Calculating planing Velocity

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

  1. masrapido
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    Hi Tom,

    My was a comment from my personal experience some years ago, working on a testing with various underwater forms for the navy.

    As a navigation officer, I was driving the glisers towing the models we were investigating and models included concave, convex and flat deep V (we tested 20 degrees and 32 degrees shapes since 22 and 24 degrees are already well documented.)

    Concave shape exibited better directional motion, softer and smoother ride and greater drag. Convex shape was behaving more erratic than flat deep V.

    There is no conclusion jumping here, just a visual observation and what we have learned from the experiment.

    And while our naval engineers and architect did expect the suction, it did not happen. After analysing the possible explanations as to why we did not measure any suction, the engineers concluded that the only reason is in basic physics demostratable with the spoon. Fluid passing a concave shape longitudinally does not interact with the void.

    Not only that, the reason why the concave shape goes up faster can also be explained with the lack of suction. The fact that the concave shape goes up faster is exactly because there is no suction force, which would be by default a downwards force. Downwards is opposite to upwards (repeating for my own benefit here) so if a boat goes UP, that implies that NO downwards pulling force exists.

    Now, these are my comments from the memory, given the distance of the time since the tests. No hard data to display and I do not have enough of engineering knowledge to debate your statement that concave shape is rocking more and convex less. But these were our findings back than and I see from the initial posts and from james' last one that his conclusions are similar to ours.

    For the argument's sake we can treat the concave shape as two half tunnels left and right from the keel. By default the water exerts three forces: one upwards, one away from the boat's keel at a right angle and one opposite to the movement of the boat. The resultant force is angled and upwards, pointing to behind the boat.

    The boat shows two forces: downwards (the weight) and forward. The resultant force is angled and pointing forward. The water moves horizontally and upwards, not away from the hull which would result in a suction. And boat presses the water, not allowing any type of a vacuum to form. Assuming that you are right, where the suction force would come from?

    It took me a bit of imagining to visualise this:
    "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."

    I managed to see what you are saying, but that did not happen in our tests. Greater surface of the flat and the convex shapes always slammed harder. We concluded that the greater buoyancy was the reason for harder rocking of the two shapes and the greater exposed surface was producing more lift, keeping the boat on top rather than letting it fall in.

    I'd be interested to know more on the physics and dynamics that confirm what you are saying, if you have some data, graphs or images? Anything.

    I thought I would buy or built a boat with concave shape ever since then. But in the light of your comments, maybe more of technical reading is in order.
     
  2. masrapido
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  3. tom28571
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    tom28571 Senior Member

    I have had trouble understanding what you intend but think that we do agree on much of it. One problem is undersanding what you mean by "flat" without explanation of which surface is being described as flat relative to which plane.

    The claims of the Millenium concave hull run counter to what I have been taught and counter to my reasoning in regard to smooth riding in waves. As the deadrise reduces near the chine, it is bound to create slamming in the same maner that a horizontally flat hull would. Much of the video footage is in mirror smooth water and the part in waves does show what looks like slamming as the chines hit the water.

    I'm hopefully open to learning new principles or discarding old beliefs but this does not make sense to me.
     
  4. masrapido
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    Hi Tom,

    Trying to explain oneself in a foreign language is often a minefield...Hopefully here's a couple of images that will show the meaning. This page has on top the cross section of the "flat" deep V shape (the image called Deep Deadrise Hull), as I call it.

    http://www.tropicalboating.com/powerboating/deadrise-hull-angle.html

    Also, it talks about another concave variable deadrise cross section, which is a clever crossbreed between curved and flat surfaces. The Sea craft web page (the shape designers),

    http://www.seacraft-boats.com/about/hull.cfm

    also claims superior handling to standard deep V shape. And these claims, just as the ones from Quintrex are consistent with my experience during the tests with the three shapes.

    In the end our naval architects did choose the shape you indicated as the softest ride, the one I call "flat" (the deep V deadrise). Their choice was guided by the lowest drag, which is more important in military applications (fuel economy) than the comfort.

    I realised there was a bit of theory on your page and you are right, we do agree on just about all of the technical aspects. I think we only diverge on the suction part and rocky ride comparison among the shapes. I read your explanation of how you see it happening under these hull shapes but just couldn't visualise it convincingly. I readilly admit my own knowledge and other limitations which may be the reason for that.

    One thing I have learned working with professional naval architects is that hull shapes are designed specifically for a particular purpose and/or a particular speed. Outside of that very narrow objective, the hull will perform from average to bad, subject to conditions in service. If one wants to have it all, one has to fly above the water...

    Diego
     
  5. Willallison
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    Willallison Senior Member

    Masrapido - One could be flippant and suggest that there are very few spoon-shaped boats...;) As Tom suggests, and as you have pointed out yourself, there are a great many considerations when it comes to planing hull designs. But when it comes to convex sections, I'm struggling to think of a situation wher they can be in any way beneficial.
    Let me quote directly from one of my text books:
    "In section view, none of the hull bottom sections should have any concavity. Even a small amount of cancavity in section can pound in certain conditions (sometimes in many conditions). etc etc"

    I have driven a number of the Quintrex boats that feature the Millenium hull - indeed my father-in-law owns one! It's true that in some conditions - usually heading directly into a small surface chop - the hull produces quite a good ride (when compared to other aluminium boats of the same type anyway). But outside of those conditions they can be most uncomfortable. The motion is exactly as Tom described it: Initial entry is quite soft, as you would expext given the high deadrise. But the rapid change to low deadrise makes them pound badly. Also landing with the vessel heeled in any way can be very unpleasant too. I put the Qintrex example down to clever marketing rather than any great leap forward in hull design.

    There is also the strength aspect to consider. A convex shape is by its very nature stronger than a flat or concave one. And whilst this can usually be overcome by the addition of increased scantlings, the enemy of (almost) any good planing hull is weight, so any advantage that can be gained "for free' is a welcome one in my view.
     
  6. tom28571
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    tom28571 Senior Member

    masrapido, Will,

    This topic is more involved than we have so far seen. Masrapido (we need a new and simpler name) has brought up a design that I remembered from a designer that I have a lot of respect for. I remembered seeing some information in a past issue of Professional BoatBuilder and went through my stack and found it. It's in issue 91 from Oct-Nov 2004. It is not a simple concave section though, although it does have decreasing deadrise in transverse section. The basic patent is #3,237,581 dated 3/1/1066. Carl Moesly utilizes longitudinal reverse laps on the bottom of a deep v hull. That is not new and has been done by many previously but his use of a deep step at each lap and deadrise that varies from high at the keel to very low at the chine does have some characteristics of a concave hull. The performance of Moesly's boats in rough water has been impressive and I must wonder why others have not followed his lead rather than doing more and more complicated transverse steps and strakes. Maybe patent royalties was the issue.

    Perhaps air entrainment overcomes the slamming tendency of the usual concave sections. Moesly's intention was to utilize air entrainment to cushion the ride in rough water and low deadrise near the chine to avoid the poor stability of the usual deep V. By all accounts, he was very successful in ocean races with this hull.

    I did not look much further at this design at the time since I only build or design in wood and wood is not the best medium for a complicated hull bottom to be built at home. Also, my boats are too lightweight to utilize a deep V bottom. However, this concept could be used in the high deadrise sections near the bow of a lightweight boat. Most all deep V's add longitudinal strakes in an attempt to accomplish the same results as Moesly but I doubt they can work as well.

    masrapido, Don't worry about your excellent English. Far better than our command of any other language and better than many others here exercise in their native English tongue.
     
  7. masrapido
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    Hi Tom,

    I thought I left a hint when I signed my post with Diego..:)

    With my dreams "shattered" I ran some tests in the freeship plus for the three hull shapes. Interestingly the Moesly's "concave" hybrid came on top with the lowest resistance.

    Now, despite the flashy names and numbers for the mathematical formulas behind the calculations of resistance, I do not know whether they are accurate or not, but I thought to try and compare them anyway as that is the only software I have for hull resistance calculations.

    The "flat" (no curvature) shape came the second and the convex ended up as the shape with the most hull resistance. My "favourite" the regular concave shape, came close to but still behind the "flat" one, ending up as the third.

    I modified the motorboat.fbm as it is already a planing boat, with slightly convex shape.

    I am on the ship and due to 4 hours on, 4 hours off shift, I am pressed with time a bit so did not keep the files, just wrote numbers after the tests, to compare the final results. If you are interested, I can re-do the shapes and post them a bit later.

    I can see that being possible, as the water around the Mosley's hull would part to the left and to the right of the keel and, thanks to the shape, it would flow over the steps, creating voids and reducing the contact between the hull and the water, resulting in lower drag.

    Was there anything in the article that was addressing other aspects of the boats' dynamics, in the spirit of the original theme of the thread?

    Diego
     
  8. tom28571
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    tom28571 Senior Member

    Diego,

    I completely missed your "nom d' plume".

    Moesly claimed better stability at both high and low speed compared to the usual deep V's, less sensitivity to load movement, an easier ride into waves and lower power requirements. Because of the zig zag shape of the bottom, it would naturally be stiffer for equal material weight than most other bottoms.

    I would like to see the comparison of the tested shapes. Can there be complete sections for each? That would greatly help visualizing the shape.

    If you send me your email, I will copy the pertinent part of the article for you.

    Tom
     
  9. masrapido
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    Hi Tom,

    Could you send me a personal message with your email please? You have no contact details so I couldn't send you my email. I'll re-do the files in a day or so and post them here.

    Diego
     
  10. tom28571
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    tom28571 Senior Member

    Diego, I sent you a PM. Also, if you left click on a members forum name, several options including sending messages come up.
     

  11. masrapido
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    masrapido Junior forever

    The files attached. I noticed that in my first playing around I kept draft always the same and that gave me the numbers which created my first impressions.

    That is not quite accurate because the shape determines the draft and consequently displacement, which affects the drag most dramatically.

    Some fiddling with the shapes is suggested, if you are after the optimums. Files were done in Freeship plus 3.04, using motorboat.fbm sample.

    Ultimately the differences for the same displacement are minimal. In fact to small to tilt in favour of any of the shapes decisively, although they reflect to a certain point current knowledge and thinking. But, if one plays around with the shapes, one can arrive to a very wide range of conclusions in favour of any of the shapes. Optimising all shapes and then testing them in the tank would probably give the final answer.

    Diego
     

    Attached Files:

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