Open source 12-15m high performance/semi-cruising catamaran design

Discussion in 'Multihulls' started by groper, May 10, 2017.

  1. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    If you zoom in you will see that the lines are many small lines joined together - the dimension you have is for 1 very small line of 6cm length.
    The lines generated are a series of polylines. Each one is very short. Hundreds of these small polylines make up each section. Make sense?
     
  2. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    Screen Shot 2017-05-22 at 10.16.24 PM.jpg better resolution but still something off. Stn 9 bridgedeck -- what is the overall width? scaling factor should be consistent:)
     
  3. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 49, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    1. Ok fixed the coach roof to be more like the SIG with the tilt, makes a big difference for the entry, no need to cut out deck area which is an accident waiting to happen.

    2. It's a race boat so what is an extra step, made the aft beam span all the way across boat and tied a really wide bench into that. Now you can sleep holding the tiller and you get nice headroom below. ))

    3. I made sure the door to aft cabin clears when opening in toward outboard side. It's looking okay back there so moving forward...
    15_20.jpg 15_21.jpg 15_22.jpg 15_23.jpg
    15_24.jpg
     
  4. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 49, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    One small detail, bench and box should come together with an angle so nobody busts their head getting into bed.
    15_25.jpg
     
  5. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    section 9 dimensions; This opens fine for me in autocad - no scaling or anything required from the last file i uploaded and pulled these dimensions straight up. If your having trouble, it must be the settings in the software at your end thats messing it up.
    [​IMG]

    Jorge - i orginally had the coach roof like that but upon looking at the boat in profile and from other angles - it juts up too high and looks silly/out of proportion - not to mention the extra windage and upwind performance penalty. It pushes the entire roof line up just so you can walk through a door without 1 step in front of it and then having the roof that high once your down the step it then becomes overkill/higher than nessesary once inside. I know you hate steps - but steps are an absolute necessity in a design like this to keep the windage down as much as possible. If you want upwind performance - you MUST have low windage.
    I know your struggling with the concept of small spaces - but thats what performance boats are - tight spaces. its challenging to make the most efficient use of the small spaces available - but consider this - my last boat has MUCH lower volume than this in the hulls - approx half the width!
     
  6. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 49, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    I think windage would be about the same, the outboard side is right where your old coachroof was but it does give a lot of extra height inside. I will drop it and add a step. Yeah I don't, at all, like steps. Had one boat that was all steps and benches to climb over, hated that thing with a passion.

    Fridge and Freezer fit
    15counter.jpg
     
  7. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    So this is the resistance prediction when flying a hull. 6.5 tonne displacement to approximate sinkage, 10 degrees heel, and -1 degree pitch.
    [​IMG]

    i think ill take the middle of the road prediction as shown by Mr Fung`s method even tho slender body theory should be more accurate. So if we assume we are flying a hull on a beam reach @ 25kts, we have a driving force from the rig of approx 8kN.
    Center of effort of the rig @ 10m above waterline gives us an approximate moment of 80kN.m.
    Thats one hell of a moment which is why im considering the forward foils to help lift the boat and reduce forward pitch under power.
    Now if would like to lift 50% of our displacement on the foils, we need to generate approx 3 tonnes / of lift or a lift force of approx 30kN. I need to figure out (cant get my head around it right now) how far forward to place the foil so that we end up with 50% displacement when lifting 30kN and the boat back to a level or +1 degree pitch up trim.
    I could trim the 3d model to pitch up +1 deg then reduce DWL until i have 3000kg displacement - and see where the LCF of that condition lies. Then position the foil so its equidistant from the static/level trim LCF. Then i need to move it slightly further forward from there to account for the rig driving force moment - im lost at this point - anyone care to help?

    Also what i would like to know is - whats a reasonable dynamic load factor on these types of foils to account for wave action and hull accelerations in a seaway?
     
  8. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Ok i think i have this - can someone please check i havnt goofed up big time?
    [​IMG]

    So the rig driving force was taken from the previous posted resistance predictions to be 8kN.m.
    I recalculated the hydrostatics @ 50% displacement on a single hull (assume flying 1 hull) and gave the hull a 1 degree pitch up which leaves the stem just kissing the new trims running waterline. This new trim moves the LCB 1.2m aft of the static waterline and so this is where i centered the moments from.
    Assuming the foil is lifting 50% of the displacement = +30kN. i assumed 80kN.m from the rig as the center of effort is approx 10m above moment center @ 8kN.
    To balance out the forces, i have the lifting foil positioned 2.66m ahead of the moment center - the running trim LCB. This determines my foil longitudinal position @ 8.96m from aft perpendicular or 59.7% DWL.
    Have i goofed?
     
  9. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    Yes, you've goofed.

    First question is that the image shows a shift in LCB of 1m, but the text says 1.2m.

    Either way, taking moments about the original LCB/LCG (arbitrary, but eliminates one term), then you have:

    80kNm (rig moment) + 30kN x (1.0 or 1.2m) (from buoyancy) = 30kN x X (from foil), where X is forward offset of foil lift from original LCB/LCG

    This gives X=3.866m for 1.2m LCB shift or X=3.667m for 1.0m LCB shift.

    Remember, X in the above calc is from the original LCB/LCG, NOT from the new LCB as shown in your figure. Someone should check my figures, too.

    Edit: I also wanted to mention that your original LCG position looks a bit far forward for a typical design?
     
  10. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    For clarity - the LCB moved 1.2m aft of static DWL LCB when lifted and 1deg pitch up trim. The 1m shown in the diagram is for the 80kn.m rig moment, 80kn @ 1m - not the distance to the original LCB.

    The original LCB is close to 50% static DWL as is the mast step location you can see in diagram - it looks a little FWD in the pic because the boat is rotated 1 degree in pitch. The new waterline shown in yellow is for this new pitch up and reduced displacement condition ie- running at speed with 3 tonnes lift on the foil and 8kN rig driving force @ 25kts boat speed.

    My approach was to assume that everything is in equilibrium at this pitched up/reduced displacement condition - the rig will be producing 8kN and the boat will be flying a hull at 25kts with a 1 degree pitch up trim.

    Therefore - i went to the trouble of rotating the hull and lifting it out of the water 50% to find the new LCB via CAD model. If this new LCB point is now assumed to be the center of moment, i dont need to account for bouyancy moments of the hull anymore - ie, bouyancy is centered around this new LCB! So this leaves only the rig moment and the lifting foil moment to be balanced in the equation?

    So as we are calculating the moments in a fast sailing condition with all forces present - not static at anchor - is not the static DWL and static LCB now irrelevant?
     
  11. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    The new buoyancy is centered at the new LCB, but if you take moments about this point then you must also include the moment created by the mass at the LCG, which is still acting at the original LCB location, not the new one. This is your error.
     
  12. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Ah yes, thankyou! - the CoG hasnt moved with the CoB.

    Ill have to do another calculation at a later date which accounts for the crew weight - there will be ~300kg of people close to the transom when sailing like this racing.
    Ill also have to get to the weight analysis and see where the static CoG is going to be. It will probably have to be designed with the CoG aft of 50% DWL.

    Im starting to understand why the current trend of racing multihulls has moved the rig well aft :)
     
  13. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    so i changed the hull below waterline to move the static CoB aft (and hopefully the CoG will also work at this location - still yet to be confirmed) and also it is now a warped flat plate bottom, deep V forefoot, with hard chine - for experimentations sake and 100% developable surface plating for flat panel building.
    With the CoB moved well aft - the foil configuration and the previous moment forces calculation will now work.
    The penalty with this hull shape is 2 sqm more wetted surface area for the same displacement @ 6 tonnes.
     
  14. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 49, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    I read the V requires less dagger to point. As to the mast being further aft, I noticed this too, should I be putting my mast further aft?
     

  15. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    No - the reason for moving everything further aft - not just the mast - is to get the center of gravity of the entire boat further aft to get the foil assist system to work and account for the rig driving force moment.
    There is a complex balancing act going on here- you cant have the foil too far forward or the trim changes too much with variation in rig power, boat speed, and added cargo/people etc. You dont want to be changing the foil or rudder AoA too much as there is only a modest range built into the structure id want to be moving the boards and rudders within and a modest range of AoA that the foil operates at best lift to drag ratio. There is large variations in driving force simply from variations in wind pressure on a gusty day and you dont want too much trim effect happening or it will make the boat unsettled and unruly.

    So the foil shoulnt be too far ahead of midship or there will be larger required inputs to move the foil AoA on different points of sail, larger trim variations for different boat speeds, variations in wind pressure etc and a narrower envelope in which the foils will be beneficial to performance when they start to operate outside their best L/D ratio.

    In order to get the moments of rig force and foil lift to balance and achieve the desired running trim at a speed which i think is reasonable for this type of yacht, the CoG for this particular design, should be approx 1.2 aft of midship whilst the lifting foil is about the same distance ahead of midship.

    None of this applies to a big ole slow cruising boat :)
     
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.