Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

  1. kengrome
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    kengrome Senior Member

    Here's something else to think about ...

    What if it takes more energy to "bring the water back together" behind the box keel than to simply allow it to flow straight back from each of the side tunnels? In this case the boat in this picture might be even more efficient than the Atkin tunnel-stern designs:

    [​IMG]

    This one uses twin propellers, one in each tunnel, to provide the thrust.

    When Fred asked earlier about how to possibly accelerate the water in the individual side tunnels even more than it is already accelerated, I said a couple of smaller props in those tunnels should do the job. Now I'm thinking that this design might not even need the main propeller if two side-tunnel propellers are used instead ... and it might be even more efficient than the Atkin designs.

    With two small props instead of one large one, the tunnel section can be shallower and the boat will still have the same draft -- but with less lifting of water up into the tunnel section. Of course means more power available to push the boat forward and thus more efficiency, right?

    But will the widening side-tunnels aft create less efficiency?

    Instead of funneling the water to a smaller area this design APPEARS to let the water that is trapped in each tunnel spread out a bit toward the aft end of the boat. I would think that the best efficiency might be found in just the opposite condition ... where a wider tunnel narrows down so the prop can be located in the narrowest part of the tunnel.

    Or maybe it just "looks like" the tunnel gets wider aft when really it doesn't?

    Another issue to look at here is that the box keel extends all the way to the transom -- which makes it longer and more efficient, right? So how much more efficient does this make the boat compared with one that has its box keel 2-3 feet shorter as in the Atkin designs?
     
  2. FAST FRED
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    FAST FRED Senior Member

    This indeed might recapture the energy in the wash , but the surface avilable for the prop pressurized water is far smaller , so there would be less lift aft , and perhaps the stern would not be held up.

    Long and slender is wonderful when there is lots of SURFACE wave making. With the box keel supplying much of the displacement , I'm not sure the added skin friction would be an improvement.

    Looking at the submerged surfaces of other boats that get buoyancy from under water surfaces , most don't seem to be very long .

    Perhaps Navy research on the many under hull protrusions on modern warships could answer the question , although most go over the 20K envisioned .

    *

    Now in Oswego NY , enjoying a day off , abet with 4 hours of "field day" (general task & clean up time). Again free Wi Fi fills the air waves , to my delight!!
    Monday its off to Canada to pick up Trent Severne waterway.


    FF
     
  3. tom28571
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    tom28571 Senior Member

  4. Excalibur
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    Excalibur Junior Member

    I'm not at all sure Fred is wrong. Boundary layer water is created regardless of whether the energy is recaptured or not. I think the word "free" is what is throwing folks off. The energy contained in the moving boundary layer is not "free", but if it is recaptured as it flows through the prop, it's a net gain.

    As far as pontoon boats are concerned, there are different kinds. Some have pretty much "pure" displacement shapes (ie, round bottomed logs), but others are designed with 3 logs and lifting strakes. Those are the fast ones.

    Check the pic in this article

    http://boatingworld.com/Articles/2007/04/Boats/Lowe_Suncruiser_Trinidad_220XL.html_491904626.html

    You can clearly see the optional lifting strakes tacked to the outside.
     
  5. tom28571
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    tom28571 Senior Member

    I put this question to a couple people who should know, if anyone does, and did not get a definitive answer. The question being whether the forward moving water from the boundary layer feeding into the prop was a net positive or negative effect?" I don't know yet but feel that it may be positive.
     
  6. Excalibur
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    Excalibur Junior Member

    There are conflicting forces in play here. Generating boundary layer laminar flow creates drag. The Atkins design has more surface area than a round bilge boat with the same beam and displacement would have, so drag should be greater. This is born out by the tank tests that reported that the Atkins hull had more drag than a conventional hull when towed. The only way that the design could be more efficient under power is if some if the the energy used to generate the boundary layer is recovered by the prop. Someone hurry up and build one so we'll know!

    The twin tunnel design, BTW, has at least one significant difference that will not enhance efficiency. The hub of the prop generates no force, and is in fact a source of drag, as are the struts and shaft. The Atkins design has no strut, and puts the prop hub right at the end of the box keel where water can most easily flow around it.
     
  7. kengrome
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    kengrome Senior Member

    Tom, it has to be positive. It's the same as motoring downstream. When you're motoring downstream you're moving in the same direction as the water and you're going faster than if you were motoring in still water ...

    With these boats (and with any other boats for that matter) the boundary water is already moving in the same direction as the boat, so the effect is positive. But in the Atkin designs much of the moving boundary layer water is being directed into the propeller where it can be "further accelerated". I think this is the main thing that makes these boats more efficient.

    I can finally understand what Fred was trying to describe when he kept saying that the water is accelerated before it gets to the prop. It is already moving in the direction of the boat because of the skin friction effect which drags some of it along. The main difference between the Atkin designs and most other motorboats is that Atkin takes advantage of this moving boundary layer of water instead of ignoring it.

    In this respect my twin-tunnel will do the same thing -- take advantage of the moving boundary layer of water by trapping it beneath the hull and directing it toward the propeller(s). Obviously there are other factors going on in my design that will result in more or less efficiency, but there are so many potential things going on here that it seems only a test will tell us whether or not the twin-tunnel is "more efficient" than the Atkin design. I suspect that it is not.

    However ...

    If the boat must accommodate twin engines, my twin-propeller design may be the most practical even if it isn't quite as efficient as the typical Atkin tunnel-stern designs. It should still be more efficient than the vast majority of other semi-planing hulls out there these days which of course do nothing to use the accelerated boundary layer water to their advantage.

    As I have discovered recently, there are not many cheap or easy ways to turn a single screw with twin engines either. This makes me want to design a boat that is powered by the twin 10 HP diesel engines that I can get over here in Asia for $200 each. I think they are a great solution for small boats that needs 15-20 HP to reach low-end planing speeds and want to have two engines for redundancy. These engines are already EPA-certified too, so they can be imported to the USA -- a big factor for me since this is what I would like to do sooner or later (probably later).

    Personally I like redundancy on the water, especially in a boat that's going places far enough from land that a potential disaster might be avoided with more than one engine. If one engine dies in this system you can still get home on the other one with no problem -- and it will NOT cost you an arm and a leg to get one of these engines fixed either. If worse comes to worst you can simply pull out the bad engine, throw it away, and bolt in a new one for a few hundred dollars.
     
  8. tom28571
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    tom28571 Senior Member

    I also think the accelerated water in the boundary layer is a positive effect on a propeller but am afraid your example is irrelevant. A boat in a steady current is completely unable to tell whether it is in a current or not without some outside reference.

    In any case, the effect will be most prominent in a displacement boat and all but insignificant in a planing boat. The easiest way to prove or measure this effect would be to calculate the slip of the propeller on an Atkin boat and compare to a comparable case on another well established "normal" hull where the boundary layer effect on the propeller is minimized..
     
  9. kengrome
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    kengrome Senior Member

    Understood, but I was actually trying to get at the relationship of the boat with the ground, not the boat with the water. Obviously I didn't explain this well enough which is no surprise to me. I know what I'm thinking but getting it down in writing so other people can understand it is not always the easiest thing. Maybe an example will help:

    Let's take a look at the theoretical performance of Boat A and Boat B:

    Boat A - an Atkin tunnel-stern or similar design
    that funnels the accelerated boundary layer
    water to the propeller

    Boat B - a typical boat that does not funnel
    the boundary layer water to the prop

    My guess is that when both boats are run in completely calm water Boat A will reach a speed of 14 knots under the same power that Boat B will only reach 12 knots.

    Why?

    Because the prop in Boat A is running in the accelerated boundary layer water, and that water is already going 2 kt faster than the water being pushed by the propeller on Boat B. So the Atkin design simply ADDS the speed produced by the engine/prop in still water to the speed of the moving boundary layer water. The result is that the overall speed is higher in the Atkin boat.

    It is common knowledge that for the best efficiency and performance propellers should be run in water that has not been disturbed by the boat. But guess what? I believe this "common knowledge" is wrong, especially for boats designed to run at low-end planing speeds and slower ...

    I think Billy Atkin somehow realized that *IF* the prop could be made to run in the disturbed (and accelerated) boundary layer water -- without cavitating and without any other problems -- he would have a more efficient boat. His tunnel-stern keeps the air out of the tunnel so it seems he eliminated the biggest potential problem right there. But that's not all ...

    He also managed to prevent the stern from squatting at the same time, which of course suggests that the boat should be more efficient in the speeds between displacement and planing since it doesn't have a big "hole in the water" to climb out of like most other planing powerboats do when they actually start to plane. Smaller holes (or no holes at all) will mean less energy to push the boat up and therefore more energy to push it forward.


    Granted, and since those Jersey Seabright skiffs were originally nothing more than displacement boats in the first place, Atkin seems to have found a way to modify them in such a way that he was able to make them plane ... while also using the accelerated boundary layer water to great advantage.


    I don't know about prop slip as a measurement, how is this done? My other question is how do you determine which normal boat to use in the comparison? Any suggestions for one that is similar?
     
  10. kengrome
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    kengrome Senior Member

    At what speeds?

    I don't think you can talk about drag unless you also talk about speeds at the same time. They are related to each other, aren't they?

    There may be a cross-over point where the less efficient hull at lower speeds becomes the more efficient hull at higher speeds. But even if not, I think you're missing a point about these long slender box keels ...

    Personally I think the long and narrow box keel supporting most of the boat's weight will reduce the total drag on the boat over that of a round bilge hull that (in my opinion) has nowhere near as efficient an underwater shape.

    My gut tells me that when most of the weight is held up by a long skinny underwater section the boat is simply going to move more efficiently -- at the low-end planing speeds (or semi-displacement speeds) we are talking about -- than any other type of hull except for a catamaran. And catamarans generally have two "even more efficient" hulls than these box keels, with no other hulls to drag on the waves at the surface.


    I never saw any of those tank test that writer reported. Some of his conclusions did not make sense to me either, but that's really beside the point:

    We are not talking about towing these hulls, we are talking about powering them. Thus those tank test results are (in my opinion) rather useless with the possible exception of this conclusion:

    I agree with this statement but not because of those tank tests. I agree because I think I understand what's going on with the water under and around the hull in these boats. If I'm wrong I will know sooner or later I guess ... :)


    Robb White built one but no one seems to believe what he said about his. Yes I KNOW he did not built it precisely according to Atkin's plans -- but who cares if he didn't follow the plans exactly? His son said he built the tunnel section precisely according to plan, and it was a VERY efficient boat according to both of them. I guess this only confirms the design if you believe them.

    Nevertheless, I would like to build TWO BOATS -- one with the Atkin style tunnel-stern and one without. This doesn't mean I am actually going to do this. After all, it costs money -- not as much in the Philippines as in other places -- but it is still an expense that I do not feel comfortable shouldering on my own at this time.


    That's for sure!

    I think the factors that may work to counteract this drag and improve efficiency in this design are the longer box keel --- arguably one of the primary reasons why these boats are reported to be so efficient in the first place -- and the straight flow of water aft from the boat's mid-section rather than making it bend around the box keel as much as in the Atkin designs.

    The twin-tunnel looks like it will have more drag from hull friction as well, although to tell the truth I haven't calculated the surface areas and I'm not sure I will bother at this point since it is just a theoretical design.

    My basic goal is to build hulls and test them on the water once I come up with a design that appears to have a favorable combination of features. The labor and materials are both inexpensive here (relatively speaking of course) and I really think there are few places on Earth that would be better for prototyping and testing boats.

    I only wish I had a wealthy benefactor to cover my costs for such testing. Oh well, maybe in the next life ... :)
     
  11. Willallison
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    Willallison Senior Member

    I think you guys are over-analysing this. No doubt, Aitkin was a clever fella, but I think he developed a boat who's primary goal was shoal draft operation with relative economy. The boats' clearly evolved over time... elements like the "side-skirts" and kick down at the transom were different from boat to boat, suggesting this was more of a trial-and-error approach than some kind of high-tech boundary layer experiment.
    Nothing wrong with that of course - most boats are improved this way. And it may well be that the end result did indeed re-use the energy expended in creating the boundary layer in the 1st place, but I think it'd take some pretty serious testing to prove one way or the other.....

    The boat has a number of other benefits - particulary for a containerable craft like FF's.... hurry up and build the bloody thing will you.... we are all eager to examine the results!

    ps ... Tom... did you get my PM, or email...?
     
  12. charmc
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    charmc Senior Member

    Fred and Ken,

    Earlier, Fred, I said you were wrong about the net energy effect of boundary layer drag. As this discussion has progressed, I've started thinking that the boundary layer effect was not the only answer, and that there may be more even to boundary layer effect than I realized. My original analysis had the layer of water dragged forward adding to the energy requirement in two ways: first because it had to be accelerated to the hull's speed, drawing energy, and second because it had to be stopped, then reversed, which would use more energy. I was wrong about the second way (not so hard to say, after all :) ). That analysis would be correct only for a stationary prop. For a vessel underway, however, the prop is moving forward at the same velocity as the hull. The difference in velocity (delta V) between water and the hull/prop is equal to the vessel speed minus the water speed, normally zero. So for a vessel moving forward at 10 knots, delta V = 10-0 = 10. For that portion of water in the boundary layer, however, there is some forward velocity imparted by drag. Assuming boundary layer average velocity is 50% of the vessel velocity, delta V = 10 - 5 = 5. Boundary layer water will offer less resistance to the prop, so will require less energy. In addition, focusing a channel of water to the prop without any aeration reduces cavitation and adds to efficiency.

    At least one patent has been granted, I found, for the design of a tunnel to channel and focus boundary layer water into the prop.
    "This dynamic boundary layer arises at the interface between the hull of the vessel and the water. Part of this dynamic boundary layer has the speed of the vessel and energy is required in order to obtain this acceleration. According to the present invention provision is made that this dynamically accelerated water is as far as possible concentrated at the propeller, as a result of which an improvement in efficiency can be obtained." (Bouwe Prakken, June, 2006) http://www.patentstorm.us/patents/7066776-description.html

    Another aspect, mentioned by Ken, and by Atkins, is the fact that the stern does not squat. I believe this may be because the tunnel geometry actualy imparts lift to the aft section of the hull. In a conventional V hull, the deadrise angle represents a deduction from lift (why deep V hulls need strakes and other aids to run on plane; the ideal planing surface is perfectly flat). A tunnel, however, regardless of its precise shape, functions as an inverted V, with lift added by the reverse deadrise angle and by the increased surface area supporting the same weight.

    An example of an efficient cruiser incorporating a variation on the Atkins design is found in Walter Schulz's Shannon 38 SRD (Schulz Reverse Deadrise), a semi-planing design which reaches 20.5 knots max and 18 knots cruise with less than half the power of typical cruisers of the same hull length. What Walter calls reverse deadrise is actually an inverted V forming a tunnel leading to the prop (actually there are 2 tunnels, as the boat has twin diesel power). Design benefits include the progressive and level transition from full displacement to planing mode, level ride, excellent seakeeping, and 2 mpg fuel economy.

    http://www.shannonyachts.com/38SRD.html

    BTW, Fred, I thought you'd be interested in Walt's straight bow and hull length, although his boat has way too much beam to fit in a container!
     
    Last edited: Jul 5, 2007
  13. tom28571
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    tom28571 Senior Member

    Yes, I do think this issue is being over analyzed but hopefully some light will shine in here somewhere. It already has with the general agreement that the boundary layer water can provide some advantage to a trailing propeller.

    I am shocked that a patent has been granted on this phenomena that has been known for some time. Lindsay Lord has quite a bit of information on it in chapter 12 and I think I read about it in Gerr's books.. I was a bit skeptical but this discussion has helped to reinforce the idea of positive contribution of the boundary layer. This has been hard to come by since I know that the energy in the boundary layer has to be supplied by the propulsion system, engine or sail, and detracts from boat speed.

    I also wonder just what Schulz's patent contains. Negative deadrise has been around for a very long time. His hull bottom does resemble Atkin's in principle. I have been using a similar shape on the aft bottom of my boats for about 10 years. My design arose from material I read from Weston Farmer. I guess it could be looked as a double tunnel as mentioned above. There is very little new in boat design.

    Most of what we see as "new" rests on work or suggestions done by others earlier.

    Will, I don't check my PM often enough. The at-rest CB of the 24 (dry hull) is calculated as 57.7% aft of LWL. This is close to what you are shooting for. Adding engine and fuel aft of dry hull CB, two crew, water, stores and other gear forward of dry hull CB should keep it close to that.

    Fred, Sorry I missed your PM also. Like most of the rest of you, I wish I had the loot and time build all these designs we talk about here. Discussing them here does no harm even if some of the thoughts are probably completely off the wall.

    Added: OK, I looked up Schultz's patent. It looks a lot like the famous Hinkley patent fracas in that it is a combination of 6 claims that taken alone are already used or known by the boating fraternity. So it appears to me that it could be infringed only if someone built a boat including all of these claims since there are existing boats that already have the features of some of the individual claims. http://www.patentstorm.us/patents/6994049-claims.html Any patent experts here?
     
  14. Willallison
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    Willallison Senior Member

    Yes - sorry - I re-read my post and it comes across as a criticism of the discussion in general. I didn't intend it that way by any means - the pursuit of improved efficiency is of course a valid one....

    I haven't done any research on this, and I'm sure it varies from boat to boat, but I wonder just how far from the hull surface the boundary layed extends. If it's but a few inches then it's doubtful the much if any of the prop is running in this pre-accelerated water.

    Oh - and thanks Tom
     

  15. Tad
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    Tad Boat Designer

    I guess the ultimate development of a reverse deadrise hull would be the Hickman Sea-Sled.

    That a patent has been granted to Schulz doesn't really mean much until he has to defend it in court. So far no other builder has seen the need to challange this patent. When the SRD story was first published in 2004 I ran some numbers looking at the transport efficency and could not find anything special about the performance of the boat.

    I don't believe the claimed 13,500 pound displacement for the SRD 38' so I upped it to 15,650 at half load. At 21.5 knots with 320 HP I get Et = 3.22 at a Volume Froude Number of FNv =1.02

    An American Tug 34 (1980's semi-displacement hard-chine Lynn Senour design) at slightly lower FNv =0.79 achives a Et = 3.34

    A 40' traditional wooden Lobsterboat designed by Pete Kass and myself achives Et = 3.88 at FNv = 0.93.

    Looking at the Et of Rescue Minor is dificult because of the hazy numbers quoted by Rob White. He mentions 550 pounds as weight with 2.5 gallons of fuel, presumably that is without crew. So I'll guees at a weight of 750 pounds with one person aboard. He mentions 22 knots but claims a useful speed of 18.6 for a FNv = 3.67. He does not mention how this speed was measured. Unfortunatly the power required is unknown, though he mentions it takes "about half the available HP". I don't really believe that, but if it takes 12 HP to achive 18.6 knots Et = 3.56, or if it really did only take half the installed 18 HP, Et =4.75

    At a FNv = 2.82, Whio's Et = 2.27

    Looking at some really efficient boats.....

    Harry Bryan's Rambler (16') reportedly runs 5.2 knots on 3.2 HP for an Et =7.998. Small and slow equals efficient.

    LF Herreshoff's Piquant topped out at 20 knots with twin 16 Hp gas engines for an Et = 5.88. Long (47') and light (6700 lbs) equals efficient.

    W. Garden's Clam uses 10 HP for 6 Knots, Et = 17.13 at FNv = .34. Very long (40') and slow equals efficient.

    Phil Bolger's Slicer design achieves 18 knots with a 15 HP outboard for an Et = 6.26 at FNv = 1.8. A very light and narrow hull can be fast and efficient.

    All the best, Tad
     
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