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Old 02-06-2014, 03:24 PM   #21
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"...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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Old 02-06-2014, 03:37 PM   #22
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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
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!
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Old 02-07-2014, 06:46 AM   #23
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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.

Hull speed is never efficient , a 20%-30% reduction may double the boats range
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Old 02-07-2014, 08:26 AM   #24
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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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Old 02-07-2014, 09:11 AM   #25
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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.
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Old 02-07-2014, 09:47 AM   #26
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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.
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
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Old 02-08-2014, 09:13 AM   #27
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Quote:
Originally Posted by RickB View Post
"...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?
sounds good to me
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Old 02-08-2014, 09:52 AM   #28
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"...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?
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 --------
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Old 02-08-2014, 11:50 AM   #29
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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?
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Old 02-08-2014, 12:42 PM   #30
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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.)
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Old 02-08-2014, 09:39 PM   #31
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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
Fuel efficiency has nor ever been in my case a reason to run a twin as a single. That being said my experience over the years regarding shaft locked vs unlocked in several boats with shaft brakes has been I think a 5 to 10 percent efficiency loss. To put that into perspective hull slime,wind, current, foul props, and waves have had more bearing on nmpg than anything else. 1700 on a single in flat water, shafts locked and no wind is 6 knots at 2.5 g/hr. 1700 on twins 7.5 at 5.5g/hr. Freewheeling 6.1 kits at 2.5 g/hr. Pick your poison. This data is at about 3/4 load. At 1/2 load it's better and at full load it's worse. My conclusion is ...It depends and forget about efficiency.

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