seattleboatguy
Senior Member
Anyone know which transmissions can be left to freewheel without damaging the transmission?
freewheeling propeller
- Thread starter seattleboatguy
- Start date
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Borg Warner Velvet Drives.
psneeld
Guru
Most of the Twin Disc transmissions I have run could be freewheeled for a certain period of time...after that they recommended you run the engine to redistribute the tranny oil and also check the oil periodically. Some were start every 10-12hrs or so I think so dragging it all day wasn't an issue.
With our twins I only run either screw singularly at considerably slow cruise speed (4.5 - 5.5 knots - 6 max) in calm waters. This decreases gph (by .75 to 1 gal) as well as reducing hours use and therefore less wear on each engine. Although BW Velvet Drives (per factory specs) are supposed to be OK for many hours at slow speed with freewheeling prop drag... simply for good measure (i.e. common sense) I alternate engine use each hour to make sure that both trany stay well lubed internally. This run-schedule also allows the off engine to maintain enough temp so that warm up period is not required for the newly started engine to become the driver at such slow speed; engine that had been driving can therefore be rather quickly shut down (I do let the restarted engine idle in neutral for a minute + to be sure its oil channels have become filled). About the only drawback I see is minor increased wear and tear on starters. With engine to be restarted still a bit warm and in good tune it takes just a second for starter to get it firing. 
Also, engine noise level is kept at minimum (my Admiral loves that!). Our Tolly has 350 cid, 255 hp gas engines... so... in a well insulated, well vented engine compartment when running at or below 1,800 rpm there is very little noise anyway, on one engine the noise is almost nonexistent. I imagine with twin diesels when running similarly as mentioned above there would be remarkable noise reduction... seeing as single diesel would only need to run at considerably reduced rpm to maintain slow cruise speed. That said, there are some models/brands of diesel that are not designed to run at low rpm for extended periods... check with manufacturer. Always better safe than sorry.
One more thing... twas four (4) decades ago in New England coastal waters where I did some sport fishing. We never ran on single engine of a twin while trolling and I do not feel I would do that now if I decided to go off shore trolling. I run on twins whenever in congested canals, in populated bays, and whenever docking. Control baby!! I’m a boat maneuver control freak Captain and I love the control that co-operating twins constantly afford!
Happy Boating Daze! - Art
Also, engine noise level is kept at minimum (my Admiral loves that!). Our Tolly has 350 cid, 255 hp gas engines... so... in a well insulated, well vented engine compartment when running at or below 1,800 rpm there is very little noise anyway, on one engine the noise is almost nonexistent. I imagine with twin diesels when running similarly as mentioned above there would be remarkable noise reduction... seeing as single diesel would only need to run at considerably reduced rpm to maintain slow cruise speed. That said, there are some models/brands of diesel that are not designed to run at low rpm for extended periods... check with manufacturer. Always better safe than sorry.
One more thing... twas four (4) decades ago in New England coastal waters where I did some sport fishing. We never ran on single engine of a twin while trolling and I do not feel I would do that now if I decided to go off shore trolling. I run on twins whenever in congested canals, in populated bays, and whenever docking. Control baby!! I’m a boat maneuver control freak Captain and I love the control that co-operating twins constantly afford!
Happy Boating Daze! - Art
Edelweiss
Guru
Somewhere I've got a copy of a shop Velvet Drive reference manual that breaks down the shaft RPM by gear ratio at which Velvet Drive series 71 and 72 can safely freewheel. A transmission shop gave me a copy of that section of their manual. You have to count or measure the rpm of the free wheeling shaft because it's dragging, so it turns at a slower rpm than the powered shaft. But they said you're golden freewheeling at sailboat speed.
This question comes up so often, if I can find it, I'll post it and maybe it can be sticky noted to the forum.
That would be great to see... if you come accross it. None of the lit I have on my 71 BW VD is specific to that point. It's calls I made to BW HQ, my decades experienced trany specialist's recomendations, net/forum accounts from private boat owners, and my own experiences that have let me feel comfortable in allowing freewheel at low rpm / slow speed. Maybe I'm too cautious in making sure to change which engine is in freewheel every hour... Maybe I can run all day on one with no bother to switch to the eother?? I simply do not know for sure - yet. Your shop-copy may clear things up on this topic!
seattleboatguy
Senior Member
Do you do a lot of traveling at 4.5 - 6 knots? I had the impression that trawlers generally cruised in the 8-10 range. 4.5 to 6 is down into my sailboat range. The slower speed must be very wallet-friendly when it comes time to visit the fuel dock.
psneeld
Guru
Funny...was just on the dock talking to a Monk 36 owners about my trip back north and he was surprised at how slow I ran...he runs around 1800-1900 RPM and gets 8 knots...
My sweet spot for my threshold of pain fuel consumption is 1650RPM, 1.9 gal/hr, 6.3 knots and that averages out to be around 3.3NMPG over my last 4000 miles....
My sweet spot for my threshold of pain fuel consumption is 1650RPM, 1.9 gal/hr, 6.3 knots and that averages out to be around 3.3NMPG over my last 4000 miles....
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Hi sbguy
We often travel with twins running at 7 to 7.5 knots (somewhere just below our Tolly's calced hull speed of 7.58 knots). Then we get between 1.75 to 2 nmpg
.
On really low-key occasions, when we need to get no-where fast; I cruise on a single screw at the 4.5 to 6 knots and average around 2.75 +/- nmpg.
During cruises when we have some distance to cover and time is of the essence I plane her out at 16 to 17 knots. 1 nmpg is the average then.
Upon emergency (only happened twice in last several years) I'll push her twins up to just below WOT for a relatively short period. Then she's clipping through the water at around 21 knots. That speed sure is fun while piloting up on the bridge! BUT – She’s then using OMG who knows how many gph (near or at WOT I believe 45 +/- gph would be a good estimate). I’ve never left her at that speed long enough to do any sort of fuel use calc on gph... and don’t plan to either!! - LOL
Happy Boating Daze! - Art
