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

    That way you can have your cake and also the means with which to eat it.

    But to serious matters...
    I'm very interested in your work with small hulls. Are any of the results available in publications or elsewhere?

    Leo.
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 207, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Leo I'll PM you, otherwise we'll take over Eric's thread.
     
  3. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hello Eric,

    Appologies for the absence (here but not all there like the English say...:D)

    Da Meterical stuf.... Yuk, I was under the impression we were working in metrix, and here you guys bugger around in inchies and galliengs again. You can imagine how dissappointing it was when Richard halved my boats performance and it was going to take me double the time when I flee to .AU :(


    Ok, changing the CP to 0.7 was a most interesting exercise. I sat down today and tried to see if I can improve the CP of my hull from 0.61 to 0.7. While at 0.61 was nicely shaped before with a rocker to the bow and to the stern. The bow rocker ended just below the draft water line, the stern rocker ended obove the waterline (of course).

    I got to 0.7 fairly easy. What was interesting is that the bow area is now less sharp, the foreward ~ third of the hull widens somewhat quicker and the hull is flat there, not rounded.

    The area aft of the foreward third seems to favour higher CP if it tapers down slightly to aft. I haven't done much in this area.

    Also the foreward rocker draft is now more, at around 400mm of the total of 500mm draft (it was at ~100mm before). This won't be much of a problem since the bow has less buoyancy than the stern. I haven't calculated the added stern buoyancy, but it probably won't be very much. As it is now it defenately pierce the water and will not attempt to go on the plane like before.

    Little problem however.

    It seems that Freeship does not support boats with a CP of more than around 0.6 :D My resistance graph is not working any more, unless it's now off the chart :D Fat chance.

    How can I check the hull to see if what I have altered is any kind of gain ?


    The Longitudinal center of buoyancy 5.164 [m] and the length of submerged body is 9.903 (LOA = 10m)
     

    Attached Files:

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

    Fanie Fanie

    I have another question wrt power on a displacement hull.

    The hull speed has now in theory moved up from 7kn to 11kn. If propulsion is done with ie an outboard, then there will be a hump at 11kn, up to which accelleration was fairly easy.

    What happens beyond the hull speed ? Double the power doubles the speed or has the hull a different power graph slope after the 11kn hull speed ?
     
  5. capt vimes
    Joined: Apr 2009
    Posts: 388
    Likes: 14, Points: 18, Legacy Rep: 247
    Location: Austria

    capt vimes Senior Member

    fanie

    if freeship is not calculating any resistance curves - go to the results tab and you will find a line there:
    "Cp is outside valid domain of 0.48 .. 0.64"

    i was a little puzzled when you mentioned a CP of 6.1... if you mean the prismatic coefficient - the highest value there is, is 1 and only a completely plane log would have that... ;)
     
  6. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,004
    Likes: 209, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    I resent that statement.

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

    Eric Sponberg Senior Member

    First, Capt. Vimes is correct, your Cp numbers are off by a factor of 10, having misplaced the decimal point. Cp by definition cannot be more than 1.0.

    As for determining what the power requirements are, you will have to do more study in powering requirements (it is a complex topic), compare similar designs, get empirical data from other full-scale designs, or go to the model tank. Typically, as the length-to-beam ratio (L/B) goes up, the size of the hump in the resistance curve diminishes. And after hull speed, the power required generally follows a cubic curve; that is, the power is proportional to the cube of the vessel speed. So if it takes you 100 hp to reach a speed of say, 15 knots (to pick some numbers out of the air) then to reach double that speed at 30 knots, the power required will be something on the order of 100*(30/15)^3 = 800 HP. That's putting it in simplistic terms, but you get the idea.

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

    Fanie Fanie

    The factor was corrected, I saw too late, sorry.

    So that is the drawback a displacement hull has, it remains bound by the fact that it's a displacement hull, the advantage being up to hull speed it is relative economic.
     
  9. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Any one want to offer an oppinion on the hull pictures I uploaded ?
     
  10. apex1

    apex1 Guest

    Better not.:D :D
     
  11. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Oh BS !

    If you see anything obvious that needs attention then say so.

    Or would you rather I make a mistake there ?

    I'll just blame you if you don't point it out :D
     
  12. apex1

    apex1 Guest

    I should have just added this :p or this::D I was joking:) :rolleyes: :cool:
     
  13. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    You'll get over it.

    Naval architects obviously need to have a wide range of engineering knowledge and how to apply them, but they can't all be experts on (among many other specialities), rheology, tribology, vibrations, fluid dynamics, etc etc
    I.e. the steak knives, cherry-pitters and oyster forks in the same drawer :p
     
  14. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 247, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    How about seing the NA as a cook who uses the above-mentioned tools for preparing a nice meal? :)
     

  15. TeddyDiver
    Joined: Dec 2007
    Posts: 2,579
    Likes: 123, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    for a group of diners having lactose intolerance, diabetes, coeliac and veggie :D
     
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.