Proa questions...

Discussion in 'Multihulls' started by Inquisitor, Jun 9, 2010.

  1. Guest625101138

    Guest625101138 Previous Member

    This will give you an idea:
    http://www.boatdesign.net/forums/design-software/flotilla-demo-2-07-a-32276.html
    Flotilla gives the trim, lift and squat at sub planing speed.

    I have tested the Flotilla results on my V14 hull and they compare well.

    The new V15 hull is similar to V14 but with a modified deckline that should drive up easier when submerged than the flat deck on V14. The seating position on V15 is also a little higher than on V14 so I can drive through larger waves without get a wet backside.

    Rick W
     

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  2. Inquisitor
    Joined: Nov 2005
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Not that I want to get between you two love birds...

    I see Rick's point - All airplanes have zero buoyancy (static lift) and work only on dynamic lift, damping and pitch oscillations do exist.

    And Terhohalme's point - Predicting and counting on them for anything but a hydroplane racer seems well beyond the realm of the nursery hobby-horse.

    I know... lets call in the rocket scientists and fubar it all up.

    P.S. I could have imagined myself doing something like your yellow "boat" when I was in high school and rocket sciencing it to the hilt in graduate school. But these days, I would rather have my boat's behavior not noticeably change whether I had bacon and coffee for breakfast or not.
     
  3. Alex.A
    Joined: Feb 2010
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    Alex.A Senior Member

    Thanks.
    Much of this i find logical but then something that i thought i understood gets explained differently and i get confused again....
    I am again happy with my first understanding and after some changes am back on target - going to make a model and play on the water to see.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Analysing pitching is complex but the largest contribution to the moments that produce hobbyhorsing is the reserve buoyancy in the bows of the hulls. If you remove the reserve buoyancy you reduce the moments substantially.

    The attached shows the bows of the hulls on BMWO. Take a look at their shape. These are designed to drive through waves without generating high pitching moments. The wide, high flared bows of Wharram hulls do the opposite. The lost energy in the constant vertical accelerations of the hull adds considerable drag - can be 20% higher than calm water drag.

    Rick
     

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  5. Inquisitor
    Joined: Nov 2005
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Ellison's tax exemption...

    Hi Rick,

    I think you understand that I agree that it can be done... And you are succeeding at using it... and BMW/Oracle is certainly using it. And you're proving my point completely.

    (1) Ellison probably had the best AE's in the world at his disposal. I doubt he even had to pay for them. I can imagine they would have given the first born to him to help on the project.
    (2) Even Ellison admitted that he was worried that the thing might not hold together in the unusually rough condition (above 10 knots true wind) What a laugh. Although, it was mainly due to structural optimizations, I'm sure there were quite a few fluid dynamic optimizations also. No one in their right mind would think about using that thing in a trans Atlantic record attempt.

    So let's take this school yard brawl back into the classroom. If it is not proprietary to your research...

    How would you go about applying these theories to a cruising/racer caliber of boat that is 15+ meters long? For this thread, and at the moment, my interests would be a Proa. Speed and efficiency are VERY important, but not at the expense of safety. Comfort is important, but its subjective and negotiable.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    If you really wish to understand what is pitching and what affects pitching, rather than endless ‘bar room debates’ that go around in circles, you need to breakdown the components into each subsection analytically. Since we know how much some of you only like analytical solutions, despite the endless anecdotes and empirical nature that is only ever provided in return when questioned.

    Ok, here is the class room version.

    Pitching is broken down into:
    1) Inertial Moment
    2) Damping Moment
    3) Restoring Moment
    4) Exciting moment.

    Thus, the equation of motion that describes pitching is:

    a[(d^2.theta)/dt^2]] + b[(d.theta)/d.t] + c.theta = M.cos.wt

    The first term, from left, is the inertial moment, the second term the damping moment, the third the restoring moment, and lastly = the exciting moment.

    The coefficients are required to solve these equations, viz:

    To establish the first term, a, you need the (mass + added mass) x radius of gyration^2
    The second term is established by strip theory by integrating the wave profile along the length of the hull and finding the associated force per strip.
    To establish the third term is proportional to the waterplane area
    And finally
    To establish the forth term is found by finding the unbalanced moment caused by waves about the transverse axis of the boat. It can thus be seen that the exciting moment fluctuates with the encounter frequency of the waves.

    So, it matters not the size of the boat. The equation of motion that describe pitching is the small for all boats.

    When pitching in calm water the term is = 0.

    Thus, the pitch period can be obtained (after solving the equation) by the following:

    Period = 2.pi x sq.root[ waterplane inertia/(displacement x GML)]

    When pitching in waves, the equation can be reduced into the “magnification factor”, which indicates the onset of resonance, owing to the encounter frequency of the waves.

    This can then be used in further analysis to establish the motion of vessels in waves, by means of an RAO, as are the other linear and rotation motions of a boat.

    And on it goes…….have fun :)
     
  7. Inquisitor
    Joined: Nov 2005
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    All is fair in love and war...

    You might as well be jotting down E=mc^2 and saying it solves all the worlds energy problems... and then go on to say its left for the student to fill in the blanks. :rolleyes:

    Anyone who knows and actually works with these kind of differential equations quickly learns that anything solvable has so many simplifying assumptions that the equations aren't really worth solving and won't tell you a damn thing. And that anything without those simplifications and thus worth solving, goes into numerical solutions.

    As you so eloquently dissected the problem, I'll try to simplify without losing something valuable.

    1) Inertial Moment - We can assume the boat is a rigid body and ignore its flexing and deforming without losing much significant accuracy. ;)
    4) Exciting moment - I guess we can assume a steady state wind condition. But with the rocking of the boat (pitch and roll) this might actually be too much simplification with the sailing load vectors changing all over the place.
    3) Restoring and Damping moments - This will be basically the pressure distribution along the hull as a function of time. Just imagine the pressure being zero on some infinitesimal area of the hull out of the water and then finding its pressure as it dives into a wave front as a function of time. It will have normal and tangential speed and finding its dynamic pressure (along with its static buoyant pressure) (along with its skin friction)... AND... then integrating it over the entire hull for every other infinitesimal area sounds like a daunting task. READ - IMPOSSIBLE!

    Although I have been away from the numerical analysis fields for a while, I feel pretty certain no one claims to have a Computational Fluid Dynamics program linked with a transient analysis (possibly Finite Element Analysis (overkill)) to determine the next "time slice" and dynamically restructure the mesh with different boundary conditions. Yeah its possible, but not even Ellison is going to fork up the money for the super computer CPU year needed to run that analysis. Besides, CFD USAGE has too many simplifying assumptions in it. Does anyone think the Maltese Falcon required real-world tuning after their extensive plane-jane, STATIC, steady state CFD analysis? Oh Hell yeah it did!

    So back to reality...

    Rick, I would be very interested in your real world observations even if they aren't well formalized. It sounds like you have been very systematic and quantitative. I frankly would trust your rules of thumb (or even glimmers) over a million dollars worth of numbers.
     
  8. DarthCluin
    Joined: Mar 2009
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    DarthCluin Senior Member

    Actually airplanes have very negative buoyancy. Airships are generally trimmed to neutral buoyancy, though in flight they are trimmed to slightly negative buoyancy to compensate for the lift developed by their shape.
     
  9. Guest625101138

    Guest625101138 Previous Member

    Rob Denny's harryproa is hard to beat as a concept. The lee hull for going fast and the windward hull for ballast.

    I have not given much thought to the features of the windward hull.

    For the lee hull on a 15m+ boat I would base the underwater design on minimum drag for 20kts and displacement of 1t. I expect this will be closer to flat section than round section but would force a flat section similar to what I have done with my V14 and V15 hulls. Both these are minimum drag for 6.5kts at 90kg with 6m and 5m length constraints respectively.

    The deckline would start just above the waterline at the bows. There would be enough buoyancy to cater for squat without full immersion in calm water or under pitching moment from the sails.

    The decks would form inverted Vs at the bow and rise to about 8% of the boat length in height to meet a flat deck.

    I will draw up something if I get time over the next couple of days so you can see what I mean.

    Rick W
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    My bad, I thought you wanted to understand what is pitch and hence the mechanisms. Since if you understand the mechanisms, then you understand what drives pitch.

    No need to calculate the coefficients etc, that is just obtaining numbers or pretty pictures. What influences each term? What are the mechanisms, in other words, if I change XXX will it affect the pitching? It is not necessary to perform such complex calculations, just an understanding of the roots of each term that describes the motion of pitching. (only when a student or ones specialised of hydrodynamics). So, what are the influencing factors that affects pitch, referring to the points above in the equation of motion:

    #1 The virtual added mass….what affects this?....the displacement and the underwater shape.

    #2 Vessel length…since the wave profile is integrated along the length.

    #3 The second moment of inertia of the water plane

    Which if you look at the simplified formula for the natural period of pitch I gave above:

    It only has 3 inputs!
    Thus, the factors that affect pitch are governed by

    1) The waterplane area…..i don’t think this is difficult to calculate
    2) The displacement….again, I don’t think this is difficult to calculate
    3) The Longitudinal GM,….again, I don’t think this is difficult to calculate.

    Which surprisingly, is the same as the more complex formula above, but by simple analysis of the terms (or real physical quantities). That is to say, by understanding the mechanisms.

    Thus I can only conclude that you prefer to debate “feelings” and “thoughts” of what affects pitch rather than investigate the 3 known physical parameters that actually do.

    Terholhalme was pointing you all in the right direction:

    That is the GML term, again, easy to compute.

    But you have all selected to ignore this. Thus again, I think terholhalme is correct:

    So I too, like terholhalme, shall leave you to debate stars and planets rather than what is established and known. Simply because it does not align with the stars today.
     
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  11. Guest625101138

    Guest625101138 Previous Member

    The lowest drag hull for 1t and 20kts ended up 12m with almost round section. The viscous drag dominates. I have not looked at how this changes with displacment or design speed.

    Anyhow I forced a hard chine and came up with the attached to give an idea of what I would start as a preliminary idea for the lee hull of a harryproa.

    I would use this as a starting point for discussions with Rob Denney. He has spent more time working on proas than anyone else I know.

    This hull takes 1.2kN to get to 20kts. Given that it is only 12m it might be possible to build the overall boat displacing less that 1t.

    I have not examined the trim at speed but expect it will trim bow up and there will be some net lift at 20kts. The pitching moment from the sail will obviously counter the bow up trim.

    Rick W
     

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  12. rob denney
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    rob denney Senior Member

    Thanks Rick. Could you post the drag curve of this hull against the hull I am building, please. It is 15m long, 400 wide, semi circular sections becoming Vee'd for the last metre on each end. Sides are parallel for 13m, then gently rounded to pointed bows. No rocker, prismatic approx 0.95. Assume same weight.

    I have not used flat bottoms since the early days of harrys. For the ww hull due to the slapping and slamming when it comes down after a hull fly (although the flat floor does mean lower hull height if you don't want to walk on a curved bilge). For the lw hull as I figured it would generally be slightly nose down so the flow around the chines would be messy. May have to rethink this!
    Ta,

    rob
     
  13. daiquiri
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    daiquiri Engineering and Design

    Eh... That's a nice contradiction you have here. ;)
     
  14. DrCraze
    Joined: Apr 2010
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    DrCraze Junior Member

    Don't bash the guy for choosing logic over number crunching and theory's:rolleyes:
     

  15. Inquisitor
    Joined: Nov 2005
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Well, your observation of a mild contradiction is perfectly understandable. I'll explain further, although its off topic. I have a small career's (in another field) worth of hind sight in my comments that no one here cares to hear. And, I have seen a million dollars (literally) worth of theory and numerical analysis performed on one structure. And I've seen that same structure fail because the analyst wasn't analyzing for the worst case in the worst place.

    And further off topic... like the Locheed engineer who specified the strongest steel he could for the wing bolts holding the C-5 transport's wings onto the fuselage. Well... the story goes... the bolts reached their fatigue life on the taxi to the runway where the wings promply fell off during the engine run up. Can you say, clean the cockpit time!

    Finite elements is only as good as the analyst.

    From a structural or vehicle dynamics standpoint, I know the theory and I practiced it successfully... most of the time. But when I go to design my own boat, I won't be trying to analyze the last pound out of it. I recognize boats have some of the worst and varied conditions to deal. The load cases can not be bounded as readily as planes and satellites. And although Ad Hoc is perfectly correct, I don't think we can even remotely define the boundary conditions as a function of time to enough certainty for the results to have any more validity. Yes, they may make suggestions toward a direction as he did point out in his second post.

    And my response to him did come off differently than I intended.

    So if it came down to me actually attempting the detailed analysis that vehicle dynamics suggests OR listening to Rick's mixture of quasi static analyses augmented with real world, seat of the pants experience... its a no brainer to me! IMO.
     
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