gemini workboat antislamming

Discussion in 'Multihulls' started by lucdekeyser, Dec 7, 2013.

  1. lucdekeyser
    Joined: Aug 2004
    Posts: 137
    Likes: 3, Points: 18, Legacy Rep: 51
    Location: Belgium

    lucdekeyser Senior Member

    This is the underside of a GRP workboat from Gemini Workboats ltd. meant to prevent slamming in heavy weather from what I gather. Would it be better to have a different hull form prevent the need for such "outgrowths"?
    [​IMG]
    Luc
     
  2. TANSL
    Joined: Sep 2011
    Posts: 5,803
    Likes: 203, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  3. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    I guess ther ain't no bridgedeck ?

    pogo
     
  4. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    is the boat running in planing or fast displacement mode ?
    Having single chined hulls it should be planing mode, narrower and longer sprayrails , at the tailing edge about 20cm above CWL and angled 3Degrees might help. Sprayrails not only deflekt the water, they also generate better trimming angle.
    Here, somewhere in between, you'll find a pic of sprayrails, but, yours should lateral be negative angled , alt least 90 Degrees (similar as outer stringers on planing motorboats)
    http://forum.yacht.de/showthread.php?98827-Farr-goes-retro&highlight=farr goes retro


    Running slow in displacement mode-----I have no idea.

    pogo
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,913
    Likes: 338, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    This hull appears to be a cat, not a tunnel, so water is being displaced into the tunnel underway. Tunnel height seems not to be that great. Are you sure these appendages are designed to reduce tunnel slamming ? Are they something the boats always carried, or appeared later ?
     
  6. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    Planing, good trimming angle.
    Notice spray between hulls ! Generated either by poorly designed deflectors ( see above) or much too low bridgedeck, perhaps both.

    http://www.bluewaterboats-plymouth.co.uk/southboats-catamarans-photo-gallery.htm

    Sprayrails on the inner side of the hulls should be designed similiar as ( to ?) those on the outer side ( as described above).
    Outer stringers on the Kimm --the edge of the hulls - should help also.

    Sprayrails and outer chinestringers
    http://www.fishyfish.com/img/sprayrail_hull3.jpg

    pogo
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,913
    Likes: 338, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    If the boat is skating along on its tunnel, because it is low, and/or the boat heavy, nothing much will help the slamming. I am a little intrigued by those appendages, which have the characteristics of foils to some extent, looks more like an attempt to generate more lift at speed when the bows drop in, such as running down-sea, even a way to get more lift for the lower, "lee" side when stern quartering. I'd be interested to hear what forumite Alik thought about them.
     
  8. lucdekeyser
    Joined: Aug 2004
    Posts: 137
    Likes: 3, Points: 18, Legacy Rep: 51
    Location: Belgium

    lucdekeyser Senior Member

    found the following in marine-world.com
     
  9. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante


    :rolleyes:;)

    Wat für'n Schiet !!
    vor de patat en bierkonings: Quelle merde !!
    4 yanks and John Bull: What a ******** !!
    ( 8 stars can be translated-- Bockmist)

    pogo
     
  10. lucdekeyser
    Joined: Aug 2004
    Posts: 137
    Likes: 3, Points: 18, Legacy Rep: 51
    Location: Belgium

    lucdekeyser Senior Member

  11. lucdekeyser
    Joined: Aug 2004
    Posts: 137
    Likes: 3, Points: 18, Legacy Rep: 51
    Location: Belgium

    lucdekeyser Senior Member

  12. Ad Hoc
    Joined: Oct 2008
    Posts: 6,168
    Likes: 399, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    There are ostensibly only 2 ways to prevent slamming:

    1) Increase the wet deck clearance beyond the sea states the vessel shall encounter
    and/or
    2) Reduce you speed when the sea state increases....i.e. exercising good seamanship.
     
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,913
    Likes: 338, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Well, instead of my just wondering what Alik might think, we now have an opinion from cat expert Ad Hoc. Have to say the clearance does not look to be that good.
     
  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The image above suggests a bandaid was applied after the first few hulls popped out of the mold and where tested. At certain speeds, I can see these deflecting portions of the inner bow waves, possibly to correct wet deck slapping, though also possibly to address other issues. The fact these are mechanically fastened is telling and maybe they need to reevaluate their multihull design team, more so then apply crutches to the inside of the hulls.
     

  15. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    Yepp, you got it.

    Here again the manufacturer's statement :
    " Gemini are still the only cat builders in the UK that address the slamming problem and we have eliminated it on all our vessels."
    Quelle merde !

    pogo
     
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.