Calculating planing Velocity

Discussion in 'Boat Design' started by member 14989, Oct 16, 2006.

  1. member 14989
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    member 14989 Junior Member

    Is there a way of determing the theoretical planing velocity of planing Hull?
    I have a copy of Savitsky's Demo Planing Hull program which I have slightly altered to suit a Variable Deadrise Hull of 21-33 degrees, and would like to know if there is a certain point where you can help me determine the "Theoretical Planing Speed".

    I know that it is very difficult to model planing hull anyway, but, by using Savitsky or any other methods that may be avaibale e.g. A certain change in resistance, draft, LCG or any of the coefficicents that would give an indication of this point (or more realistically a range of speeds)

    I have done some testing of 2 models (see below) and the Variable deadrise did plane at a lower speed (this was done visually by inspection) - YAY! - but would really like to confirm this analytically.

    Any ideas at all would be great, regardless if its just somebodies unproven theory or thoughts.

    This for my Final Prroject for the B.E. in AUT, Auckland
     

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  2. member 14989
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    member 14989 Junior Member

    My constant and variable deadrise model hulls
     

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  3. Willallison
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    Willallison Senior Member

    It's a little difficult to tell from the pics, but it looks like you're not actually using what's known as 'variable deadrise' at all.
    Variable deardrise refers to a vessel whose deadrise changes as you move aft, and continues to do so to the transom. So at amidships the angle might be 30 degrees, at the transom it might be 15 degrees.
    What you show (or so it appears) are two hulls with constant deadrise - otherwise known as a monohedron. Here the angle remains the same aft of about station six (60% of waterline length). The first is a hull with straight sections, the second with concave sections.
    It is generally accepted these days that a boat with slightly convex sections is in fact the most efficient - and significantly stronger too.
     
  4. member 14989
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    member 14989 Junior Member

    Thanks for your response Will. There seems to be a common illusion as to what people seem to associate a Variable Deadrise Hull to(with respect to their global location). I refer to the concave shaped hull as the "variable deadrise" soley due to the fact that is the name given to it by two powerboat companies, Qunitrex and Haines Signature call it, that is my reasoning for giving it that name. When they explain it they talk about the angle of deadrise changes from 21deg at the chine to 33deg at the keel, and is measured at the transom - and all the boating circles I have spoken to (here in NZ and Australia) it seems to be the common convention.

    Regardless of the name given to it - I do agree with you though! It is far more efficient and our research concurred with our experimental results.

    Do you have any ideas on finding the theoretical planing velocities?

    Cheers,

    James
     
  5. Willallison
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    Willallison Senior Member

    Ah - yes - but these are terms given not by the designers, but the marketing departments at both Haines and Qintrex.:D
    I don't know of any simple formulae, or combination thereof that cover commencement of planing. Some very expensive FEA might do it. Problem is you have so many variables to consider.
    I think most would simply accept that in order to lower the planing speed you need to incorporate lower deadrise, a CG that's not too far aft, and low bottom loading (ie light weight and a reasonably large paning surface area)

    If you come up with anything - do let us know - I'd sure like to see it!
     
  6. member 14989
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    member 14989 Junior Member

    I most definitely will, I'll try looking into it a bit further - I would ask my lecturers for a bit of guidance but unfortunately their nautical knowledge is somewhat not existant with the exception of a couple of keen yachties - thus making the project almost completely self-directed! Which does have positives and negatives..... I have enough trouble trying to explain the differences in Haines Hunter and Signature let alone the theories of planing and resistance.... a learning curve for us all.

    I have also emailed Dan Savitsky on his thoughts - so if there is any methods Im SURE he out of anyone will know! He has helped me out a couple of times already!
     
  7. Gerard DeRoy
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    Gerard DeRoy Junior Member

    see Principles of Yacht (second edition)

    Hi,
    In this book by Lars Larsson and Rolf Eliasson, chapter 10 High Speed HydroDynamics may help you. It is using Savitsky algorithm.
    This is the most advanced I have found, that I could understand and I think that practical calculation could be made to design something practical.
    May be you are ahead! Good Luck.
    Gerard Deroy
     
  8. Verytricky
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    Verytricky Large Member

    Gosh - concave hull!

    I was told that this was a (not good?) thing to do. Something to do with the water pressure.
     
  9. tom28571
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    tom28571 Senior Member

    Ah, a convert.:D
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    James,

    The planing potential of any boat is dependent primarily on its length because length governs the creation of waves as the boat moves from displacement mode to semi-planing and then to planing. Displacement mode is any speed below a speed-length ratio, V/Lwl^0.5, of 1.34. V is boat speed in knots, and Lwl is the length on the waterline in feet, and it is raised to the 0.5 power, that is, the square root. For boat speeds from 1.34 to about 2.5, the boat is in the semi-planing (or semi-displacement) regime. That is, not fully planing, and not fully in displacement, just somewhere between the two. At speeds above 2.5 or so, the boat is, or should be, fully planing.

    This all comes from the work of William Froude in England back in the 1870s. He deduced from experiments that wave speed was a function of a dimensionless ratio called Froude Number which was V/(g x Lwl)^0.5 where V is in feet/second and g is the acceleration of gravity, 32.17 feet/second^2. A water wave travels at a Froude Number of 0.4. Therefore, for any given length of wave, you know its speed. Since we typically use "Knots" for speed units, and "g" is a constant, we can rewrite Froude Number to be the speed-length ratio that we are familiar with. At Froude Number = 0.4, speed-length ratio is 1.34. That's where the term "hull speed" comes from.

    A boat travelling at hull speed is creating a wave system with a primary wave length equal to the boat's length on the waterline. If the boat tries to go faster than hull speed, the wave length gets longer and the boat trims up by the bow because it is trying to get over the top of the bow wave it is creating. It will finally get over the top (the "hump", or hump speed) at about speed-length ratio of 2.5 or so. At speeds faster than that, the boat is fully planing.

    All that said, there are exceptions. For example, very long multihull vessels (L/B ratio greater than 10 or so) can travel in full-displacement mode at speed-length ratios greater than 2.0. This is because, depending on the shape of the hull, only very small waves are created and wave resistance is very small. There is little or no wave hump. Also, submarines can travel very fast submerged because they are not creating any waves at all.

    So all you need to know is the length on the waterline and the ratios above to determine the theoretical planing speed. Variations in hull shape do influence the actual speed where planing occurs, such as you have already discovered with your tests. L/B ratio, B/D ratio, deadrise angle and bottom shape are all influencing factors and can make that speed-length ratio limit at 2.5 vary slightly up or down. The only way to pin down precisely when planing occurs is to conduct model tests. But the guidelines herein should get you close to where you want to be.

    Eric
     
  11. member 14989
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    member 14989 Junior Member

    Thank you all for the advice, especially Eric, I had just discovered that method last night redoing my Froude calcs. Anyway a bit of useful information, Dan Savitsky replied to my email as per below, may be of some interest to everyone.

    ""To the researcher dealing with the hydrodynamics of
    planing craft, the inception of planing is associated with complete
    separation of flow from the chines and transom and full development of
    the stagnation line at the forward edge of the wetted bottom area. This
    will occur prior to the so-called hump speed when the trim angle is
    maximum. My 1964 paper in Marine Technology discusses this.
    To the average yachtsman, planing is taken to occur at
    speeds higher than the hump speed when the craft runs at trim angles
    smaller than the hump trim. They refer to this as "getting over the
    hump and running on plane".
    As a hydrodynamicist, I use the first definition.
    Good luck with your studies,
    Daniel Savitsky ""
     
  12. member 14989
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    member 14989 Junior Member

    VeryTricky,

    Not a good thing, I hope not! haha.

    You are right about one thing it is to do with water pressure. The concave hull allows more water to be kept underneath the hull and since water is considered incompressible then the pressure (or force) that the water exudes tangentally onto the hull gives it a large amount of lift, thus the transition to planing mode is alot quicker due to this lift.

    Bear in mind that water is still pushed out from the hull but in a fashion of "down and out" rather than "up and out". We did actually witness this in our testing, where we had stop half way through because the bow wake of the 21 degree V was so large it was filling up the concave hull! nearly 3 litres of water
     
  13. Willallison
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    Willallison Senior Member

    Tom - no point being here if I can't learn anything!;)

    Eric has given you a concise description of how to appoximate the point of planing. But as your own tests have shown (spuriously perhaps, but we'll get to that in a minute...) variations in shape, weight etc can have quite dramatic effects on the point at which a vessel could be considered to be on the plane. For instance my old 27ft Searay had a similar wl length as Tom Lathrop's Bluejacket 24. Yet his boat gets onto the plane at about 10mph, mine didn't really make till about 18. All down to bottom shape, weight and its distribution.

    As I suggested in my 1st post, and VeryTricky suggested in his, you have a concave bottom shape - this is generally accepted as bad news. There are three main reasons (that I can recall off the top of my head)
    One is you will be creating an area of low pressure under the hull (lower than if straight of convex sections) which will tend to 'suck' the hull down onto the water.
    Convex sections tend to throw the water clear of the hull, reducing wetted surface - drag
    Convex sections are stronger
    And 4th - bow sections with concave sections tend to suffer more from bow steer.
     
  14. member 14989
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    member 14989 Junior Member

    Will,

    I find many people seem have different interpretations or theories of the hydrodynamics of different hulls. Up until the last year my engineering understanding of hulls has been of novice standard at basically just learning as I go, which is why Im on here now, to learn! Very appreciative of everyones insight though.

    I can see what you mean in all 3 points. The suction forces would be increased due to the cavity under the hull.

    The whole idea into the project was to test and see 'how' the concave hulls work, and how they plane at lower speeds?

    I have been out on a Signature many times and definitly noticed lower planing speeds of around 12-14kph. When I compared to a 21 deg V (Buccaneer) of same size, weight and same hP, albiet different model outboards the planing speed ranged from 16-19kph. As you suggest the concave shape would produce a low pressure suck they boat into the water, this would invariably increase the resistance. In your opinion, is it possible that they are sacrificing top end speed, say 50mph, for the lower planing speeds and better stability and ride (which I believe is due to the deeper keel to cut through waves) and more efficient mid range speeds of around 35-40mph?

    What was noticable in the testing which also supprts the low pressure idea, is that the concave didnt rise out of the water as much when planing and stayed at a slightly lower trim to the V, but the concave did definitly plane sooner. Our force meters on our models - i think you can see them in the pics - also supported the increase in resistance by about 15%.

    One thing I have also noticed about the Signatures is that they do not need as much hP to get onto the plane as other similar sized boats. My boating experience and many Boat tests in both Aust and NZ have supported this aswell.

    Basically Im just trying to justify hull efficiency in terms of running and powering costs, which is one of Haines' and Qunitrex's marketing claims, that they are more efficient.

    PS>(I now understand Qintrex actually has a Variable Deadrise as you described first, the deadrise aft is straight at 15deg and then concaves towards the bow. Signatures VDH is there own patented variation)
     

  15. member 14989
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    member 14989 Junior Member

    NB: the force meter on the V hull (the right one) did break before these photos were taken
     
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