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Tunnel Mounted Pod Drives?

Discussion in 'Technical Discussion' started by theav8r, Jan 14, 2012.

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  1. theav8r

    theav8r New Member

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    I've been reading a bit about pod drives and have noticed that ZF pods (pusher) are tunnel mounted and Volvo IPS pods (tractor) are generally not tunnel mounted (except in triple engine applications .... i read that somewhere ....)

    why tunnel or no tunnel ??
  2. Boatbuilder

    Boatbuilder New Member

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    The Volvo pods are mounted 'normal' to the hull, which is also perpendicular to the deadrise angle. The older Zeus and other pods needed to be plumb to the world, or perpendicular to the waterline. So a tunnel, or fairing, has to be developed into the underbody of the hull to give a flat surface for the pod to mount to.

    Clear as mud?
  3. theav8r

    theav8r New Member

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    Gee ... you're right that is clear as mud ......

    Are you saying that the ZF pod has to be mounted on a shaft that is perpendicular to the waterline and the IPS drives can be mounted on a deadrise ??

    why ?

    the only thing I see is that the props should be aligned along the relative flow direction .... the props don't know that they are supported on a shaft that is straight up and down or on a cant (deadrise) .......

    I read also that triple ips installs are in tunnels ..... is that right ? ..... why ?
  4. Boatbuilder

    Boatbuilder New Member

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    The only 'why?' that I am aware of is that's the way Zeus/ZF wants it to be. With the Volvo system there is a little bit of up and down vectoring of the prop thrust as the pods turn port and starboard when in joystick mode, resulting in a bit of rocking motion. However, with a bit of practice this tends to go away as the tendency when first using the joystick is to over correct, with familiarity the motion becomes much smoother.
    The other issue that can come up with the Zeus/ZF vertical installation is that some boat layouts don't have enough depth to install the pod, as it tends to be higher with the tunnel pushing it upwards.

    I don't have any experience with triple IPS installs.
  5. theav8r

    theav8r New Member

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  6. NYCAP123

    NYCAP123 Senior Member

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    Just tried. Good luck,Zzzzzzzz. The tunnel's job is to lessen draft,allow for a more efficient running angle with shaft drive, and funnel the water to the prop. However the running angle isn't a consideration with the pods as they're mounted to the appropriate angle. The biggest problem with tunnels is that it can upset the flow of water past the prop if not calculated properly. I suspect that the forward facing props might run into a problem with that unless they completely redesigned the tunnel. Why do that if it isn't necessary? Zeus may see a potential for the refit market and also an advertising advantage in offering a reduced draft by fitting to a tunnel. Their reverse facing prop won't require a redesign of the tunnel, although I have to wonder if there isn't some sacrifice to either handling or efficiency because the articulating pod isn't going to take the flow equally as it turns near a tunnel. Sounds like a trade-off.
  7. PacBlue

    PacBlue Senior Member

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    IPS came to the market first and I believe that the development team at CMD (Cummins Mercruiser Diesel) had to dodge a few issues on the Volvo Penta patents - hence the tunnels for the Zeus/ZF product which provide a flat mounting surface. Other areas that required patent "tweaking' were the attachment joint and seal of the units to the hull. I am sure there were a few more....

    I would not read much more into the tunnel design requirement ;)
  8. Karl2

    Karl2 New Member

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    Tunnel or no tunnel with pods ? Covered pretty well in the posts; Patent issues, marketing spin or ?

    Shaft angle is no issue with the pod - Correct. Draft gain with tunnels ? Maybe, maybe not. The tunnel shape will result in a buoyancy loss. Will the net result be less draft ? Not always.

    Efficiency loss with tunnels - Always. You will lose some because of the water flow in the tunnel. With a good design this can be kept to a minimum but cannot be totally eliminated. Secondly, with a tunnel shape you will always be dragging more gel coat in the water = efficiency loss. This cannot be eliminated.
    In triple installation Volvo uses a tunnel shape or a flat on the keel line. If in a tunnel Volvo's owner’s manual states that the engine driving the pod in the tunnel will turn 40-60 rpm less at WOT compared to the two outboard units (without tunnel) = Efficiency loss.

    Karl
  9. captaintilt

    captaintilt Senior Member

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    Pods and Mounting

    In regards to Pods (ZF/Zeus) I was one of the main Test Captains / Program Managers for this project in Charleston for just over 3 years, and worked on a variety of builds, Grand Banks, Cabo, Hatteras (QuadZeus), SeaRay, Cobalt, Maxum to name a few, if you have any questions about it, please feel free to PM me.

    Thanks!
  10. NYCAP123

    NYCAP123 Senior Member

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    Welcome to YF Cap. It sounds like you may be the "expert" on this, at least where the Zeus is concerned. Have you run Zeus without tunnels for testing or was it a patent decision to run tunnels from the start? If they did test without tunnels, what was the operating difference between with and without the tunnels?
  11. Capt J

    Capt J Senior Member

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    Not all Zues boats have tunnels. The 40' Cabo zues does not have tunnels.....
  12. theav8r

    theav8r New Member

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    On the 40' Cabo ...... are the drive mounted straight up and down or perpendicular to the deadrise like the Volvo IPS ??
  13. Capt J

    Capt J Senior Member

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    Straight up and down if memory serves me right.
  14. Karl2

    Karl2 New Member

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    If they are installed straight up and down they have a "shape" in the bottom that adds surface area and reduces buoyancy. Call it tunnels or.....

    Karl
  15. captaintilt

    captaintilt Senior Member

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    NYCAP, In response to your question, the only difference that was noticed on boats not equipped with tunnels, such as the Cabo 40' Express, which doesn't have the tunnels like a SeaRay or Grand Banks. When maneuvering with the joystick, it tends to make the boat "roll" side to side. It definitely takes some adjusting to stop that tendency, it can be corrected with about a days work around the docks.

    As far as testing performance, we did test a few boats with round bottoms or no tunnels, and the performance was not much different. It took a little longer to get on plane with the auto trim tabs, but if you manually adjusted them when coming out of the hole, there was no difference.

    Lastly, the boat "QuadZeus" which is powered by quad QSC 8.3 (600HP) on a 60 Hatteras S/F was only about 3 knots slower and consuming about 100GPH at WOT (3000RPM) than a normal CAT C-32Acert 60' that was pushing 3000+ HP.
  16. PacBlue

    PacBlue Senior Member

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    Cummins QSC8.3's rated at 600 hp burn 32.5 gph per each engine according to their website, so WOT fuel burn would be 130 gph for 4 x engines. Depending on Which CAT C32 engine you are looking at, thefFuel burn runs at 82.1 gph each for 1600 hp up to 94.2 gph for the 1825 hp rating. (I don't believe the 1925 hp C32's were in the mix at the time of testing. So you are looking at 164.2 - 188.4 gph at WOT for the CAT's.

    I was told that the CAT's were more efficient at cruise than the Quad Zeus. Certainly this project was a challenge to equal or improve performance on an existing design that was never intended to ever be powered by Pods ;)
  17. CaptSteve

    CaptSteve New Member

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    I don't have an answer, but a thought....with the rotation of the pods, you would think a horizontal relationship to the water line would be beneficial. With hull mounting, the angle of the thrust would change with the rotation, for better or for worse.
  18. captaintilt

    captaintilt Senior Member

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    Thanks for the input PacBlue, in reference to the QSC 8.3 burning 35GPH, I am in total agreeance with you as many of the Zeus Powered boats with these engines are burning right around that number, obviously they need to be at 98-100% load and still turning the full RPM's. My reference to QuadZeus burning 100GPH is accurate, within a few gallons, (it has been over 2 years since the project was completed, and I don't have my notebook with me to reference), but QuadZeus was not fully outfitted with furniture, TV's, Fighting Chair, Riggers, etc when I picked it up at the factory to return to Charleston. Hence it was an operating "shell" but still had full tankage, etc. As far as comparison to the C-32's the operating range at cruise was about the same with the Quad QSC's. There was some scuttlebutt about staggering triple M-11's (punched out to 800HP) in a 60GT, but it never happened due to the economy falling short on rec. boat sales.

    Overall, I really like the concept of Zeus, and as far as mounting them into a boat, a huge reason for the "tunnel" concept was due to the mounting grommet that needs to be in place to properly seal the hull. It is much larger than the IPS grommet and also designed to sheer the drive off and not break the watertight integrity of the hull. I have firsthand experience of performing this "breakoff" test on a hull before the trim tabs were lengthened to protect the transom from the propellers. Needless to say, it was quite a ride / swim!!!
  19. Capt J

    Capt J Senior Member

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    The 40' Cabo zues, burns 43-45GPH at 80% load for both 600hp Cummins QSM's, or 21.5-22.5 GPH per engine. It burned 65.5 gph for both at 3050rpms WOT or 33 GPH per engine. I have to re-size the pics to show the drive relationship to the bottom......etc......My opinion is that a twin IPS or Zues application you see very good benefits in fuel economy over traditional shafts, with a triple application you still see a pretty decent fuel economy over shafts, but on a quad application the amount of drag from the drives in the water and having 4 sets of props pretty much is very little benefit over a twin inboard traditional shaft application.
  20. Opcn

    Opcn Senior Member

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    Something is unclear to me here; Increased rocking was mentioned as a drawback to the IPS (though one that can be overcome by a steady hand).

    Is this due to the tunnels on the Zeus displacing buoyancy laterally resulting in a hull form with greater initial stability and roll dampening, or is it a result of the IPS being out of plumb with gravity?

    If it's the later, or some third factor would you care to take a shot at explaining just how that works?