Maximum radius on spray rails

Discussion in 'Hydrodynamics and Aerodynamics' started by Olav, Dec 12, 2011.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Not at all.

    You need to think about the application of the appendage.

    If a spray rail is downward facing and 100mm wide and with a 1mm rad at the ends, is this an effective spray rail? If the rad is now increased to say 10mm (so leaving 90mm flat) is this effective?...now, if you increase the rad to 100mm, ie it is just a semi-circular section, no flat, is this effective?

    What is defined as “effective”?...redirection of the water flow in a downward and positive manner.

    Clearly the first two examples do, the last, the semi-circular rail will not. So, the radius is a function of its size relative to the amount of flat, or width of the rail.
     
  2. Joakim
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    Joakim Senior Member

    No, you are confusing fluid dynamics!

    Whisker spray is not spray (= cloud of droplets) until it leaves the surface of the hull or spray rail and is broken into droplets. It is a sheet of water, which has air on the other side and hull on the other. Just like your finger touching water flow from the faucet (not at the exit, but on the free stream). Both streams have kinetic energy. One comes from the pressure in the pipe system and the other from the moving hull, which "throhws" this sheet of water in front of it (flat bottom) or at an angle region to the sides (prismatic hull).

    This sheet of water will try to attach to the hull. If the edge is sharp, it can't.

    Whisker spray drag comes from friction of this sheet of water against the hull. A light high speed boat can have whisker spray darg of ~10% of total drag even with very effective spray rails and much more without spray rails or with ineffective spray rails.

    If you have a rounded edge in spray rails, there will be drag at that rounding. If the water sheet hits the hull again there will be friction drag.

    The spray rails in Clements paper were horizontal, 17 mm wide and 6 mm high due to deadrise. 1 mm radius was too much, while it was only 6% of the width or 17% of the height.

    Wider and/or more angled spray rails will work better with the same radius, but no matter how wide the rail is the sheet of water will try to follow the curvature at the end of it just the same. It is just the added distance from hull and added thickness of the sheet that helps to minimize the damage.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    You are mistaken in the understanding. I see you’re from Finland, so perhaps English is not your native language. But you are mistaken in your comprehension of what “whisper spray” is, from reading the papers.

    It is simply the water that is attached to the hull that is fwd of the stagnant point. Aft of the stagnation point one generally uses this running WSA etc for calculations. There is no allowance for that sheet of water that is attached to the hull, but is fwd of the stagnation point..that is all it is, nothing fancy at all. So what area is this…how do I calculate it..and what effect does it have on total drag??

    Simply put, if you place your hand in front of you, then into the water..and then at an angle a positive attitude, and push your hand fwd, there is water on your hand which is ahead of where your hand and the water intersect.

    So, all that paper describes, is a simple method to calculate the amount of area fwd of the stagnation line to estimate the amount of drag it adds. Nothing else.

    I fail to understand how you bring spray rail geometry into this?? As you now seem to equate this whisper spray with spray rail geometry and the amount of radius on them…why?..end radius on spray rails etc has nothing to do with whisper spray. Whisper spray is dependent upon running trim and deadrise angle not radius of spray rails. Spray rail location locations effect the amount of whisper spray, but has nothing to do with the radius etc of the rail. That is a separate issue. Since what is the purpose of a spray rail?...yet combine it with selective locations, it doubles up as reducing the whisper spray.

    This comes as no surprise when you obverse any planning hull on the water which has no spray rails. However if you add spray rails, what happens, you minimise the effect of whisper spray and on higher Fn boats you can improve the lift/drag ratio of the hull as the hulls ride on the spray rails as their lifting surface rather than the whole hull…that’s what racing power boats do (it increases their L/B ratio)…again, watch any planning hull on the beach it is isn’t rocket science to see why, all you’re doing is minimising that sheet of water, which appears to be called whisper spray by some.. That is what spray rails are supposed to do!. But to put this simple phenomenon into a technical paper, one ends up not seeing the wood for the trees!
     
  4. Joakim
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    Joakim Senior Member

    English it not my native language (is it yours?), but I have no problems reading papers in English. I have studied English 14 years at schools and university and all the books we used at the university were in English.

    It's not whisper spray, it's whisker spray. The name comes from the look of the spray, which resembles whiskers a cat has.

    That is covered in the paper by Savitsky et. al. from 2007, which I linked earlier (http://www.stevens.edu/ses/cms/file...f_Whisker_Spray_in_Performance_Prediction.pdf). I have the whisker spray drag model in my own VPP.

    Spray rails do not effect the amount of whisker spray, they jut separate the whisker spray earlier from the hull and point the spray to a different angle. You may have less visible whisker spray, but the total mass flow is not affected.

    The radius of the edge of the rail (not the end) has a big effect on how well the whisker spray is deflected away from the hull and how much energy is wasted to the deflection of the spray.

    Yes there are two functions for the rails, which often run all the way from the bow to the transom. Near the bow (or quite close to transom at very high speeds) they work as spray rails deflecting the whisker spray away from the hull as early as possible. Near the transom the rails work as chines allowing the actual chines come off the water at high speeds and greatly reducing the wetted surface. In some racing boats there are actually no real chines at all. In Finnish these rails are often called "lifting rails".

    My VPP takes both of these effects into account and can thus predict accurately resistances at high speeds as well, which is no true for most Savitsky based programs.


    There is not much difference whether the rail function as "a replacement chine", a whisker spray reflector or is just under water. It still needs to have a sharp edge to minimise drag.

    [​IMG]
     
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  5. Olav
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    Olav naval architect

    Wow, what a discussion! :)

    First, thanks to Joakim for the Clement paper. There's a good chance that it's the investigation that was cited in the paper I had in mind, but it's an interesting and enlightening read anyways.

    Regarding the discussion whether the radius plays a significant role or not I admittedly hesitate to object to Ad Hoc, but all investigations I am aware of say the same: Make the spray rail edges as sharp as possible. Quotes from Savitsky's paper on whisker spray drag (also brought up by Joakim):
    and

     
  6. baeckmo
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    baeckmo Hydrodynamics

    The sheet of fluid with a limited thickness that constitutes the whisker spray layer may be described as having a longitudinal velocity plus a transverse velocity. Any rounding of the detachment "line" will be projected in the absolute flow direction as a "longer" rounding. Its impact on the sheet flow is to allow an upward deflection along a parabolic trajectory, in fact the flow is breaking up into a droplet spray. This is a combination of flow diffusion and wave breaking phenomenon (think critical Froude no based on "depth" of spray sheet).

    This diffuse spray is forced to reattach to the hull, since there will be a reduced static pressure just outside the rail, due to the droplet impulse exchange with the ambient air (same process as in an ejector). The result is first an increase in hull friction from the reattached flow, and second an increase in losses due to three-dimensional flow.

    The radius should probably be related to sheet thickness rather than to spray rail width, but I have not been able to find any recommendations. Even when using extruded profiles with R~0 for spray rails, reattachment can be an issue, generally there is a recommendation to use a downwards deflection of 10 to 15 degrees in the transverse plane. More than that will create heavy spray when the deflected flow hits the surface, creating a "wet boat". So: the smaller the radius the better!
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, English is my first language. I struggle badly with Japanese, so you way way ahead of me on that score!

    Yeah..my spell check kept picking up the wrong word. I didn’t notice until I finished writing, and couldn’t be arsed to amend it,…you knew what it was being referenced.

    But even Savitsky himself flip-flopped with the definition. In his 1964 paper he called it “spray root region” after Wagner!..times are a changing eh?!! But in some of his other papers he does not even mention such effects, other than in passing. So much for being important!

    But he does always state, as all definitions do agree on, in the purpose of a chine. The classic function of a chine is that they ensure separation of flow. That is all that is required. Simple.

    Also, you have to consider the gross systematic errors in such experiments, which have been based upon “estimations” too. The estimations for simplification to one side for a moment, and model testing of spray rails….how can you effectively create and measure such small differences of radius and at model scale??..the device would required to be a 6 axis machine with laser precision, to begin with. (Not available in those days!!) And then faced with projecting these results them into full scale without such gross errors? You can’t, the systematic errors are too great to arrive at meaningful conclusions. So I would be highly suspicious of claims of major improvements when the radii at model scale would be very hard to measure accurately (in those days), as well as the results themselves based upon empirical assumptions. But that is your prerogative if you choose too.

    And as Baeckmo, the wise man of the north, also noted:

    Beyond that, don’t get bogged down into absolutes and don’t become a “can’t see the wood for the trees”. Take what is interesting but do not read it as an absolute, especially when you read the assumptions and methods used in such papers.
     
  8. Joakim
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    Joakim Senior Member

    I think the Clement paper clearly shows that spray rails and also sharpening the edges of them have a beneficial effect. Measuring the radius of the edges of rather straight rails needs nothing but a radius gauge and several samples. These state +-0.03 mm accuracy at that radius level, thus no problem at all to measure a clear difference between 0.5 and 1 mm. http://www.ttsjapan.com/measuring/radius.htm

    The absolute radius is not even important and that was not the purpose of that paper. They just noticed that spray rails need to be sharp and measured the radiuses. 0.5 mm is still far from razor sharp, which may have been even better.

    Yes there are many assumptions in the measurements and especially the radius effect may not scale. But at model scale the measurement was clear and the phenomenom is well known from rather basic fluid dynamics. Thus I wouldn't count on scaling the radius (=allowing bigger radius for bigger scale) nor having the same drag increase at different scales, but I'm certain that radius has an effect.

    Most planing boats are heavy and rather slow and thus have only minor whisker spray drag component even without spray rails. Who would even notice 1-2% (or 10%) increase in drag in normal use? At higher speeds and especially in racing (or other applications with high efficiency demands) the radius really is an issue.
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    That aspect is not in question. But you need to question the validity of the results.

    Firstly, the %’age increase is only given as a total. So, where are the original figures to check they are not cherry picking? Has an allowance been made or corrected for the increase in WSA owing tot eh strips…if not that also brings into question the validity. Nice smooth curves are shown, are we to believe that there are a near infinite number of data points? Fig 10 shows a large number of data points for the strips fwd, but for the smooth none shown. Thus are we to assume the same applies to the spray rails strip data in fig 4? Too many unanswered questions. (It is also not clear to me in the Clement paper, the measured radii, on the model or boat??)

    In their conclusions they also state it si for conventional deadrise and anything greater than 20 degrees would differ. Hardly a major endorsement.

    But the telling part is the spray rail itself. Shown here:

    spray-1.jpg

    And the one from Savistky paper here:

    spray-2.jpg

    Notice anything??

    Well, as noted by overleaf and by Baeckmo…research to date has shown that spray strips with the nominal 5-10 degrees downward are far more beneficial, why?..because they redirect the flow downwards in a positive manner. Those used in both Clement’s and Savtisky’s paper, do not! As shown in typical spray rails used today here:

    spray-3.jpg

    The spray rails used in the Clement paper can be excused, owing to its age. But the Savitsky paper is poor, because it is using data from 1991 and what is considered “incorrect” anyway, as those have a zero or worse positive angle, not negative as commonly used today. Even in the Satvistky paper he states other NAs may have different preferences….that is not rocket science and also not an endorsement of his own work!

    The angle with horizontal has a far greater influence, than minor changes in radius. Not saying ignore it, but as noted above a semi circle, as an extreme, one would not use for obvious reasons.
     
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  10. Joakim
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    Joakim Senior Member

    Yes I notice. Most importantly they are from different areas of the hull and for a different purpose. The latter ones have about 90 degree angle to the hull. The flow will not follow the face of the rail, it will more like reflect from it and thus radius is not that important. That kind of rails in that area of the hull have a purpose of keeping the boat dry when it hits waves or is driven with long waterline. There is some added lift from the "stopped" whisker spray. There will be high pressure on the face of those rails and added drag as well.

    The ones Savitsky (and I) is talking about are used to minimise whisker spray drag by deflecting the spray efficiently away from the hull. For that you do not want to use close to 90 degree angle. Maybe up to 30-40 degrees. The smaller the better as long as they do their job. The same applies to "replacement chines" used near the transom.
     
  11. yipster
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    yipster designer

    great conversation, allmost all on that trailing edges radius tho, good illustrations too but straight lined all around
    yet a hollow or parabolic whisker flow to the rails edge as i've seen in polyhulls should give a less unobstructed flow
    that flow be in a 5-10 degrees downward direction too but figger more things like waterlines, mold etc play a role
     
  12. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Your 5 meter rescue boat will one day be serviced by one of these marina clowns who give a **** for all your science. And so they will happily sand (or even grind?) over your well designed shape for an antifouling, or they will scratch your perfect surface on a trailer support. When you make a work boat, you better make it tough and fool proof.
     
  13. Landlubber
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    Landlubber Senior Member

    ....Hi mate, I am one of those marina clowns, and I object to you suggesting the part be made fool proof....that sort of thing could put us out of work!
     
  14. HakimKlunker
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    HakimKlunker Andreas der Juengere

    :) Your reply qualifies you. Sorry if I was not clear enough: Of course not all servicers are 'clowns'. I was rather thinking of one sort of-emm: cowboys?- that we often find in marinas. THEY will rather put you into work I reckon.
    Fool proof: Work boats are meant to operate. Down time reduces their efficiency. If we now put too much 'technology' into them, I think that we make them vulnerable to failure (or reduced performance). This will not change the fact that you still have your business - at last all boats need maintenance.
     

  15. daiquiri
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    daiquiri Engineering and Design

    Regarding the question about spray rail edge radius, I believe that the situation is essentially similar to the problem of a two-phase flow over a rearward-facing step, treated by textbooks and papers about open-channel hydraulics. Perhaps there is some research around about the effect of the edge-rounding on the flow separation. Just thinking out loud, for those who like scientific methods and proofs... Cheers.
     
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