Hull Rocker

Discussion in 'Multihulls' started by basil, Feb 12, 2013.

  1. basil
    Joined: Apr 2005
    Posts: 154
    Likes: 8, Points: 18, Legacy Rep: 55
    Location: aUSTRALIA

    basil Senior Member

    G'day Guys,

    I've a question that relates to both multi hull's and mono hull's. It has to do with hull rocker. I'm talking about fast/planing mono's and fast multi's here.

    Why does maximum hull rocker in a multi hull usually carry on a long way past the centre board before kicking up to the water line?
    And why does a mono hull's rocker starts heading towards the stern water line as soon as it passes the keel?

    I'm aware this is not always the case but generally it seem the way it is.
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,557
    Likes: 684, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There are a couple of reasons. One is that the stern rises on a monohull to allow clearance for the propeller. The other is to get a low rating under some racing rules.
     
  3. basil
    Joined: Apr 2005
    Posts: 154
    Likes: 8, Points: 18, Legacy Rep: 55
    Location: aUSTRALIA

    basil Senior Member

    Sorry - forgot to mention I'm talking about sailing here not motoring
    Bas.
     
  4. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Another reason might be to allow the boundary layer to expand a little more
    gently. A short run might induce separation a bit earlier in the adverse pressure
    gradient. Maybe, and I'm guessing a bit, it is a little like the underside of a car:
    it is poor design to have it perfectly flat. It's better to allow a gentler exit
    geometry, a bit like an inverted airfoil surface. How much difference that makes
    to the total drag of a yacht is debatable. Maybe DCockey can contribute here.
     
  5. John Perry
    Joined: Nov 2003
    Posts: 280
    Likes: 30, Points: 28, Legacy Rep: 129
    Location: UK

    John Perry Senior Member

    I can think of a couple more possible reasons.

    One is that there is a trend to move the main accommodation and payload weight in cruising multihulls further and further aft. This may be partly a styling trend, I think people like the look of long light bows, but it probably does also help to avoid pitch poling and lee bow burying. If the main weight of the accommodation moves aft, the designer has no choice but to increase bouyancy in the aft part of the hulls and this tends to mean extending a deep keel line well aft then turning the keel line steeply upwards to avoid having immersed transoms. I think that some of Richard Woods' designs show this trend quite clearly. On the other hand, with a monohull, the weight of the accomodation tends to be more evenly spread along the hull and the ballast keel is often the heaviest single item and this is likely to be about amidships or forward of amidships.

    I think another reason is to do with the beam to depth ratio of the immersed part of the hulls, this is normally lower for multihulls than for monohulls. That means, in broad terms, that, by comparison with monohulls, rather more water tends to flow round the sides of a multihull hull rather than underneath, this in turn means that a steeply rising keel line in the aft sections is less likely to cause a problem with boundary layer thickening and flow separation than it would be with a typical monohull hull form.
     
  6. Richard Woods
    Joined: Jun 2006
    Posts: 2,207
    Likes: 162, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    The major thing to bear in mind is that a hull is a 3D shape, not 2D. So you cannot take any one line in isolation, you have to look at the shape as a whole

    So if you look at a deep V hull, for example, it will have a very different rocker shape than a round bilge hull. Similarly canoe and transom sterns have different keel lines

    Then the others are right. Most people add weight aft, not forward, so you have to have more buoyancy aft to compensate. Monohulls tend to have wide aft sections to get in the double berth(s) owners now want. So the rocker can be less because the boat is wider. Obviously a fast monohull will try to plane and that needs a wide flat run aft

    On offshore boats I prefer to have Veed forward sections, rather than round bilge, to help reduce slamming. That means there is a deeper forefoot on offshore boats.

    Boat design is naturally a complicated subject, you could probably have a whole forum dedicated to it

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  7. Sand crab
    Joined: Feb 2011
    Posts: 92
    Likes: 3, Points: 0, Legacy Rep: 23
    Location: Montana

    Sand crab Junior Member

    Ha. Good one.
     
  8. cavalier mk2
    Joined: Mar 2010
    Posts: 2,121
    Likes: 54, Points: 48, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    The Farrier tris also turn up farther aft, he found that the suction with the curve helps keep the bows up as the stern is pulled down letting more sail power be used.
     
  9. Manfred.pech
    Joined: Apr 2010
    Posts: 596
    Likes: 81, Points: 28, Legacy Rep: 319
    Location: EU

    Manfred.pech Senior Member

    That`s expelling the devil with Beelzebub or Satan.
     
  10. DCockey
    Joined: Oct 2009
    Posts: 4,734
    Likes: 326, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    I don't think an analogy with the flow under a car is particularly useful in understanding boat bottom hydrodynamics. Major difference between a boat and a car is the car operates in relatively close proximity to the ground, the car has wheels and tires which reach down to the ground and are surrounded by open wheel houses (unless an open wheel car), and most cars do not have smooth bottoms. Also, "diffuser" ramps at the rear of a car underside are typically intended to produce down-force or at least provide the appearance of being designed to produce down-force rather than reduce drag.
     
  11. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Thanks for clearing that up David. I knew I was stretching the analogy way
    past breaking point by only considering deep water and not finite depth or
    (especially) small gap flows.

    I agree that the diffuser-like ramp geometry is sometimes there just for
    how, but there are also some other aerodynamic benefits, for example by
    reducing the strength of trailing vortices. See, for example,
    Tuck, E.O. "Irrotational flow past bodies close to a plane surface",
    J. Fluid Mech., 1971, pp. 481-491.
    Potthoff, J. "Air resistance and lift of modern vehicles",
    1st Symposium on Road Vehicle Aerodynamics, 1969.
    I think Sheila Widnall's group also produced several papers on the subject too.

    I suspect you have much more recent work on that topic! :)
     
  12. Richard Woods
    Joined: Jun 2006
    Posts: 2,207
    Likes: 162, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    Maybe relevant to catamaran bridgedeck design though?

    Richard Woods
     
  13. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    It might be, but it would probably be better to account for the waves between
    the hulls. Tuck wrote a few papers that would be helpful with that problem and
    I know he was working on similar problems between about 1980 and 1990:
    http://www.maths.adelaide.edu.au/ernie.tuck/
     
  14. Richard Woods
    Joined: Jun 2006
    Posts: 2,207
    Likes: 162, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    yes of course wave interaction and reducing the wave slamming are the priorities, but as you said, it wasn't until car designers really investigated the airflow under a car that they realized how important it was

    I remember when I was with Derek Kelsall in the late 1970's we did discuss air flow under the bridgedecks and did some rough calculations, but then moved onto other things

    Richard Woods
     

  15. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Can you remember the rough estimate of the proportion of total drag that the
    deck would contribute? 1%, 5% 10%?
     
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.