High Performance MPX Foil/Self-righting Trimaran-The Test Model

Discussion in 'Multihulls' started by Doug Lord, Dec 28, 2010.

  1. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    All: when I design a hull like this I do the design so I can build it and I use the minimum lines to describe the shape. The curve, if any, to the bottom in a hard chine shape such as this is done with a plastic curve(or by hand) to insure a similar shape at each station. Same with the deck crown. I don't do any lofting because the sections are faired right on the strongback if necessary. Using this method no "buttocks" or "waterlines " are required. Normally, I would use 10 stations and run the simpsons to get CB and displacement.Lb per in(or half in) immersion and trim moments calculated, if required. If any corrections to the section areas below the waterline are required the affected station is checked and the simpsons redone. Normally that is not required. This is a fast and accurate way for me to design and build a hull to precise targets.
    This particular design is based on an earlier hull so the only under water differences are from station 5.5 forward. No additional simpsons was required and the calculated additional volume forward moved the CB forward one foot.
    On this design the topsides are new above the "old deck line" and the bottom as above. This hull is being modified to save money but also because the hull with the mods is perfect for the Trapwing and SRT. The stern will be changed to allow an adjustable gantry just like the MPX which extends the foil footprint for better pitch control. The daggerboard/main foil combination will be the same with mods for ballast. The rudder will be the same as on the original boat-so will the initial rig.
    Numerous different tests will be accomplished with the model as described in previous posts.
    The ama for the SRT model will be hand shaped to a target CB and L/B ratio. Once the ama shape has been found that works with the SRT a minimum lines plan will be drawn for full size building. There are likely to be several iterations of the ama to get its role and the role of the "curved piece"(ama attachment system described earlier) to work well for the SRT. The dihedral of the cross arms will be completely adjustable on the model and the final angle will determine exactly how the foil assist system will work.
    --
    One other note: before any working plan is drawn there are probably 50 pages of notebook calculations, preliminary sketches, and more calculations including comparative ratios and a lot of research. Reviewing and updating that information goes on constantly on a project like this where several elements of the project have never been tried before on any successfull tri(as far as I know: the MPX foil system and the self-righting system). Constant research and development....

    PS I'll post the hull sections for the model(and full size) when I can.....
    PS and the bow to the left side of the page because I've done it that way since I was 9-never could get a proper "feel" with the bow the other way....

    pictures-working plan, station 6, 12" model of same hull-click on image and then again on resulting image:
     

    Attached Files:

  2. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Could I build a hull?

    Paul - Bugger - I thought I knew what the shape was but then on closer look I would have got it wrong. Doug, I would have built the boat upside down from the first sketch alone. That bottom got me confounded first up.

    Will look closer next time

    Doug - Why go fat?

    cheers

    Phil
     
    Last edited: Mar 24, 2011
  3. Gary Baigent
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    Gary Baigent Senior Member

    Paul, yes, I drew the main, central, double bulkhead full size, then the other bulkhead and half frames modeled off that, a bit like Dick Newick's process where he uses a similar curve line - in my case the curve getting tighter as it went forward and aft. The transom also dictated the after shape (I wanted a fine stern because of the lifting IT rudder foil).
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Phil, I have the hull( the fullsize hull to be modified)-all carbon and very light-its a situation I can't afford to not take advantage of. The modified L/B is the same as my first planing hull tri 34 years ago and ,as it turns out -the same as a Weta. The main hull is lifted on foils at about 6-7 knots boat speed and thats the last she'll see of the water. Also, the boat will be used to test the Trapwing movable ballast system. Without the foils-given the great beam-she wouldn't lift the main hull before at least 15 knots of boat speed. The foils (with a wand) control the sailing angle of the boat. An interesting twist is that from about 10 knts of boat speed thru about 15 the main foil completely unloads save for its role in sailing angle control. The main foil + rudder foil continuously augment pitch control allowing a really powerful rig.
    Building the model rightside up seems far simpler to me given the shape.....
    ----
    Comparison to semi-circular hull with the same displacement shows that there is a very small difference in wetted surface with the fat hull pitched down and heeled over a bit which it could be with a mobile crew or using the trapwing ballast system. For the SRT heeling and pitching down will be impossible without a mobile crew but the negative repercussions are only from 0 to about five knots boat speed when foil lift begins to substantially reduce wetted surface. Take off of the main hull is at 6-7 knots boat speed and therefore wavemaking resistance due to the smaller L/B vs a skinny hull is pretty much moot. Light air performance will be affected to some degree vs a skinny hull but in wind over about 6-8 knots the boat comes alive flying the main hull. Thats a key "ingredient" of the MPX system: light air main hull take off before it would be remotely possible w/o foils. The foils begin to unload very shortly after takeoff as the heeling moment grows reducing the lift required by the main hull foils. That is one of the unique aspects of the system. In other words, the faster the boat goes the less the foils have to work-not the way foils are normally thought of...

    pix-Trapwing -single seat-click on image:
     

    Attached Files:

    Last edited: Mar 26, 2011
  5. Paul B

    Paul B Previous Member

    I would have got it wrong as well. The curved bottom is not defined in the original drawing, so I would have assumed a straight line. Also, I assumed the "old" sheerline would be superceded by the "new" sheerline. The actual idea uses both.

    I would bet no one would have drawn that section based on the information presented in the first drawing.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    The curved bottom is there in the first drawing - but what I didn't pick is that the later cross section sketch reveals quite extreme tumblehome ... why have you done that, Doug? Could you have not reduced the waterline beam? I'm sure you have done the correct displacement calculations but the hull does look overly fat to me.
     
  7. Paul B

    Paul B Previous Member

    Where in the first drawing is there any indication of the athwatships (sectional) shape, with curvature or not?

    As I pointed out earlier, there are a small number of points defined, but there is nothing defining the shape between those points.
     
  8. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    High Performance Self-Righting Trimaran: The Test Model

    -----------------
    No. The hull on which this project is based already exists. The "tumblehome", as you call it, makes the design-it just looks cool- if I do say so myself(but I'm not the only one). The deck opening is just wide enough for a single seat. The weight of the modification to the topsides is significantly reduced -given the fixed waterline beam- using the angled in topsides. Oh yeah-did I mention that some say it looks cool?


    Images of the SRT main hull: posts-2, 6 , 16 , 17, 22, 31, 34 and in my gallery: http://www.boatdesign.net/gallery/showgallery.php?cat=500&ppuser=31218
     
  9. Gary Baigent
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    Gary Baigent Senior Member

    Yeah, you're right, Paul, it could just as easily be flat - but I figured that would have looked a bit weird in the bow sections (looking at the model) so assumed there was a curve.
    Doug, referring to tumblehome, you should checkout the main hull of the ancient proas from Santa Cruz Islands; they had just enough room to get your legs inside - they also carried internal stone ballast. The semi-decked in shape was to reduce water and spray ingress and their cabin was set high on a major thwart set amidships - in many ways similar to your ideas ... but they are a thousand years old or so.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    =======
    Sounds great....timeless design. A worthy goal...
     
  11. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    High Performance Self-Righting Trimaran: The Test Model

    Here are the sections-full size for the model-ready to be traced onto the balsa. They're not perfectly lined up because if I touched one they'd all slide.
    Stations 1, 2, 3 had mistakes that were corrected.

    click on image NOTE THAT THESE SECTIONS ARE RIGHT SIDE UP!:
     

    Attached Files:

  12. catsketcher
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    catsketcher Senior Member

    Sorry to be flippant Doug but to me it looks like a nice 60s Spencer design like Ragtime. Then again I would build it the wrong way up.

    cheers

    Phil
     
  13. Doug Lord
    Joined: May 2009
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    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

    High Performance Self-Righting Trimaran: The Test Model

    =============
    Flippant or not it's great to have the SRT compared to a "nice 60s Spencer design like Ragtime" right side up or upside down. And if that's what you get out of all the detail I've posted here in response to your question-well, that's the way it is. Both Spencer and Ragtime are impressive and favorites of mine.....

    picture-maybe, a little :
     

    Attached Files:

  14. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Self-Righting Trimaran(SRT)-Full Size / Updated Preliminary Specifications and Sail Plan-3/27/11(original in post #1):

    Length: 18' / 5.49m

    Beam: 22' / 6.7m (foldable for trailering)

    Ama LOA: 10'

    100% ama buoyancy: 337lb.

    Ama attachment(curved piece ) buoyancy: 320lb

    Total Buoyancy for ama + curved piece attachment: 657lb

    Draft w/boards Up: 6.4"

    Draft, daggerkeel down: 4' 8"

    Mast Length: 28' / 8.3mm

    Sail Area:
    --284 sq.ft. upwind SA
    --575 ft2 / 53.4 m2 downwind SA

    Total Boat Weight incl. ballast and crew : 750 lbs

    Crew weight: wide range-for these numbers 175lb.

    Ballast: (at the juncture of the daggerboard and main foil)- 175lb.

    Boat weight minus crew and ballast: 400lb

    =====================

    COMPARITIVE RATIOS:

    Bruce Number: SRT=1.86 F18=1.66

    SA/WS:
    -- not flying-SRT=5.73/1 F18=4.77/1

    -- flying main hull-SRT=11.14/1 F18(flying one hull)=6.03/1

    SA/D: SRT=55 F18=44.16

    W/SA-smaller better): SRT=2.64 F18=3.29

    =====================

    click on image:
     

    Attached Files:


  15. Paul B

    Paul B Previous Member

    Very few people would consider a statement that their design resembles a boat that is turtled to be a compliment.

    Not to mention your hull shape does not resemble the shape of Rags, inverted or not...
     
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