DIY tunnel drive

Discussion in 'Surface Drives' started by CDK, Nov 29, 2007.

  1. powerabout
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    powerabout Senior Member

    CDk
    Was the prop calc using the whole blade area or just what shows under the tunnel?
    Your max blade load is usually getting over the hump so what works below and above this might be fine but you have to get there.

    My guess would be on a planning 28' cruiser if you had 2 stern drives running at propshaft level that there wouldnt be enough blade area to get on the plane due to the weight of the boat..?
    The advantage of a drive out back is its gets water when getting over the hump and then the static water level goes down as the boat levels out.
    Cheers
     
  2. baeckmo
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    baeckmo Hydrodynamics

    CDK, that test is a beautiful validation of the statements given in post #46! It also provides a few additional observations to be made, not to mention your faiblesse for wet jobs, beer and cigars……..(I enjoy those as well..)

    Observations with additional cover:

    1. Max speed ~9 kn, compared to ~11 before.
    2. Max rpms ~2400, instead of ~2100.
    3. Increased stern lift
    4. Rudders have less directional authority

    First, we now have two non-ventilating operating points to use as benchmarks for control of propeller calculation. One is the previous bollard-pull test that gave maximum rpm of ~2100 rpm, the second is the 2400 rpm at ~9 knot. When calculating the propeller power for the later, it is obvious that you have an extremely disturbed inflow to the propellers, even when not ventilating. Using an inflow velocity of 60% of the ship speed for demonstration, we can plot prop power versus rpm for constant speeds 0 kn, 9 kn and 18 kn, see graph below. The dotted blue line shows performance for these props with a more normal inflow velocity of 90 % of ship speed and 18 kn, which probably is fairly close to the suppliers original calculation.

    From the plot you find that the shaft power is far lower than expected from the original VW power curve. Twisting the inflow factors to a total stagnation does not increase propeller power to any considerable extent above the figures here, so the engines are NOT producing full power (or, heaven forbid: your rpm measurements are inaccurate……….?). In addition to this, the inclined inflow to the prop (see below) will increase the prop power slightly (this effect is not included in the prop calc.). This propeller calculation normally is reliable within ~+/- 7 % in power (or ~3 % in rpm.); much of the uncertainty a question on wake factor and inclined flow.

    Second, the observed trim behaviour is supporting the conclusion that the flow is nearly completely stagnant in the tunnel in front of the propeller. When operating non-ventilated, the upper half of the prop is working with an axial inflow velocity close to zero. This situation will produce maximum swirl (=tangential velocity composant) in the outflow. The integrated outflow from this section of the propeller will have a strong downwards composant. For the lower half of the prop, the inflow is less extreme, and the resulting swirl is less intense. The combined overall effect of this is an upward stern force causing the bow to trim down and a drag increase. With the propeller aerated, most propeller work is done by the lower half and the swirl from this part is dominating the outflow. Consequently the vertical force is reversed, causing a bow-up trim moment.

    Third, until you reach a speed, where the transom clears from water, all the ventilating air enters the propeller from behind. To undertand this, you have to realize how the flow behaves around the propeller. Imagine the inflow as a ”sink flow”; there is a primary inflow from all directions, like what you find when you empty you bathtub. In addition to this, there is the longitudinal velocity that varies with speed. The resulting streamlines resemble the magnetic field lines near a magnetic pole. Your parallell tunnel sides will cut off all the flow entering from the sides, causing an energy loss to the inlet flow. On the outflow side, the final axial velocity increase occurs over a finite length; the fully developed flows are found approximately one prop diameter forward and aft of the disc.

    If we study the 9 knot, non-ventilated case, the mean inflow velocity is 2.8 m/s. The mean outflow is 8.0 m/s, and the mean flow through the propeller disc is 5.4 m/s. Now, the effective outflow diameter will be ~290 mm, and the required inflow dia ~500 mm. If you mark this outflow dia along the shaft CL on your ”Medusa Deflector sketch”, and try to find a 500 mm section 400 mm in front of the prop and connect the upper and lower extremes with fair lines through the corresponding propeller tips, you will see:

    A/ there is a stagnant low velocity pocket between the ”hood” and the dividing streamline in the outflow,
    B/ in order to feed the propeller from the tunnel inlet section, the flow is really tortured. It is in fact difficult to find enough area to fit the 500 mm min requirement, say at the 45 degree position one dia forward of the disc.

    ”A” will cause a flow detachment from the tunnel aft of the prop, letting air leak from behind into the low-pressure area in front of the prop. Two things happen: 1/ the blades ventilate and due to the high angle of attack there will be a stall on part of the blade area (=loss of thrust), and: 2/ the pressure in front of the prop increases, reducing the inflow, but also slightly reducing the ”back-suction” on the tunnel inlet ramp. This reduction of back suction is the reason for the slightly higher ship speed with the aerated propeller in your case.

    ”B” is one of the disturbances to the inflow; the others are: too steep ramp, blockage due to shaft tube and double bracket and finally: leak flow at inside junction tunnel side – transom.

    The flow picture around the propeller has strong three-dimensional composants, in fact stronger than normal because your propeller is smaller than optimum for the load it carries at hump speed. A ”standard dia” would have been 16”, ie 30 % more area. Unfortunately, this limitation makes any inflow disturbance extra fatal, and to be quite frank, I do not see any easy solutions to the problems. To make the engines a bit happier, you could reduce the pitch substantially. The loss of thrust in ventilating operation can be reduced by cupping the blades from 70 % radius to the tip, but the tunnel arrangement itself is really bad. Considering the efforts you have put into it, I think you should not waste more time there, but redo, and do it right. Sorry, it is a sour medicine, but I can’t lie to you, it leads nowhere. I would be happy to assist in getting it right though, so please keep in touch and keep me posted.

    PS The long tunnel extension may now be substituted by an "interceptor"; a square/rectangular, sharp-edged profile (say 15 mm high x 20 mm wide; height to be adjusted) installed just inside the tunnel exit. It will create lower vertical force and have less influence on rudder action. With less pitch plus cup you might keep going another year before looking at post 46 again...........DS
     

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  3. baeckmo
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    baeckmo Hydrodynamics

    Looking at the pic in #193, I noted that the inner (twds boat CL) tunnel sides still do NOT reach down to the bottom level! I mentioned this in one of the earlier notes. It would be much better if you corrected this, rather than designing a pocket with horizontal baffles. This leakway is causing early detachment of the flow from the inlet ramp; the low pressure zone that makes the inflow follow the "roof" is destroyed. Seal it and the props will be happier!!

    BOTH TUNNEL "SKIRTS" (INSIDE AND OUTSIDE) MUST GO ALL THE WAY DOWN TO THE LEVEL OF THE BOTTOM WHERE THEY ATTACH TO THE TRANSOM!

    The horizontal baffles won't work well when planing, you will have air entering the inlet. The result will be total detachment; the props will then be truly surface piercing, but with far too small disc area!
     
  4. CDK
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    CDK retired engineer

    That is quite a lot to digest Baeckmo. But there is plenty of time, any changes can only be made after the boat is on dry land in the autumn.

    The area between the tunnels certainly isn't a design prize winner, but I couldn't find a better solution. The box I made to extend the hull follows the bottom profile and since it is a V-hull it renders this awkward shape.
    Making the tunnel wall longer is no option because the two inside rudders are located there.

    What I plan to try is change the prop rotation by exchanging the props. The gearbox control is electrical and can be adapted by changing just a few wires.
    It would mean that the props draw more water from the sides which are covered by platforms.

    That is something I could probably do with the boat in the water, try it out and if it has an adverse effect undo it again.
     

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  5. DennisRB
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    DennisRB Senior Member

    Interesting thread. I hope you sort it out.

    I read somewhere you were thinking of getting more turbo pressure by replacing the wastegate springs. There is an easier and much cheaper way to get more turbo pressure that is also adjustable. All you need to do is bleed away some pressure to the wastegate actuator. You just need to fit a bleed valve in the air line that supplies the actuator. This tricks the actuator into acting like there is less pressure being made and the wastegate stays closed longer increasing the total turbo pressure.

    Devices like this as well as turbo pressure gauges are cheap on ebay. ($30)

    [​IMG]

    You can get more advanced electronically controlled programmable versions of these turbo pressure control devices (boost controllers) that work by pulsing solenoids. You can use these to say increase boost at only certain RPMs or for certain times, IE enough time to get on a plane etc. I have computerized controller on my car which only cost $200 secondhand. You will probably need to up the injection as well to take advantage of the extra air pressure or you wont see high power gains.

    Dennis.
     
  6. CDK
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    CDK retired engineer

    Thank you Dennis.
    I found a similar solution on a German VW forum, where people squeeze 125 hp out of an 85 hp engine.

    But as things are right now, the props draw air with the current engine output. I need to solve that problem first.
     
  7. Bglad
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    Bglad Senior Member

    Drawing air may not be a problem. Have a look at this: http://smithboats.com/pdf/SmithBoat1.pdf

    Maybe yours is suffocating?

    I sent you a private email also regarding some pics I have of one of these boats I surveyed a few years back.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    The problem with this comparison is that the Smith boats are designed with ventilated propellers (and considerably more power per weight than CDK's rig). These have more disc area for the thrust delivered, ie proportionately bigger diameters.

    In CDK's arrangement, due to the restricted tunnel diameter, his thrust loading (in "thrust per disc area") is much greater than a ventilated propeller can produce, particularly at hump speed. In order to use this technology, he would need considerably bigger props and as a consequence lower prop shaft rpm's.

    If you compare the thrust characteristics of ventilated and non-ventilated propellers, you find that at low speeds the ventilated prop has a constant, low thrust up to a certain speed. In the same region the non-vented has a steadily rising thrust with reducing speed. This means that the ventilated prop must be checked very thoroughly for hump performance. A short, fat and rather heavily loaded vessel, such as CDK's is a difficult task for a ventilated prop; it can be done, but you have to know what to look for!
     
  9. Bglad
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    Bglad Senior Member

    FYI the 42' I looked at was said to weigh 24,000lbs, had 2 x 440hp Yanmars, 2.2 reduction gears turning 21.25" x 32" six bladed propellers.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Yes, I got the information. The key loading expression is "kt/J^2", which boils down to "Thrust/(fluid density * speed^2 * diameter^2)".
     
  11. CDK
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    CDK retired engineer

    Quite early in the development of this project I thought ventilated props would be the way to go, but I was confronted with so much vibration I feared everything would fall apart, so I discarded the idea.

    Several people maintained that using multi-blade props in cleaver style would run quiet in an air/water mixture, yet I have never been aboard a vessel with such a propulsion that did run quietly.
    Recently there was an over-sized RIB here with 2 V-8 engines and surface piercing props. The guy had a similar boat last year, equipped with twin Mercruisers; his business is "exporting" goods from the Balkan to Italy, where he sometimes is forced to navigate in very shallow waters to avoid encounters with patrol boats.
    The stern drives limited his possibilities once he was forced to throttle back, hence the new boat. He immediately admitted that no quiet ride is possible, even with 5 blade cleavers, but quietness was not what he aimed for.

    That doesn't comply with my ideas about recreational boating. When anchored in my favorite sandy bay I see and hear a lot of distant traffic: without looking up from my book I can tell how they are propelled.

    With today's knowledge I would have constructed my tunnel drives slightly different. A more elongated shape to keep them water-filled, or a much heavier construction with 2" shafts and surface piercing props.

    But when experimenting, making mistakes belongs to the territory.
    In this case I am not prepared to start all over again because I'm afraid I'll be running out of time. Small improvements so I'll be able to use full throttle without reducing speed is what I aim for.

    And with a maintenance-free propulsion system that is more efficient than any same sized boat in the area, I must have done something right as well.
     
  12. Bglad
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    Bglad Senior Member

    That is certainly a reasonable plan and Baeckmo seems to have the experience and skills to help you get there:)
     
  13. Firemedic833
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    Firemedic833 New Member

    Mr. CDK
    I have a 26" wooden Crab boat that has about a 12" v in bow and about 1" v in stern. as of now I have a 1984 140hp Evinrude, a real gas hog. I was thinking about putting in a 4cyl diesel inboard. I was trying to think of a way to put a tunnel in it so I can run shallow. I was thinking about taking a metal pipe slightly bigger then the prop I will use, cutting it in haft welding a flange on it and bolting it to the bottom. Searching the inter net for ideas I came across this forum and read some of your threads on your tunnel drive. Your design looks more of with I am wanting due to the prop is easier accessed for object removal and I don't have to cut a hole in the bottom of my boat. Can you tell me how yours worked out?
    The prop on my outboard is around 18 to 20" below the water line and most likely be more with the heaver inboard. Do you think your tunnel design will work for my boat? http://www.boatdesign.net/forums/attachment.php?attachmentid=73828&stc=1&d=1346461838
     

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  14. CDK
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    CDK retired engineer

    I don't think you'll be happy with the result because the pipe will cause a lot of drag and makes steering harder.
    If you want to make a tunnel to run shallow you need to make a cutout in the bottom. In your case the beams must be cut, weakening the whole structure of your boat. A conventional prop shaft and a ring around the prop (Kurt nozzle) instead of a flat rudder seems the best option to me.
     

  15. Firemedic833
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    Firemedic833 New Member

    Thanks for your reply CDK, I did want to cut out for tunnel for that reason you said about weakening the bottom. I had an old jacuzzi jet drive i did think about using but I don't think it would be a good match up with the 4cly diesel, so I plan on using it in a smaller boat with a 140hp inboard. But back to my problem. I am thinking your right about using a conventional prop shaft and I know you know more about this then I do. But it is impotent that i get it to run as shallow as i can due to the need to place and run my crab traps in sallow water at times. I am thinking that maybe I can combine a conventional prop shaft and what you did to your boat and use some form of and prop enclosure like a Kurt nozzle. What you think CDK?
     
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