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Old 04-18-2018, 10:18 AM   #61
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I'm sure locking a prop so that freewheeling does not hurt transmission can become PIA if done often and not having specialized, easy to actuate shaft-lock apparatus.

Luck for us locking is not needed. BW Velvet Transmissions our Tolly has is fine for freewheeling without hurting it; especially at the low speeds we travel when purposefully using just one engine. In addition, I alternate engine use on regular basis during the very slow cruise to keep use-hours similar as well as to not constantly let just one trany experience free wheel stress... no matter how OK it is according to BW experts.

And... because freewheeling is no problem... the freewheeling prop causes less drag than prop locked rigid in position. Which is another assistance to getting near 3 nmpg at 5 nmph, on single engine use, slow-cruise in a planing boat that can also easily cruise at 16 to 17 knots - or above.
Years ago, on this forum in its infancy or on the old Passagemaker Mag Forum, I don't recall which, we had a fellow in Europe with a GB42 who provided actual data to make a true comparison between his boat running at displacement speed on both of his twins OR on only one of them. IIRC his data was collected over a long enough time span to be meaningful.

He proved conclusively that trying to gain efficiency by running on one of a pair was a fool's errand. A single, pushing from the corner, lost efficiency through the drag of the still or freewheeling prop and the rudders having to be angled to keep the boat running straight.
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Old 04-18-2018, 10:54 AM   #62
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I’ve heard of it done by some commercial guys in the PNW taking a fuel guzzling charter boat up to SE Alaska from Seattle. A long haul so a slight savings can tally up.

For general purpose it isn’t a good idea and there’s almost nothing to gain.

Actually Art is right for some boats. A long and narrow boat, one w the prop shafts close together, outboards that can kick up the lower unit and probably other characteristics can make running one on a twin a half assed but fairly workable practice. But very few trawlers or similar boats are narrow or have their props close together. So it’s basically silly.
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Old 04-18-2018, 02:49 PM   #63
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I’ve heard of it done by some commercial guys in the PNW taking a fuel guzzling charter boat up to SE Alaska from Seattle. A long haul so a slight savings can tally up.

For general purpose it isn’t a good idea and there’s almost nothing to gain.

Actually Art is right for some boats. A long and narrow boat, one w the prop shafts close together, outboards that can kick up the lower unit and probably other characteristics can make running one on a twin a half assed but fairly workable practice. But very few trawlers or similar boats are narrow or have their props close together. So it’s basically silly.
It takes a certain and unavoidable amount of horsepower to drive a particular hull of a particular weight through the water regardless of how many engines you use to produce said horsepower. Simple physics, efficiency comes from the designers drawing board not wishful thinking. If a person wants multiple engines for some other reason, maneuvering, redundancy or some other that's fine, it's their nickel and perfectly acceptable in every way.
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Old 04-18-2018, 09:55 PM   #64
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It takes a certain and unavoidable amount of horsepower to drive a particular hull of a particular weight through the water regardless of how many engines you use to produce said horsepower. Simple physics, efficiency comes from the designers drawing board not wishful thinking. If a person wants multiple engines for some other reason, maneuvering, redundancy or some other that's fine, it's their nickel and perfectly acceptable in every way.
I think the point being made is that a given boat will require more horsepower to push it through the water if it is only driving one prop (off center) when it’s set up as a twin.
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Old 04-18-2018, 10:19 PM   #65
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I've been running twins in different hulls since about 1963. Ships, boats, props and jets. I'm sold, so biased opinion.
Twins usually are smaller engines than a single in the same hull. They use 10-20% more fuel and almost double the maintenance. Except in sport fishers, you don't push twins as hard as a single. But you have much more reliability, especially with the new electronic controlled engines. If you travel to remote places, one set of spares covers both engines. In an emergency or in trouble shooting you can swap parts between engines. You have more options for engine accessories - different pumps, alternators, power take offs. If you learn how to maneuver twins, you don't need a bow thruster. (But I don't need a bow thruster with a single). Fishing I run on one engine unless tuna trolling. I've come back on a single twice (not counting military service) in 55 years, but I ran the most reliable 100% mechanical marine engines made (opinion).
The fishing on one engine comment made me wonder: does anyone happen to know if it's better to leave a velvet drive transmission (on a FL120 from the late 70s) in neutral (assuming the prop freewheels) or drive or reverse on the dead engine.
I know every transmission is different...
Edit: drippless shaft seals if that makes a difference
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Old 04-18-2018, 11:15 PM   #66
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For a velvet drive to oil itself the oil pump needs to be spinning. (engine running) I would avoid freewheeling in any gear as the planetary gears are turning in an any gear, with no oil circulation. The same presumably applies to dripless seals. In a pinch, and only if necessary I would tie down the shaft to keep it from turning.
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Old 04-19-2018, 12:29 AM   #67
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Nopistn,I seem to remember from the times this has been discussed that operating at trolling speed with one engine not running was not harmful to the transmission of the rested engine. I think someone found it in the Velvet Drive Manual. Unkindly,trolling speed didn`t seem much different to the speed many of our "trawlers" make. Best look it up.
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Old 04-19-2018, 02:21 AM   #68
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But couldn't you just leave that transmission in gear to prevent anything from turning? Is there a drawback to this?
Not sure if the same physics apply to boat, but on propeller airplanes a 'freewheeling" prop produces infinitely more drag than a stopped on, even if the stopped prop isn't feathered.
With boats having much wider and slower moving blades the same may not apply, I'd guess the difference in drag between a locked prop and a freewheeling one would be small.
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Old 04-19-2018, 02:52 AM   #69
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But couldn't you just leave that transmission in gear to prevent anything from turning? ....
People used to talk about locking it,by restraining the propshaft from turning.
I`m not sure leaving it in gear would work. There was a cruise ferry in Sydney which for a time, was started by towing it, in neutral,and throwing it into gear. Like running a stick shift car downhill and releasing the clutch! Wonder what happened if it stalled while cruising...let`s not go there.
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Old 04-19-2018, 03:46 AM   #70
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Strange concept I know but several years ago I actually contacted twin disc regarding a gearbox used on a cat build I was involved in.

The consensus was that it could happily be run in neutral for several hours at a time, water lubrication to the shaft bearings could be an issue but slots in the bearings should allow enough water inflow.
If really worried about it a small 12v pump would take care of it.

I guess what I am saying is you could always give someone a call.
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Old 04-19-2018, 04:54 AM   #71
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I think the point being made is that a given boat will require more horsepower to push it through the water if it is only driving one prop (off center) when it’s set up as a twin.
And while I stated that a few postings ago and it is included in this attempt to simplify by the words "particular hull" which was meant to represent resistance, which of course includes the condition you mentioned.
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Old 04-19-2018, 05:23 AM   #72
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People used to talk about locking it,by restraining the propshaft from turning.
I`m not sure leaving it in gear would work. There was a cruise ferry in Sydney which for a time, was started by towing it, in neutral,and throwing it into gear. Like running a stick shift car downhill and releasing the clutch! Wonder what happened if it stalled while cruising...let`s not go there.
Haha that's hilarious. I'm pretty sure it would be the mass from the spinning running gear that started the engine. Next time I'm out in my boat I'll leave one engine off and in forward l, and see if 7kts created by the other engine is enough to turn it.
I would never put it in gear while the shaft is spinning though, that's gotta be hell on the transmission/dampening plate
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Old 04-19-2018, 08:22 AM   #73
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Lepke wrote;
“Twins usually are smaller engines than a single in the same hull. They use 10-20% more fuel”

That’s because the twin engines are almost always bigger. In the case of most trawlers they are twice as big. So the usual comparison is w a 120hp boat and a 240hp boat w about 700 cu.in. Now wonder they burn more fuel but it has nothing to do w the fact that they are twins. A boat w 200hp installed should burn tha same amount of fuel as a boat w two 100hp engines. That is about 98% true IMO. There are small variables.
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Old 04-19-2018, 09:20 AM   #74
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For a velvet drive to oil itself the oil pump needs to be spinning. (engine running) I would avoid freewheeling in any gear as the planetary gears are turning in an any gear, with no oil circulation. The same presumably applies to dripless seals. In a pinch, and only if necessary I would tie down the shaft to keep it from turning.
BW tells us that the Velvet Drive can be allowed to freewheel without harm.

In practice though: I had one engine down for a whole summer. I put a shaft grabber (honkin big pipe wrench) on the shaft to prevent it turning. I left it a few inches off the stringer. After running several days it hadn't moved so I removed it.
My "single engine " boat would cruise at 6.5 knots at 2100, where with twins I cruise at 8.2 knots at that rpm.
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Old 04-19-2018, 12:10 PM   #75
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We had an engine out and I clamped the shaft for the long tow back.
I’ve heard it should be thus.

But I wonder how a set of gears in a box w oil in it could not lubricate itself. Unless the oil level was below the gears. Otherwise there would be oil slung all over inside. But the clutches are powered by the oil pump so perhaps a BW could damage itself by clutch slippage w/o the engine/pump running. My BW behind a 37hp engine dosn’t need much oil pressure to keep the clutch plates engaged w/o slippage. Harbor Marine rebuilt the BW and recomended I use a much smaller gear (about 1/3 the size) for the pump and that the standard pump takes 5hp to run at high speeds. But re the freewheeling shaft it may engage the clutch enough to keep the shaft from turning. Would only take a minute to find out on a twin.
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Old 04-19-2018, 12:24 PM   #76
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But couldn't you just leave that transmission in gear to prevent anything from turning? Is there a drawback to this?.
The oil pump providing pressure to engage the clutches is in the input shaft, so if the engine is not running a hydraulic gear will be in neutral regardless of gear selector position.
If you have an old manual gear of course, it is in gear when the lever is in gear.
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Old 04-19-2018, 12:41 PM   #77
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I believe heat is the issue with free wheeling on a Borg Warner Velvet Drive. At high speeds the running gear will be turning but the cooler can’t do its job with the engine off.
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Old 04-19-2018, 07:26 PM   #78
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Thats correct. I just read the BW manual. They call for 110 deg C max sump oil temp in the freewheeling text.
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Old 04-19-2018, 09:24 PM   #79
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We have anchored or moored at times in areas of fast moving current. One example is St. Augustine. I have seen my shafts free wheeling simply from the current. The first time I tied them off. Now I just ignore it. BTW putting them in gear would not stop the rotation.
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Old 04-19-2018, 10:49 PM   #80
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BTW putting them in gear would not stop the rotation.
Well from my understanding they are essentially the same as an automatic gear box in a car.
You wouldn't put a car in park to tow it so why would you lock a shaft?
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