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

    That allright Will, we are all here to try an uncover an obvious myth in the planing of concave hulls!

    As you stated earlier John Haines may have a few things up his sleave that we do not know about - hence the patent on the hull, and my reasonsing for testing the shape without any of his 'special features such as the ski plank or spray rails. Also bear in mind that we that the planing for each hull was determined visually and there was absolutely no more than a second in it - given the size of the models as well as the accuracy of the models, they were built pretty quickly and the concave did not have a 100% accurate shape along the longitudinal buttock lines.

    We did 12 runs of the models as the wind was getting up. The concave planed within 0.5-1second of the constant 9 of the times out 12, the other 3 about the same time.

    But, the results did showearlier planing, yet determing of this planing was not as obvious for the concave as the constant. The constant tended to trim alot more to get over the 'hump' and then settle down and move with what appeared greater force aft. The planing of the concave was alot smoother and had less trim before and whilst planing. When we slowed it was noticable that the constant trimed up again and sank aft - the concave just slowed and appeared to ride on top of the water for longer - possibly "keeping on the plane" as alot of boat tests (no less than 20 from NZ and Aust magazines) have written.

    Better and larger scaled models would provide with some more substantial results - maybe for another project - but Im glad this topic has taken alot of ineterest. A tank test with underhull shots may be the only way to find out, however our budget of very little didnt allow us to fly to Tasmania and use their tank in Launceston.:(

    Guillermo: The draft of both the models was the same which does decrease the volume of the concave, however we ballasted them both out so that they were the same overall weight - but the displaced volume would still be less. Im not sure of the volumetric differences in the Signatures models compared to regular vees though. I tend to lean towards your idea of the force being generated into the sides. If there is low pressure point inside the cavity, producing a suction force, as Will eluded to earlier in the thread, then this may in turn allow water to enter underneath the hull from the sides and around the stagnation line that was not previously there. This would undoubtley mean than Savitskys method would not apply for this extra lift unless you added in another... hull factor for a certain amount of concavity on the lift. We already know that the resistance increase due to the wetted area - and assuming the results I got are correct, then the only way to counter or overcome this extra resistance would have to be have more water underneath the hull. The concave definitly travelled alot smoother than the constant.
     
  2. Willallison
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    Willallison Senior Member

    James, your observations would tend to back up my theory about the pressure distribution.
    As you say, further testing, maybe with larger models, set up to tow from the generally accepted place (which I think is the LCB...?) would be interesting. Particularly if you were able to build a 3rd with convex sections and measure the resistance of all 3 at various speeds

    Also, your thinking on weight / displaced volume is a bit screwy. A vessel will ALWAYS immerse to the point where it displaces a volume of water equal to its own weight. ie a 1000kg boat will displace 1000kg of water (1000l of salt water weighs approximately 1026kg btw).
    So if you made the boats weigh the same, then they displaced the same amount of water (at rest: once hydrodynamic forces enter the equation it becomes a whole lot more complicated). Or put it another way, if the volume of the bottom of your concave boat was less than the straight one, then it would sink further.
     
  3. PI Design
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    PI Design Senior Member

    I agree with Will about your displacement calcs. If the two models weigh the same they must displace the same. Due to the different shapes this would mean that the concave model has a deeper draught. For what its worth, I think comparing models of same displacement and differing draught is the right way to go.
    It would be good to compare the concave hull with an equal but opposite convex one.
     
  4. tom28571
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    tom28571 Senior Member

    I don't think you can accurately evaluate the trim of models while towing from the stem. Particularly with such a short tow line. I tow from the CG with a bridle. Not the CB, as that changes as soon as the run starts. Probably not as good as the fancy tank rigs but much more accurate than towing by the stem. A third line to the stem that is long enough to control large excursions, but not under tension which would interfere with normal motion, is needed to get the run started.
     
  5. tom28571
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    tom28571 Senior Member

  6. Willallison
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    Willallison Senior Member

    Oops! yes - I meant the CG, not the CB....
     
  7. member 14989
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    member 14989 Junior Member

    Yes Will I think proper tests would prove better results, I havent actually investigated convexed hulls at all. I would assume that they would tend to ride on top of the water a little more than vees? maybe? im not too sure I dont know, but I would think that they would have to push through the water and up onto it and thus slowing the transition into planing - I just have a vision in my head.

    If I do some tests again Ill spend a bit more time than a week to build, setup and test and try and get some "quality results" that we can a more informed discussion about.

    Anyway, of course you are right, a 1000kg boat will displaced 1000kg of water when static, I was trying refer to the changes in displacement during the transition to the dynamic stage for the concave and the constant - if I wrote or implied something i apologise, I think it was late night after a long day!

    Tom : "Liz" looks great, the models you made look good would produce some pretty good results. I did notice the strings on the chines as well, I might incorporate that into my next tests whenever those will be.
    My models were 1200mm waterline length (48inch) as yours is, scale 1:5
     
  8. innomare
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    innomare Naval Architect

    convex versus concave sections

    I wouldn't go for convex bottom sections in the aft. It's actually convex surfaces that create a "sucking-down" effect. I think you'd need even more power to get it planing. But as we say "meten is weten" (transl: you have to measure to know), so no harm in trying if you have the time and money to spend.

    Interestingly, the flat sections required less power to go at equal speed than the concave sections. And I think that's what you should be looking at. I wouldn't care too much if a boat is planing or not at x knots, what I would care about is how much throttle (fuel, emissions, noise etc.) you have to give it to go at a certain speed.

    James, any chance you could post your resistance/speed graphs of both hulls here for everyone to see?

    Bruno
     
  9. member 14989
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    member 14989 Junior Member

    Bruno,

    Thanks for your input. Would your basis on the sucking down be due to the increased transom area exposed compoared to the concaved area at all or along the section of the hull in contact with the water?

    And yes I 100% agree with you that particular speed at which planing begins isnt of huge importance but the ability to stay on the plane at lower speeds would be as this is where yoiu do the majority of your travelling, at planing- as this would save you on fuel consumption based on RPM etc.

    Due to the lack of resources and time the easiest way for us to test whether the concave does infact "stay on the plane" at lower speeds (thus saving power) was to determine when it they begin to plane, is was at this speed that we used, assuming that this is critical planing speed, although in reality there will be a greater speed to stay "on the plane" and above the hump of the wave.

    In terms of our resistance graphs for the testing we unfortantely do not have anything incremental as we tested from a full sized boat and were forced to use the boats analogue speedo, which was not particularly helpful as it only started at 3mph so any incremental speeds/resistances up until there were not helpful. These are crude results but gave us an indication.

    However what we did do was determine that both models begun planing around 4mph, there was no more than 1 second in between planing for each - of which over 10 runs, the concave 'averaged' 3.75seconds and the constant was 4.5seconds. In terms of resistances the boats wernt too far off from my Savitsky Calcs.

    There were many other variables such as
    -wake from each of the boats,
    -extra induced drag on the concave hull due to wider topsides (although beam at the waterline was the same)
    -deflection of the boom
    -accuracy of the resistance meters
    -precise weights, dimensions and shape of models (as they were made of ply)

    I am having trouble uploading my Excel file so if you would like a copy I can email it to you or anyone else interested. Else I will try to upload it again tomoorrow

    Cheers

    James
     
  10. tom28571
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    tom28571 Senior Member

    James,

    I think Brumo is right about convex surfaces aft. In the longitudinal direction, this would be rocker, which everyone knows is very bad for efficiency in a planing boat. Transverse convexity aft is also bad because the apparent water flow there has a transverse component as the water tries to get out of way of the boat. Perhaps this is where your concave hull gets the extra lift it must have if it is going to plane quicker. The transverse water flow encounters an exponential flattening surface and the lift component normal to the hull surface is more vertical.

    If the water flow were truly longitudinal, I would think the sum of all vertical components of lift would be equal for both models. It may help to think about what the water under the boat is actually doing. Descriptions of hydrodynamics under the boat all show the water flowing aft. While this is mostly mathmetically correct, it might obscure what the water is actually doing. The water can only react by following Newton's laws of action and reaction. The moving boat imparts momentum to the water exactly equal to the forces acting on the boat and perpindicular to the surface it encounters. If the hull is flat, the momentum is almost all down and forward. The forward component being the result of the hull trim angle. As the hull takes other transverse shapes more of the momentum is to the side and less remains to provide lift. Thus a flat plate is most efficient of all planing surfaces.

    If the water flow encounters a convex surface in the direction of flow, some energy is lost in changing the path of the water toward the curving surface which is the source of the "suction". If the surface is concave in the direction of the water flow, the water gives up energy to the surface.

    Is this making sense in explaining the difference between your two models:?:
     
  11. innomare
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    innomare Naval Architect

    Here's how Savitsky explains the suction effect of convex surfaces:

    Hold a spoon by it's tip and keep it dangling next to the waterflow from a tap. If you let the round part of the spoon touch the flow, it will immediately get sucked-in entirely.

    I agree totally, Tom. It's making sense. The concave hull would provide some extra lift in the aftship from the transverse flow component much in the same way as a flat section + reversed chine does.
    The question is, how much transverse component is there really? We just did a CFD for a fast round bilge hull and the streamlines were almost perfectly aligned with the buttocks.

    Bruno

    PS
    As for low-budget tanktesting, how about a bicycle with digital speedometer, a long fishing rod and a channel with cycle lane next to it (on a windless day)?
    You would tow one model at a time from the fishing rod which is attached perpendicular to the bicycle.

    You could attach the fishing rod to a rigid pole at the bases and from the deflection of the fishing rod, you should be able to deduce the force acting on it. A videocamera would be very useful too...
    Has anyone tried this?
     
  12. tom28571
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    tom28571 Senior Member

    Bruno,

    My test rig used a postal scale with a lever pressing against the input arm as the drag measuring element. I have since made and calibrated a metal spring steel arm which operates a dial indicator to do this better. Like you, I made several doodles of using bicycle gears to get a range of speeds while towing the model. I got on into building the boats and never followed this up but may get back to it. There is considerable "hunt" in the drag force so some visual integration is called for in arriving at the recorded force. I have looked at a damping device to help this integration out also.

    If the hull is towed fast, the transverse component of flow will not only be proportionately less but should be less in absolute terms since the boat is higher in the water and your round hull will present a proportionately flatter section to the flow. That is, the round chines, being more out of the water, play a smaller part in what transverse flow remains. Therefore the apparent flow will be more alligned with the buttocks than at the slower speed of the transition hump.
     
  13. member 14989
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    member 14989 Junior Member

    Updated

    Hello all,

    I have since significantly advanced the research into concave planing hulls, with testing conducted at the Australian Maritime College tow tank. The model (Rhino pic attached) had a very deep vee and of dimensions as per below:

    NB: a plain pure concave hull with sharp keel was selected intentionally. Essentially I was testing a SHAPE and not neccesarily a HULL.

    LWL = 1267mm
    Beam= 327mm
    draft=130mm
    deadrise 22deg-54deg.

    speed range 0.70-4.0m/s (Fnbeam=2.22)

    Experimental dynamic force for the concave, from experiment, was greater than a Savitsky modelled and equivalently loaded constant deadrise hull of 32.5deg.

    Resistance was high, however formulation and conclusions have determined that the logitudinal component to spray or bow wave was significantly larger. Natural trim also increased drag significantly. By conducting a full power and resistance analysis, I will be able to find out any advantages. But as of now the jury is still out!

    render4.JPG

    Speed range picture.JPG

    Anyone who is still interested, I can post some results!
     
  14. masrapido
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    masrapido Junior forever

    One very interesting topic. I would like to comment from my experience with ships, which should be applicable to smaller boats too. Predominant opinion expressed here is that the constant V is the most efficient. That depends on a definition of efficiency. Let us not forget that a small boat is built exclusively for transport of people and that the comfort is one of primary concerns fro that purpose.

    From that perspective the flat or convex shape are horribly inefficient to say the least. Concave shape provide a lot smoother and comfortable ride. They slam and rock a lot moe than concave shapes, skim the surface with the full area of the hull and tend to fly out of the water at higher speeds, slamming even harder when landing back on surface. There is no way that they'll re-entry isoftly nto the water due to their fairly flat and exposed underwater shape.

    Secondly, the example with the spoon actually demonstrates the opposite of what majority is saying. The rounded (convex) shape will attract the spoon, not the concave shape. In fact there is no sucking efefct produced by the concave shape for that exact fact. Easilly tested: take the spoon and place it close to the water flowing from the tap. The convex shape will suck it in, the concave shape won't.

    And with the boat shape, the principle is the same. The water is travelling in the same direction, passing the hull horizontaly, just as the water from the tape is passing the spoon. If anything, as Guillermo had said, the water will create upwards (vertical) force and that is why the boat goes up into planing earlier. Turn the concave side of the spoon and place it under the water from the tap and strong pushing force will splatter the water away from the spoon, or push the spoon away from the water flow (depending how strongly you are holding the spoon)

    The same happens to the boat except that the force is pushing the boat from both sides and the boat goes up as a result. The sucking effect of concave shape can only happen if you pull the shape away vertically from the sea/water surface which creates the vacuum under the hull. As there are no vertically sailing boats just as yet, the sucking is not a problem here.

    The only problem with the concave shape is the resistance resulting from larger wetted area.But, between the two shapes, I'd always go for concave shape for smoother, more comfortable ride and for better steering.

    The choice really depends on what do you want from the hull.The smooth ride, or to save a few litres of fuel.
     

  15. tom28571
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    tom28571 Senior Member

    masrapido,

    Many of your statements about the effects of concavity and convexity are accurate. However, these truths do not support your basic conclusions. I think you may be mixing longitudinal and transverse convex/concave shapes and their effects on hull performance. It's true that flow over a convex shape will produce suction but the hulls in question have no convex shape fore and aft but only transversely. It's false that a transverse concave hull produces a smooth ride in waves compared to a convex one. Just the opposite.

    When the concave shape enteres a wave, it does so smoothly at first but, as the entry progresses, the buoyancy increases nonlinearly and the hull will slam. This can cause structural problems in addition to a jolting ride. The amount of slam is directly related to the degree of concavity. A bit of concavity in the area of the chine is useful in directing spray away from the boat but, too much will produce the problems noted above.

    When a convex transverse section enters a wave, buoyancy also increases but the rate of increase decreases with depth of entry because of the convex shape. This produces a smoother ride. The initial buoyancy at entry must be kept low or the hull will slam though. Therefore, in either case, deadrise in the forward part of the bottom must be made high or the boat will slam.

    What appears to be either convexity or concavity to the eye may not be so, depending on the angle at which the hull is viewed. The moving boat in the water may create an entirely different shape to the water than your eye saw.

    The problem of a boat moving through or on water is more complex than the conclusion drawn from watching what happens to a spoon under a faucet. You are on the right track but perhaps, jumped a bit too far to a conclusion.:)
     
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