Semi displacement hull

Discussion in 'Powerboats' started by Steph357, Nov 15, 2009.

  1. dskira

    dskira Previous Member

    I think Peter Thornycroft knew very well the work of Cooper, which was essentialy in high speed planning hull in hard chine and round bilge, but he was also very well award of the quality of the round bilge hull, designing a lot of small speedy military craft for recovery and other task.
    And of course every body at the time knew the precursor, Tamahine, from Laurent Giles, also they knew the work of Saunder, Du Cane, Uffa Fox with Black Maria and other. It was a very numerous bunch of great designer who was interrested in different state of speed, depending the use of the craft. All with beefy contrct from the Royal Navy. So yes it is possible that Peter Thornycroft designed Nigella in 1959 mixing all these knoweldges. She became the Nelson Class.
    I don't think a design come alone by itself, I think it is the result of a knoweldge of predecessors, and with this knoweldge a personal design is created depending of the circonstances in which the boat as to sail (motor)
    Personaly reding this thread I mellow, and I start to understand better why the appelation semi-displacement. Still bizarre, but I read great explanations.
    Just before Uffa Fox death, I went to Paris were he resided half the year, been maried with a French woman and then to Cowes and spend a lot of time with him. He talk at length of planning hull (Avenger of course) and other speed boat saail and motor. His great passion (beside singing!).
    I was with my fiancee. She feel in love with the great man! He told us all the story of the other designer, his great involvement with Saunder Row, the design of is own high speed motor boat, and also his joy of rowing!!
    Coweslip was hanging on her crane in front of his home.
    And the splendide enormous all brass mechanical integrator.
    The story of the thickness of the fin of the FlyFifteen, and coutless other stories. It was the Summer of my life.
    Cheers
    Daniel
     
  2. HJS
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    HJS Member

    Displacement and/or planing

    The attached illustrations may help to understand the relationship between displacement and planing hulls. The graph is created by Marine Director Börje Huss. Together with Hadlers graph from series 62, 65A, and 65B you may get a good picture of a boat's properties to begin with.

    Then of course there are a lot of other parameters to get an accurate picture of the boat's characteristics, such as center of gravity, bottom width, deadrise, entry angle, prismatic coefficient,………..

    Good luck.


    hjs

    www.sassdesign.net
     

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  3. HJS
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    HJS Member

    cattumaran

    The word "Cattumaran" is tamil. It is a common "raft" in Tamil Nadu, India.

    hjs
     

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  4. Ad Hoc
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    Ad Hoc Naval Architect

    HJS

    Those are great pic's..they show the "technology" very well.

    I do find it strange that, as you correctly say, the word is Tamil (I personally thought from Sri Lanka, not India), yet always associated with the pacific Polynesian islands.

    Since good old Capt. Cook et al when arrived in that part of the pacific was shocked at the speed of the proa's and the other similar two hulls boats, the generic term used for those boats with two hulls in the Polynesian islands, was catamaran. Did Cook use the Tamil word, or were the Polynesians using the same word, or even give the word to the Tamils (they did travel all over the pacific, why not west?).

    Was it just that those in India/Sri Lanka were just not fast and so no one really took notice until Cook arrived in the Pacific?

    Yet, strange that the origins being in "India" for the word (the method of construction -tied wood) but the commonly known reference of two hulls being used, is always Polynesian. (Well has been that I know).

    I've never been able to fully establish which came first and where and why, you??

    Sorry slight digression from semi-planning hulls.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Yup, let's deviate for a while; interesting how the term has migrated. Friend of mine, NA who worked for UN in Bangladesh and India also said the "Katu maram" was a standard phrase for the vessels Jürgen refers to. But what is the connection to Polynesia, and how did it occur??
     
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  6. Willallison
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    Willallison Senior Member

    You are a lucky man, Daniel... a lucky man...
     
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  7. dskira

    dskira Previous Member

    Yes I know, and I never forget it. :)
     
  8. tom28571
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    tom28571 Senior Member

    If the focus of this thread is to promote discussion and the spread of knowledge about hull characteristics and their effect on performance, then it is proving very useful. If it is to generate definitive numerical formulas to describe the action of hulls in the transition range, it must fail, because no one has ever satisfactorily defined when a boat is considered planing in the first place. There are lots of individually preferred ratios, etc., but not everyone or even the "experts" will agree with them.

    Tad offered lines of two boats and called one semi-planing and the other semi-displacement. He must have been smiling when he did that since it added yet another layer of confusion. If these are not one and the same, all bets are off.

    If we take a hull designed for full planing with completely parallel aft buttock lines and start to drive it forward, it will immediately experience some small dynamic force. By the time the speed reaches "hull speed", significant lift is exerted on the hull. As it moves faster, more and more dynamic lift appears. Unless it goes fast enough to become airborne, some of the hull will always be immersed and buoyancy will be part of the force holding the boat up. I believe the first hulls that can be called semi-planing were developed by William Hand. The work of a continuum of other designers and builders resulted in the range of hulls we have today.

    It is quite possible to have a hull that develops a high level of dynamic lift and is considered to be planing before it reaches what would be normally called hull speed. Long and light hulls do that with ease. Then there is the slender multis which throw another wrench in the works.

    My point is that these terms mean nothing without reference to hulls within a narrow range of characteristics, Therefore general definitions are impossible. They do lead to much argument from varying viewpoints though.
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    How about German WW2 "E-boat"?

    About 130' long, 30' wide and able to hit 40 knots....but said to be 'displacement' and leaving far less white foam trail than a PT boat, and able to operate at fairly high speed in seas that a planing hull like a PT boat couldn't?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Precisely!...since there is plenty of documented evidence of "multihulls" being used for centuries by Polynesian islands, long before Cook. Some even suggest also long ago as 2000 years. Whether millennium or centuries, how come the "word" used for each is the same, yet the "boat's" origins are many thousands of miles/kilometers apart...and many attribute the usage of the boat, not the word, to the Polynesians.

    I've not come across any evidence that pins this down...it is intriguing.
     
  11. Willallison
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    Willallison Senior Member

    You know the Polynesians invented the wave-piercer too....;)
     
  12. Wayne Grabow
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    Wayne Grabow Senior Member

    Good question and, by now, you have received a variety of expert answers. I (an amateur) have been interested in the same question. Some of the best answers I have found come from looking at plans, descriptions, and museum exhibits of turn-of-the-century "high speed" powerboats, say about 1900 to 1925. Back then, the power-to-weight ratio of most engines was not favorable enough to permit true planing. By necessity, designers were optimizing hulls to perform in the semi-displacement or semi-planing performance range. Some characteristics I noticed in such hulls were slight rocker in the keel, variable deadrise continuously decreasing aft, a transom which was only slightly immersed at rest, a pronounced immersed forefoot (usually a plumb bow), and a B/L ratio significantly higher than today. Even within this group, characteristics varied depending on whether the displacement or planing part of the performance spectrum was favored. The listed characteristics may only illustrate the limitations of naval architecture research then or fashion trends of that period, but I am sure that much practical knowledge was also included. Some of the stated speeds attained with moderate power are remarkable, if true.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Will

    Yes, i am particular impressed with the way the heave and particularly the vertical accelerations are dramatically reduced by the amazing thin slender revolutionary wave piercing hull. Well isn't that what everyone "thinks" and "feels" so, it is surely true :p

    (this should really be on your thread...sorry)
     

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  14. Tad
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    Tad Boat Designer

    I disagree Tom.......While Hand is generally acknowledged as inventor of the vee bottom, that's also (IMO) a bit of hyperbole. He did take a form that already existed (the Chesapeake deadrise hull), lifted the chine forward and dropped the fairbody, to create a better handling boat.

    As to developing semi-planing hulls.......that's a bit like claiming Columbus "discovered" North America. Long, lean, round-bottom powerboats were running at above "displacement" speeds before Hand came along.

    What I was trying to get across in various posts above was that we're dealing with physical constants, it matters not what we call them.....which is all marketing. I've heard hulls called Penetrating, High-speed Displacement, Double Wedge, Semi-Planing, Semi-Displacement...etc. Unless one adopts and defines the precise criteria, it's all meaningless.
     

  15. tom28571
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    tom28571 Senior Member

    Tad,

    You may well be right about that. I don't know enough of history to argue that. I'm not sure the long lean powerboats should be part of the same discussion though. I was thinking more of semi-planing as referring to boats of more "normal" L/B of 3.
     
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