Questions on keel design for jet drive tunnel hull?

Discussion in 'Jet Drives' started by DSR, Jul 4, 2017.

  1. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi everyone;

    I'm finishing up the final revisions for my Mod-VP hull design and I would like to gather some insight regarding possibly using a "drop keel running pad" in the design?

    From my research, jet drives in full-size high speed tunnel hulls using center pods with jet drives (such as Eliminator's Daytonas and other similar hulls) respond well to modifying the area in front of and at the pump intake by utilizing a "drop keel" to mount the intake below the keel of the center pod. From what I can gather, this modification allows a more "laminar" flow of water to the pump intake at higher speeds and in rougher water conditions to keep the pump loaded. As I understand it, the drop keel allows the intake access to solid water below the cavitated boundary layer present at planing speeds with the low-deadrise center pod designs with a minimum of induced drag. I presented this idea to my father, who is a retired mechanical engineer (and a pretty sharp guy) and between the two of us, we came up with what I call a "drop keel running pad".
    Now, neither one of us are well-versed in hydrodynamics, so I wanted to present this to the experts here on the forum to gather insight and feedback on this idea....

    Specifically, the center pod of the hull is a constant 12" wide from bow to the end of the drive extension pod. The drop keel as shown is 5' 6" long by 11" wide with the bottom surface being ~.75" below the keel centerline. the pad is level with vertical side walls 39" forward of the aft edge and then gently tapers and slopes to a point with the side walls angled as shown in the pics. The pump intake is 6.5" wide x 15" long from the lead-in to the front edge of the shoe assembly (rear of intake). The front edge of the shoe is 5.5" from the aft edge of the extension. The pad is dead flat as is the pump intake and surrounding mount flange.
    Any idea if this drop keel design will work as intended? If not, what needs to change to make it work? Would it be needed at all with this hull design?
    Any and all help with this would be greatly appreciated. This is pretty much the last piece of my little puzzle before I start building.

    Thanks!!
    Dave

    DSR Jet TNT drop keel1.jpg DSR Jet TNT drop keel2.jpg DSR Jet TNT drop keel3.jpg
     
  2. Ben Landgren
    Joined: Jun 2017
    Posts: 19
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    Location: Newcastle Upon Tyne

    Ben Landgren Junior Member

    Hi Dave,
    To mount the intake in the center pod is a good idea in this hull form according to my experience. At least in terms of waterjet performance. This hull form might ingest certain amount of air underneath it on speed which would then in normal application be sucked into the waterjet causing ventilation, which will result in loss of performance. As you know, air is lighter than water so the air bubbles tend to move towards water surface, in this case bottom of hull. By using the central pod and the drop keel the air bubble ingestion to waterjet will be probably minimal since the air travels along the bottom area which is closest to water surface, in other words the air will go around the waterjet intake in your design.
    The center pod will result however in some additional drag and bow down moment.

    Hope this helps
    Ben
     
  3. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi Ben,

    I appreciate you responding to my question
    I have gained a little bit of ground on this issue and from what I can gather with this, the original "NACA" shaped keel I came up with wouldn't work well with the transitions to it from the original keel being rather abrupt and would disrupt the laminar flow to the intake (basically making the issue worse instead of curing it).

    It seems that the preferred setup is to add a radius keel area in front of the pump intake that is the same width as the intake opening (6.5" wide in this instance) with a very gentle transition in the front and slopes down at the rear blending into the front edge of the intake opening. This evidently provides the best balance of pump loading and water drag characteristics.

    What I haven't been able to come up with is what kind of depth the cross section apex (basically, the thickness of the center) of the radius should be? 1/2"? And with the keel center line at the transom being 9/16" lower than the bottom edges of the sponsons, would adding the radius to the original keel be deep enough? or should I add a drop in between the original and the radius to lower it further in the water? With the fact that this design is what I would call a "negative" keel (with the keel level and the sponsons and tunnel roofs angling up going forward) as shown in the attached Linesplan, my gut feeling is that just adding a radius keel to the original without the additional drop would do the trick, but I really wanted to get some feedback from those who know before I start experimenting at a high rate of speed........ :D

    What do you think Ben?

    Thanks,
    Dave

    DSR mini jet mod-VP linesplan.jpg
     
    Last edited: Aug 9, 2017
  4. Ben Landgren
    Joined: Jun 2017
    Posts: 19
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    Location: Newcastle Upon Tyne

    Ben Landgren Junior Member

    Hi Dave,
    Sorry it took me ages to respond. Honestly I have to say that I know few cases that use somewhat similar approach for waterjet inlets and these were designed to avoid the air bubble ingestion in intake but in northern Europe this kind of design is not common at all. I think it would be good to run some CFD to understand the flow around the intake better in your design. It seems to me that there is unfortunately only little to gain with this kind of design. I found this link that might be of interest to you Edge Marine Defined http://www.edgemarine.com/edgedefined.htm .

    Best of luck and keep me updated
    Ben
     
  5. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Ben,

    I appreciate you getting back to me.
    It seems that this last little detail on my project is something that few people have experience with unfortunately.

    I have finally stumbled across a few pics of the radius keels done to similar jet drive tunnel hulls that give me a little more insight to the problem though. They don't give me all the answers, but l think they'll point me in the right direction.

    From what I've gathered, I also think that a lot of the problem with this seems to be that the solution varies quite a bit depending on the shape of the hull, whether the keel positioning in the design is negative, positive or parallel to the tunnels and sponsons, and the performance target for the boat. I'm attaching the pics that I found ......

    I've done a couple possible designs in this direction and I think that your mentioning doing a CFD analysis on the hull might be a very good notion. Do you have any recommendations regarding a person, or company that could help me with doing that?
    Or possibly a free source program that might do the trick?

    Thanks again Ben!
    Dave

    Keel-Mod-small-300x225.jpg Tunnel keel mod2.jpg
    Tunnel keel mods3.jpg
     
    Last edited: Aug 13, 2017
  6. Ben Landgren
    Joined: Jun 2017
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    Location: Newcastle Upon Tyne

    Ben Landgren Junior Member

    Hi Dave,

    Those designs look very interesting indeed.

    CFD is vast and complex field and to master it will take a lot of time. There is however an open source software called openfoam, which you should be able to install to windows or Linux. It works with C++ programming language, and that is what stopped me in the first place. But I would say you have to be very confident with principles of CFD before trying openfoam.

    You could also try contacting the university near you, there are always people interested in carrying out projects that they can then add to their resume.

    Ben
     
  7. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Ben,

    Yeah, I kinda figured that CFD was going to be a can and a half of worms all by itself, judging by the information I found.

    I do like the idea of contacting someone at the University level to see what they have to say. Thanks for the tip!

    This whole deal is getting pretty interesting. A little more digging revealed that the use of the added radius keel section leading into the pump inlet is based on the "Coanda effect", with the lower pressure generated on the back side of the radius keel "steering" the water flow up into the pump intake.
    I have found instances to indicate that the "flat" drop keel (similar to what I originally started with) can be made to work very well up to 70-75 mph. From there on up the flat drop keel can get dangerously unstable. Evidently, the radius keel is not as efficient at generating hydrodynamic lift at lower speeds (which makes sense to me, in that the flat keel acts like a Delta, or "running" pad) but the radius keel remains much more stable at speeds above that 70-75 mph mark (along with better intake loading).
    I'm assuming generalities within all of this, but some of these tunnel hulls with the radius keels are running up to 125-130 mph.
    I'm definitely not looking for that kind of thrill, but performance analysis of my project did conservatively generate 80+ mph estimates with my powertrain in stock tune.

    Thanks for helping Ben. If you have any other ideas please let me know! :)

    Dave
     
  8. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi everyone,

    Well, after a ton of research and bugging the hell out of anyone I could contact, I may have come up with something that'll work for the keel, but I wanted to show it here to see if any of the members here would be able to give me any idea if I'm headed in the right direction?
    As Ben suggested previously, I do have a University locally that I will be contacting regarding possibly running CFD tests on the design, but I'm not going to hold my breath on the when, or even if they could get to it.....

    I ended up with an added radius keel to the original keel on the 12" wide center pod that is 6.5" wide (matching the width of the lead-in for the pump intake) and about 5' long from where the transition starts to the end where the radius blends into the intake. the center of the radius is 1/2" below the original keel.

    I'm posting a couple pics of a crude rendering that I think gives a pretty good visual of what I'm thinking.

    Also as a side note, what's not shown is that there will be intake fins on both sides of the intake and the ride plate the I will be fabbing for the hull.

    As always, any and all feedback is greatly appreciated!!

    Thanks,
    Dave

    DSR Jet TNT radius keel1.jpg
    DSR Jet TNT radius keel2.jpg
    DSR Jet TNT radius keel3.jpg
     
  9. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    The intake shapes shown will give trouble to the jet; cavitation and high losses. It must be understood that any flow into the intake opening is the result of a pressure difference between the free flow along the wall (bottom) and the ducting. This is an example of of the "classical" potential sink flow. If you imagine the flow into an opening in the wall, and no advance speed, the flow will enter the opening symmetrically from all sides. When you add an advance flow, the streamlines will shift gradually to an inflow dominated by the advance direction. BUT: there is always a considerable transverse flow, even at higher advance speeds.

    This is where the inlet shapes you have shown will result in trouble, because the transverse flow will have to "climb" across longitudinal steps, which creates vortices and high flow losses. In doing that, it will also bring a lot of the air/water foam that flows between side-hull and center hull.

    To show what is looks like, I have arranged three pics from a CFD simulation of the flow into a tank opening. Never mind the 90-degree bend (it is from an industrial application), the important thing is what the incoming flow looks like. With the shown relation between advance velocity and duct velocity, the streamtube width of the incoming flow (at the bottom) is roughly double the duct diameter.

    The main problems occur at acceleration and at the hump speed, where the advance speed is low, but the jet is supposed to work with maximum flowrate. Then even more of the inlet flow will come from the sides of the inlet. Funny thing is, that when advance speed is higher than duct speed, the streamtube width is not changing much, but its height will be reduced. The transverse area of the incoming flow is close to a semi-oval.

    So, single jet in a tunnel hull or catamaran is a tricky business (...and I have designed some for catamarans). In order to get it right you have to start with a set of operating requirements and go from there to end with a working design. I feel that you are doing some backwards engineering here; you have dreamt up a bottom shape (have you any calculations on resistance and operating WL to base the jet positioning and its size upon?), and the jet inlet comes as an afterthought.
     

    Attached Files:

  10. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Baeckmo, I appreciate you stopping by.

    I guess I'll start at the end of your response.....

    Yes, I've definitely come to the conclusion that a jet drive in a tunnel hull is definitely tricky, hence the reason for my post here to try and get some help to make sure that it is as "right" as I can make it.

    For a statement of requirements, this boat is a design / build exercise to build a unique, lightweight, fast 1-2 person toy for playing in calm protected inland waters (what I would call a lake racer).

    I'm not sure what you mean by "backwards engineering"? The design is based off of an existing low-deadrise warped bottom design for outboard power that I modified by adding the tunnels to create the current mod-VP bottom. The hull is also lengthened and the drive extension pod added to the transom to accommodate the engine and jet pump from the Honda PWC that I've acquired for the build, and provide the best static and dynamic C of G within the hull when complete and loaded. And now I am currently working on the keel design to best utilize the Yamaha PWC cast aluminum intake duct that I'm adapting the Honda pump to and for the expected performance. Based on all the research I've done, this type of conversion is kind of a black hole as far as info is concerned.

    I've run simulations on the design and it shows planing before 20 mph, at 45 mph (before transition into aerodynamic lift), WL is 6.25' with total drag of 616 lbs. The aerodynamic lift transition zone starts at 48-50 mph and the design shows decent stability past 86 mph with the center pod WL at 2.1' (another reason to move the pump intake as far aft of the transom as I did) and total drag at 348 lbs. At this speed it also shows aerodynamic lift at 278 lbs., or 32% of total lift.
    I also appreciate you sharing the CFD images, that definitely helps me understand what's going on down there. Something that I didn't show on the renderings, and I'm attaching, is that the intake will also have fins running along both sides of it and I wondered what effects they would have on the transverse flow and the streamtube that's present?
    The radius drop keel and intake fins that I've posted was the result of conversations with a couple of great people that had a lot of experience setting up and modifying full-size versions of this style of hull for drag racing with speeds up to 125-130 mph, and what they found to work the best. I also know from reading your posts here on the forum, that you have a lot of knowledge and practical experience with jet drives, so I would greatly appreciate it if you have any ideas or suggestions on what would do the trick with my design and it's intended function?

    Thanks Baeckmo!
    Dave


    DSR Jet TNT intake fins1.jpg
     
    Last edited: Aug 26, 2017
  11. DSR
    Joined: Mar 2017
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    DSR Junior Member

    I just realized that I haven't shown the intake that I'm planning on using for the build and it's certainly relevant to the situation. I apologize....

    The intake will be mounted Into the 12" wide center pod and is flat both front to rear and side to side with outside dimensions of 7" X 24". A very subtle lead-in curve at the front of the opening starts 4" from the front edge, which places it 8" before the transom, and is 6.5" wide. The opening tapers to 5" at the beginning of the shoe, which is 11" from the lead-in, and also ramps down to 5" wide heading deeper into the duct at the front. The center of the shoe radius is 15" from the start of the lead-in, which places it 7" aft of the transom and 5" before the rear edge of the drive extension in the design.

    (As a side note: I also plan on fabbing an adjustable ride plate extending back under the pump from the rear edge of the shoe assembly)


    IMG_20170724_215804078.jpg
    IMG_20170724_220240405.jpg
    IMG_20170724_220021447.jpg
     
  12. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Any further thoughts or suggestions regarding this Baeckmo? Or, anyone else who might be familiar with this particular problem?

    Thanks
    Dave
     
  13. jim lee
    Joined: Feb 2007
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    Location: Anacortes, WA

    jim lee Senior Member

    The guy that understands this kind of thing is Jeff Bennett. ibsurfin@comcast.net was his eMail about 12 years ago. Shaping jet boat bottoms was his deal and he was/is VERY good at it. We set the world speed record for PGJ drag class on one of his bottoms. Or was it PCE? Whatever. 132 mph in the quarter mile. All the boats that were winning at the time had his shaped bottoms.

    Jeff had a database of bottom shapes that had been tweaked and tested over the years. The difference between a bottom that can win and one that just does not perform, can be really small. I don't think you could model this very successfully because people don't understand how it all interacts well enough. Basically it was all about saving data from every boat possible and the numbers on how each boat performed at the track. From that pile of data he could get you a baseline bottom that would usually work ok. From there, looking at your race data, he could suggest changes to help improve your times. Sometimes it would be, remove .010" thou here etc.

    Good luck!

    -jim lee
     
  14. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Jim, thanks for stopping by.

    I believe I actually found Jeff, during a previous internet excursion, at Bennett Analytics ( judging by the information available on his website, I have to assume this is the same Jeff Bennett that you're referring to?)

    Bennett Analytics ยป Home http://www.bennettanalytics.com

    Thanks,
    Dave
     

  15. jim lee
    Joined: Feb 2007
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    Location: Anacortes, WA

    jim lee Senior Member

    Yes, That's him! (Recognized his picture)

    When he was on our race team, we won everything. When he "retired" from our team, much to my dismay, we lost everything. Good job finding his website. When we parted ways he didn't have one.

    [​IMG]

    That's him on the left. I'm standing next to him with the embarrassingly ugly goatee. We're picking up our trophies in Vegas for Pulling First in class and setting the speed record.

    [​IMG]
    The boat. That's Keith, team owner, tall blond next to me in picture driving. Phoenix AZ.

    -jim lee
     
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