Displacement vs Planning

Discussion in 'Sailboats' started by cmarrero, Aug 20, 2015.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    Gee, I would have argued that this much rocker would not allow a boat to plane. The diagram from SukiSolo's link is attached below.

    How do you explain that? How is the water releasing from the hull? Is the stern wave somehow conical in section, tapering away from the chines towards the v-bottom? Does that imply that any boat with rocker has the potential to plane?
     

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  2. gggGuest
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    gggGuest ...

    The dynamic lift when planing comes predominantly from the forward and mid immersed sections of the hull, not the stern. Its pretty complicated stuff, which is why, as noted above, there's not really a magic bullet for the original proposition.

    Almost any hull shape will sort of plane given great enough power, low enough weight and appropriate trim, just drag any toy boat behind a fast launch!

    Uffa's writings on planing have confused many over the decades, because although he was a brilliant intuitive designer, a great free thinking sailor and a good descriptive writer, he wasn't a hydrodynamics engineer, and a lot of what he thought about how his boats worked was incorrect. A flat bottom hull is better for planing lift than a V bottom hull for instance, but a V bottom hull with 5 degree deadrise sailed heeled at 5 degrees is better for planing than a flat bottom hull sailed at 5 degrees, let alone 20 degrees. To my mind Uffa's real breakthrough with Avenger wasn't the hull shape, but that he was the first person to figure out how unballasted dinghies should be sailed, and he sailed his 5 degree (or whatever) deadrise hulls heeled at 5 degrees when everyone else was letting their flat floored boats heel right over. A modern dinghy sailor would sail one of the flat floored U section Morgan Giles boats absolutely dead upright, and ditch the heavy centreboard which would make life much more difficult for Avenger. But this is too much of a digression: sorry OP.
     
  3. SukiSolo
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    SukiSolo Senior Member

    BTW that 12 pictured was 7'2" wide in the beam (12s' are now limited to 2 meters) so had a lot of sitting out power (RM). Quite a V shape towards the stern to let the water go, the Final Chapter is diferent but still comes in a bit with quite sweet diagonals.

    Your right ggg, some of those Morgan Giles shapes are pretty good under the water, much closer to modern shapes. Of course with decent kickers and the extra sail power, and sailed flat, they would have taken off just as easily or more so than Avenger.....;)

    My dad once actually sailed in Avenger some time in the late 40s'.
     
  4. SuperPiper
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    SuperPiper Men With Little Boats . .

    Whoa, slow down. I'm having trouble keeping up with you guys.

    I've attached a diagram of Uffa's N-12 with lots of rocker. I was expecting that heeling this boat might have presented a flatter surface to support planing. But the sheer profile in plan view suggests that at any angle of heel up to 90-degrees, the rocker would be the same or MORE. I'm having a hard time imaging this boat planing.

    Can any boat plane? I thought that a displacement hull was more likely to become a submarine if it was driven (towed) too fast.
     

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  5. SukiSolo
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    SukiSolo Senior Member

    Those Uffa Kings are pretty tippy, I've sailed a couple of them. There's even a new one been built recently see the link, but so was the much later Design 6...

    http://www.national12.org/cgi-bin/Blah/Blah.cgi?m-3541/

    They will plane, just there is quite hump ie forced mode before it will pop up on the plane. Pretty rolly on the run as they are so narrow on the beam, but not too bad in displacement mode.

    You will find the later Proctor Mk8 was a lot quicker to plane, then the March Hare, China Doll, Bouncer, Baggy Trousers, Feeling Foolish, Dead Cat Bounce etc which kind of brings us up todate.

    Don't forget that dynamic weight ie crew have a big influence on trim. Note that things like 2.4 Meter keelboats (as an example) tend to nosedive because the crew has to stay in a more or less fixed position. Note also if you get very fine in the bow it exacerbates the nose diving, you may find a picture of N 12 Subversion, which soon got called Submersion.....
     
  6. SuperPiper
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    SuperPiper Men With Little Boats . .

    If this boat was planing level, supported by the hull at maximum beam, then the transom would be elevated above the water. Only the rudder's bottom tip would be immersed.

    I'm going to conclude that the boat assumed a significant bow-UP pitch in order to plane. It was planing on the rocker (see photo).

    I could believe that.
     

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  7. gggGuest
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    gggGuest ...

    Planing, being a dynamic lift phenomenum, requires angle of attack.
    If you have negative angle of attack on the water at the stern it will suck down rather than lift, so that counters the lift from the front of the boat.

    So given on one extreme a boat with no rocker at all then it will lift early and plane like crazy at the right angle of attack, but as soon as it pitches down its going to head for the bottom of the sea, and as soon as it pitches up it will drag its stern in the water, and it will be very unstable in pitch.

    At the other extreme a boat with loads of rocker will have negative aoa aft, and will be towing a damn great stern wave so will have way more drag at dynamic lift speeds. On the other hand the stern "suck" will tend to prevent it from pitching down, and pitching up will just expose a different area of hull at the optimum aofa and the boat will be much safer and stabler in pitch.

    So its design compromise as ever. Practically speaking in a very low rocker boat you keep the bow as close to the water as you can without pitchpoling, and you will go very fast between wipeouts. In a high rocker boat you will want the bow waving in the air, and you won't go nearly as fast, but you won't p*** it in nearly so often either.

    Take a look at this horrendous over simplification...
     

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  8. Barry
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    Barry Senior Member

    gggGuest
    Why do you think that the area indicated in red for the lower two pictures on the right hand side for the rockered hull will create excessive drag. Your picture shows is being ventilated to atmosphere and hence 0 psig and hence no "suck"

    Your comment "if you have a negative angle of attach on the water at the stern it will suck down rather than lift, so that COUNTERS the lift from the front of the boat" appears to be questionable as the lift generated from the front of the boat will produce a counterclockwise moment and your negative pressure " suck down rather than lift" at the stern will also produce a counter clockwise moment. Two moments in the same direction do not counter each other

    If your definition of "suck is less than 0 psig (psi gauge) then the area would merely ventilate.

    Therefore any water in contact with the bottom of the boat at the stern on a curved negative angle of attack provides a positive pressure though approaching 0 psig until it ventilates
     
  9. SukiSolo
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    SukiSolo Senior Member

    Super Piper - The 12 you show would have some trouble planing on the rudder only BUT it may be possible as the 12s' now have T Foil rudders and IF you could get both crew aft of the transom it is technically possible!!! but I doubt anyone has done it yet.

    On these relatively short two man boats they are very sensitive to fore aft trim. Your picture is also in a wave situation, probably a sea venue. Different in flat smooth water, but you would still be aft a little. Very design dependent, I've sailed some that were great and others which were a lot more tricky. Yet to fully go down the mine in one though....;)

    Planing is leaving the wave train created by the boats own length behind. Somewhere around 1.34 X square root of the Waterline length of the craft. I personally don't believe you need to change the hull angle at all to get in a planing mode, the hull will lift dead level in some circumstances. However in many cases it is better to use the hull shape by trimming to help the lift, especially in waves where the momentum of 'dropping down' a wave can promote an early break out into planing mode. Often on a really quick plane the boat will in fact run pretty level, just a bit elevated, regardless of the hull rocker IF the boat is correctly designed to plane. Very very wide sterns, simply for example promote nosediving because the crew cannot get far enough aft to prevent the bow being depressed on a puff.
     
  10. SuperPiper
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    SuperPiper Men With Little Boats . .

    Despite being a horrendous over simplification, gggGuest's diagram describes my understanding of the requirements for planning. If there is rocker, the hull needs to be trimmed aft to create an AoA.

    SukiSolo, you mentioned a keelboat with rocker that might be able to plane. What model is that? Does it trim aft like the N-12 to create a lifting surface?
     
  11. SukiSolo
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    SukiSolo Senior Member

    There are loads of keelboats that plane. A current one with a reasonable amount of aft rocker is the XP33 and that's just a cruiser.....;)

    https://www.youtube.com/watch?v=jhS-bVoZC_k

    I'll let you find the side elevation and other drawings. Interesting eh? 15 Kn is somewhat beyond displacement speed for this hull length.....:)

    Keelboats are different in that you don't get the options of moving ballast as much, as you can in a dinghy or foiling type boat ie Moth.
    Be careful of assuming the back of all 12s' are flat, they definitely are NOT but the crew would be in a similar position. Partly dependent on
    what I call 'dynamic' buoayancy, the effect of the shape above the waterline. I can assure you a Baggy Trousers is er er interesting for the
    crew on a run in heavy weather - the tiller needs to be short to prevent wearing out his/her back!, the crew has to kneel astride the C/L with
    his/her back scraping the tiller to get far enough aft. A Bouncer would be and is a doddle by comparison.

    The hull only needs to be trimmed aft, IF that will promote earlier planing, it is not always necessary and less so with a good design.

    BTW a 12 with no rocker has been tried - it does not work.....
     
  12. SuperPiper
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    SuperPiper Men With Little Boats . .

    Great video, SukiSolo.

    Here is the link to the XP33 website:

    http://www.x-yachts.com/range/xp/xp-33/

    And some pix from the website are attached below.

    This totally upsets everything I thought I knew about planing. The transom shot shows the water releasing from the rockered hull forward of the transom. I absolutely did not realize this was possible.

    Can someone help explain this please?
     

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  13. SukiSolo
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    SukiSolo Senior Member

    Don't fret about that photo, most likely she's not quite planing or the wave pattern (presumably Swiss Lake) is just doing what it does. Water won't always go to the end of the boat if it wants to find it's own level before reaching that point. If you have ever capsized in about 1 second to windward on a run in a small boat, you'll know there was NO water supporting the hull at that point....;)
    even if you were planing....I've had to straighten the odd bend in the mast from a couple of these incidents.
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    The Xp 33 hull just looks typical of many production hulls. What characteristics allow this boat to plane?
     

  15. SukiSolo
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    SukiSolo Senior Member

    Aaahh now that is what you have to learn...

    Not all reasonably current boats are the same shape, lots of things define how they behave as well as the SOR ie cruising, racing, ocean crossing etc. But small dinghies allow lots of cheap experimentation to try ideas. The maths is fairly straight forward on these small boats so you need a 'feel' for what has worked before and where things are going as well as fluid flow. That is what the pros have - many years of experience and knowledge a lot locked up in their heads. I'd suggest sailing as many boats as possible, especially of the length similar to ones you are interested in.

    BTW here are links to a couple of boats quite a bit different to the XP. Admittedly the Sigma is a lot older (the 33 is better than the 38 FWIW) and I have raced them. So there is actually quite wide variance and thinking, funny how quite a bit of development class racing is so close....;)

    http://sailboatdata.com/viewrecord.asp?class_id=7732

    http://www.yachtsnet.co.uk/archives/sigma-33/sigma-33.htm

    Enjoy exploring what makes a craft tick, you'll never stop learning with an open mind...;)
     
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