Friction Coefficient

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Sep 30, 2015.

  1. Joakim
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    Joakim Senior Member

    I have hard time understanding what you are writing, but I don't think I agree with you. The pressure due to "Bernoulli effect" will not be lowest in the aft section, but before the midship. Thus it will not cause the trim you see. The bow wave starts at the bow, it will not be near midship as long as LWL doesn't start near midship.
     
  2. jesdreamer
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    jesdreamer Junior Member

    Bernouli and the Bow Wave

    I would hope that anyone coming into the thread at this point will read at least a couple pages of recent posts to get an idea of where our discussion has been going last few days --

    I was somewhat in jest per the comment on surfing down the bow wave's front slope -- but someone's earlier post did locate it mid hull when at 2x hull speed.

    Joakim's comment above per lowest pressure before midship seems to confirm my continuing contention of some flow velocity induced pressure reduction -- in contrast to several other posts which seem to deny existence of any pressure reduction along flow path.

    Since this aft increase in draft at each stroke of the C1 paddles in races on YouTube seems to be prompting a debate going around in circles, I'd like to go back to friction for awhile as questioned in first post to this thread --

    Typically friction is predominant source of drag on a canoe or kayak type hull (all displacement hulls??) as speed increases until up toward hull speed the wave resistance gradual climb becomes more steep and surpasses friction at hull speed and above. Now these YouTube C1 and K series hulls under new lack of beam restriction are starting to look very narrow. It appears that the C1 beam might be down toward 12" or even less -- just wide enough to accommodate paddlers knee and foot when placed longitudinally in-line. I can see little or no evidence of a bow wave, other induced wave(s), or wake in the turmoil of these C1 races. I can not believe the flow to be laminar for any significant flow length, and I visualize an increasing turbulent boundary layer thickness which in the LWL involved must lead to separation well before stern. With turbulent friction 5x that of laminar and a lot of energy going in to growth of boundary layer thickness & even backflow -- could we get some reduction in hydrostatic support?? And with probable separation well ahead of stern might we even have some cavitation & related reduced hydrostatic support?? all allowing dramatic increase in rear end draft at the sudden surge of each stroke??

    I haven't blindly plugged numbers into the separate friction and wave drag formulas but might these extremely long an narrow hulls yield predominately friction and very little wave drag -- even at 2x hull speed -- so might the transition to turbulent and turbulent boundary layer growth as well as separation play more than the usual level of contribution to drag??

    I remember years ago that the greater friction of a longer wetted area when compared to that of a shorter/wider wetted area of equal square inches and it's related greater drag led to the stepped hydroplane hull -- Might the back end bobbing of these unusually long and narrow C1 hulls also be telling us something?? And if so just what??
     
  3. Barry
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    Barry Senior Member

    There has been quite a few comments about the Bernoulli effect "sucking down the aft section of a boat" and the OP has said that this effect reveals itself in planning and displacement hulls

    Regarding planning hulls with a constant deadrise in the aft 2/3 of the hull.
    As the boat speed increases, the back of the hull appears to drop below its normal static waterline. It appears that the OP takes this change as the back of the boat being "sucked down" due to the Bernoulli effect.

    At rest the hulls center of buoyancy ( and I am going to call this the center of lift) and the center of gravity are at the same point, otherwise there would be a rotation of the body. As the boat increases in speed the center of lift, a combination of buoyant forces and hydrodynamic forces from the water impacting the hull moves forward from the static position. With movement comes a high pressure line very near the front wetted surface line (stagnation line) and the center of lift moves toward say a position 1/3 back from the wetted surface.

    The bow then moves up due to the front upward location of the center of lift.

    If you walk up to your kayak when it is sitting in the water and merely lift the front of the boat say 6 inches, the back drops, the actual static pressure increases at the back of the boat as some of the front buoyancy is taken away from you lifting the front.

    If the OP is correct and the Bernoulli equation creates a suck on the bottom of the hull, then the faster a displacement hull moves, eventually it will just sink. IE the pressure on the underside of the hull goes below atmospheric, lift from buoyant forces evaporate and the boat sinks.
     
  4. Barry
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    Barry Senior Member

    You only have to google "pressure distribution on planing hulls" to see actual data on the pressure distribution on a hull for confirmation
     
  5. jesdreamer
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    jesdreamer Junior Member

    I have heard all kinds of war stories of just such swamping result of towing or even paddling too fast -- could they all be result of the CB moving ahead of CG as Barry has stated?? -- or are they in fact a result of reduced pressure supporting hull as flow velocity increases?? (as with the spoon in faucet flow)
     
  6. jimburden
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    jimburden Junior Member

    Hover craft and surface effect ships SESs are usually designed to have floors above their bow and stern flexible air chamber seals that are above the highest waves they can run across at a fast speed or are made short enough to cross over larger swells. If a conformal side seal was created to descend into the wave troughs with less penetrating surface area then these SES boats could have lower frontal areas or lower upper hull air drag. If the bow and stern seals were smooth at their trailing edge and made a smooth spray free release into the air chamber of the water that passed under the seal in a long length SES there would be almost no water entrained air lost from the chamber or lower pumping losses. If these surface effect ships were as slender as railroad trains are at maybe two cargo containers wide and high then they might have little more normal operating frontal area air drag than a conventional streamline railroad train. 40,000 HP could move several thousand tons at 200 MPH to 300 MPH counting the propulsion efficiency losses of large ducted fans and about 1,000 to 4,000 pounds per linear foot supported by an expanded air ride wave conformal seal base just above the wave tops. If these now slender hulls were divided up into many possibly hundred foot long sections and pulled from the bow unit or provided with rudder center boards and wing sails centered on each barge unit then they might form power sea trains as fast per unit of power to weight ratio as land trains since the water drag is almost eliminated. In some points of sailing these longer sea trains might form sail trains almost as fast as ice boats less the drag of variable depth low aspect ratio forward swept centerboard rudders between each barge section. Imagine 100 MPH ocean crossing of sailing container sea trains and passenger and LCL cargo sea trains powered by conventional turbofan engines now having at sea level some 110,000 pounds thrust could carrying a thousand tons of freight and ten thousand passengers at less than the cost of operating a single 380 Airbus at. Their TBO is shorter but worth it under such conditions and GE has salt water resistant versions for fast military ships as turboshaft units. Turboprop fast Ekranoplan fast sea trains can not be jumbo jet large in frontal area to be fast in the three to four times denser air at sea level. This is $50 tickets from New York to Paris by water in eleven hours, take your car along for $300 dollars on the global high speed seatrain ferry boat network. This is less fuel burned per ton mile. from Jim Burden, Lincoln Nebraska.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Development of this idea is already in train :)
    See, for example:
    https://higherlogicdownload.s3.amaz...28-9885276d3c1b/UploadedImages/Feb24brief.pdf
     
  8. jesdreamer
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    jesdreamer Junior Member

    Last 2 Posts????

    I found last 2 posts to be interesting but so what?? I thought we were talking about boat hulls immersed in water --
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The sidehulls of SES are in contact with water: more at take-off and low speed,
    almost none at very high speed in calm seas.
     
  10. jimburden
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    jimburden Junior Member

    Testing surface friction for very long boat hulls.

    This proposal involves using an out drive lower unit housing with a tension load cell mounted in the gear hub. Plastic or metal pipes weighted for neutral bouyancy of the same inside diameter as the lower unit housing, of various surface finishes and lengths, could be towed to test surface drag. A base line non-cavitating following cone could also have a radio signal load cell to separate form trailing drag from surface friction. Pipe sections could be screwed together with a flush joined surface to add and subtract lengths to get various total lengths. Hundreds to a thousand or more of feet of pipe could be towed off center on a side mounted frame by a large fast boat to learn about mostly out of wake and prop wash area surface drag at length. Jim Burden, Lincoln Nebraska
     
  11. philSweet
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    philSweet Senior Member

    Too late to edit previous post, so I'll post a retraction here. Earlier, I wrote

    So it turns out, as I only just learned, this is a bit overblown. In potential flow (ie irrotational), Bernoulli's eq. works everywhere in the flow field.
     
  12. DCockey
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    DCockey Senior Member

    Also, where gravity effects are significant such most analysis of water flow around a boat, the gravity "head" term needs to be included in Bernoulli's equation
     
  13. PI Design
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    PI Design Senior Member

    Hi,
    Can anyone point me towards the transactions of ITTC 1957? I am trying to better understand the limitations of the friction coeffient calculation. I variously read that it is a friction calculation, or a correlation line implicitly incorporating some form factor. I am not clear on how this can be so, as I assume the experiments are based on 'formless' flat plates but if it is true, is it only strictly applicable to type-formed ships of 'normal' proportions?
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    http://ittc.info/download/proceedings
     

  15. PI Design
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    PI Design Senior Member

    Thanks Leo,

    I found that, but it appears just to be a table of contents (at very high level), with no way of seeing the actual content. The Archive of Recommended Procedures also does not provide links to any documentation.
     
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