Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

  1. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I know about such tests for sailboat hulls/appendages, also for double body barges and pontoons; unfortunately those results are not with me now. Some are published in Russian also :)
     
  2. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    To be fair, average outboard with high RPM and small prop is very inefficient on displacement craft or planing craft at displacement speed. Take heavy LWL=10m boat; 40HP outboard. Efficiency with optimum prop (given high RPM) is 30%, optimum prop diameter is 320mm. In reality, 11 1/2" prop is only available for Suzuki 40HP, so efficiency will be even less.

    For sailboat with 40HP inboard, two-blade prop diameter is likely to be 4...4.5% of LWL, so it is 400...450mm. Given better PPM at shaft, the efficiency will be higher compared with outboard - 40% and it will grow to 50% for 0.62m propeller (if it is enough torque). Of course, folding props on sailboats are not so efficient, but...

    So reverse comparison of resistance using power should be done very carefully.
     
  3. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    ""Quote:
    Originally Posted by Perm Stress View Post
    Boats of similar length, and displacement:
    a) yacht, with ~3-4kW/t and folding prop (inefficient by definition, figures as low as 25% are published) do the same or even more speed as
    b) planing-shaped boat with ~8-9kW/t and real propeller (efficiency not less than 40%).
    It should have something to do with really big difference (certainly more than ~20-30%) in resistance. Or am I still missing something?""

    "To be fair, average outboard with high RPM and small prop is very inefficient on displacement craft or planing craft at displacement speed. Take heavy LWL=10m boat; 40HP outboard. Efficiency with optimum prop (given high RPM) is 30%, optimum prop diameter is 320mm. In reality, 11 1/2" prop is only available for Suzuki 40HP, so efficiency will be even less.

    For sailboat with 40HP inboard, two-blade prop diameter is likely to be 4...4.5% of LWL, so it is 400...450mm. Given better PPM at shaft, the efficiency will be higher compared with outboard - 40% and it will grow to 50% for 0.62m propeller (if it is enough torque). Of course, folding props on sailboats are not so efficient, but...

    So reverse comparison of resistance using power should be done very carefully."
    ___________________________________
    Please note, that here I talk about wrong application of hull form for wrong speed. Not about underpowered planing boat with high speed outboard engines.


    In this case, she (motor boat) has quite normal, heavy inboard diesel with max RPM ~3000 or so. And a real ~0.6m diameter, ~80% blade area three-bladed propeller. So propeller efficiency is unlikely to be below 40-50%. Probably more.
    Please note, she is not a planing boat as such. She is a displacement boat with planing boat hullshape. And she is not alone. I know several other similar boats. Transom is almost equal in submerged area with midship (by the way, THIS is what I mean under "planing hull at displacement speed" here; if transom is less than say 40-50% of midship, it is closer to semi-planing shape). Bow area (which in planing mode would be above the water :) ) is very bluff, WL entrance ~45 degrees, no less.
    While sailing boat has folding prop.

    So, for the first approximation, (making BIG error in your favor), propelling efficiency could be considered as roughly equal.

    Than, 2-3 times power for same speed mean similar differences in resistance. If we do account that folding (or feathering with flat blades) is most likely to have less efficiency, resistance difference will be somewhat more.


    ________________

    I feel it is necessary to repeat:
    when comparing good displacement hull with pure planing hull (transom at rest has same or greater area as midship), at FnL ~0.3, differences in resistance are very likely to be in the order of 2 or sometimes 3.
    However, the greater L/B and L/(D^1/3), the less this difference will be.

    ____________________
    With further consideration, such dramatic resistance increase is much more likely to happen for monohulls, than for catamarans. For simple reason, that cat hulls are much more slender, while for slender hulls, resistance differences (in conditions, as described above) tend to be smaller.
    And differences will be still smaller, as soon, as hull speed (FnL=0.4) is reached and exceeded.
    ___________________________________________________

    ""Trend lines show difference ~1.75, still in the order of 2. close to what I did post at very start of discussion.""
    "But I said is below 2, so who is right?"
    You said 20-30%, so who is closer ? :)

    Quote:
    ""Drop out points, although always suspect, (and I understand it perfectly well, thank you) can indicate possible extremes. In general, no one ever is 100% sure he will NOT get accidentally taken to extreme of this kind by some trivial reason.""
    "Why do You reference extreme points then???"
    Because extreme points still fit "up to X ...." definition well enough? :)
     
  4. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Yes, agree, for inboard option.

    Maybe You are talking about very unsuccessful or badly overweight design. On our planing designs, we never have 45degrees of WL at bow. I believe for 'brick shaped' pontoon situation would be even worse :)

    If You are discussing particular boats, pls post particulars and calculations for those boats.
     
  5. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    No, because for those points other parameters are not considered; they are unlikely to be same.

    Have You found Robinson's paper? I can email if You need.
     
  6. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    It would be helpfull, thank you.
     
  7. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I said that? Pls refer to particular post :)
     
  8. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    "Maybe You are talking about very unsuccessful or badly overweight design. On our planing designs, we never have 45degrees of WL at bow. I believe for 'brick shaped' pontoon situation would be even worse "

    Yes, it is very unsuccessful design. And badly overweight. And it all-too-easy to build overweight: cheaper but less strong materials, so you need more of them, heavy instead of lightweight interior (cheap v expensive) ... ..10-15cm lower, WL entrance is closer to 30 degrees.
     
  9. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Need You email if so...
     
  10. DCockey
    Joined: Oct 2009
    Posts: 5,091
    Likes: 556, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    The April/May 2010 issue of Professional BoatBuilder has a polite letters from Alik and Gvidas Misiunas questioning the inclusion in the interview with Donald Blount in Professional BoatBuilder No. 128 of Series 62 data starting at FnL of 0.25 rather than starting at higher speeds. Alik in his letter asks whether the data at lower speeds is reliable when it was not included in the series 62 data in PNA. Strong doubts about the reliability of the Series 62 low speed data from the interview article were expressed earlier in this thread.

    Donald Blount provided a fairly lengthy response to the letters. The models were tested from low speeds, and the test data in tabulated form and graphs was published in the original paper in SNAME Transactions in 1963. His response includes graphs of low speed resistance to weight ratio for three displacements of Series 62 hull with a length-to-beam ratio of 3.06 for three displacements. Blount estimates the accuracy of the Series 62 data as "about +/- 2% at planing speeds and about +/- 6% at displacement/hull speed". He believes the Reynolds Number for the tests was sufficiently high based on plots of the residual resistance coefficients vs FnV.

    Blount mentions that "Keuning et al. have published similar data in International Shipbuilding Progress in 1982 for 25 deg deadrise hulls, and 30 deg hulls in 1993." He also mentions the 2009 paper by Blount and McGrath "Resistance Characteristics of Semi-Displacement Mega Yacht Hull Forms" and includes an unpublished plot of resistance at FnL of 0.4 for the three hull forms considered in that paper.

    While PBB may not be a technical journal it occasionally has good technical information and discussion.
     
  11. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    The only issue is: I never wrote any letter to PBB on this matter...
     
  12. DCockey
    Joined: Oct 2009
    Posts: 5,091
    Likes: 556, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    There is a letter published signed by:
    Albert Nazarov
    Naval Architect, PhD, MRINA, MSNAME
    Albatross Marine Design Co., Ltd
    Nongprue, Banglamung, Chonburi, Thailand


    I thought that was you. It's a very well written letter.
     
  13. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Yes, but that was letter direct to Donald, not to PBB... :)
     
  14. DCockey
    Joined: Oct 2009
    Posts: 5,091
    Likes: 556, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    In message #199 above Perm Stress reports an email he sent to Donald Blount. That email appears also appears in PBB as a letter "To the editor". Perm Stress posted in the same message the response he received from Blount, and what is published in PBB April/May 2011 issue from Blount is a slightly expanded version of the response to Perm Stress.

    There's nothing fundamentally different in the PBB response beyond the three additional graphs of resistance coefficients at lower Froude Numbers.
     

  15. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I just believe that publishing private correspondence in magazine (or in forum) is not polite, without getting permission from sender... OK, for me no problem.
     
Loading...
Similar Threads
  1. zstine
    Replies:
    8
    Views:
    353
  2. zstine
    Replies:
    8
    Views:
    578
  3. Furkan
    Replies:
    7
    Views:
    867
  4. Leo Ambtman
    Replies:
    24
    Views:
    2,869
  5. Claudio Valerio Parboni
    Replies:
    3
    Views:
    838
  6. dustman
    Replies:
    78
    Views:
    4,579
  7. Surfer Naval Architect
    Replies:
    4
    Views:
    1,006
  8. anuprdk
    Replies:
    3
    Views:
    1,310
  9. Alexanov
    Replies:
    6
    Views:
    1,832
  10. Federico Ferretti
    Replies:
    3
    Views:
    1,987
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.