Efficiency: Hull interference vs length on catamarans

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jun 3, 2021.

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 7,188
    Likes: 1,109, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Yes, here:

    upload_2021-6-4_11-24-20.png

    Then you're going to say...er??..that is not 30 foot.
    Correct...it is 45m.

    But the NUMBERS of the hull are exactly the same.
    L/D ratio = 7.5
    Fn = 0.9

    So it matters little what the actual size of hull/boat is...it is all about the numbers of the hull in terms of its hydrodynamics.
     

    Attached Files:

  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,029, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Fallguy's skoota cat will do over 15 knots
     
  3. dustman
    Joined: Jun 2019
    Posts: 147
    Likes: 15, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    Oops, I should define the 20 to 1 as length to beam of the individual hulls.
     
  4. dustman
    Joined: Jun 2019
    Posts: 147
    Likes: 15, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    If this question was for me, it will be a power cat, with no accommodations in the hulls.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,029, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    No, I was querying fallguy. I assume he was talking a sailboat
     
  6. fallguy
    Joined: Dec 2016
    Posts: 5,506
    Likes: 1,063, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Right, but not considered a displacement cat.

    So, definitions....
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,029, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    What does he call it ? Semi-displacement ?
     
    fallguy likes this.
  8. fallguy
    Joined: Dec 2016
    Posts: 5,506
    Likes: 1,063, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Frankly, I'm confused.

    Sailing cat or power cat?

    Displacement or semi-planing?

    20:1 l/b?

    calling it a nite
     
  9. fallguy
    Joined: Dec 2016
    Posts: 5,506
    Likes: 1,063, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Yeah, or semi-planing...
     
  10. fallguy
    Joined: Dec 2016
    Posts: 5,506
    Likes: 1,063, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    If you go to extremes on the l/b, at some point; things go haywire. You lose displacement for one. That pax ferry doesn't have that problem, but it ain't 20:1 that I am aware, or at 45M, her hulls would be only 2M or so. I'd love to see that engine room if so.

    For a 30' cruiser to operate at 20:1, wife is gonna get tired of husband telling her to only pack the bikini.

    At some point, pushing the l/b past a certain point must have/has diminishing returns.

    Anyhow, I need to rest.
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,029, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    certainly is getting to the extreme end of matters for a smallish boat offshore, on a river a different matter
     
  12. dustman
    Joined: Jun 2019
    Posts: 147
    Likes: 15, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    If I'm not mistaken some of the large racing catamarans are in the 20:1 range and sail to extreme.
    Isn't it relative to other factors, like weight distribution, reserve buoyancy, etc?
     
  13. Ad Hoc
    Joined: Oct 2008
    Posts: 7,188
    Likes: 1,109, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    The pax ferry is 18:1, so, almost the same.
    As I noted, it is about the numbers in terms of scale. It is why model testing works...using geosims...exact copies of the hull at different scales all at the same length beam displacement and speed - scaled to suit of course.
    Hence whether the cat is 3 feet 30feet or 45m matters little ..if the scaled numbers of each hull are the same...it is the same hull in every respect.

    This:-

    upload_2021-6-4_13-21-37.png
     
    fallguy likes this.
  14. Dejay
    Joined: Mar 2018
    Posts: 721
    Likes: 137, Points: 43
    Location: Europe

    Dejay Senior Newbie

    So presumably what Woods calls semi-displacement is what ad hoc describes as "ventilated transom" with flat flow aft? ("My Skootas use semi displacement, non-planing, asymmetric hulls.")
     
    fallguy likes this.

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,029, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    For starters, sailboats don't go into the waves typically, you could easily find with slender hulls and the wave encounter frequency, and rounded hulls and the the modest boat length, your experience may be less than agreeable. More so if the ends of the boat are "kinda" symmetrical, which is why the transom might be desirable
     
Loading...
Similar Threads
  1. big_dreamin
    Replies:
    19
    Views:
    3,457
  2. AndySGray
    Replies:
    29
    Views:
    7,531
  3. dustman
    Replies:
    31
    Views:
    2,056
  4. dustman
    Replies:
    6
    Views:
    1,118
  5. jbo_c
    Replies:
    26
    Views:
    2,331
  6. ross whitaker
    Replies:
    32
    Views:
    4,444
  7. SVLB
    Replies:
    0
    Views:
    1,412
  8. Phancy
    Replies:
    5
    Views:
    8,044
  9. daiquiri
    Replies:
    68
    Views:
    11,558
  10. manon
    Replies:
    31
    Views:
    6,286
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.