Hull speed

Discussion in 'Boat Design' started by HJS, May 9, 2010.

  1. HJS
    Joined: Nov 2008
    Posts: 372
    Likes: 69, Points: 28, Legacy Rep: 288
    Location: 59 45 51 N 019 02 15 E

    HJS Member

    In this forum often is written about hull-speed relative to waterline length.
    It often ends up in a confusing mess of arguments.

    Should we relate to something more relevant and yet easily accessible?

    When I did some investigations on two rowing boats that would go in the critical so-called "hull-speed", I found that the resistance curve changed drastically, if I had a transom. The larger width of the stern revised the longitudinal metacentric height very considerably.
    One advantage of studying the longitudinal metacentric height is that we also include the boat's shape above the waterline.

    Has anyone studied how the longitudinal metacentric height is proportional to the resistance in speeds between FnL 0.3 and 0.5?

    js
     
  2. BlueBell
    Joined: May 2017
    Posts: 1,787
    Likes: 504, Points: 113
    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Oddly interesting that this has garnered no response in 11 years and 1200 views.
     
  3. jehardiman
    Joined: Aug 2004
    Posts: 3,071
    Likes: 574, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    It is probably because correlation is not causality. There are many factors that can be calculated for any hull form that have very specific uses, but are meaningless when compared between hull forms. Thus it is naval "ART-ca-tecture".
     
    DogCavalry and fallguy like this.
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 7,127
    Likes: 1,070, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Interesting observation! :cool:

    Then in your research one assumes you discovered that there is no such thing as "hull speed" :oops: It is a misnomer used by non-NAs.

    You can read more about it HERE and HERE.

    As JEH notes:

     
  5. HJS
    Joined: Nov 2008
    Posts: 372
    Likes: 69, Points: 28, Legacy Rep: 288
    Location: 59 45 51 N 019 02 15 E

    HJS Member

    In the introductory post, I have used the wrong term by mistake.
    The longitudinal metacenter height shall rightly be the longship metacenter radius.
    That is a significant difference.
    Sorry
    JS
     
  6. gonzo
    Joined: Aug 2002
    Posts: 15,511
    Likes: 1,045, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I think that "hull speed" is used informally as the speed at which the power curve has an inflection point upwards. It has a practical application for design.
     
  7. HJS
    Joined: Nov 2008
    Posts: 372
    Likes: 69, Points: 28, Legacy Rep: 288
    Location: 59 45 51 N 019 02 15 E

    HJS Member

    What I am pointing out is that "hull speed" is normally related to the waterline length, which is misleading. I believe that longship's metacenter radius is a much better dimension.
    JS
     
  8. gonzo
    Joined: Aug 2002
    Posts: 15,511
    Likes: 1,045, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I think that hull shape has the most significant influence. For example, straight buttocks and a full stern can generate sufficient dynamic lift to get the vessel on planing mode. On the opposite extreme, a circular currauch will have the bow wave break over the bow and sink with increased speed.
     
  9. TANSL
    Joined: Sep 2011
    Posts: 6,811
    Likes: 499, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The power-speed curve is a parabola corrected by various factors and with various addends in the total equation. Therefore, for positive velocities there cannot be an inflection.
    Hull speed, to put it quickly, is nothing.
    It would be very frustrating if the hull shape did not have "significant influence".
     
  10. Ad Hoc
    Joined: Oct 2008
    Posts: 7,127
    Likes: 1,070, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Why?
     
  11. sharpii2
    Joined: May 2004
    Posts: 2,092
    Likes: 231, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Actually, IMHO, Displacement Length (D/L) ratio is just as effective and easier to calculate.

    Low D/L boats make less waves than high D/L boats do.

    And, because of this, they can typically out sail the traditional "Hull Speed", which is usually defined as: 1.34 * ((LWL ft)^0.5).

    It is easy to have two boats, which have the same Hull Length and Displacement, but have much different D/Ls. This is because it's the Waterline that is counted, not the Hull Length.

    Not only that, but the D/L is a cubic relationship, just like a longitudinal meta center is.

    A method I use to determine the D/L of a boat is: (28.6 * DISPL. cu) / ((WL lu*0.10)^3).

    cu = cubic units
    lu = linear units

    A D/L of 200 or less is considered low.
     
  12. clmanges
    Joined: Jul 2008
    Posts: 500
    Likes: 93, Points: 28, Legacy Rep: 32
    Location: Ohio

    clmanges Senior Member

    Do you mean that you're measuring the perimeter of the waterline instead of it's length?
     
  13. jehardiman
    Joined: Aug 2004
    Posts: 3,071
    Likes: 574, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    No, he's using the waterline length, but cubing it to get the same volume unit as displacement. This makes the factor dimensionless.
    Anyway, standard practice for all small craft hull forms and propulsor types was mostly settled back in 1970's with the 1971 publication (and 1977 revision) of Small Craft Engineering, Resistance, Propulsion and Sea Keeping, Pub No. 120 from UofM. While some more modern paper dive into the niches and specialty hull forms, this publication covers all the basics and even has a section on waterjet performance by Jacuzzi.
    Small craft engineering: resistance, propulsion, and sea keeping https://agris.fao.org/agris-search/search.do?recordID=XF2016073850
     
  14. Ad Hoc
    Joined: Oct 2008
    Posts: 7,127
    Likes: 1,070, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    If they are conventional hulls and not planing hulls, that is not correct.
    The datum is always the Lwl.

    The datum for comparison remains the same (one does not mix and match), which ever hull 'type' one is using. Ergo, same displacement and same "length" = same LD ratio.
     

  15. jehardiman
    Joined: Aug 2004
    Posts: 3,071
    Likes: 574, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Sharpii2, Ad Hoc;
    OK, let us set some rules here. Ad Hoc is exactly correct here...IF...(big if) you use standard convention of D/L (i.e. long tons/(0.01 *LWL)^3...it is actually dimensionless for standard rho but shifted). Same length on waterline and same displacement means same L/D of D/L ratio. However, you use a non-conventional formula for your D/L ... neither better nor worse... assumes some "typical" hull shape. This is a problem for "fitted" data... much like Taylor's Circle "C" or the Admiralty Coefficient; they lose meaning the further they get from the "standard" data set hull form.
    So two very different hull forms could have the same L/D of D/L ratios. What can be different though, is the shape of the Sectional Area Curve. However even this is not a good indicator of hull form, as identical Sectional Area Curves could have very different hull shapes (i.e. a plank on edge yacht and a skimming dish, that displace the same, could have identical L/D ratios and sectional area curves). This goes back to the infamous "it all depends..." statement. Dimensionless ratios and curves of form need to be evaluated against the profile, bow lines, and stern run to have any meaningful significance.
     
    bajansailor and Ad Hoc like this.
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.