center of flotation calculation and implications?

Discussion in 'Boat Design' started by capt vimes, Jan 7, 2010.

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

    Leo Lazauskas Senior Member

    I totally agree with you regarding the longitudinal location of the centroid, at least for small yaw angles. However, the same argument does not necessarily apply to the *vertical* location of the centroid and its effect on the attitude of a boat at speed.

    In the calculation of the trim, there is a term involving Zw*Rf, where Zw is the vertical location of the centroid of wetted area, and Rf is the skin-friction.

    Now, Rf is clearly important, otherwise you would not be concerned with minimising wetted area to reduce it. That is indisputable.

    My argument is that if Zw is not small, then the moment, Zw*Rf, is also not small. I am happy to concede that in many cases Zw will indeed be small, but I want to see an actual calculation to be completely convinced. Hand-waving arguments and guesswork are not always reliable. OTOH, unnecessary calculations are a stupid waste of time :)

    There are also unusual cases (e.g. SWATH and SLICE) where Zc could be quite large, but the moment Zc*Rf is small compared to other terms in the trim and sinkage equations.

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

    Leo Lazauskas Senior Member

    Then I have achieved my purpose :)
     
  3. fredschmidt
    Joined: Jan 2010
    Posts: 152
    Likes: 4, Points: 18, Legacy Rep: 73
    Location: Natal - Brasil

    fredschmidt Naval Architect

    Leo

    Undoubtedly......doubt.

    Fred
     
  4. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 147, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    In the case where the hull is heeled so the wetted surface is asymmetric, the asymmetric drag would result from the effect of lateral flow especially across chines.
     
  5. Brent Swain
    Joined: Mar 2002
    Posts: 951
    Likes: 35, Points: 0, Legacy Rep: -12
    Location: British Columbia

    Brent Swain Member

    The biggest drawback of assymetric hulls is not wetted surface , but changes in centre of buoyancy when heeled , which leads to loss of directional stability.
     
  6. TeddyDiver
    Joined: Dec 2007
    Posts: 2,579
    Likes: 123, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    No Brent.. The center of buoyancy stays where the CG (and the vertical vectors of the heeling forces) make it to be.. It's the position of the hull and alignment of the wetted form compared to sail and keel/rudder alignment that changes. If that change has an effect to directional stability is another matter but not allways negative.. thou reckon you meant helm balance but that's not so simple either :)
     
  7. capt vimes
    Joined: Apr 2009
    Posts: 388
    Likes: 14, Points: 18, Legacy Rep: 247
    Location: Austria

    capt vimes Senior Member

    trim angle?

    as explained so well by eric - if a boat heels the CB, CF tends to move aft (the wider the stern the more they do so) - causing the hull to trim...

    what would be an 'acceptable' trim for a ship at - lets say - 30° of heel?

    considering the vessel is designed as an 'all arounder' and should have good compromise between down- and up-wind performance...
    i have no doubt that 10° would be way too excessive but i also have no clue what would be 'in range'...
     
  8. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,004
    Likes: 209, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Ideally, trim should be near to zero no matter what as the boat heels. In particularly, trim down by the bow, is undesirable. But that is the ideal and we do not live in an ideal world. As stated above in a prior post, some trim down by the stern is OK, which, on a sailing boat, tends to lift the hull and point the keel to windward. In modern designs that does not happen too much. In a powerboat, you want some trim down by the stern to help start the boat planing, and then once the boat is on plane, you don't want trim to be very much, maybe 2-3 degrees by the stern. Hydrodynamic drag is directly proportional to trim--if trim doubles from 2 degrees to 4 degrees, drag doubles, at least according to most hydrodynamic calculations.

    In sailing yacht design, I typically check my heeled waterline at 15 degrees heel without correction for displacement and trim to see where the CF moves to momentarily--it gives me a snapshot of what will happen to trim before the hull settles into a steady state of flotation. And 15 degrees heel is a normal and comfortable heel angle.

    Eric
     
  9. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,004
    Likes: 209, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Up-coming notes:

    To the class,

    The plan for the following few weeks will go as follows:
    • Speed/Length ratio and A/B ratio (these are not related, but they are both relatively short topics so that I can get them out of the way, in preparation for a more detailed discussion on:
    • Displacement-Length ratio, then:
    • Sail Area-Displacement ratio and Sail Area-Wetted Surface ratio, which will then lead us to a surprise topic....
    • The S number--a neat, different, and very sensible way to rate boat performance.
    • The Multihull Ratios: L/B ratio and the Bruce Number.

    Stay tuned!

    Eric
     
    1 person likes this.
  10. apex1

    apex1 Guest

    Nice thread!

    Danke Herr Kollege.

    Thanks Eric for taking the time!

    Regards
    Richard
     
  11. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hi Erich,

    The cat I designed has a Prismatic coefficient of 0.6103, LWL is 9.889m and the WWL each hull is 0.892m.

    You stated that catamarans and trimarans (which have long narrow hulls) have very high Cp ratios. Is 0.6 in this case low, average or high ?

    This is where the caper resistance calculation is about as low as I get it, it is around 210N at 7 kn.
    Displacement is at 2000kg each hull.
     
  12. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,004
    Likes: 209, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member


    Fanie,

    I don't know what you mean by "caper resistance calculation"--that is an unfamiliar term.

    As you probably know, there are three major components of resistance for a boat traveling through the water: Frictional resistance, Form resistance, and wave-making resistance. At slow speeds, below hull speed (speed-length ratio V/Lwl^0.5 <= 1.34, or Froude Number V/(g*Lwl)^0.5<= 0.4) Frictional and Form resistances make up the larger part of the total. Above this limit, Wave-making resistance overshadows frictional resistance and becomes the major component--in most conventional designs. It's possible to break through that 1.34*Lwl^0.5 speed barrier if the hull is light, long, and narrow. Such hull shapes have been shown to achieve speed/length ratios greater than 2.0 without planing--that is, still in displacement mode. Such hulls have very small waving making resistance, and part and parcel to that is that they also have a Cp of about 0.70. This is a limit--generally to achieve even higher speeds, Cp does not have to go any higher--it levels off. All you need to go faster is to add more power.

    So, in your case, it depends on how fast your boat is intended to go. At Cp = 0.6103, your target speed is at or just below hull speed. If you want to go faster, your Cp should be larger, according to the chart I posted earlier.

    Eric
     
  13. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hi Eric,

    The caper resistance is a chart in Freeship that indicates I guess how easy your boat would move and displace water, so frictional and form resistance.

    I doubt any one designs a hull to be slow so if you want to ask how fast the boat is intended to go, when the pirates appear it has to have unlimited speed :D
    If the fish bites it must remain right where it is :rolleyes:

    I'm trying to visualise what the interaction of the water with the boat would be when the CP changes, say from 0.6 to 0.7
    Does this prolong the following wave making when a certain speed is achieved so a higher speed can be achieved before wave making becomes more than the friction and resistance
    or
    does it merely improves the friction and form resistance ?
     
  14. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Well, how about that, I actually answered myself there :eek:
     

  15. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,004
    Likes: 209, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Form and friction resistance go up with the square of speed, but they stay pretty much the same proportion to each other. Wave-making resistance is inhibited with longer narrower hullforms. We see this all the time in vessel design--long narrow hullforms have lower overall resistance because of lower wave-making resistance. The flow interactions that play with changes in prismatic coefficient--the ends of the hullform are fuller, have more volume--are very difficult to visualize outside of a model tank, or outside of a full-scale side-by-side comparison of differing hullforms.

    Eric
     
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.