DIY tunnel drive

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

  1. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The same would apply to all the outboard engines, sterndrives and jets on this planet. I'm sorry, but I have more urgent problems to take care of.
    Btw, between the TWO outer seals there is a chamber filled with silicone oil: water repellent, non-toxic, not (quite) petroleum based.
     
  2. PetterM
    Joined: Sep 2004
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    PetterM Senior Member

    Very impressive project. :)
    Your rudders look a bit small for the speeds you are doing.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Thank you for your admiration.
    That the rudders are too small became manifest in the first turn, adequate steering is only possible with engine rpm.
    I already ordered some ss plate and shaft material and will make new rudders with 40% more surface. These I can enlarge by welding: the current ones are GRP with an ss insert to attach them to the steering pins.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The new stainless steel rudders are in place. They really make a difference: running on one engine it is now possible to stay on course and even make a port turn with only the port engine engaged. With both engines at the same rpm, steering is sufficient for all situations except cramped spaces; with one engine in reverse the boat can be turned on the spot in any direction.
    So far we've made only short tests covering 26 miles and there is still some work to be done on the engines and instrumentation, but it is already clear that the efficiency has vastly improved when compared to the stern drives the boat was originally equipped with. Not 10 or 20% better, but probably over 50% more mpg!
     
  5. PetterM
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    PetterM Senior Member

  6. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I'm sorry, no. I was so focussed on improving the performance that I didn't even pull the boat out but installed the rudders while submerged. A hell of a job because every small piece you drop is lost.
    They are just stainless steel plates, corners rounded and a piece of shaft welded at 1/3 of the 12 inch length.
     
  7. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Update on project

    Did the flanking rudders ever work out? I am build something similar but only one rudder per prop, thinking of putting more traditional in middle. How did reverse work out. Would love to put rudders on side to reduce vibration etc. But I am afraid of low speed handling, your input is appreciate since you have done it.
     
  8. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Props in tunnels make a boat feel that it is on rail lines and they do not want to go right or left.Any type of rudder gives very little turning effect especially at speed.
     
  9. Frosty

    Frosty Previous Member

    Sony drive owners are now cutting off one side of the tunnel to have one leg like the flexitab drive. I have done this with amazing results with 3 knots on the cruising speed and incredible improvement on cornering.

    Apparantly according to my friend Rudy at levi drive this is a well know modification on twin drive vessels.

    I can not emphasize just how much of an improvement it is.
     
  10. PetterM
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    PetterM Senior Member

    3 knots from the reduced rudder drag? what's your boat speed?
     
  11. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Which do you cut


    Do you cut the inner or outer rudder and what is rotation of blades?
     
  12. Frosty

    Frosty Previous Member

    I have a cat with a hydraulic tie bar I had no choice but the cut off the inner side or I would need to change all the hydraulics.

    This is opposite to what a mono hull would do to keep rudder in the water when turning.

    Rotation is left left--right right

    I used to cruise at 17KNT at 3400RPM 4200max.

    I can now cruise at 20.2 at 3400.

    With the old tunnels I would turn into a corner and would need plenty of rudder and it would pull it down to 13 -14. With the cut offs it bites into the corners and looses little speed.

    There is no difference to the feel of the steering or is it heavy, the forces on the one legged rudders are ballanced out.
     
  13. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    There are stuffing boxes that use shaft packing as the bearing too.

    These usually will have 15 to 20 packing rings , and do require lubrication , usually a turn of a grease cup at the end of a run to stop drips .

    With the new graphite style packing no lube is ever required as they run very very cool.

    Perhaps an even simpler solution to solve a problem?

    FF
     
  14. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Sorry for my slow response time.
    We've bought a motorhome and made a 10 day trip trough Europe to check it out.

    I've added two pictures of the latest modifications, with a better shape for the tunnel entrance.
    Unfortunately one of the rudders has been severely damaged before I could make the first test run to measure the increase in top speed. A violent storm combined with high air pressure pulled more water from the bay than ever before, so the rudder hit a rock that has always been there, but until then with enough water over it. The climate change.....

    The rudders I attached last year and made several day trips with them. You cannot compare their performance with stern drives or outboards because the boat is much more reluctant to change its course. With the original Mercruisers, slow cruising meant steering continuously to stay - more or less - on course. Now you can walk around or drink a beer and hardly see a change at the compass when you come back.

    Running on one engine, it is possible to make a 360 degrees turn to both port and starboard, but a starboard turn with only the starboard engine has quite a radius. Also, with one engine you can maintain a straight line, so you can get home.
    With both engines running you don't need any rudders, just push the throttle of one engine a bit to increase the rpm and pull it back if the desired course is reached. Much easier and more effective than turning the steering wheel.

    Reversing with two engines goes perfect, again without touching the steering wheel. Turning the boat around without moving forward and positoning the bow in any direction with one engine fwd and the other in reverse is a piece of cake. Reversing on one engine is not impossible, but the steering response is very slow so you need a lot of space.

    I hope this answers your question.
     

    Attached Files:


  15. Frosty

    Frosty Previous Member

    Convetional stuffing boxes dont work with surface drives ,--not enough submersion in water, How do I know this?

    You have to have an oil bath system. Simple to make--a bronze fitting with oil seal in both ends and oil feed from a static resoviour and a cooling outlet from the engine into the stern tube.
    Been on 4 years now.

    Also stuffing boxes allow very very little movement to accomodate the movement of the engine. Exessive shaft wear will be the result.

    PS I also use a special cutlass bearing of 10 inches to accomodate side thrust from the prop. Its all easy stuff if you start right.

    Ive never seen a stuffing box with 15 -20 rings,--usually 4-5
     
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