Critical speeds for Semi-Planing

Discussion in 'Boat Design' started by Leo Lazauskas, Feb 18, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

    redu
    I started this thread a while ago as a reference thread for my electric boat data:
    http://www.boatdesign.net/forums/showthread.php?t=20445

    May be a suitable place to add your data.

    I can help with a better prop if you want. I now have a machine shop in Canada able to mill to my design. If you post details of what you want on the other thread I will follow up.

    JavaProp is great for prop design if you can use it.

    Rick W.
     
  2. FAST FRED
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    FAST FRED Senior Member

    "If the propeller operates in accelerated flow, does that say that the driving force will be greater than if it operates in static water?"

    It would seem to me that the weight ,mass, volume of accelerated water pushed out the stern would be the total driving force .

    On old jet engines the burning gases exiting was the thrust , today the fan produces the majority of the thrust , far more efficiently.

    The fan moves large amounts of slower air than the "pure" jet.

    Perhaps the large volume of already accelerated bounadry layer water is accelerated more as a mass, by the prop operating in this faster water. The narrowing of the sides at the stern should accelerate the water even more.

    Atkin actually proposed large more efficient WWII transports (hundreds of ft long) but the Not Invented Here mentality of the military burorats prevented even one from being built.

    An interesting question is with todays superior hydronamics If an even "better" version could be built.

    Better would be good fuel mileage at higher 15-20K cruise speeds. Top speed SL 3 or so.
    There is no question to go 75K+ ,a conventional hull design would be best.

    As Rick noted skinny is in! , but there are limits for seagoing stability and mere volume for living space.

    Our plan to build a boat to stuff into a shipping container gets rid if the ocean crossing endurance requirement , only the ability to handle a really nasty day near shore is required .

    FF
     
  3. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy Rick,

    How do you simulate the acceleration of water due to the propeller accelerating the flow?

    Is that implemented in Michlet? I've never spent enough time to get it running.

    Best wishes
    Michael
     
  4. BOATMIK
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    BOATMIK Deeply flawed human being

    Thanks for that Ken,

    I wasn't aware there were so many of his design with the tunnel.

    However to me it does sound like just the sort of feature that every boat should have.

    So it really bugs me that the concept died - it doesn't seem rational that it should.

    The thing to realise is that a small increase in speed means quite a large reduction in drag. So if that is true we are talking about something of real significance. It means commercial boats can go further on the same fuel or make more paying trips per lifetime.

    Maybe it was largely co-incident with the improvements of power to weight ratios for engines?

    Can you see my problem? I struggle to find out how something that increases speed by 15% (a boat that could do 18 knots without a tunnel can do 21 knots with the tunnel) has reduced drag by roughly 25% was overlooked. It looks way too much for it to be an historically overlooked effect - it is massive.

    If it was that big every one of Atkin's boats would have had it and his boats as a group would have outperformed every boat they met.

    Sorry if I sound like a doubting Thomas here - but I just don't get it - and I worry (but only a little bit) that no-one has really done the science or careful testing.

    When someone does and the improvement is there I will be as happy as anyone - but I will fall over backwards if even a 5% dividend has been overlooked.

    It is possible but it strains my credulity.

    Best wishes
    Michael
     
  5. Guest625101138

    Guest625101138 Previous Member

    I use my own software for propeller calculation but the basic theory is the same as what anyone can use with JavaProp. This theory assumes the prop is operating in streamline flow. This is an approximation but with small error because the boundary layer is not that thick for a relatively short hull.

    The acceleration of water is not very high. The more efficient the prop the less velocity change in the water. A big prop works on a larger area so velocity change is small.

    Also, with the canoe underbody there is not the usual transom drag.

    So there is nothing magic with the hull it is just that it offers a number of benefits that compound to give a more efficient hull/propulsion set up than most other boats of current design.

    It was around the early 60s that powerful, lightweight outboards became available. Before that there were many boats using discarded car engines to power planing hulls. Hence the need for efficient hulls simply got passed by. There was heaps of power and, even I can remember, fuel at 2 shillings an imperial gallon. Hell we used to burn 35 gallons between two boats on both days of the weekend between fishing and water skiing. We got premixed fuel by the 44 gallon drum. It cost nothing. We always had diesel at home for washing hands and cleaning parts.

    So people have got used to seeing deep V planing hulls and considering these are the only real power boats. Everything else is odd. Heavy boats to handle the slamming loads, deep V to improve ride comfort but increase drag, small props to reduce draft meaning heavily loaded and low efficiency, huge engines that need a lot of fuel so weight is compounding. This is the world we have come to accept as "normal" with power boats.

    The only no-compromise power efficient hulls in common use are rowing sculls. EVERYTHING else is a compromise as far as power efficiency goes. One of the most compelling constraints of course is the need to stay upright. There are other requirements like useful accommodation that a rowing scull will not easily meet.

    So the closer the hull is to a rowing scull the more easily driven. A rowing eight displaces about 1 tonne and does around 11kts with 5kW at 70% efficiency. Now if I said I had a 1 tonne boat that did 11kts for 50nm/USgal most people would say impossible. But that is what a rowing scull will do with a good engine and prop because it is an efficient hull. There is the target. Now what compromises will be made for a practical boat and what are the costs.

    I think Atkin had a good grasp of all these factors. Ever since Fred pointed out the design to me I have been impressed with its merit.

    Rick W.
     
  6. kengrome
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    kengrome Senior Member

    This is probably nothing new to you, but a company in Canada used to make a mechanism that was in production for quite some time, and very popular from what I understand -- The Disappearing Propeller Boat Company:

    http://www.disappearingpropellerboat.com

    [​IMG]

    [​IMG]

    This type of drive system could be fitted to many different boats, and with electric, gas or diesel powerplants. It is no longer in production but I think if it were it would be popular with many small boat owners.
     
  7. tom28571
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    tom28571 Senior Member

    Guys,

    I do not in any way question what Robb White may have experienced with his version of Rescue Minor. The problem there as well as in all other reports is that there are no independent data and no design details. Most especially, there is no explanation of the design details and proof of what the effect of these details are. Robb left us no plans and did not let us in on what he was thinking when these details were decided on. He did deviate from Atkin's plan and Atkin did different designs on the same theme. How and why were these different designs arrived at?

    I want the tunnel to be a demonstrated success as much as anyone. How is this "success" to be repeated if no one knows how it was achieved. Ken is building his own version as well as Renn Tolman and his version. More power to them but I must think they are building somewhat in the dark without a clear plan. I do applaud the willingness to invest their time, effort and bank account to experiment while the rest of us speculate. To be of really useful, these need to have "control" models to evaluate against.

    Asking these questions does not mean that we think there is no merit to the tunnel. We should not be afraid of these questions and until there are answers backed by data, we are still speculating. The observation that a later design shows the aft chines immersed deeper than than an earlier one is good.

    I have a friend in SC who built a 40 something footer with a tunnel that I have driven. It seems to work well although there is no comparison to judge it by. One drawback that can be frustrating is that it is not very controllable in reverse. It ventilates in reverse even though the stern lip is below water level.

    Ken, I don't think the DisPro was meant to operate with the prop raised so it is not applicable to the discussion of tunnels. I found one of these in a shed in Ontario a few years ago. Not in the best shape but certainly worthy of restoration.
     
  8. kengrome
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    kengrome Senior Member

    I've only seen a fraction of them myself. I think the Atkin Co. had a natural disaster that destroyed lots of their plans at one time, so unless the hulls still exist somewhere I imagine lots of those designs are gone forever.

    No, I think there are better ocean going vessels than tunnel stern Seabright skiffs. Fuel economy isn't everything, sometimes you have to focus on other performance characteristics. But I do agree that there seems to be a far wider application for these hulls than they have ever enjoyed in the past.

    That's the problem with the real world. Sometimes the best ideas do not 'make it' to the top. It bugs me too, but there's not a whole lot I can do about it as an individual ... although in the case of these tunnel-stern skiffs I am trying since I believe they offer a unique combination of features that are becoming more and more appealing these days, at least to some boaters.

    Agreed, but if the designs are not adopted by "those who have the money" we cannot do much about it, can we?

    I think these boats were developed to serve a specific purpose, and that purpose was not fuel efficiency -- but this efficiency was 'discovered' after-the-fact by the folks who built and used them.

    Fuel was really cheap back then, plywood was just gaining a foothold as a viable boat building material, lightweight outboard engines were coming on the scene, and then all of a sudden 'fiberglass' invaded the boat building industry and turned everything on its head.

    I think this particular innovation got lost in the shift from inboard to outboard power, the decline in numbers of small working boats along the jersey shores, and any number of other events which conspired to dramatically alter the previously 'traditional' industry of boat building in the USA.

    You're only thinking of fuel economy, but the folks in Atkin's day thought fuel was cheap ... so this was not an innovation that they took to simply because it might save them money on their fuel costs. I think they liked the boats for other reasons -- reasons which were more important in that day and age than fuel economy. This is what I believe anyways, I could be wrong.

    I think the guys who used the Atkin boats in the days when he designed them were motivated more by their ability to get through the surf and on and off the beaches with loads of fish than how much fuel they used. These boats could get through the surf safely in both directions, whether heavily loaded or light, and use the beaches for launching and retrieving rather then being constrained to using harbors, piers, and the like. That's where these boats found their utility I believe. Fuel economy was just an incidental benefit, not one of the primary considerations back then. But today, fuel economy would be considered one of the prime reasons for having a boat with this hull type.

    I don't really know how these boats compare to similar sized boats in terms of fuel economy either, but I can tell you that they offer lots of attractive features other than fuel economy -- very shallow draft, easy beachability, surf-safe handling, sitting upright when out of the water, protected props and rudders -- these are just a few that come to mind at the moment.

    Atkin claims they are great sea boats too, although he does say in some of his writings that some of his other designs are better sea boats -- which is to be expected of course since you cannot have the best of everything in a single boat.
     
  9. tom28571
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    tom28571 Senior Member

    Up to a couple years ago, including inflation, fuel was cheaper than at any time in my life. That is, back to the 1930's. Considering the much higher standard of living now, fuel was pretty dear back then.

    I forgot to add. Most of recent discussion of enhanced performance seems to be based on accelerated flow to the propeller. The boundary layer does accelerate a measure of water near the hull surface and some of this water does impact the propeller. However, relative to the propeller, this water is actually retarded in velocity. It is accelerated only in relation to the ambient water the boat is moving through.

    I get hung up between the momentum and screw theories of propeller action. If the propeller is rotating in water that is slower relative to the blade surface than the ambient water, it would seem to be more toward the momentum theory. A propeller driving a fast boat in undisturbed water would seem to be operating more like a screw. Does this offer any solution to the possible enhanced performance of the Atkin tunnel?
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    If you extend the hull back to fill the hollow without disturbing its shape, then you will not create any additional waves. You would be placing the hull along existing streamlines.

    There are several reasons that boats make small stern waves. One is because they are travelling at Froude numbers where (roughly speaking) the stern wave is cancelled by the bow wave.

    Another reason (and one that I've just managed to model efficiently) is that viscous effects near the stern damp bow waves.

    All the best,
    Leo.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The problem of modelling a speed-dependent geometry is fairly difficult, but all you really need is fast code to predict squat. Unless I misunderstand what you meant by "speed-dependent geometry".

    And, no, I haven't come up with a super-sekrit wave-breaking formula! I can predict roughly where waves are likely to break and then adjust the wave pattern to modify crests and troughs to comply with a simple wave slope criterion. Unfortunately that's all I can do - I certainly can't predict the additional drag due to wave-breaking.

    Leo.
     
  12. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy Ken,

    Agree completely but for one problem. 25%.

    Fuel economy, Range (which has been a hard nut to crack - probably the biggest single struggle of Power boat design), Top Speed, weight reduction (mostly from the fuel).

    Maybe the combination that makes the concept less attractive is the boats where the performance was reported needed to be kept light and narrow so had a limited payload - which limits applications.

    I find it very hard to imagine a design field that is forever in search of a 3% performance improvement would miss a 15% one.

    Francis Herreshoff, Bolger, Hickman all played with light long and narrow boats and produced high performance on very low power.

    If the performance improvement is not there for similar boats with a tunnel, then the other advantages are available to boats with an outboard and the defect that Tom mentions as well.

    In the end I don't have any data at all.

    Good luck - and I hope I'm wrong - it would be a great thing. And someone who gets the information together will gain huge commercial rewards in the approaching economic conditions.

    However they will need to gather some good evidence - or it will look like a marketing spiel.

    Anyway - I will leave it there. I've made a strong enough case here should anyone want to read it.

    Best wishes
    Michael Storer
     
  13. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy Rick,

    Thanks for your contribution on all this - you have pushed my thinking all over the place and I have both learnt and relearnt some things which I think are pretty valuable.

    I might be completely misunderstanding here ...

    There are claims of higher efficiency for the Atkins Tunnel.

    This efficiency is because of some type of interaction between the prop and the hull.

    So if the Michlet model doesn't include this interaction - doesn't include the effect from the prop but still finds a reduction in drag from the tunnel - then the drag reduction occurs with all hulls when fitted with the tunnel, whether or not the prop is inside.

    Or are you modelling without the tunnel and we are seeing that the drag of a long slender and light canoe sterned boat is quite low.

    Best wishes
    Michael
     
  14. Guest625101138

    Guest625101138 Previous Member

    Michael
    I am not modelling any magic interaction.

    If you want an easily driven hull make it long and slender with nice clean entry and exit with no appendages.

    If you want an efficient prop use high aspect blades (meaning large diameter or lots of blades), have it lightly loaded and run the shaft horizontal.

    Atkin managed, quite cleverly, to achieve the combination of these features with little compromise.

    Godzilla is able to produce the optimum shape for a given speed and stability constraints but Atkin is not far off the money for a given set of conditions. What you will see is that as the design speed goes up relative to hull speed the ends will get fuller. This was discussed on another thread this week. This is evident in the hull image I attached a few posts up. From memory it is 40ft optimised for 15kts - well above hull speed.

    Rick W.
     

  15. FAST FRED
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    FAST FRED Senior Member

    I think the Atkins problem was that the boats were optomized for a PARTICULAR speed. Or at least Speed Ratio (SL)

    As engines got bigger and higher speeds were easy , with cheap fuel, who wanted to go 15K when you could go 30 , or 50?

    Now that modest , above displacement speeds , with minimum fuel burn are again of interest , and 30K is pricey for all, the box keel and reverse deadrise may yet be rediscovered , as the Austrians and Shannon seem to have done.

    FF
     
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