  6. Vega
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    Vega Senior Member


    Incredible....30 years ago!!!! That's why he has already a place in the history of Naval Architecture;)
     

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  7. Tad
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    Tad Boat Designer

  8. Ike
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    Ike Senior Member

    Whatever happened to the canoe body double ender? I served on 2 Coast Guards ships built in the late 1930's. They were both 327 foot, with a canoe body stern and basically a little squarer in the bilges than a canoe body. They could go to sea almost forever in all weather. They were the most sea kindly ships I have ever been on. I went through three hurricanes (one of which was Beulah 1967) They had a nice slow roll as well.

    It sometimes seems we spend a lot of time re-learning what we already know.
     
  9. RHough
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    RHough Retro Dude

    That would surely explain the news being full of reports of boats pitch poling every time they race through the Southern Ocean ... :D

    It would also explain the terrible reputation that Farr has for design of ocean going yachts. :p

    Perhaps your "real world" is different from mine?
     
  10. Guillermo
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    Guillermo Ingeniero Naval

  11. RHough
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    RHough Retro Dude

    I'm only taking that one shot. :D Some people clearly have different points of view. It makes the world an interesting place.

    Randy
     
  12. Guillermo
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    Guillermo Ingeniero Naval

    Absolutely! :)
     
  13. Tim B
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    Tim B Senior Member

    The plumb bow has become more common in recent years, and there is no doubt that it reduces wave drag. I would question whether the "axe" bow is much different.

    I wouldn't say that making the bow finer and the stern fuller would make the motions any more comfortable. In fact, I would suggest that full sterns actually make the boats rather uncomfortable in adverse conditions, and probably more prone to broaching.

    Plumb bows and full sterns have their place, but I would say it's on performance yachts which are going to be sailed fairly hard most of the time. It's certainly not advantageous for a cruising yacht, I think it would be particularly uncomfortable.

    Cheers,

    Tim B.
     
  14. jehardiman
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    jehardiman Senior Member

    As I have said before, there is NO single best hull form. While it is possible to optimize a hull form for your design point, you give up some things when you have to operate off design. Farr's wide transoms are a prime example. There are some conditions that they excel in, and others that they become complete dogs. Depending on what the wave conditions were where you first met a wide transom boat shades your opinion.
     

  15. Vega
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    Vega Senior Member

    I agree. All boats are compromises. But of course, we should discard all compromises that as not to do with the boat's performance, but only with being more competitive under a restricted set of rating rules .
    In that case what you get is not a boat that is optimized for sailing, but for best performance only under a determined set of rules.

    Unless the boat is designed to a very particular set of conditions, a boat should be designed as the best compromise to the most frequent sea conditions, not forgetting a particular emphasis on safety.

    From this general view point it seams to me that large transoms (not necessarily beamy boats) have come to stay. If you compare modern boats to 20 or 30 year old boats, larger transoms are a constant.
     
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