Discussion in 'Boat Design' started by sailingdaniel, Jul 14, 2011.

  1. sailingdaniel
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    sailingdaniel Junior Member

    DLR vs SLENDERNESS? If u make o boat longer an slimmer and the weight and sail area stays the same ,then the SAD also stays the same. DLR goes down and the boat can perform better.. I would think that the slenderness also play a big part? How big?. I know that there is no simple answer and there are many different factors etc but it would be interesting to here your opinions??

    Im thinking of a 16m wl and 3.3m bwl sailiboat whit a DLR of around 90 traveling at a SLR of 1 or higher .. I know SA/WS and other ratios changes etc etc.. But im interested in your "general" opinion of how much (if any) the slenderness of a hull like this would matter compaired to DLR...
    Again , Im not looking for a exact answer but im interested in your general opinions.. I dont have any VPP ore the knowledge to use them but maybee someone of u have tried this i a coputer?

    Thanks and Fair winds to everyone , Daniel
  2. daiquiri
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    daiquiri Engineering and Design

    I didn't understand if you're looking for a mathematical relationship between them, or something else?

    Slenderness Ratio (SR=Lwl/Vol^0.333 ; SI units) and Displacement-to-Length Ratio (DLR = Displacement/(Lwl/100)^3 ; Imperial Units) are both telling the same thing and are inversely proportional to each other - with the complication of the former using the SI units while the latter is using the Imperial ones. There's also a further complication of the former using volume displacement while the letter is using the weight displacement.

    But converting them is pretty straightforward. Consider that:

    Lwl (meters) = 0.3048 Lwl (feet)
    D (long tons) = 1.016 D (cubic meters)

    By substituting these into the two formulas (for LDR and DLR) and doing some math, you get the relationship::

    for fresh water:
    • DLR = (30.32 / SR)^3
    • SR = 30.32 / (DLR^0.3333)

    for standard sea water (1025 kg/mc):
    • DLR = (30.57 / SR)^3
    • SR = 30.57 / (DLR^0.3333)

    Hope that helps.
  3. jehardiman
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    jehardiman Senior Member

    It really doesn't follow that performace increases. If you make a hull longer but keep the weight (displacement) the same, then the wl beam and/or draft has to get smaller if you keep proportions similiar. Both of those decrease the ability to carry sail unless you increase the BG seperation (vertical or transverse). If you don't keep proportions similiar, then there is no comparison and you can't say that performance is either increased or decreased...i.e. skimming dishes and log canoes have similiar lengths, DLRs, and SAD ratios...but very different hulls and ways of generating sailpower.
  4. sailingdaniel
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    sailingdaniel Junior Member

    Thanks both for your reply..

    I know that DLR and LDR is the same thing expressed in two different ways. But two boats whit the same length and weight , and therefor same DLR/LDR can have different forms. One could be wide and shallow. the other one narrow and deep, and the Cp the same.. From my understanding a boat whit lower DLR/LDR can reach a higher SLR in a displacement "mode". not paning or semi planing..

    Lets forget about stabilety , sails , rig etc. I know everything has o to whit everything and so on and so on... So lets say this is a power boat whit a small engine. Does the more "slender" or the "fatter" boat need more power to reach a speed around hullspeed? By looking at kajaks , kanos , catamarans etc i think a slender boat need less power. And therefore i wonder , how much of this is because the SLENDERNESS vs DLR/LDR.. ??

    Im sure there is something wrong in my expanation but i hoop u can understand what i meen.. Im not after numbers ore formulas, more youre general ides aboute this?

  5. sailingdaniel
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    sailingdaniel Junior Member

    Asking my question again, hopefully in a better more clear way..

    Whit Length and beam i mean WL. Whit slim i mean L/B

    If u take for ex a 12 m long, 3 m wide, 12 ton boat and make 15 m and 12 ton, (metric ton, tonnes?) The beam stays the same , 3m. The hull the gets shallower.. The Cp keept to an ""optimum""..

    The LDR goes from 5.2 to 6.5 ( DLR ca 200 to 100)

    The SA/WS might go (guessing) from 2.5 to 2. The SAD stays at 20.

    I assume the shorter boat will do better, be more effiicient at low speed beacause of WS .

    The slimmer ( i avoid slender as it can be missunderstod) will do better at higher speed and be able to reach higher speed in disp.mode. Beacause of LDR an of course Length.

    I also assume in this case that being "slimmer" has a positive effect on speed. If so how much in relation to LDR????
    And when/were/what speed(Fn ore Slr) is the crossover of positve effect in between a higher LDR and a Lower SA/WS? At a given SAD..

    I ASSUME a lot and i know it makes a *** of U and ME... :)

  6. j.harkin
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    j.harkin New Member

    i think long and narrow is definitely the way to go. just look at mini 650s, they are 21ft long and 10ft wide but need a 36ft rig to maintain 8+ kt averages. whereas a 25yr old 60ft long and 12ft wide solid block of foam yacht (yes, it was carved out of a solid block of foam) with a 68ft rig can keep up with the very latest all carbon cookson 50. i know there is a huge size difference but the principle is still the same
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    FAST FRED Senior Member

    After the boat is slimmer than 6-1 it is far less hampered by wave making resistance.

    That's why many cats are 8-1 or better.

    For a lead sled compensating for the slimness may make the draft too deep.

    Light is great , but gets expensive . And cruisers usually NEED load carrying capacity.

  8. groper
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    groper Senior Member

    I *think* i understanding what your eluding to... my take on your question goes like this; talking displacement hulls only of course...

    Your wondering how long and how much beam to give a hull of a design displacement?
    Your wondering at what point do you get diminishing returns with respect to increasing length and narrowing beam, which will give better DLR and length/beam ratios for a lower resistance/ more efficient hull, before the increased WSA limits your gains?

    If this is what your getting at, then i suggest learning to use GODZILLA and michlet and look at what results you get.... if you dont have the patience, then i can tell you that the most efficient shapes are VERY long and VERY thin and VERY slender... to the point where the shapes are not very practical for real world boats - hence the compromises we all have to deal with.

    As far as putting this into real world practice, you will keep getting better efficiency advantages from increased waterline length beam ratio even beyond +20:1, godzilla usually stops increasing length around 20-22:1 so id say this is the point where WSA limits further gains. A 20m long hull that has a WL beam of 1m is already pushing the boundaries of whats practical, the only practical uses for such hulls is racing multihulls and similar such designs with virtually no accomodation in the hulls limiting their usefulness. And the same in a monohull will be almost impossible to stabilize in any useful application so you run into problems there.

    The same thinking applies to the DLR, the more the better until you run into practicality limitations...

    However, when working within a given set of constraints, its usually better to reduce the waterline beam and increase the draft for the same length and displacement with regard to lowering resistance only, all other considerations (of which there are many) ignored.

    So in summary, for your 12tonne displacement design, make it as long in length as possible until you reach practical limitations, then narrow the beam until you reach practical limitations, then set the draft to give you your design displacement - in that order - to get your minimum resistance 12 tonne displacement hull.
  9. Motivator-1
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    Motivator-1 Junior Member

    Great! Now how about hull form or shape?

    Thanks sailingdaniel,
    I have posted before about similar topics, but I like the way this thread is going, and I'd like to see it go just a little further.
    Thank you as well, Groper, for summing it up so well in your last post. Now with all of this said in very broad terms, what type of hull entry and what type of hull exit would further define a Max DLR? The last posts would indicate a hull with a full Prismatic coeffecient of 1.0...essentially a long square timber with ends as full as it's midships...A piece of lumber, if you will.
    I am at this stage in finalizing a hull for a Multihull that I am designing, but my question is:

    For the same given waterline length, and in keeping with "General Rules", What sort of Hull entry and what sort of Hull exit would be best for a hull as Groper defined...(1) As long as possible...(2) As narrow as possible...(3) As little Draft as possible? In other words, should the hull be "Fat at the ends with Parallel Midsections, and if so, Pointed at both ends...or Pointed at the Bow and perhaps a submerged Transom?
    It is the factor of the Submerged Transom that puzzzles me the most. If submerged, then how much? What percentage of full Midsection? Also, would this hull be a full displacement with flat sides and bottom, and a sharp chine as the rule would suggest? Does a Submerged Transom even have an effect at these 1,2,3 parameters? Is there a relationship for an optimum Submerged Transom based on LDR, etc.?

    I suppose that the missing element is what power is going to be applied to drive this hull, and I want to let this be a general rule once again. My intent is to overpower the hull to push it a bit past what the hull would do under conservative or just adequate power.

    I am essentilly asking for Groper's # 4, #5, and #6 perameters beyond Length, Beam and Draft. Thanks very much, Groper, for attracting my attention to this thread.
  10. groper
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    groper Senior Member

    there is no one rule solves all... when you consider #4 #5 #6 etc etc, you need to be more specific in your design goals...

    for example, the typical speed or froude number range the vessel is to be operated at will influence what type of transom or aft run you employ, same goes for the prismatic coefficients or "fullness" of the aft and forward sections... and changing these parameters influences more than just the hulls efficiency, it effects the sqat and heel behavior, pitching properties just to name a couple...

    but using your example, if your designing a high speed (operating at high froude number) catamaran, with no other considerations other than speed and efficiency, then you want a very high length/beam ratio hulls, immersed transoms of a very full section (high Cp) similar to the fullest section of the midship, super fine entry bows (low Cp), very flat rocker line and lowest weight possible (minimum DLR). If the same boat was optimized for slower speeds and low froude numbers, then you would have more curvacious rocker line with transom above the waterline of semicircular section for minimum WSA, a low Cp sharper exit stern looking similar to the bows... You still want a high length beam ratio, very fine entry and minimum weight tho. Then things change a bit depending of whether this cat is a sailing cat or power cat due to the different thrust vectors, so you need to design her with this in mind, sailing cats generally push the bow down in high winds so typically have more rocker in the hull to keep the buoyancy further forward and help alleviate this, powercats do the opposite and sqat under power so a flat rocker line and more buoyancy aft helps them trim better etc.

    But in all practicality, other seakeeping qualities or required accommodation spaces will prevent you from having a minimum resistance hull anyway. The golden rules for efficiency are minimize weight, maximize length/beam ratios, keep the bow entry as fine as possible and design your aft section/transoms/rocker depending on the speed of the vessel...

    This is what minimum resistance high speed hulls looks like, note the above parameters we have talked about... If this cat was a powercat instead, it would maintain the fullness right to the transom instead of the slight taper you see, and the rocker line would be flat from the midship aft. [​IMG]
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  11. sailingdaniel
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    sailingdaniel Junior Member

    groper , thanks for good answers..

    A L/B ratio of 20 ore 10 is really extrem. In my above exampel the L/B is about 5. If i would "pull" that hull from 15 to 30 m i would get a L/B of 10.. In that case i guess the SA/WS would be just above 1. I think that a hull like that might be more efficeient in really high speeds. But the added surface friction in such "long" hull must have a huge negetive inpact on normal speeds (Fn number) I understand that at the same FN number these two hulls would go at different speeds , but still ???

    I found this from Ad Hoc in an old thred..

    However as the slenderness ratio increases, for a constant immersed volume, the surface area also increases, which increases the Cf. But, the adverse pressure gradients over the aft section decreases as does the wake, hence Cpv is less. Contrary to this, when the ratio is reduced both the wake and Cpv are increased. The optimum ratio has been defined as 6, by trials, by Arentzen & Mandel in 1960.


    But it was for me a bit complicated to follow the discussion there. They also talked about submarines and im not sure what it was meant for..

    Earlier in the same thread i think PAR said someting like "after L/B 5 ore 6 to 1 everything goes out of the window" ( not sure this was the exact quote, but something close i think)..

    So if i streach my exampe even more , from 15 to 17 m...

    The L/B goes from 5 to 5.7.
    The LDR from 6.5 to 7.4
    The SAD stays at 20
    SA/WS goes fro 2 to 1.8 (guestimating)

    what would a good GUESS be to the performance. Not thinking Froud nr but reall speed.
    When would this longer hull whit more suface aria and higher LDR start to go faster then the shorter..

    Im thinking:.. the longer hull will go faster in knots but be slower in Fn/slr number. But at low winds the shorter hull might go faster both in knots and Fn/slr number..??

    Ore is it that the longer hull whit higher LDR wins eaven in really light winds due to lower drag at lower Fn etc...??

    I dont yet have knowledge to "try" this in a computer ore the program to do so. But any thoughts would be interesting.. Ore If some one have some "graphs" from some tests or calculations..

    I remember seeing this 5-1 ore 6-1 im many places but i cant remember where.. Is there some ''magic" hump there???

    Im a bit "over my head" here , But i still think its interesting. So please to see the big picture.. As i might explain things in strange ore wronge way.. And i Understand there is no exact answer.



  12. groper
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    groper Senior Member

    i would need to see the context in which you quoted adhoc and PAR... however i can guesstimate the "ideal ratio of 6" most likely would have been for a specific boat design type and froude number operating range, of which it would most definitely not be a high speed displacement vessel.

    "After a L/B of 6:1, everything goes out the window..." - this is referring to when a different set of hull theory comes into effect, namely slender hull theory. This is the theory that michlet models its simulations from and because of this, it doesnt accurately predict resistances for hulls of low L/B ratios. its not a magic hump, its just when a different set of rules becomes more accurate at predicting how a hull will behave.

    The longer hull always "wins"... so a good guess as to performance will be that it always goes faster if you make it longer, provided everything else remains constant.

    You can see examples of this everywhere, all high speed and/or high efficiency displacement vessels all share 1 visibly obvious thing in common, they all have extremely long slender hulls.
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