Semi displacement hull

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

  1. apex1

    apex1 Guest

    Well Crag, so completely wrong it was´nt! The industry misused the term quite often enough.
     
  2. dskira

    dskira Previous Member

    crag I realy don't understand what you said. You seams contradicting your former post.
    Embelish a simple concept? I don't get it. Since when these boat are wrong in the water. I am not talking naval architecture again, I designed (looks at my gallery the 50' motor boat) it is a semi displacement she goes to 20 knots and do not plane. I designed and built a lot of work boats, they are not automatically SD thanks God, the fuel bill will be a killer! so I know what I am talking about. The term is in my view wrong. I repeat, the term.
    Cheers
    Daniel
     
    Last edited: Nov 27, 2009
  3. Crag Cay
    Joined: May 2006
    Posts: 643
    Likes: 49, Points: 0, Legacy Rep: 607
    Location: UK

    Crag Cay Senior Member

    I have quoted the part I said was factually incorrect. Twice!

    The term was not invented by boat builders to excuse their poor designs. To suggest that is wrong. It may have been used on numerous occasions for this purpose but to say it was invented solely for this purpose is to dismiss all the real work that went into refining and exploring the semi-displacement hull form.

    If you read Douglas Phillips-Birt's Motor Yacht and Boat Design from the early 1950's, you will see his characterisation of 'fast power boats' only stretches from a speed/length ratio of 1.1 to 1.8. His dismisses faster boats as only being suitable for the calm waters of the USA and the Riviera. This association of true planing vessels with being un-seaworthy for UK waters persisted for years (still persists?) and lead to thousands of hours being spent refining the idea of semi-displacement hulls, predominantly for the work boat market. (This idea didn't only have supporters in the UK - West Palm Beach and Fernandina Beach pilots were both good clients).

    The RNLI are also locked into the search of this holy grail, unlike the Dutch who have adopted full planning for their boats.

    I myself am not convinced that the difference in rough weather performance is clear cut between the semi and full planing hulls. There are many other factors that seem to be more important and can be incorporated into hulls of both types. However there are thousands of Pilot boat skippers and Lifeboat coxswains running their semi displacement hulls in all weathers, that are convinced that their boats are the way to go, and are not using these boats because they were fooled by the marketing strategy by some poor boat builder. Fifty years of daily service in ports around the world would have rumbled these boats if they were nothing but an illusion.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 914, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Daniel

    The word "catamaran" is French Polynesian. It comes from katta-maram, which means tied wood.
     
  5. dskira

    dskira Previous Member

    Every day worth living since I learn something!
    Thank you, I didn't know the meaning of catamaran.
    Thanks again
    Cheers
    Daniel
     
  6. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    KC Barnaby's Basic Naval Architecture of 1954 did not mention semi-displacement or semi-planing. He did however refer to a "double wedge form" of high speed round-bottom hull.

    In 1934 George Crouch refers to "Round Bilge Wide Transom" hulls. In 1965 GN Hatch titles a paper "Fast Displacement Craft and the Transition Zone". Nither Du Cane (1964) or Lord (1963) refer to semi-anything. But about the same time Edwin Monk Sr. was calling some hulls "Semi-Planing Round Bottom".

    In the late 1970's Tom Fexas called them "Penetrating Hulls", these were of round bilge form but incorporating a number of lifting strakes. Tom also wrote that no hull was truly planing until water broke cleanly from the chine full length and the transom was completely dry, often a S/L of 4!

    My personal view is that a Semi-Planing hull is one designed to lift substantially, and it's probably hard chined aft for maximum lift. A Semi-Displacement hull is one designed to Penetrate....ie slide through the water and (at least) not sink as a full displacement hull does. Semi-displacement includes (In my mind) the Maine Lobster Boat form for example.

    A Semi-Planing Hull
    Douglasfircolour.jpg

    A Semi-Displacement Hull
    Whitepinecolour.jpg
     
  7. baeckmo
    Joined: Jun 2009
    Posts: 1,269
    Likes: 240, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Hey, gentlemen try to focus on the subject of the thread! The guy asked for knowledge and info on "semidisplacement hulls". The term is widely known, and technically correct. To be precise, the prefix "semi" per definition says that the boat is operated in such a way that its weight is carried 50 % by buoyancy, and 50 % by other means (normally, but not necessarily, by hydrodynamic lift), end of discussion!

    Most of us can associate to the type of vessel he is looking for, so why are you flipping his nose with a dozen of irrelevant notes on linguistics? The towing tank archives are full of reports on these hulls, and there are certainly numerical attempts galore, so come on with the real stuff and skip the "semibullshit"!
     
  8. dskira

    dskira Previous Member

    Thank you for your order my captain.
    And thank you for your sense of open discussion.
    Semicheers
    Daniel
     
  9. Willallison
    Joined: Oct 2001
    Posts: 3,590
    Likes: 130, Points: 0, Legacy Rep: 2369
    Location: Australia

    Willallison Senior Member

    You just pipped me to the post in regards to the appropriateness, or otherwise of the terminology. A displacement boat is held afloat entirely due to buoyancy. A planing boat as a result of dynamic lift. A semi-displacement, or semi-planing, boat is supported by a significant proportion of each.

    Now, as to the question itself. I fear I can be of little assistance beyond that which has already been given... sorry
     
  10. gonzo
    Joined: Aug 2002
    Posts: 15,246
    Likes: 949, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Any hull that moves through the water produces waves. That is caused by the pressure of the hull against the water. It is impossible to move a hull through the water without some component of dynamic lift. Also, it is not possible to have a hull plane and have no component of displacement.
     
  11. baeckmo
    Joined: Jun 2009
    Posts: 1,269
    Likes: 240, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    ....yes, and.......?

    That is common knowledge here, Gonzo! Still, I (and the thread creator?) am awaiting info on the vessel type that is operating with about half of its "lift" (or at least a substantial part thereof) from displacement buoyancy, and the remainder from hydrodynamic forces, ie semidisplacing. What are the important features of such hulls, where can we find knowledge? That is what this thread is about!

    BTW, it is completely possible to design a hull that is producing zero or negative hydrodynamic lift in the vertical plane, even if there will be dynamic forces acting on the hull as a result of movement. And what do you call a hull that has 50 % aerostatic lift assistance from a "lighter-than-air" vessel?
     
  12. gonzo
    Joined: Aug 2002
    Posts: 15,246
    Likes: 949, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    This is the first time you define what semi-displacement is for you. Some people define it as more than 1.8 hull speed but less than three. By your definition, a hydrofoil qualifies.
     
  13. baeckmo
    Joined: Jun 2009
    Posts: 1,269
    Likes: 240, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Formally yes, as long as the basic requirement is met; roughly half lift from buoyancy, the rest from a combination planing lift plus foil lift, which occurs at some speed before coming onto the foils! That is not to say that it is a good semidisplacement configuration as such, though.

    My point here is that a definition relating to wave system and Froude number is adding a restraint that may be misleading, because you can have a pure displacement hull working far into "planing territory", provided it is slender enough, just as you can have extra hydrodynamic lift assistance at lower speeds.

    But if you use the definition in the original sense of the phrase, we can analyse the two "lifting systems" regarding their individual optimum shapes, in terms of some performance parameter, and what possible trade-offs that may result in a combined optimum, that is better than the "flat" sum of the individuals. The problem with this analysis is that we are depending on different tools for the two (at least for the moment).

    The regression methods mostly used for displacement speeds fail when it comes to speeds where dynamic lift is coming into play, and the empirical algorithms for planing are of little use in the intermediate speed range. I have some hope, that numerical methods for resistance prediction (see Almeter f.i.), may be developed to a "working tool" level, substituting empirical algorithms like Savitsky's, but until then we only have towing experiments to lean on for the semidisplacement range. Unless Leo L or Ad Hoc or ??? has something up the sleeve right now, huh..?
     
  14. gonzo
    Joined: Aug 2002
    Posts: 15,246
    Likes: 949, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Which comes to the original discussion, what is semi-displacement? Baeckmo got kind of pissy about it, but unless it is first defined, we don't even know what we are discussing.
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 914, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Not sure why stating basic facts is seen as a pissing contest by others?

    One of my very good friends and I often debate this simple questions, what is a the definition of a semi-displacement hull. In simplistic terms:

    But beyond that is speculation, since other factors come into play which cannot be defined by a simple root x1.8 or whatever simplistic term one has been told to us or learnt from a book.

    Exactly, fully concur.

    Take the lines of the boat posted by Tad above:
    http://www.boatdesign.net/forums/powerboats/semi-displacement-hull-30169-2.html#post318948

    If i were to have seen these lines and asked for my opinion, i would have suggested that it may be, but depending upon several other factors may be not too. Since the waterlines, in plan view, are not conducive to what one would normally design to be "semi-planning"...but that is the point.

    Pushing a boat through the water or riding a bit on top, both can be achieved in totally different ways, with both operating at the same speed. Both shall not be compliant with any "formula" either.

    Generally looking at the boat and seeing how she runs too, one can say, ahhh yes....that is semi-planning, but as soon as you attempt to make it prescriptive, you've missed the point.

    The person i referred to above, having this endless debate, simply because it amuses us, has been referred to here already. My good friend Arthur.

    Finally, it was a long time ago, but i was given a "reason" for the evolution of a semi-displacement hull. Can't recall by whom now too...so long ago.

    I was told that Thornycroft was getting fed up slow boating across the Solent from the IoW to Portsmouth/Southampton commuting to work every day. The solent can be rough, short sharp chop at times, so a fully planning boat is out especially in the winter times. So he discounted this type of design. So he decided to make a "half and half", semi-displacement. Whether this is true, i have no idea....
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.