Tunnel Dimensions

Discussion in 'Boat Design' started by helluvaboater, Jul 17, 2015.

  1. helluvaboater
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    helluvaboater Junior Member

    I am converting a twin 27 foot express cruiser into a single screw diesel. To make the boat easier to trailer with less draft, I am going to build a tunnel.

    Boat pics:
    http://boat-building.wix.com/wrangler

    Boat Specs:

    Finished Gross Weight: 9000lbs
    LOA: 27 feet
    LWL: 25 feet
    Engine: Single Cummins 6BT210
    Gear Ratio: 1.56
    Prop: 22 x 20 3B prop
    Tunnel Diameter: 28in (3" clearance for prop)

    The tunnel will be recessed about 8 inches deep from the lowest part of the hull. I don't want the tunnel to be too long to lose too much displacement, but I don't want it too short that it loses thrust efficiency.

    I have been reading this paper by Donald Blount on tunnel design and I think I have my tunnel design mostly figured out:

    http://www.boatdesign.net/forums/at...2693-prop-pocket-design-propeller-tunnels.pdf

    There is one major thing I can't figure out:

    How much length should I have between the prop and the front of the tunnel? In other words, what is the ideal length of "X" in the first attachment?


    The PDF says on page 3:

    The propeller region of the tunnel has significant impact on the achievable propulsive
    efficiency. Of particular note is the longitudinal distance between the propeller and the
    tunnel entrance, Koelbel (1979) and Denny (1980a). Should the propeller be placed too
    near the downward-sloping tunnel roof, its low pressure field (suction force) can act to
    increase tunnel resistance. Figure 3 provides design guidance based on thrust loading and
    longitudinal location of the propeller relative to the tunnel entrance. This guideline is
    intended to show a balanced trade-off between added drag and loss of buoyancy.


    I plan on using a flat-roof style tunnel entrance because that is what is used for faster boats and it looks like it gives a better uninterrupted flow over the intersecting cylinder-type tunnel entrances. (page 12 of the pdf)
     

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  2. helluvaboater
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    helluvaboater Junior Member

    A couple more pics...
     

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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Is it not desirable to radius the intersection of the tunnel with the bottom ? A sharp transition could give you more turbulent flow into the prop in a turn.
     
  4. helluvaboater
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    helluvaboater Junior Member

    When you think about the tunnel and prop as a single propulsion unit like the PDF suggests, the most important transition is the fore to aft transition, which is what I am assuming you are referring to. With a flat tunnel roof, it looks like the water immediately only has to move upwards, rather than upwards and inwards with a cylindarical tunnel entrance. (see attachment)

    Or, am I off base here? Is the cylandrical tunnel entrance the best way to go for boats in the 20-30 knot range?

    The PDF says that the transition angle should not exceed 15 degrees and I plan on designing the tunnel with that in mind. I do plan to have a radius on every corner, but how big I have yet to figure out.
     

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  5. messabout
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    messabout Senior Member

    I am going to guess that a flat paneled tunnel will make more noise than a semi-cylindrical one. Likely more turbulence too, which will increase the noise and vibration factors.
     
  6. helluvaboater
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    helluvaboater Junior Member

    So a semi-cylandrical tunnel entrance is the best way to go?

    How do I make the roving conform to the shape of a semi-cylandrical tunnel entrance? I plan on making a mold for the propeller section of the tunnel, but a semi-cylandrical tunnel entrance has so much roundness that I don't think I am going to be able to lay heavy fiberglass in large pieces. Can I hand lay it somehow? Cut the fiberglass into the largest that pieces I can get to stick and stagger the joints? Would a chopper gun be the best?

    If my rudder is 16 inches wide and my prop is 4 inches wide, how much space should be between the rudder and the prop? Then, how long should the prop section of the tunnel be?
     
  7. messabout
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    messabout Senior Member

    Some functional tunnel hulls have been around for a long time. One that comes to mind is from way back designed by William Atkins. The name of the boat is Rescue Minor. Explore on the internet to find some of the details of that hull and its' tunnel. You can also scout around the forum with the search function to find more commentary about the subject.

    I have seen one of the Rescue Minors in operation. It can run in very thin water that would stop a conventional hull in an instant. It was using a little Kubota tractor diesel engine for power. I was impressed.

    In my post above, I said that I would guess that a cylindrical tunnel would be best. That was only a semi educated guess so I will defer to some of the guys who has more authoritative information.

    You can make the tunnel any shape that you like. I would presume that you will make a plug and lay up your glass over the plug. It would be grafted to the boat later. You can do it with accuracy that way.
     
  8. PAR
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    PAR Yacht Designer/Builder

    The Atkins designs typically employ a few more tricks than simply a tunnel. Most incorporate reversed deadrise (inverted V shapes) too. The shape of the tunnel is a pretty big consideration, if you want the best of the setup. Getting the flow dynamics right isn't very easy. It would also be wise to delineate the difference between a tunnel and an inlet (partial tunnel). Pressure differentials and shapes can make or break these things and it's really easy to screw the pooch, without a pretty solid understanding of the the dynamics involved.

    You should seriously evaluate the available power (210 HP?) and the abilities of the hull (speed potential), which I suggest will barely make it above the the mid 20's, possibly as much as 27, but not much more. This suggests a circular tunnel section, not squared. The math, as Blount shows isn't very difficult, though you'll need to work up the appropriate Froude numbers, you expect for it's operating range, along with the anticipated performance envelop you desire, before making any decisions about tunnel shape, length, entry and exit, etc.
     
  9. CDK
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    CDK retired engineer

    Crucial for the performance of your tunnel is a smooth transition from hull to tunnel entrance. There is no ideal radius, just make it as large as possible to minimize turbulence.
    The tunnel is a cavity you want to be filled with water having the same speed and pressure as the water under the hull, but to achieve that the flow has to change direction. There will be turbulence at the entrance but also at both sides where water flows into the cavity.
    This is something you cannot completely prevent, but you can surely try by making the transition very, very gently.
     
  10. PAR
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    PAR Yacht Designer/Builder

    The entry is an important part of the equation. I've had some luck keeping the flow attached with a asymmetric wing just after the entry, to keep flow attached and pressures up, with less turbulence. The shape of the entry and ramp angles need to be scaled to the speeds the boat's performance expectations. If the OP runs through the suggestions in the Blount paper, he'll do fine, though it does assume some understanding of what's going on. I've also played with a modified NACA duct to keep up pressures.
     
  11. Village_Idiot
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    Village_Idiot Senior Member

    Good advice given above. Also, i recall reading that the clearance between propeller tip and the tunnel should be minimized as much as possible, although that may limit your future propeller choices.

    As others have mentioned, you want to minimize turbulence within the tunnel. I've often thought that perhaps having a bulbous protrusion ahead of the tunnel may help somehow, to displace the water temporarily, and as it bounces back, it is sucked up into the tunnel in such a way to help minimize turbulence. However, that is just a gut feeling and I have no scientific evidence to back me up.

    Either way you go, you will end up with some turbulence in the tunnel. A prop with a large blade area or heavy cupping on the blades can help to get a better "grab" onto the aerated water, and increase performance/efficiency.

    I deal mainly with tunnels in outboard boats, not inboard. Having said that, a modification that many builders make to their outboard tunnels is installation of a vent at the leading edge of the tunnel, usually behind a baffle. This vent is routed, via piping, to the helm, where the operator can open/close a valve to let air into the tunnel. Sounds counter-intuitive to allow air into a system where you are trying to reduce turbulence, but the process does have the effect of breaking the vacuum that so often accompanies tunnels. Users report that opening the vent feels like you are pulling up an anchor, ad performance is drastically increased. They often report gaining 3-5mph top speed at wide open throttle, which is a huge performance gain. So why not leave the vent open all of the time? It does detract from running in very shallow waters, which most outboard tunnels are designed to do, particularly with planing hulls. Since you are dealing with displacement speeds and likely not shallow water, there may be no need for a valve on the vent. Just the same, it would be interesting to see if it makes a difference with the valve open or closed.
     
  12. CBD Boat Design
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    CBD Boat Design Junior Member

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  13. c1steve
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    c1steve New Member

    Helluvaboater,

    I like your idea, and have plans to do something similar in the near future in a new boat. However with a 1.5:1 transmission, I believe you would be best off using a smaller diameter propeller. Based on my past experience I would think you could use a 19" prop. A smaller propeller diameter would reduce shaft angle, which would increase efficiency.

    Also it appears that you are installing the engine aft of the fish hold/storage space. This would be best for reduced engine noise, but I believe that the shaft angle would be less if you could put the motor farther forward. This would make the tunnel would be longer and more efficient, and thus provide many benefits.

    Whatever you do, plan on insulating the engine for noise reduction as much as possible. Switching to a diesel dramatically increases sound levels. Running with a noisy engine greatly fatigues the operator and leads to numerous problems.

    -Steve
     
  14. helluvaboater
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    helluvaboater Junior Member

    OK, after all of your input I have decided to go with the intersecting cylinder tunnel. I plan to make the tunnel 9 inches deep and taper it like the attached diagram.


    I plan to make an oversized plug and graft it to the boat once completed, but I don't know how to make an accurate template to trace onto the hull so that I can cut an accurate hole for the plug.

    View attachments to see what I am talking about.

    Any ideas?

    Once I install the new stringers, I can can cut out the propeller/rudder portions of the tunnel because those lines are symmetrical. That will enable me to position the prop, rudder, transmission and shaft. Then I can figure out the actual length of the tunnel.

    But, my question again, how do I transfer a matching line from the plug to the hull?
     

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  15. SukiSolo
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    SukiSolo Senior Member

    A large profile guage, you may have to make one. Alternatively line the plug with polythene and mould a bit of F/glass, plaster of Paris, Body filler (Bondo?) around shape, remove and use as template. Brace if you need to.
     
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