SWATH speed and stability opinions

Discussion in 'Boat Design' started by jon haig, Jul 25, 2019.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    You're getting a bit confused here.
    The struts are symmetrical, which means with zero AoA, there is no lift.

    Secondly, the boat you cite:
    Is not a Swath - it is just a slender catamaran. As you can read here.

    It is an attempt - I say an attempt - to create a hybrid or a half way situation between a catamaran and a Swath. Many call this a 'semi-swath'. But no such thing exists.
    You can dig a hole, but you can't dig half a hole. You can't have half a swath, either it is or it is not a swath. Hydroponically the numbers do not stack up for anything less. Thus it is more a semi-catamaran than anything else.

    What you most likely experienced was the fact that the hull form and the design created a high KG which was not by design but perhaps more by lack of design control.
    Any high speed boat with a high KG will eventually exhibit exactly that which you cite, any high speed boat.
     
    Last edited: Jul 27, 2019
  2. jehardiman
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    jehardiman Senior Member

    Correct, COBRA is not a SWATH...it is a high submerged volume displacement cat...sort of like '95 America's Cup support boat SWATH which had all the motion/structural problems...it falls into those "not good" areas for SWATHs. Really, if you want a vessel to liken to "Concept B" you need to look at Lockheed SEA SHADOW...but good luck finding open source data on that...let's just say she was optimized for stealth, not sea keeping or speed.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Or more like the Ghost.

    BTW..I find it odd that in the header of their website they show a series of revolving pictures of vessels.
    One of them - is our design/build!!!....WTF is all that about then?
     
  4. Yellowjacket
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    Yellowjacket Senior Member

    No, not confused at all. Obviously in the straight ahead position there is no lift from a symmetrical foil with zero incidence.

    But the problem I am referring to occurs when turning. Go back and read the my post I am only referring to the condition when turning. I didn't refer to traveling in the straight ahead position. When turning some lateral slip (yaw angle) is present. There always is, and there has to be. Lateral yaw produces an angle of incidence of on vertical surfaces when turning, this is what prevents the boat from simply skidding instead of turning. When turning the vertical struts in Design A produce lateral force toward the inside of the turn because of this slip. Because of the large area this lateral slip will be small for design A. But make no mistake, when turning there is an incidence angle on the struts, and this is present in both the vertical and the angled struts.

    On the angled struts any lateral slip angle (yaw angle) will induce an incidence angle on the struts, just as it does on the vertical struts, but with design B there will be a component of that lift in the vertical direction because of the strut angle. The direction of this force depends on the angle of the strut. When the boat is turning the outboard strut in the turn will see an incidence that will create a force in the downward direction. The inboard strut will see an angle of incidence that will produce a lifting force. As speed increases and the turn radius decreases these forces get higher. That is, the more lateral force you create there will be an additional rolling moment created in the B design because of the angled struts.

    For this reason there will be an additional outboard rolling moment that is created by the angled struts when turning. This outboard rolling moment is greater than just the rolling moment that is created by the CG being above the surfaces where the lateral forces are created as is the case in design A. Since the purpose of a SWATH is to travel at higher speeds, then this effect becomes more important.

    Your assumption that the angle of incidence on the struts is zero when turning is incorrect.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    I fully understand that and I fully understand what you’re referring to.


    No, there does not have to be....and now you’re getting confused with directional stability, and the purpose of a rudder. A brief intro is noted here.

    This is true for just about every vessel, since the central of lateral resistance is generally below the waterline (half draft roughly) and multihull vessel have their CoG around the raft/main deck height which is above the waterline. The roll angle is a function of the GM.

    And what enables it to travel at higher speeds… a motion control system. This maintains a level attitude throughout the turn anyway.

    I have made no assumption, unlike you have.

    If you consider the COBRA a Swath as you previously noted, when it is not a Swath, it is no surprise you’re getting confused and making many assumptions without foundation.

     
  6. jehardiman
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    jehardiman Senior Member

    Guys, the OP wanted an opinion of slanted struts on a SWATH. While we can argue all the issues of computational water verses real water, the real situation is "don't do it, it doesn't work". There are only a few of us who have had access to most of the data and studies and have done a preliminary design of a SWATH. SWATHs are only viable in 3 sizes (because of construction and propulsion reasons) and none of them have slanted struts. Go see my old response to someone that wanted all existing SWATH research papers...read them and you will understand why you don't slant the struts, i.e. the whole idea is to make a SWATH's response VERY peaky, not spread it out.
     
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  7. Ad Hoc
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    Ad Hoc Naval Architect

    JEH,
    I'm curious by this statement:

    Care to elaborate?
     
  8. jehardiman
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    jehardiman Senior Member

    Let's see if this works....from an old thread...
    Torpedo hull

    EDIT: Yes it worked!
    Anyway, the sizes are divided based upon the construction materials and propulsion and how they fit the optimum SWATH performance characteristics. In order to make the structural, weight, and cross deck clearances, the smallest are composite with cross deck mounted IC engines. The next size up are AL with gas turbine cross deck mounted engines. The largest group which includes the T-AGOS-19 class and my senior design project which are steel and have MS diesels in the demi hulls. It is really a weight, strength, and arrangement issues as opposed to an absolute size issue.
     
    Last edited: Jul 29, 2019
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Aahh..i see what you referring to now as 3 "sizes".
    Small
    Long Range
    Ocean Capable

    Interesting take on it, but one could say ostensibly this is the same for any vessel, 3 sizes! namely (Inshore, offshore, ocean going).
    I take a different view with respect to size/speed/performance.
     
  10. jehardiman
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    jehardiman Senior Member

    True, but I can make a 8m sailing mono hull that is ocean going...not so much a 8m SWATH...<shrug>… "Different horses, different courses ''. It's what makes this so interesting :)
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    Fully concur :D
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    Gee, I don't know where to start. No I'm not confusing dynamic stability with turning, perhaps you are... But I'm sure a person of your intellect could never be confused... Let's keep this simple, and not relating to dynamic stability, just steady state turning of a system with inherent stability... That is, the simple case of executing a turn. No discussion of transient response or stability.

    From basic Newtonian mechanics, we know that in order to deviate from a straight line, an object, any object you have to generate a force component perpendicular to the direction of travel. Without the presence of that force the object will just continue in a straight line. This force is equal to the mass of the object times the velocity squared divided by the radius of curvature. The direction of this force is toward the center of the turn and is commonly called the centripetal force . Since according to Newton F=MA (force equals mass times acceleration) we can no-dimesionalize this by dividing by the mass and calculate lateral acceleration.

    Since to turn a boat without generating a lateral acceleration you would be violating at least one (or perhaps more than one) of Newtons more popular laws, and you mentioned "center of lateral resistance" you seem to recognize that a lateral force is present... Now could you please explain to us, the great unwashed, how this force is generated on a symmetrical hull in the water with a rudder in the conventional position (at the stern of the hull)? I'm sure a person of your great intellect can do this in just a few equations.

    Once you do that please show us your calculations as to how this force is created and what surfaces the forces act upon and how that force is generated by a symmetrical hull with an aft rudder without first creating a deviation from the direction of travel at any instant (i.e. yaw angle).

    We're waiting...
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Nor me. Since if you think this:

    upload_2019-7-31_13-34-22.png

    ....is a Swath hull form, it is no wonder you're getting confused and attempting to create a debate about a non sequitur issue. Any attempt is just showing ignorance of what is a Swath hull form.

    And it seems to continue:-

    But then you say:

    If that is the case, why are you introducing control surfaces into your argument? o_Oo_O

    Im somewhat aghast at this statement (which perhaps suggests why you're already confused with what a Swath hull form looks like) - please state where i have said it is not present. Which tends to explain your over zealousness in creating a non-argument over what....??...im not sure. Go figure... :confused:

    Ok, so...a series of numbers and calculations will suggest what to you...or is this just another attempt to move the debate sideways for some unknown reason?
    Since if one understands what a Swath hull is hydrodynamically, words alone are more than sufficient, since the numbers/calculations merely follow the words and understanding of said words....

    However, if you wish to waste band with with cut and pasting basic equations and forces of what are produced on a vessel during a turn, from any text book ...please do. But I fail to see how this resolves your dilemma of misunderstanding what is a Swath hull.

    Who is "we"...and waiting for what?...

    Most odd reply to a simple issue.
     

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  14. jehardiman
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    jehardiman Senior Member

    Yellow, I hope you are familiar with the control characteristics of zeppelins. Yes, an airship and a water borne vessel turns because an AoA on the hull is developed by the rudder. When turning to the stbd, the vessel first moves to port caused by the rudder force. Then the vessel begins to yaw to stbd, picking up the lateral drag force based upon the CoA. What happens then depends on weather the vessel is fully immersed in the fluid. For fully immersed vessels like airships and submarines, the vessel then rolls INTO the turn because the CoA is ABOVE the CG. For partially immersed vessels like most ships and especially SWATHs, the vessel rolls OUT of the turn because the CG is ABOVE the CoA. (please construct the FBD's to satisfy yourself). Rolling out requires less rudder input both to option A and option B, with option A generating more lateral force because of geometry (barring some weird edge of envelope CoA/CG stuff). Unless the angle between the two struts is some structurally unacceptable obtuse angle on option B, the inside strut will never generate a force causing additional roll out like an aircrafts wing.
    Also, ships "skid" in turns, they do not "slip".
    Finally, I notice this is a X-post with Ad Hoc...
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    Exactly... While lateral force can be generated by rolling, depending on the geometry of the hull and the placement of the CG, there is always a yaw angle present when turning. What I was describing in this particular case (the version B of the OP's design) is commonly described as "adverse roll" in aircraft that is created by anhedral wings. While the surfaces in question here are symmetrical, the effects of yaw (skid) are the same and are present. The magnitude of the forces would have to be determined (based on speed and yaw angles), but there is no question that these forces would be present with a design as shown in version B of the OP's design. In this case the outboard struts, as a result of their angle have a negative incidence when yaw is present. The inboard struts have a positive incidence. This is fundamental to the geometry. Imagine a very large yaw angle. Like 45 degrees. The outboard strut is digging into the water and the inboard strut is acting like a lifting surface. This is a gross overstatement, but those same direction of forces are present at any time there is a yaw angle present.
     
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