diy surface drive?

Discussion in 'Surface Drives' started by moTthediesel, Apr 19, 2006.

  1. jmiele3
    Joined: Jan 2012
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    Location: Philippines

    jmiele3 Junior Member

    Our drives can trim +/- 7 degrees... That is sufficient for 99% of conditions. You are also thinking solely in terms of recreational vessels. In most military applications, vessel load and sea state vary considerably, and those conditions need to be taken into account.
     
  2. Frosty

    Frosty Previous Member

    Thats the trim of the shaft not the prop. Sea state behind any boat is always the same when your 3 feet from the transom on a planing vessel, doe'snt matter how big the waves are.

    The height of water behind the transom is not altered by pay load when in planing mode
     
  3. jmiele3
    Joined: Jan 2012
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    jmiele3 Junior Member

    This discussion was related to drive, rather than prop trim, though... and sea state will impact you on planning if the slamming becomes excessive. Trimming the drive becomes quite useful in those situations.
     
  4. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    If the sea state is excessive wouldn't drive oscillate between air or full submergetion, and boat would have to slow down or in and out could damage drivetrain.

    Also can SD be over ventilated, in other words, too much exhaust being sent down leaving props in air.
     
  5. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Ventilating Surface Drive is not necessary with a planning boat,it happens automatically as speed increases if it is set up right a Surface Drive ( or subsurface drive) boat should be able to go all day with prop fully submerged at any RPM.
    Sea conditions always change behind the transom of a planning boat especially in following seas when you can not go faster than the following sea which can easily top 25 MPH.
    It makes no sense to install a shaft drive in a boat that can not be adjusted or retracted
    when there are so many advantages to making it adjustable and making it easier to install and increase the strength at the transom and reduce the need for fine tolerances in alignment. Fully rubber mounting makes a much more smoother drive unit and reduces the risk of damage.
     
  6. Grumpyrelick
    Joined: Nov 2011
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    Location: Ottawa, Canada

    Grumpyrelick Junior Member

    Tom,
    Your comments are on the money.
    A quick review why surface drives are so good:
    1. You want a ventilating prop because cavitation doesn't happen in ventilated water. Cavitation will erode props causing loss of performance.
    2. You also do not want any parts in the water that do not contribute directly to the forward motion of the boat: Struts, shafts, intermediate brackets, prop hubs and trim tabs create drag and water is 700 times more dense than air. Drag is a big deal.
    3. You want your expensive drive and prop to clear the bottom of the boat when grounded on a beach.
    4. You want to be able to replace a prop without diving equipment or having to dry dock the vessel.
    5. You want to control the effective diameter of your prop for peak efficiency of the propulsion equipment with "trim".

    You can only do all the above with a trim-able Surface Drive. The fixed SD
    I built was only to see if I could build one. It was not fancy but it worked. Now if I could find a cheap Allison 250 Gas Turbine...
     
  7. Grumpyrelick
    Joined: Nov 2011
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    Location: Ottawa, Canada

    Grumpyrelick Junior Member

    I have a question for those of you who have Surface Drives:
    Do you run constant pitch or variable pitch props?
    (By variable I do not mean controllable pitch prop)
     
  8. stupidbaker57
    Joined: Jan 2011
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    Location: Lakeville Ma

    stupidbaker57 Junior Member

    Altho my boat is still in the building process and it hasn't gotten "wet" yet, I hope the prop I choise will work as I hope. It is a 7.25 dia SS cleaver with a 12 pitch comming in at 10 and going out at 14.
     
  9. Grumpyrelick
    Joined: Nov 2011
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    Location: Ottawa, Canada

    Grumpyrelick Junior Member

    What I was getting at is this:
    1. Props are divided up as a percentage of the radius. Eg. .1R, .2R, .3R, etc. to .975 R. (There is no prop to really measure at 1.0R (the tip)
    2. Propeller pitch is determined by measurement only at .7 Radius.
    3. Old props (early last century) were constant pitch props where the pitch was the same at all radii.
    4. Modern props are all variable where the inner radii has more pitch than the outer radii. This was done to lessen the loading on a prop near the tip.
    5. You cannot see this difference with your eye. You can only measure it.
    6. If you have a variable pitch prop on a Surface Drive you believe you would be losing pitch as you raised it out of the water.

    So my question... Do you run variable or constant pitch props?
     
  10. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Question for you eggheads... lol... I understand that SD are more efficient than a fully submerged prop, but why? It would seem that prop fully in water would have a better grip on water. Yes it drags more but it also pushes more. If the hub is barely in water then only side of prop is working. It would seem it takes less torque to get a Surface prop to a certain rpm, it is traveling in air not water. So guys enlighten me what is going on because I know air doesn't push anything.
     
  11. Frosty

    Frosty Previous Member

    The basics of the SD is different, the drag is reduced because all the 'stuff', bearing and shaft is out of the water.

    An SD prop will be twice the size of conventional, yet the same blade area in the water, usually 4.

    The SD prop is a different shaped blade, concave.

    Its more like a rotating sculling device.
     
  12. Grumpyrelick
    Joined: Nov 2011
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    Location: Ottawa, Canada

    Grumpyrelick Junior Member

    1. "To every action there is always an equal and opposite reaction:" Isaac Newton. You can't negate this law. A prop blade grabs a handful of water and slams it back hard enough to punch a hole in a boat. (Check out hydroplane race boat damage)
    2. It does this about (4 blades X 3000 RPM) 200 times a second. So 4 ounces X 200 = 800 Oz/second. Divide by 64 oz = 3.125 us gallons/second. If your pitch is 36 inches... 3 feet X 50 RPS, The stream is pushing 110 miles/Hr. That is a lot of speed and volume. It is so powerful that the stream is driven out of the water in a rooster tail.
    3. It does not matter whether or not the spray is in the water, air or outer space. Rockets do not push against anything yet they move. Jet boats also shoot the water into the air and they still move.
    4. Note that 2 bladed props are not used on Surface Drives because when they are parallel with the surface, they would both be out of the water. This would cause damaging vibration. Surface drives usually have 4 to 6 blades to steady the loading.
    Oh yeah, Props do not push or pull the boat. They do not "screw" the water. They are lifting surfaces - like an airfoil.
     
  13. Grumpyrelick
    Joined: Nov 2011
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    Grumpyrelick Junior Member

    Frosty,
    This is true (I have 2 props custom made for me in my garage) but it still does not address whether, say Arneson props are constant or variable pitch.
     
  14. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    In theory adjusting trim should basically adjust the pitch somewhat.
     

  15. Frosty

    Frosty Previous Member

    Irrelevant discussion it has boiled down to preference.

    My shallow cat needs no adjustment your military stuff will no doubt be different if you like the complications of external rams , pipes and barnacles, so be it. I don't.

    If I had a 100MPH cigarette that I blasted across the bay on a Sunday afternoon and then hauled it out and had a slave to wash it I would have an Arneson with bells on just to make huge rooster tail for the girls at the bar.

    But-- since it sits in the marina and is too big to haul out to a trailer I prefer less stuff getting fouled up.

    If there is one chore I hate its diving on the boat cleaning barnacles.

    antfoul does not stay on SD very long You build em you should try owning one.
     
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