effect of close clearance tunnels on gear ratio and wheel selection.

Discussion in 'Inboards' started by Dan V, Nov 5, 2011.

  1. Dan V
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    Dan V Junior Member

    I´m looking for advice concerning the effect of close clearance tunnels on gear ratio and wheel selection. I was very dissatified with the performance of a boat I designed with a tunnel. I found research of the late Donald Blount and redesigned the tunnel as per his reccomendations. The boat is 32´ and about 14,000 lbs. Cummins 450C w/ 1.48 gear. Upon relaunch w/ a 22 x 26 four blade (medium cup), the top speed was 32 knots but top rpm was only 2350. Calculations show a slip of only 5%, so the tunnel w/ its .375" tip clearance was doing its job perfectly, but that restriction had to be hard on the engine. So thinking hydrodynamic friction, I started whittling away at pitch, then diameter and back to pitch again. I´m finally down to 20-1/2" x 22" and just am getting 2600rpm at wot. Performance sucks: top speed is 24k, 20k cruise at 2200 and I´m burning 15% more fuel than before. I think I have to go back to filling the tunnel w/ propeller to get the system to work as designed and will need to change gear ratios to protect the engine. How do I determine if I need a 1.79:1 gear, 2:1 or something different? Or is there something else I have overlooked?

    Thanks,
    Dan Varner
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Please state rated engine power @ rpm's and main tunnel dimensions, including propeller position. Then verify engine running rpm's using a calibrated tacho to the crankshaft.

    And lets keep the discussion in one thread (this one) please.....
     
  3. Dan V
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    Dan V Junior Member

    Thanks for your reply, Baeckmo. The boat is overboard now and I'll have to pick up the drawings from work to get accurate answers on the tunnel dimensions. The engine produces 453hp at 2600 rpm. Here are test numbers from some prop experimentation:
    Here are the test numbers, always w/ medium cup.
    22 x 26 prop: 32kt at 2350 rpm wot
    22 x 24 : 29kt at 2400 rpm wot (2"pitch made 50 rpm difference)
    21 x 24 26kt @ 2500 rpm wot (1" dia made 100 rpm dif)
    21 x 22 25kt @ 2550 rpm wot
    20-1/2 x 22 24kt @ barely 2600 rpm wot (too much slip)
     
  4. keysdisease
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    keysdisease Senior Member

    Looks to me like you're almost there. Online looks like 2600 is max RPM, are you working with a prop shop for prop selection?

    Based on your below experience a 22 square should be close to right. I would not jump the gun on a gear change when you're this close "out of the box." Also many owners of tunnel boats end up switching to 5 blade props and dropping the diameter a little.

    Also if she's just launched and you are running these tests in light ship conditions you need to let the engine run over top RPM to allow for typical loading.

    Even DLBA will tell you tunnels can be tricky and the "art" of engineering them is not a done deal. With your tip clearance you might experience a lot of noise and you might have some friction burn on the tunnel itself.

    Not an NA or engineer, just some one that knows that a good prop shop can be a wealth of experienced information.

    Steve

     
  5. Dan V
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    Dan V Junior Member

    Thanks Steve,
    I should have tried the prop square before whittling away at the diameter. But when I went from 22x26 to 22x24, I only gained 50rpm. Another 50 would have still put me 200 under the engine mfg's recommended rpm and destroy engine longevity. When the 22" prop was on the boat, surprisingly it did not even take the bottom paint off.
     
  6. keysdisease
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    keysdisease Senior Member

    Talk to a prop shop. Ask about a 22 dia 5 blade. Pick some brains in the most experienced prop shop in the area.

    The fact that the paint is hanging in seems to indicate the tunnel and 22" might be "right." Of course i imagine this was over a short term and time will tell re tunnel burn.

    Steve
     
  7. baeckmo
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    baeckmo Hydrodynamics

    The "Blount type" of tunnel, with a cylindrical rooftop and with an elliptical connection to the bottom (often with a sharp edge), has a tendency to "starve" the prop, in particular with a high propeller loading. The reason is that the propeller is acting as a s.c. hydraulic sink. The incoming fluid has a bigger transverse area than the propeller itself. Part of the flow is thus entering the tunnel with a transverse velocity close to the propeller disc. The tunnel sides are then acting like boundary layer spoilers, creating an oversize wake in the tunnel.

    This means that the propeller is experiencing a far lower incoming velocity than you might expect from the forward speed of the boat. In your case the mean inflow speed ("advance velocity") is probably some 26 to 27 knots when your boat is making 32 knots. The fluid in the upper segment is close to stagnant. Hence your pitch was FAR too big from the beginning (cupping adding one or two inches, depending on size), while the diameter was already on the low side, resulting in a high disc load and some cavitation.

    When you reduced the pitch, the cavitation was slightly reduced as a result of a lower angle of attack and the propeller had an increased "bite". This is the reason for the small increase in rpms you observed. You should have kept the dia and gone for a propeller with reduced pitch. Remember that it is better to start the optimizing process with a slight underpitch, since a pitch increase is easier to get correct than a pitch reduction.

    Please note that even minor reading or calibration faults regarding rpm's are critical, so before scrapping another propeller, check calibration of the tacho!

    Just as Keysdisease says, your original setup was fairly closeto optimum, but unfortunately you jumped the wrong wagon..... and I am still very keen to see your tunnel shape.......!
     
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  8. Yellowjacket
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    Yellowjacket Senior Member

    I had a meeting with DLB a few months ago... I'm not sure, but as I recall, he wasn't late to the meeting and I'm pretty sure he can't be described as late right now.....

    As Mark Twain once said, "reports of my death have been greatly exagerated".....
     
  9. daiquiri
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    daiquiri Engineering and Design

    One question regarding tunnels, if you don't mind...

    It is known that, for ordinary propeller installations, there's a minimum clearance which should be kept between propeller tip and hull plating. It is typically a value of 15-20% of the prop diameter. It is done in order to prevent blade pressure-induced vibrations, and also to minimize the risk of tip-hull vortex cavitation. See this illustration, example:

    [​IMG]

    For tunnels, I see that Baeckmo's diagram indicates 2% clearance. Does it mean that tunnels do not have these types of problems?

    Cheers
     
  10. Dan V
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    Dan V Junior Member

    Yellowjacket, Don't I hate to be wrong - usually! In this case, I'm extremely happy to hear that I was (and the gentleman in question would probably be pleased I was mistaken as well.) My humblest apologies.
     
  11. keysdisease
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    keysdisease Senior Member

    Tunnels do indeed have those problems. Noise and vibration from tunnel installations are fairly common and "tunnel burn" from tip vortex does happen although not as often as unacceptable increases in structure borne noise.

    It's about performance and compromise.

    Lower shaft angle results in more efficient thrust, the "nozzle effect" results in additional thrust and throw in reduced draft for a trifecta of reasons to balance a few knots of top end speed and increased efficiency against your noted problems.

    Steve

     
  12. baeckmo
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    baeckmo Hydrodynamics

    Daiquiri wrote:
    --For tunnels, I see that Baeckmo's diagram indicates 2% clearance. Does it mean that tunnels do not have these types of problems?---"

    With a correct tunnel shape the problems are reduced if not totally gone. During the phase of rotation, where the blade is within the tunnel, the tip vortici are reduced. The critical moment is when a blade is approaching the tunnel edge from the "free position". This is where a gradual reduction of clearance is required in order to avoid blade frequency vibrations.

    With a tunnel/nozzle combination this problem is further reduced (but others are increased....). In my experience, many tunnels (far too many!!) have been afterthoughts with a bad shape and often with a highly loaded propeller (i.e. too small diameter for the thrust required) in order to reduce draft. The result is a prop working with high angles of attack within a wake with extreme velocity variations over the inlet.
     
  13. Dan V
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    Dan V Junior Member

    Sorry it has taken awhile to get you a clearer image of the tunnel but here are some pix of the original tunnel. The framing stations are 19 inches apart so the tunnel starts almost 10 feet from the transom. I built a new arc of a cylinder section about 42" long and inside dia of 22-3/4" after fairing and glassing. The cylinder was fitted nearly level with resting waterline, about 6" aft of the prop's hub, running forward. The for'd intersection of the original tunnel and the new one is at a very low angle. The triangular flat planing surface begins just ahead of where the tunnel starts. Thanks for your thoughts. Dan
     

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  14. baeckmo
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    baeckmo Hydrodynamics

    Sorry Dan, but this is a really lousy shape; it has all the characteristics needed to make life for your propeller a mess.

    If you follow an imaginary streamline roughly along a longitudinal at, say half tunnel radius from the CL, you will see that when it reaches the intersection between tunnel and bottom, it is forced to deflect upwards at a sharp edge. This generates a complete detachment of the flow, resulting in a huge wake of stagnant fluid filling the tunnel.

    For some reason tunnel designers imagine that a cylindrical surface is essential in a tunnel. It is not, in fact it is a drawback since the fluid does NOT enter the prop in any kind of imaginary circular pipe, but with a velocity distribution in space that is determined by the propeller loading and OPERATING ENVIRONMENT. In this context, the LONGITUDINAL shape is of utmost importance!

    "WATER DOES NOT LIKE SURPRISES"

    In addition to the bad inlet shape comes an outlet that very easily creates a stagnant pocket along the tunnel roof aft of the propeller. The flow is leaving the propeller in a jet with an increased velocity and a reduced diameter. I have seen a number of tunnel boats, where in fact the propeller has been ventilated from abaft, causing heavy vibrations and bad performance.

    In one case (please bear with my anecdotes...), a 12m aluminium work boat with a 350 hp diesel, constant deadrise ~13 degrees and a tunnel very similar to yours, was regularly repaired three times a year. Tunnel and surrounding structure weld repair and numerous overhauls of transmission. Vibrations were ear-deafening and speed a disaster, particularly with anything beyond light load.

    The major problem in this case was ventilation from behind. It was fixed by the addition of a simple rubber skirt, that cut off the air ingress by adapting to the outflow. The bad inlet was still there, causing inlet losses, but the vibrations were gone, as were the repeted repairs, and loaded speed was substantially improved.

    Some of the cutouts that are called propeller tunnels should rather be called recesses. To me a propeller tunnel is a flow device that is adapted to the inlet flow of the propeller (that goes for the inlet for a jet unit as well), and which is influencing the propulsor flow in a major way. This often means that the tunnel walls come fairly close to the propeller.

    A propeller recess in more of a general change of bottom shape in order to accomodate a propeller with a reduction of draft in mind. Here the roof diamater is far greater than the propeller dia, the harmful inlet edges are either carefully rounded and/or so far from the propeller that the disturbances have been substantially cancelled before entering the propeller disc. In this case, there is space enough that the flow can adapt to the requirements pf the propeller. You see this on some of Jim Wynnes designs for example.

    So: a propeller tunnel must be designed with a fair knowledge of the propeller flow; if correct it may actually increase propulsion efficiency and reduce vibrations. If shape is incorrect you are in trouble. The recess is less critical to the propeller, but has a major influence on the hull behaviour instead.

    By this I hope you better understand the roots of your experiences, and primarily the reason why you need less pitch. When there is a risk of backflow with air mixing, a propeller with a reasonable cup is a necessity, but the local angle of attack at the leading edge must be small, i.e. the blade pressure is created on the rearmost part of the blade.
     

  15. baeckmo
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    baeckmo Hydrodynamics

     
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