Light Duty Displacement Cat

Discussion in 'Boat Design' started by Saqa, Oct 4, 2014.

  1. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    G'day fellas, been trying to get my head around the displacement concept as I am fascinated by the idea of of a long narrow light cat with a small outboard to cruise around the coastal islands here in Fiji for my immediate next project. Been looking at online resources on learning the concept but I need some guidance with understanding the areas of light duty build specialized cat which is not really covered in the resources

    I am thinking narrow beam, long, narrow and light hulls. Enough deck clearance to seal the hulls and self drain the deck if ever copping a wave or something. Centre cuddy cabin to sleep husband/wife/kid and open fore and aft decks for fishing and relaxing

    The formulas result in around 8 knots for a 30ish' length. Would this be the limiting factor or will the performance be better then that when employing a pair of long skinny hulls?
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,811
    Likes: 769, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Saqa,

    Long in is basically short hand for "length-displacement" ratio. If the L/D ratio is high, i.e. say over 6.5 you can say it s a displacement hull that can run at high Fns without 'issues'.

    The "light" aspect is merely a function of what you make the hull from and what you put into it. Since you can use a long thin hull make it light..hey presto. But if you use the same hull but say its weight is doubled, thus the L/D changes, it is not so good.

    So, first thing to understand, is it only works if the full load displacement of the hull yields an L/D ratio that is favourable.

    To cruise around the islands, one assumes you will have a full deck, rather than say 2 beams, like a Proa. As such you need a good clearance from the underside of deck to the water surface, its freeboard. A poor freeboard is 5% of hull length, good is 10%.

    Good luck :)
     
  3. gonzo
    Joined: Aug 2002
    Posts: 15,126
    Likes: 899, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Displacement is the weight of the water displace by a body floating in it. It is also the weight of the body out of the water. What is called "displacement speed" is a ratio of speed to length. For very narrow hulls, the ratio can be 3 or so. That means that 3 times the square root of 30 is about 16Kt.
     
  4. philSweet
    Joined: May 2008
    Posts: 2,404
    Likes: 233, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Take a look at the old Macgregor 36 catamaran. Not exactly what you described, but it was designed to do what you describe.
     
  5. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Thats the other thing to get my head around. Having enough deck height above water. I wonder I I am thinking along the wrong lines, maybe I should think tri with a long narrow main and a pair of even skinnier outriggers of the same length. With that I could have a slotted types deck that gives me about 5' of fishing deck area at the butt end and 3' area the bow with wing types beams arching down to connect the outriggers :/ Might also remove the hassles of mounting only one outboard to a cat
     
  6. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Gonzo I am not so good with maths, I don't even know how to calculate square root. Using an online calculator here http://www.cncphotoalbum.com/technical/hullspeed/hullspeed.htm give a result for 7.39 knots for 30' and three times that is 22.17 knots. 16 knots sounds awesome and 22 be to dream for! I wonder how I can learn to calculate all the various thingys to find out what kind of outboard I will need. I have been hoping along the lines of economical efficient cruising

    From what I have been reading it looks like the local boys made some of the most efficient hulls back in the days of cannibalism
     
  7. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Yup, this is really really appealing http://imagenes.cosasdebarcos.com/b...tamaran-21186120112956565065495549524568x.jpg

    I have never sailed but wish to try is sometime. I wonder if a Fijian types looking sail will work on a cat. That would tie into my location so well :)
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,597
    Likes: 815, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    There is potentially a handicap (interference drag) with having the two hulls close together. At slow speeds anyway.
     
  9. upchurchmr
    Joined: Feb 2011
    Posts: 3,122
    Likes: 171, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Richard Woods has a line of power catamarans you might like to check out.
    They all have a cabin, but the principle of using an outboard is easy to check out.
    http://www.sailingcatamarans.com/index.php/designs/6-powercats/262-skoota-20
    James Wharram has a line of "Polynesian style" sailing catamarans which might give you some ideas. No idea if he has a power only design.
    http://wharram.com/site/selfbuild

    Wave interference doesn't matter at slow speeds, but it does slow you down at higher speeds. You avoid the interference with wider spacing on the hulls and lighter weight.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,597
    Likes: 815, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I think at the speeds he intends to run at, narrow beam will mean more drag. I should have defined "slow" better.
     
  11. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

  12. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    The scoota 20 is the inspiration for me
     
  13. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Yeah as you can see my grasp of the concept is not quite there yet. I thot that narrow was to aim for even in beam. I love beam, more the better, is 8' still too narrow for 30' cat?
     
  14. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Mate love it, something like that as a serious sportfisher for the charter is in the pipeline as a long term project in the next three years sometime. More immediate is a light family cruiser for the inshore lagoons and islands around here
     

  15. WestVanHan
    Joined: Aug 2009
    Posts: 1,374
    Likes: 56, Points: 0, Legacy Rep: 746
    Location: Vancouver

    WestVanHan Not a Senior Member

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.