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Old 06-15-2014, 04:07 PM   #1
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Different Ratios

The starboard Velvet Drive on our '87 Defever 44 was replaced some years ago with a Twin Disc. That leaves the boat with a 2.5 ratio direct drive transmission on the port side and a 2.44 on the starboard side.

My questions:

1). Is the engine synchronizer equalizing engine or shaft RPMs?

2). If the synchro is matching engine RPMs, am I not getting two different drive speeds at the props due to the different gear ratios in the transmissions and, by definition, the props are not synchronous?

3). During sea trials , we heard a mild "singing" in the starboard aft area. Could this be caused by the differing gear ratios?

4). Should I avoid using the engine synchro, think about replacing the other Velvet Drive or just not worry about it?

Thanks!
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Old 06-15-2014, 04:38 PM   #2
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I'm almost positive it's engine rpm so your stbd engine will be under more load. And more thrust on that side. If you don't have counter rotating props you maybe better off switching gears on one side to the other to counter the tendency for the boat to turn to one side depending on prop rotation.

If you do have CRP then the engines will be in out of sync when the rpm causes the harmonic vibration to make the hull drum. Rather like a twin engine aircraft slightly out of sync. The closer one is to sync the lower the frequency of the drumming harmonic vibration. Sync them w tech or w ear. No drumming when in sync. Engines must be able to maintain rpm well.

Never heard of it but theory would say you may have prop harmonics when the engines are in sync. And if you do I would think the only escape from harmonic vibration would be gears w the same ratio.
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Old 06-15-2014, 09:22 PM   #3
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Thanks, Eric. Makes sense. Another issue: one of the engines would not come up to full RPMs during the sea trial until the PO shut off the synchronizer. Then it shot up to full power. Not sure what that portends.
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Old 06-16-2014, 01:08 AM   #4
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maybe the props are differently pitched to compensate?
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Old 06-16-2014, 02:31 AM   #5
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maybe the props are differently pitched to compensate?
^this! Do you know the specs on your port and starboard props?
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Old 06-16-2014, 07:20 AM   #6
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Good thought. Both props are described as 25x19 in the survey. (The previous owner has gone radio silent, so no help there.)

I'm far from an expert here, but if the ratios (2.5 vs. 2.44) only differ by .06 to 1, it looks like the props would only be spinning at a different rate of about 100 rpm if both engines are running equally at 1800 rpm. Can a difference that small be causing the singing/harmonic and can it be accurately compensated for by re-pitching one of the props?
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Old 06-16-2014, 09:45 AM   #7
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The gear ratios are pretty close, only off by 2.4%. Should not cause any operational trouble at trawler speed. But it can cause harmonics. Props make pulses that run around the boat, so do engines. When in sync, most of the pulses are in sync, and that makes the noise more pleasant. In the OP's case, the engines will be in sync, but the props slightly off. Can make for a slow drumming (beat frequency). How annoying depends on the boat.

It won't cause singing, and it won't cause the strange rpm thing when on sync. And tweaking prop pitch won't change the noise.

At full power (off sync) what rpm do you get on each?
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Old 06-16-2014, 10:37 AM   #8
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Good thought. Both props are described as 25x19 in the survey. (The previous owner has gone radio silent, so no help there.)

I'm far from an expert here, but if the ratios (2.5 vs. 2.44) only differ by .06 to 1, it looks like the props would only be spinning at a different rate of about 100 rpm if both engines are running equally at 1800 rpm. Can a difference that small be causing the singing/harmonic and can it be accurately compensated for by re-pitching one of the props?
The 2.4% difference as a percentage of prop slip may be over 33%, and the different prop rpms, at the same boat speed, does necessarily affect prop slip. As to whether or not it could induce a harmonic, I doubt that there is any way to know for sure without making changes until the harmonic disappears. If I were you, I would talk to a prop expert.
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Old 06-16-2014, 10:55 AM   #9
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If the singing has gone away it may have been from dry stuffing box if the boat was not used much.

IMO the sync is engine speed so the props are indeed at different RPMs. A little cup would offset the load. Were the WOT RPMS equal??
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Old 06-16-2014, 12:18 PM   #10
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This situation is not that unusual: Old Velvet Drives with reduction had 1.91:1 on one, and 2.10:1 on the other. That's a pretty big difference, 10%. Prop pitch had to be adjusted between p&s.

Also some older Hurth/ZF HSW series had slight differences between p&s ratios. I think all the modern ones are the same now.
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Old 06-16-2014, 12:51 PM   #11
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Singing can even be caused by different water conditions.

I get a singing or ringing from my starboard running gear when traveling up Guemes Channel against the tide. Guemes Channel is a 1/2 mile wide channel several miles long with fairly strong tidal currents. Could be a combination of water temperature and density, current conditions and shaft load. Not overly loud, but noticeable none the less.

Attached is a Velvet Drive manual in PDF format that you can download and keep handy. It covers both standard in-line and V drive, 70 through 73 series BW units. Answers a lot of the technical questions about BW's and their operations that appear on this forum.
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File Type: pdf Borg-Warner-Velvet-Drive-Marine-Installation.pdf (1.05 MB, 32 views)
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Old 06-16-2014, 05:29 PM   #12
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Thanks, guys.

I'm using the same term, "singing," that my hull surveyor used. If I remember correctly, it was a rhymic thrumming sound that rose and fell in volume, but never very loud. You could only hear it--and then pretty faintly--in the aft cabin when standing on the starboard side. He speculated the cause might be a strut bearing and recommended getting it checked and maybe getting the prop indexed next time it was on the hard. He did not seem to think it was a major problem.

At WOT, with eight adults on board and the synchronizer off, the port engine got up to 2,370 rpm and starboard maxed out at 2,417. We used a local marine diesel mechanic who had a lot of instruments hooked up. He also inspected the linkages and said they were adjusted properly.
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Old 06-17-2014, 10:00 AM   #13
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I guess it would have helped if I understood the relationship between engine rpm and prop rpm using a reduction gear (transmission). I was multiplying the two gear ratios times an assumed engine turnover of 1800 RPMs and getting a difference of 108 RPMs. I should have been dividing the two ratios into 1800 RPMs which produces a difference of just 18 RPM or so.

My engine surveyor says it was definitely singing, commonly heard coming off a prop. he says it's nothing to worry about.

Sorry for confusing things and thanks for the feedback.
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Old 06-17-2014, 10:16 AM   #14
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Old 06-17-2014, 12:11 PM   #15
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"Singing" usually means something a prop does regardless of gear ratio and should have nothing to do with your transmissions. Singing is caused by the shape of the leading edge of the blade and/or certain water flow conditions at a certain RPM, and is easly corrected. Even a new propeller can "sing" under certain conditions.

The transmission ratios are so close, as has been mentioned, I would not consider it a problem. Did your engine guy set your tachs? If not, the "difference" in RPM may just be in the tachs themselves.
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Old 06-17-2014, 12:56 PM   #16
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"Singing" usually means something a prop does regardless of gear ratio and should have nothing to do with your transmissions. Singing is caused by the shape of the leading edge of the blade and/or certain water flow conditions at a certain RPM, and is easly corrected. Even a new propeller can "sing" under certain conditions.

The transmission ratios are so close, as has been mentioned, I would not consider it a problem. Did your engine guy set your tachs? If not, the "difference" in RPM may just be in the tachs themselves.
Thanks, Brooksie. I'm a lot less concerned about this now. The engine guy brought his own tachs and other instruments. Bags of them!
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Old 06-17-2014, 05:04 PM   #17
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I had a friend with a Heritage East trawler that had singing props. Any dolphin withing a half mile were attracted to it. You could see them making a beeline to swim in the wake.
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Old 06-17-2014, 05:28 PM   #18
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Interesting! They followed us around for a large part of the sea trial. So maybe I won't fix it.
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Old 06-17-2014, 05:51 PM   #19
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Old 06-21-2014, 10:24 PM   #20
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Join the Defever cruisers club for a measley $25 a year there are a ton of 44 owners who will help answer your concerns

www.defevercruisers.com. The forums themselves are well worth the fee.
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