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Old 12-28-2013, 01:16 AM   #1
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Most Fuel-Efficient?? Locked or Free Wheeling Prop w/ One Engine Operating

I've heard conflicting reasoning as to why a locked or a free wheeling prop may be or is more fuel-efficient for twin screw boat while operating on one engine.

Common sense tells me that free wheeling is best (as long as the transmission can handle it - such as my boat's Borg Warner Velvet Drive Trany can).

I'd like to hear input on this from TF members.

Thanks, Art
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Old 12-28-2013, 05:08 AM   #2
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How about folding props. The kind they put on sail boats.
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Old 12-28-2013, 05:33 AM   #3
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Quote:
Originally Posted by Art View Post
I've heard conflicting reasoning as to why a locked or a free wheeling prop may be or is more fuel-efficient for twin screw boat while operating on one engine.

Common sense tells me that free wheeling is best (as long as the transmission can handle it - such as my boat's Borg Warner Velvet Drive Trany can).

I'd like to hear input on this from TF members.

Thanks, Art
The issue is that the propeller of the stopped engine, moves the gearbox, which has no cooling system...
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Old 12-28-2013, 06:15 AM   #4
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Folding props for sailboats are optimised to be low drag under sail.

They will provide some push under power but the Hyde is the only unit built to provide good thrust under power with a bit more drag when stopped.

http://www.peluke.com/Propellers/propellers.html

Locked or free there are plenty of tests that validate both arguments.

Free means very very free or the drag will be HIGHER than simply locked.

My opinion is its a matter of diameter , a small 24 inch prop would only have minor drag but get up to 32 + and the drag area becomes huge.

AS on a sailboat a 2 blade would offer less drag underway (powered or not) but the diameter might become a draft problem.
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Old 12-28-2013, 07:18 AM   #5
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USCG engineers told me on the bigger boats/ships in the USCG, locked was less drag.

However...on TF there were posts that MIT or some other brain trust showed that freewheeling on "boats" was less drag.

My take on the matter is that it may depend...and not on the "get in the way" discussions of stuffing box meltdown as those are usually easily remedied....but on your particular setup in terms of prop size. blades, gearbox ratio, friction in your stuffing box, desired one engine speed, etc, etc....
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Old 12-28-2013, 07:19 AM   #6
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Quote:
Originally Posted by alberto View Post
The issue is that the propeller of the stopped engine, moves the gearbox, which has no cooling system...
According to mfg specs: Borg Warner Velvet Drive Transmissions have no problem with multi hour freewheel prop at hull speed or below. When using one screw and freewheeling the other I alternate engine usage each hour... Can't be too careful... For trany safety sake!

And, my stuffing boxes are not dripless, cooling there is OK.
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Old 12-28-2013, 06:17 PM   #7
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From my own tests I'm convinced that a locked airplane prop creates less drag than a free spinning prop.

From watching this movie, I'm convinced that a free spinning boat prop produces less drag than a locked boat prop.



I think the difference may have to do with the cord (width) of the blades. Marine propellers have very wide blades compared to airplane propellers.
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Old 12-28-2013, 09:58 PM   #8
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Here is one person's data of twin engine vs one- locked or freewheeling. It was done by Bob Lowe, a former boat yard owner in the PNW. Go to Dreamer

What it says is to run both engines and fuggitaboutit. We burn more energy talking about it than we could possibly save IF we knew the right answer. And we don't know and probably never will.

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Old 12-28-2013, 10:14 PM   #9
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Quote:
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From my own tests I'm convinced that a locked airplane prop creates less drag than a free spinning prop.

From watching this movie, I'm convinced that a free spinning boat prop produces less drag than a locked boat prop.



I think the difference may have to do with the cord (width) of the blades. Marine propellers have very wide blades compared to airplane propellers.
Yop - Hop!

I've seen that video some time before. Thanks for posting it. That video is another reason I believe (other than my own experiences and common sense) that freewheeling prop creates less drag than fixed prop in marine conditions. One reason I began this thread is because a very astute boat owner swears that fixed prop is less drag... although he has yet to prove it. Figured maybe I could find definitive proof for one way or the other... I'll send him link to that video. I'd still like to find proof from a marine engineer who works a lot with props - under all conditions.
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Old 12-28-2013, 10:40 PM   #10
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Art, the most important thing is what happens on your boat. Why not just try it? I bet your GPS can detect a speed difference between a locked and a free wheeling prop.
Try it and let us know what happens.
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Old 12-28-2013, 10:53 PM   #11
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Art, the most important thing is what happens on your boat. Why not just try it? I bet your GPS can detect a speed difference between a locked and a free wheeling prop.
Try it and let us know what happens.
I simply have not ever locked one prop-shaft, so... to date I have no personal comparison to freewheel. I may try with locked prop some day and do GPS... etc. Looking to learn other's stats before hand. If I become convinced that freewheeling hands-down wins for efficiency (such as I think it does) - then I will likely never bother to try locking one shaft!
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Old 12-29-2013, 06:58 AM   #12
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When I had my 37' sportfish I used to troll for hours single cat 3208 320hp. The other freewheeled and while I never had a solid set of numbers...I was convinced that I could double (and maybe even a little more) my mileage at 7 knots, single engine, freewheeling from 1NMPG at 20 knots to over 2NMPG at 7 knots.

Those that run their boat and think it applies to all other boats really crack me up. No one set of parameters makes all boats perform the same.

There have been a couple TFers who have tried and have SOLID numbers that single engine ops on their boats saves a bunch of fuel while others don't.

There is evidence that locked props versus freewheeling can actually reduce drag depending on variables in hull, speeds, drivetrains, etc....

To say one works and the other doesn't or one way is better or not or just forget about it is just wrong. If you are interested...find out about the requirements of your trannies and stuffing boxes and give it a try.

The only numbers that matter are the ones that you get for your boat....everyone else's numbers (unless a n exact sistership) could be off and their opinions just guesses.
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Old 12-29-2013, 07:13 AM   #13
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Originally Posted by djmarchand View Post
Here is one person's data of twin engine vs one- locked or freewheeling. It was done by Bob Lowe, a former boat yard owner in the PNW. Go to Dreamer

What it says is to run both engines and fuggitaboutit. We burn more energy talking about it than we could possibly save IF we knew the right answer. And we don't know and probably never will.

David
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Old 12-29-2013, 09:12 AM   #14
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Drag reduction or not is not what concerns me most about the situation. I want to know what damage to the drive system might be done in a free-wheeling situation??

1) proper lubrication of the free wheeling transmission/gearbox?
2) damage to stuffing box/shaft seal?
3) other?
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Old 12-29-2013, 09:21 AM   #15
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Originally Posted by brian eiland View Post
Drag reduction or not is not what concerns me most about the situation. I want to know what damage to the drive system might be done in a free-wheeling situation??

1) proper lubrication of the free wheeling transmission/gearbox?
2) damage to stuffing box/shaft seal?
3) other?
depends what you have for tranny and stuffing box...check with manufacturer...even then pretty simple steps or mods can overcome issues.
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Old 12-29-2013, 11:08 AM   #16
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If you really want to spend some $$ and get serious for locked prop and no wear/tear on stuffing box or transmission... I spoke with the owner inventor few years ago, nice guy!

SHAFT LOK INC. Since 1979

http://www.shaftlok.com/

What I'm still not clear on is which is more efficient in the long run - freewheel or locked prop. As has been mentioned by a few on this thread the answer is dependent on the boat. So... guess... I'll simply need to take the time and effort to run prop-locked tests of my own. Currently and for years past freewheel works fine for me because my BW Velvet Drive trany are OK with it and I'm certain our nmpg notably increases by shutting one engine down for cruising slow speeds in the 5 to 6 knot range.
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Old 02-06-2014, 12:26 AM   #17
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Quote:
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If you really want to spend some $$ and get serious for locked prop and no wear/tear on stuffing box or transmission... I spoke with the owner inventor few years ago, nice guy! SHAFT LOK INC. Since 1979 http://www.shaftlok.com/ What I'm still not clear on is which is more efficient in the long run - freewheel or locked prop. As has been mentioned by a few on this thread the answer is dependent on the boat. So... guess... I'll simply need to take the time and effort to run prop-locked tests of my own. Currently and for years past freewheel works fine for me because my BW Velvet Drive trany are OK with it and I'm certain our nmpg notably increases by shutting one engine down for cruising slow speeds in the 5 to 6 knot range.
I'll give one more view on this. You don't have to run twin engines in synch. You can idle one in gear at 700 rpm and run the other at say 1000 rpm. Just have to adjust with rudder. You will get home a bit quicker than synching on the limiting engine. Useful when needing to get somewhere to change a fading fuel filter. I think if I take my 46 on a real long haul, I will run slow on 2 engines, never turn them off, and buy a fuel bladder if 600 gallons won't get it done.

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Old 02-06-2014, 10:15 AM   #18
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Two summers ago we ran on one engine for about 35 hours with the down prop and shaft in freewheel. NMPG were poorer,at the same speed, by about 20% in this mode vs both engines/props running
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Old 02-06-2014, 10:37 AM   #19
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speed and boat determine whether a twin on one engine will be more efficient...anyone particular setup and what speed can vary the results dramatically.

Other TF members with twins running on one show a marked improvement.
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Old 02-06-2014, 11:44 AM   #20
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It really depends on lots of factors. One not discussed yet is that rudder drag goes up with rudder angle, and you will need some angle to offset the unbalanced thrust. Some boats need LOTS of angle.

Also, some gears and shaftlines have a lot of friction, so the freewheeling prop is only say "half" freewheeling. Those are better being locked, probably.

Locking is also a PITA. And if you suddenly want to use that engine, someone has to dive in the ER to unlock.

Sooo much simpler with a single engine!!! At least until one motor (THE motor) poops.

My second engine is a Boat/US card!!
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