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

How about folding props. The kind they put on sail boats.

Art said:

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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The issue is that the propeller of the stopped engine, moves the gearbox, which has no cooling system...

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.

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....

alberto said:

The issue is that the propeller of the stopped engine, moves the gearbox, which has no cooling system...

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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.

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.

Prop Drag Test Movie - YouTube

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.

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

HopCar said:

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.

Prop Drag Test Movie - YouTube

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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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.

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.

HopCar said:

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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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!

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.

djmarchand said:

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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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?

brian eiland said:

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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depends what you have for tranny and stuffing box...check with manufacturer...even then pretty simple steps or mods can overcome issues.

Art said:

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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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.

Sent from my iPhone using Trawler

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

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.

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!!

"...NMPG were poorer,at the same speed ..."

Well duh. Usually the point in running on one is not to achieve the same speed.

I mean, how could anyone expect a different outcome? Dragging a dead prop and offset rudder at the same speed as spinning two props and centered rudders and expecting a beneficial result is kind of silly, don't you think?

sunchaser said:

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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When running on one of the twins I slow to 5.5 to 6 knots... that gets me an added .50 to .75 nmpg as compared to running 1 to 1.5 knots faster (just below hull speed) using both engines. Never have yet, but some day I'll try running both at the slower speed and one at the higher speed to see what fuel usage happens then. Guess I'll wait till after installing my Flo Scan... should make calcs much easier!

When running on one of the twins I slow to 5.5 to 6 knots... that gets me an added .50 to .75 nmpg as compared to running 1 to 1.5 knots faster (just below hull speed) using both engines.

Hull speed is never efficient , a 20%-30% reduction may double the boats range

It's important to check the requirements of your particular boat. Some drive systems would be damaged if the prop shaft is allowed to turn without the engine running, some may not be. My dripless shaft seal is lubricated by water from the running engine so the shaft should not be allowed to turn without the engine running. Since I have only one engine, this is pretty academic unless I'm being towed.

It seems to me that if you are running a twin engine boat on one engine to save fuel, you probably should have bought a single engine boat in the first place. That would be far more efficient than hauling around an extra half ton of iron and the drag from the drive system. It will also handle much better.

Many twin engine dripless seal boats now have a T in the dripless supply so that either engine supplies both (at the Sea Ray dealership we made many conversions)...on many boats it doesn't really matter at slower speeds such as being towed or if a person is running on one to save fuel...but yes they can be damaged if you don't follow manufacturers recommended speed limitations.

A lot of people like twins for a variety of reasons....they also like saving money where they can. Buying a twin and running it anyway they like may have well been the decision of a really smart, experienced individual that has certain likes and dislikes....not necessarily a "poor" decision. Me thinks the OP really likes his boat, it's systems and has the experience to have made his choice wisely...

It seems like he's not asking "single or twins"...that decision was already made and he's happy with it. It also seems he's interested in running on one and just wants to know which is better...locked or freewheeling...and so far based on reports of the forum are..."depends"...whether shutting one down is even more efficient, whether running one at idle versus locking one make more sense or whether freewheeling or locked is more efficient.

So his thread has got him what he wanted...a lot of opinions and user reports about his options...whether he has the right boat or not, based on lot's of posts, I'm sure he's comfortable with.

WesK said:

It's important to check the requirements of your particular boat. Some drive systems would be damaged if the prop shaft is allowed to turn without the engine running, some may not be. My dripless shaft seal is lubricated by water from the running engine so the shaft should not be allowed to turn without the engine running. Since I have only one engine, this is pretty academic unless I'm being towed.

It seems to me that if you are running a twin engine boat on one engine to save fuel, you probably should have bought a single engine boat in the first place. That would be far more efficient than hauling around an extra half ton of iron and the drag from the drive system. It will also handle much better.

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You are pretty much correct Wes!

Some apparatus (trany... etc) can handle free wheel props and some can't; important info for any boater to know. Our Borg Warner Velvet Drive tranys easily handle freewheel props at relatively slow speeds – I only cruise on one engine at 5 to 6 knots and trust these 71C BW Velvet Drive trany can freewheel at up to 7 or 8 knots... IMO, any faster than 6 knots the twins should be running anyway... as I either cruise around hull speed (7.58 knots for our Tolly) or climb onto plane and cruise at 16 to 17 knots. Also, there are ways to have dual drip less shafts feed water supply off one another so that if one twin is shut down then its freewheeling shaft in drip less log can stay cool. I don’t have drip less and do not plan to again – been there done that! Drip less shaft log water lube is not possible with a single in freewheel if perchance a tow is needed; unless extravagant means of water supply are available as prop freewheels – hardly worth time, trouble and cost for once a decade or preferably never a tow. Therefore single shaft should be locked while towing if drip less shaft log is the case. Not so with twins... no need to lock shaft from turning (long as apparatus is applicable and/or drip less log is fed water) nor usually any need for a tow... due to other functioning engine.

There are hundreds (thousands?? lol) of good reasons for owning either a single or twin screw, planing or displacement, bridge or hardtop, sundeck or sedan, glass or wood, old or new, big or small, mono or dual hull, gas or diesel, sail assist or not, deep or shallow draft, full keel with skeg or naked props or pods... or... type boat. Facts on those and more (desires/reasons) have been reviewed odd-in-fin-item on previous TF threads. Reason(s) for wanting to own one type boat or another are personal and unique to each Owner-Captain! As is often mentioned: Boating and boats are simply a mass of “trade offs”...

Happy Boating Daze! - Art

RickB said:

"...NMPG were poorer,at the same speed ..."

Well duh. Usually the point in running on one is not to achieve the same speed.

I mean, how could anyone expect a different outcome? Dragging a dead prop and offset rudder at the same speed as spinning two props and centered rudders and expecting a beneficial result is kind of silly, don't you think?

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sounds good to me

RickB said:

"...Dragging a dead prop and offset rudder at the same speed as spinning two props and centered rudders and expecting a beneficial result is kind of silly, don't you think?

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In this case the beneficial result was making tidal based passages and open water crossings possible. One engine was BO until I could get to the parts store. So I learned something, between 6 to 7.5 knots running on one engine was less fuel efficient than running on two at the same NMPH. Since this question is the essence of the thread --------

A little bit off the topic of the thread, but for those of you who have planing hulls and are running at less than recommended engine RPM for extended periods of time for fuel efficiency (rather running on one engine or both), how often do you ramp up the engines to that recommended RPM and for how long? How much of the fuel efficiency you gained at running low RPM did you loose?

A big advantage of a twin-engined boat is that there is a huge supply to choose from as compared with a single. Also, one is probably getting the second engine "free" in the used-boat market. (The same with flying bridges.)