power trimaran 2 family cruiser

Discussion in 'Boat Design' started by surfbus, Dec 4, 2014.

  1. surfbus
    Joined: Dec 2014
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    Location: oahu

    surfbus justin

    Aloha everyone,,
    the wife is warming up to the idea of getting back on a boat!

    so i need some help with my conceptual drawings before handing them over to one of you fellows to engineer.

    i am trying to commission the most efficent power cruiser possible for 2 families.
    i see no problems with going long and super narrow?

    is it correct to assume the narrower and longer the more efficient or does the efficiency of a high l/b ratio at some point diverge? (my thoughts were to make (the hulls only 2' wide at waterline and only flare slightly but be 8 to 10 feet in height.)

    2nd question, beam and forward and aft spacing of amas?( i am loving playing with the layout on the bridge deck , but should do it scientifically, if possible.

    Also to keep the build cost down I was thinking to have the 3 round bilge canoe keels laid , and a hundred 4'x8' finished flat composite panels shipped to s.e. asia to complete?

    all help/advise and sarcasm will be appreciated!

    lets go surfing!
     
  2. surfbus
    Joined: Dec 2014
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    surfbus justin

    soon as i get time i will put together my estimated weight i will need to displace, housing and feeding 8 on mostly 700 mile trips....

    but i am assuming that the most efficient seakeeping and lowest fuel burn to float us would be on the submerged 2 foot wide diameter round bilge canoe hull with 6 foot of freeboard under the trimarans wing deck.

    but again , my question is does the curve of hull efficiency ALWAYS increase with the higher length to beam ratios?

    THANKS ALL?

    and does this whole plan sound awesome or what?
     
  3. groper
    Joined: Jun 2011
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    groper Senior Member

    Peak length to beam ratio efficiency occurs around 25:1. So for a 2ft waterline beam, a 50ft hull. - I assume you've Chosen 2ft waterline beam as you figured this is as narrow as you can go whilst still being able to walk through them and have a usable accommodation space? After approx 25:1, the wetted area vs displacement starts to increase beyond the point of diminishing returns and thus resistance vs displacement starts to increase again. If you look at the ocean racing sailing trimarans, you will see a common trend in the length to beam ratios all around this point, give or take a little either way...

    As to the headroom and freeboard, my take is you should make this a low as possible. Comfort dictates that this is enough for standing headroom for the people expected aboard. I would not go over 6ft6in unless someone very tall is frequenting the boat. Anymore than this is extra windage and weight and cost you don't need. Some designers compromise comfort and give up full standing headroom.in favour of increased performance and reduced weight, choice is upto the designers goals and priorities...

    The power tris by Nigel irens and similar are great boats in terms of performance and efficiency. However The main draw back is rather limited internal volume constraints for accommodations compared to a catamaran of similar length. A big effort for a boat which can't do much other than be extremely efficient...
     
  4. Timothy
    Joined: Oct 2004
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    Location: canada

    Timothy Senior Member

    I am not sure I am right but my computer tells me that a semi circular 2ft wide 50ft long hull with no rocker and volume taken as far as possible to the ends will only displace at most 3800 pounds . if both amas are to skim the surface they will only displace maybe another 1000 lbs. Eight adults and provision are at the bare minimum 1600 lbs. That leaves 2200 lbs to built the boat and power and fuel it. At the length to beam ratios of 20 to 1 and more for the hulls you will need a much longer boat to cruise with eight.
     
  5. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    After a LB of about 8-1 the efficiency and speed increase is mostly useless for a cruiser as the motion gets too violent ..the load carrying too small.

    At $6K+ each Stidd seats to lower the G forces is expensive and takes a lot of room.

    At speed on a passage you might have to sleep in the seats , or suffer spinal damage.

    At 8-1 your speed will still be far better (should you chose) or your fuel bill will be much lower.

    Offshore a modest speed , great volume and weight carrying ability is far more valuable than top , useless speed.

    Remember , its a family cruiser !
     
  6. groper
    Joined: Jun 2011
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    groper Senior Member

    Most modern cruising catamarans , the good ones, have a length to beam ratio of around 12:1 and some of the more performance oriented cruising designs go upto around 17:1. So I'm not sure why you think anything over 8:1 is useless???
    It also has little to do with violent motions also...
     
  7. surfbus
    Joined: Dec 2014
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    surfbus justin

    wow cool stuff , I'm salivating already!

    25 to 1 huh, looks like ill be going 3 wide and 75 long!I
    will do the volume on that and hopefully it will come out to the 15,000 pds I'm trying to run around with.

    now what about location and length of those sponsons? ---roughly,,,just so i can keep the dream alive and keep drawing my living spaces.

    and how should i do the math on how much those get loaded....wait sorry thats a problem later for one of you guys I pay!

    mahalo everyone -- especially you Groper ,,you saw right thru me!
     
  8. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Surfbus:

    I'm very curious as to how much you think you need to spend-from start, to splash and go.
     
  9. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    AS the LB ratio goes up 3-1 8-1 12-1 the wetted surface rises.

    Wave making is only ONE of the forces that hold the boat back. The other is skin friction .

    At higer speeds less wave making is nice BUT all the time high or low speed the surface friction bill (a product of area) is working against efficiency.

    Traveling all the time at top speed a needle would pay , for most folks 8-1 or so is the cruising compromise.
     
  10. outdoorplay
    Joined: Dec 2014
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    outdoorplay Junior Member

    Loving reading this post, full of helpful information
     
  11. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    ..unclear as of yet if he wants a coastal cruiser,or an ocean crosser-or a tri or a stabilised monohull
     
  12. surfbus
    Joined: Dec 2014
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    surfbus justin

    some one in the forum told me i can not post the same subject under two forums and to stop one----so I'm trying --albiet not very well-----if u would please go to the "Go to first new post l/b ratios: the longer the better? trimaran float placement?" under the hydrodynamics sub topic, there is quite a bit more information...thanks for all the helpful information ...
     
  13. surfbus
    Joined: Dec 2014
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    surfbus justin

    and fred ,sounds like I'm not as fast as u!......well sometimes just a little..im just trying to go 20-25 knots mostly....with some long range capabilities when I relocate to some other one of gods great creations...(warm blue and cocos)
     
  14. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    well sometimes just a little..im just trying to go 20-25 knots mostly

    No matter how efficient that speed will be 1 nm/gal or perhaps double .

    A 50 gallon hourly fuel bill is $200 an hour (here) , fine if 25 K cruising is affordable.
     

  15. Richard Woods
    Joined: Jun 2006
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    Location: Back full time in the UK

    Richard Woods Woods Designs

    Wow, that's cheap! it would be more than double that in the UK

    Richard Woods
     
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