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Old 21-02-2019, 21:51   #1
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Offset props... are they a thing?

Im goingvto dive in my berth neighbours boat tomortow to help the old guy out.
He had some rope wrapped around a prop on his twin engined launch and the diver told him that it looked like one prop was set back further than the other...
Sounds odd so I'm going for a look tomorrow for him. Just wondering... are off-set props a thing? Could be that the rope has polished one shaft and made it look like it's slipped... or maybe an anode has been knocked off.
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Old 22-02-2019, 04:35   #2
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Re: Offset props... are they a thing?

No, props set back different amounts would affect steering. The prop needs to be close to the strut for support and to the stop shaft wobble.

The rope could have been between the strut and the prop and acted like a prop puller. Pulling the shaft out of the coupling as more line was pulled into the tight space. I had that happen.
One thing nice about steam engines, when the line pull equals the steam pressure, everything just stops and nothing gets ruined.
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Old 22-02-2019, 14:45   #3
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Re: Offset props... are they a thing?

So just dived and he has offset props which are both left hand rotation. So the starboard prop is set back 30mm approx from the Port side.
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Old 22-02-2019, 14:49   #4
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Re: Offset props... are they a thing?

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Originally Posted by nzmal View Post
So just dived and he has offset props which are both left hand rotation. So the starboard prop is set back 30mm approx from the Port side.
Thatís... weird.
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Old 22-02-2019, 14:59   #5
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Re: Offset props... are they a thing?

Wing engine? Some old boats had a separate small engine for low speed use. Main engine was on center for maneuvering or high speed. Seen them on old Dutch cruisers. Cool idea, a smaller engine for areas where speed is restricted like small canals where your main engine wasn't needed.
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Old 22-02-2019, 15:14   #6
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Re: Offset props... are they a thing?

The setback would then compensate for the tendency for the boat to turn to starboard, since the props are not counterrotating?
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Old 22-02-2019, 20:20   #7
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Re: Offset props... are they a thing?

Thanks. The counter rotating was my thinking. Both engines are big old donkeys same size. Look to be about 100 up each. Not sure what they are but they are the originals from circa 1965.
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Old 23-02-2019, 08:53   #8
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Re: Offset props... are they a thing?

You might want to look to see if the engines are bolted to the stringers in the same position.
Same with the gap between the coupler and packing gland, 30mm, or 1/18" is not that much.
If there is shiny metal showing on the shaft at the cutlass bearing, it may have slipped back as mentioned.
It could also be possible that the shaft is actually abit longer than the other.
Old boat, who knows what's happened over time.
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Old 23-02-2019, 09:14   #9
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Re: Offset props... are they a thing?

Anything is possible, makers often put single props off centerline right? So the "effect steering" argument not valid. But be sure that the shaft hasn't started to pull out of the coupling on that boat, especially if it had things around the prop!
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Old 23-02-2019, 09:52   #10
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Re: Offset props... are they a thing?

The engines are mounted in line but one hull fitting is stepped back by 30mm... so it's definitely a designed issue. Boat I'd 50+ years old so it must work fine....
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Old 23-02-2019, 10:12   #11
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Re: Offset props... are they a thing?

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Originally Posted by Cheechako View Post
Anything is possible, makers often put single props off centerline right? So the "effect steering" argument not valid. But be sure that the shaft hasn't started to pull out of the coupling on that boat, especially if it had things around the prop!
There should be a visible line on the shaft aft of the coupler if it did slip or was pulled back.
Set screws should have prevented this.
Maybe look for that, as well if possible, pull those screws and see if they're sheared.
Shaft should be "dimpled" to prevent that.
Old Chris Craft I just refit with shafts and all, everything was trashed.
1968 Chevy 327Q one right hand, one left hand, rotating.
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Old 24-02-2019, 18:19   #12
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Re: Offset props... are they a thing?

He says the props are offset 30mm. More than an inch. Doubtful the shaft would still be connected to the coupling if it was yanked out that much. Then he says the actual "fittings"(assume the struts) are offset some as well. Clearly, boat was designed that way if not intentionally modified.
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