Calculating planing Velocity

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

  1. tom28571
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    tom28571 Senior Member

    I may be off the mark here but one thought is that the concave hull (variable deadrise) is also operating at variable degrees of Froude number from keel to chine. Something like the glider concept that has been discussed. Or maybe not. In the limit, the concavity becomes a flat hull with a deep keel. What does Froude say about the variation between these extremes?

    Both hulls, if they have the same waterplane should experience the same amount of hydrodynamic lift at the same trim angle although other factors like the "suction" mentioned would change the net lift. Problem is, the tests results seem to be contradictory to that reasoning. I wonder how valid it is to tow by the stem which will surely influence the resulting trim angle.

    Also it is not clear in the photos that the concave shape carries into the bow sections.

    One other puzzle is that Eric says the trim angle is max as the boat reaches the ""hump" speed and thereafter decreases. All other boats I have seen do perform that way but mine does not. The trim angle of the 24' boat mentioned by Will actually increases throughout its speed range to a max of about 2 degrees although it does not appear to change much from the onset of planing to max speed of about 24 mph. While this hull is not concave in any sense, it does have rather large reverse deadrise chine flats, of increasing width going aft, which may act somewhat in the same way. Low speed planing and level running were two of the main goals in the design.
     
  2. nevd
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    nevd Junior Member

    Concave deadrise angle

    Hello James,

    I am very interested in your work. To clarify the variable deadrise debate, and I agree with Will's interpretation, I recommend we use the term concave deadrise.

    Have your tests controlled the trim angle of the models when you concluded the concave deadrise model had lower drag than the constant deadrise model ie was drag of concave deadrise model lower at same trim angle?

    Your model pictures indicate the deadrise from chine to keel is the same on each model. Could you please confirm this is correct.

    I don't understand the earlier discussions on suction as I thought suction was caused by having longitudinal convex planing surfaces like many Australian pressed Aluminium tinnies. Could somebody explain how concave deadrise will produce suction?

    It is difficult and therefore costly to construct an aluminium boat with concave deadrise as compound curves are required in the forward stations. These compound cuves require strip planking as used by Joshua Aluminium boats or stretch forming as used by Quintrex.

    Good luck in your studies.
     
  3. Willallison
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    Willallison Senior Member

    Nevd - Wish I had my 'library' here to make sure I get this right, but.....
    water is travelling from keel to chine (not directly of course, but at an angle). The water that is closest to the bottom must travel further creating an area of low pressure.
    I may be way off the mark here as its been a couple of years since I looked at this.

    Tom - yes you would expect the hulls to present the same total horizontal surface are to the water, but the distribution of that area would probably be quite different, with a far greater proportion being aft on the concave hull due to its hollow waterlines.... I think!

    Also - and this is demonstrsted by James' experiences - the concave boat would sit lower (or at least its bottom sections would be submerged more) as the horizontal (lifting) surface area is lowest near the keel, whereas with straight sections this increase is linear. With convex sections of course, it is highest near the keel.

    James - Its been a few years since I looked critically at a Signature hull. From recollection, there's a bit more going on there than simply concave sections. Aren't the boats a bit like a 'clinker' hull, with spray rails and the like built in to the change in deadrise? Also I think the various surfaces end at different distances forward of the transom.
    John Haines is a very clever bloke, who's been around boats for a long time - maybe we should ask him...!
     
  4. member 14989
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    member 14989 Junior Member

    Nevd, I think concave deadrise is a good term that seperates itself from further confusion, as typical variable deadrise hulls are actually different as Will suggested, hence the SVDH - Signature Variable Deadrise Hull, a patented 'version' of a variable deadrise.

    Anyway, to clarify our test results the concaved hull actually had greater resistance according to our crude meters by about 10-15%, which it should do if we are talking about friction due to the increased wetted surface.

    With our models I did not want to 'pigeon hole' the Haines Signature hull as there are other companies who use other versions, such as wider chines, ski planks and strakes. To simplify things due to time constraints my aim was to compare the geometrical shape of the hull and then try and relate it back to the applications of Signature and Quintrex. To try and make the shape testing as easy as possible both hulls are the same length, beam and depth (draft).

    The draft was the important bit. Signatures and I think Quintrex do to, have a deadrise of 33-34deg at the keel which trivially means that the depth is going to be larger than that of a typical 20-21deg v shape. So, since we are only considering the shape I thought that it would be more scientific to eliminate the differences in draft so that they were equal. This ofcourse means that the concave hull has less immersed volume. As luck would have there was only 5% difference in overall weight of the two when they were displaced to their waterlines. This would have changed once we started testing as water was flying in and out of each hull (we displaced each hull using sand).

    My understanding on the suction that Will eluded to (and I did agree with him) is that a low pressure situation would take place at the centroid due to the concave shape - think of it crudely like lifting a bucket out of the water upside down - however Im not sure that this would still be true once hull was moving. The dynamic force of the water getting pushed against the bottom of the hull would react in an opposite effect, normal to the angle of the deadrise, and actually create the lift - as explained by some journalists and the people who design these hulls. There may be a suction force at the transom of the boat when moving once the water 'exits' the hull, yet, this would be there anyway for a constant deadrise hull as well. The induced drag acting on the back of the transom where this water is exiting relates to the suction forces... maybe... maybe not...

    Another thought on everything is that the concave hull tends to stick closer to the water allowing the water the move around the shape of the hull rather than being pushed away from it. A few boat test reports i have read from NZ and Australia have explained "the hull manipulates the water".

    Will, you are right, John Haines is a very clever man! and yes there are strakes and a lifting plank which all combine into a big cauldron of got knows what to try make them work. Hes had 7 generations now of these hulls and Im sure theres a reason of the patent like he's hiding something important. Now im not too sure anyone will know how they work but him - that was part of the project - to find out!

    Anyway the basic aims of the project of confirming the lower planing speeds worked with our models, against alot of the theory!
     
  5. Willallison
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    Willallison Senior Member

    Ah yes - but it's all in the interpretation.
    You said that the resistance of the concave hull was greater than for the constant deadrise. It would therefore require more power to achieve a given speed, even if it got onto the plane at a slower one....
     
  6. member 14989
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    member 14989 Junior Member

    Yes, that is the tricky part that i am trying to uncover, it should need more power to get onto the plane at the given speed but boat testers claim it needing less speed to plane, thus less power to plane. At the same speed when the concave is planing and the constant is not, the power for the constant would have to be greater again to overcome the inertia. The frictional resistance should be greater, and it is due the wetted area and will continue to increase at a greater rate, but does it have a larger effect on the planing concave compared to the not-yet planing constant? The displaced volume of the non-planing will surely be larger than the displaced volume of the planing hull. All assuming that this lift kicks in?

    I wasnt too sure, so I tried this:

    For a planing concave and a non planing constant at the same speed of say 12km/hr for agruments sake:

    Concave = Frictional drag + induced drag + displaced volume
    Constant = the same

    using my Excel spreadsheet with all my Savitsky numbers and calcs I get this:
    please excuse the rough numbers for displacement - im taking a stab

    Concave Drag (N) = 157 + 83 + 12,000 (1000kg) = 12,240
    Constant Drag (N) = 130 + 70 + 15,000 (14000kg) =14,200

    All in all I guess coming up with the reasoning for the "exceptional lift" that is created by the hull then the lower planing speeds I think would be justified, and thus less power to keep it on the plane, due to the less diaplaced volume that the concave has - yes the resistance will be increased but as a small sacrifice to the volume that isn't immersed.
     
  7. Willallison
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    Willallison Senior Member

    I suspect that what you are seeing goes back to what I was referring to before - the distribution of the lift.
    If you examine the waterlines for your concave hull you will see that for a given displacement, it has a far greater proportion of its buoyancy aft - ie the CB is further aft. I suspect that this is the main reason that you are seeing (apparently) getting on the plane at a lower speed.
     
  8. PI Design
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    PI Design Senior Member

    Eric,

    I disagree with your assertion that the speed required to plane is related primarily to waterline length. As you corerectly state, Froude's work established the concept of 'hull speed' and the relationship with bow wave formation. At speeds greater than hull speed, 'normal' hulls will experience a rapid increase in resistance if they are still in displacement mode (due to, for example, being very heavy). As you point out, some craft e.g. multihulls and submarines, do not generate significant bow waves and are therefore not affected by the concept of 'hull speed'. However, 'hull speed' is in no way related to the speed required to generate sufficient dynamic lift to be planing (assuming one can agree on a definition of planing). Planing categorically can not be defined as exceeding some selected Froude number. Very light and flat craft may plane long before 'hull speed' is reached, other craft will not plane until hull speed is exceeded (and therefore won't plane unless given a very large power supply). It is mere coincidence (or, arguably, good design) if the speed required to get on the plane equals hull speed. The speed required to get on the plane is a function of vessel weight, wetted surface area, deadrise angle, aspect ratio, water density (fresh or salt water), angle of attack and detailed hull form factors. It is this hull form factor that James is investigating, but it is a tricky area.
     
  9. tom28571
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    tom28571 Senior Member

    I have to agree with PI on this issue. The fact that most boats conform to Eric's assertions on speed/length and Froude relationships does not mean that this is an ironclad principle that always applies. Not being a NA or even classically trained in hydrodynamics, I have to get by with my own study and observations.

    I followed the idea that a properly designed hull that was light enough in lbs/sq ft of bottom area could plane earlier than hull speed formulas might indicate. I do believe that thinking about hull speed is a great deterent to hull development. What is most important is to consider how much lift is required to get a boat out of the high drag wavemaking mode. Low bottom loading means less lift required and lower trim angle required to generate that lift. Lower trim angle means longer waterline length at the transition hump and, if we think about hull speed at all, it's higher for a boat with a low trim angle than high trim.

    If a boat is planing, it's trim angle will always be just high enough to generate the lift in addition to buoyancy to hold the boat in equilibrium. If the boat sticks it's nose in the air with a high trim angle, the waterline length is very short and hull speed is much less than what the formula predicts. If the boat is at its maximum trim angle, then a bit more power will add speed which will generate more lift which will allow a lower trim angle and a higher "hull speed" and less wavemaking drag, and so on, and so on until the boat is running much faster.

    The key element in all this is the weight of the boat which determines the lift required to get it planing. So as PI said, a hull that is light enough will plane so early that "hull speed" is almost a non issue. Or that is the way I think about it without reliance on math. I am not, in any sense downplaying the importance of the formulae and mathmatics that are of such value in boat hull design. However, we can get stuck in a rut in being hog tied by the application of some rules that are, for the most part, imperical and require a variable multiplier to make them fit observation. This is the essence of my fight with "hull speed". At one extreme, there are the very long skinny displacement hulls that violate the rule and at the other extreme, I and PI say that very light hulls also violate it. For the most part, I ignore it.

    This is just the way I think about it and if anyone's principles are offended, I appologize.
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    PI and Tom both make valid points, and when taken with Daniel Savitsky's comments in post #11, we all pretty much revolve around the same issues. The point I was trying to make for James was to establish the past foundation of Froude, wave-making resistance, wave speed, and hull speed. James' models look pretty conventional, so I felt some conventional science that does apply to the vast majority of boats was in order. Certainly, there are exceptions to each situation, and as I mentioned, if you want to fine tune the behavior of any particular design to discover what other dominent factors may be influencing planing, then model (ior full scale) tests are recommended.

    Eric
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    Pressure on a concave deadrise increases towards the sides generating more lift there. Maybe this extra lift exceeds the relative loss of lift in the region closer to the keel (compared with the straight deadrise hull), thus generating a bigger total lift and so making the boat going earlier into planning mode (The outward extremes of the bottom act as if they were spray rails)

    All this if displacement remains the same in both models. If the concave deadrise one weights less (as to keep draught constant, i.e), then the earlier planning effect could be an addition of both the 'spray rail' effect and the lesser weight.

    Also: Maybe Savitsky's method is not directly applicable to such a concave hull. Read through thread http://boatdesign.net/forums/showthread.php?t=2187
     
  12. Willallison
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    Willallison Senior Member

    Guillermo - no, as I said earlier, I think its the distribution of pressure that probably changes, rather than the amount of pressure. The concave hull will surely have to present more of its bottom area in order to create the same amount of lift, as the area closest to the keel (with the highest deadrise) will present the least lift (pressure). Significant lift will only be produced by the area closest to the chine. Assuming that wl beam and average deadrise are the same for both boats, the total horizontal area presented to the water will be the same (if both boats are immersed to the chine).

    A little further research through my Westlawn texts and a couple of others reveal that it's primarily in the forefoot that concave sections are considered an absolute no-no. Here, convex sections are 'provably better' (No proof's given though!!:mad: ) In the after sections straight or slightly convex sections are considered best.
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    My dear Will: If the pressure only changes distribution but not the total amount of force it provides to the hull (vertical component), then there is no reason for the earlier planning. For an early lifting more vertical force is needed for the same speed, thus implying also a net higher pressure (acting over the same total area).
     
  14. Willallison
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    Willallison Senior Member

    Hmm - perhaps....
    But if a greater proportion of the pressure is acting on the aft sections - as would be the case with concave forefoot - then the trim angle would be less. It may be this that actually brings about the earlier planing.
    The fact that it also brings about greater wetted surface must only start to have a big influence as speed increases
     

  15. Willallison
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    Willallison Senior Member

    We are also assuming that the results as presented by James are correct (no offense James!).....
     
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