Oil Pump for outdrive leg?

Discussion in 'Sterndrives' started by decadamsinzim, Jan 31, 2013.

  1. decadamsinzim
    Joined: Jan 2013
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    decadamsinzim Junior Member

    Greetz all,

    I'm in the process of designing my own outdrive and I want to include an oil pump at the end of the drive shaft just inside the rearmost seal.
    In other words, inside the casing, just in front of the prop.

    Reasons for this are;

    1) I want oil circulation through the whole leg assembly so the CV joint at the transom and associated bearings are always running in "fresh" cooled oil.
    2) It means I can do an oil change on the outdrive without removing the boat from the water.
    3) I have a quick visual way of seeing if any of my seals are leaking.
    4) Include a magnet in the system for easy inspection for wear particles. Again, without removing the leg or boat from the water.

    So, the idea is, the pump forces oil up a pipe to the front of the system then the oil makes it's way back down the leg through all the bearings and back to the pump. All the time, being cooled through contact with the casing which is obviously submerged. As there are no gears in the whole leg unit (except for the pump if I use a gear pump), I'm thinking along the lines of running the unit on ATF or engine oil which is resonably thin compared to traditional gearbox oil.

    I have two ideas on pump design and this is where I need some opinions please.

    Gear pump or impellor pump.

    Which would be better?
    Which one would use less horsepower?
    The oil isn't under any pressure, it just has to move from the back of the unit to the front. So, all the pump has to do is circulate the oil.
    The volume needed to be pumped is minimal. A gallon a minute would circulate the oil more than enough.

    I'm concerned about creating a vacuum in the rear section of the leg and sucking water past the seals in to the leg if the oil can't return through the various bearings fast enough.

    I think the idea is a real winner though, imagine to change the oil, all you have to do is disconnect the return hose from the front of the leg assembly, place in container. Put unit in drive and wait for it to pump all the oil out of itself. Reconnect hose. Refill with fresh oil. Job done. No drain plugs, no spillage. no fuss. no boat out of water!

    Just thinking about it, I guess an impellor pump would pump oil in forward and reverse whereas a gear pump wouldn't work or would reverse the flow in reverse?

    Your Ideas and thoughts most appreciated.


    Last edited: Jan 31, 2013
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    A simple pump for this purpose would be a spring loaded plunger, actuated by an eccentric ring (cam) around the shaft. The same principle is used in every chainsaw.

    ATF in an outdrive is not a good idea because it is hygroscopic an quickly deteriorates.

    I don't think the world is waiting for your design. The reason to take the boat out annually is to remove marine growth and inspect bellows and seals, then change or add oil because it is convenient.

    But I may be wrong.
  3. gonzo
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    gonzo Senior Member

    Firstly, the current outdrives circulate the oil. The gears make it move sufficiently as the evidence of their long service life shows.
    Secondly, if you pump the oil out, the drive will be running dry for a while and get damage or at least more wear than necessary.
    Thirdly, outdrives are notoriously high in maintenance, regardless of oil circulation type. They need to be inspected thoroughly on a regular basis, and for that need to be out of the water. Boat that are in the water all the time will do better with an inboard or an outboard setup.
  4. decadamsinzim
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    decadamsinzim Junior Member

    Thanks for the input, not that keen on a plunger type setup, would prefer impellor or gear pump. Either would require only one moving part.
    A plunger setup, at least four if you include the non return valve required. I'm just trying to keep things simple. I'm leaning towards impellor. I guess I'll machine up some variations and see how well they work when spun in oil at expected prop rpm.

    The boat I'm building will be used exclusively in fresh water. Taking the boat out the water once a year isn't done here. Not much seems to grow on the hull except for a bit of slime. Probably looking at three to five years between pulling the boat.


  5. gonzo
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    gonzo Senior Member

    Outdrives already have oil circulation. Why do you claim they don't?
  6. decadamsinzim
    Joined: Jan 2013
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    decadamsinzim Junior Member

    Hi Gonzo,

    You've missed the pertinent part in my original post.
    I'm designing my own :)
    Trust me, currently, it doesn't have a means to circulate oil.
    I know this because I haven't designed one in yet :cool:

    The outdrive I'm designing just contains bearings, shafts and a CV. It's purely a way of transmitting power from the transmission, which will be inboard, to the prop.

    Whilst on the subject of transmissions, can anyone recommend a gearbox with forward / reverse rated up to 200-250 hp?
    There are loads to choose from on the net but experience counts on which ones to use and which to keep away from.
    All I'm after is a simple box I can mount between the engine and outdrive to give me the correct ratio and reverse capability. Doesn't have to be fancy hydraulic or electronic gear selection. Good old mechanical linkages will be fine. Doesn't need to be matched to any particular bell housing, I'll just put a shaft from the engine flywheel to the tranny input flange.

  7. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Oil pump for outdrive

    Some out drives have oil pumps to spray the oil to gears and other relevant parts and this does away with drag of having a large mass of oil creating unnecessary drag. Really like a dry sump on an internal combustion engine.
    James Wynne the inventor of (supposedly) the out drive had a lot of major problems with lubrication developing out drives and they burned out gears etc., at an alarming rate and cooling the lubrication.Try running your out drive with little or no oil and see how long it lasts.This is ONE of the nightmares in design of out drives.
    No oil pump on James Wynne Patent Application drawings

    I/O Dry sump image added

    Attached Files:

    Last edited: Feb 2, 2013

  8. powerabout
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    powerabout Senior Member

    Omc used gerotor pumps from 68 to 77 in 3 cyl v4/6
    They never failed
    The dry sump pump on a McM VI ( shown above) and VIII is also a gerotor style on the propshaft ( Weissman invented that style, (Merc copied it) and convert older drives as well)
    Volvo and bmw have centrifical pumps
    Other omc outboards have archimedes screw on the drive shaft as does McM 2 and 3 drives
    Not all stern drives pump it around, they just have a common passage like McM 1 drives and alpha's
    Later Bravos use the propshaft gear as a centrifugal pump
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