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Old 06-09-2011, 04:29 AM   #41
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RE: Running single on a twin to save fuel

"I think a planing hull will have 2 to 2 1/2 times as much resistance as a full disp hull run about 15% below hull speed ".

I believe the plaining shape, transom drag and prop drag only add about 50% at SL x1 , trawler ,speeds.

THis is why the "fast trawlers" , or old fish killers have such poor range at usual trawler speeds.
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Old 06-09-2011, 08:40 AM   #42
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Running single on a twin to save fuel

I'm going to quote someone on BoatDesign and hope it's OK to do that. This person is a very highly respected poster on BD and I believe he is a NA and an engineer. By the way I am known as "Easy Rider" on BoatDesign.

"I'm arriving a bit late to this interesting discussion.

Easy, you should google for Van Oossanen's paper "Motor Yacht Hull Form Design for the Displacement to Semi-Displacement Speed Range". At the page 632 (don't worry, the paper starts with the page 629 ) there is a graph which shows a linearly increasing correlation between the immersed transom area and the resistance (expressed through the Telfer coefficient, explained in the paper). The graph is a result of a significant number of towing tank tests performed by Wolfson Unit on a number of different displacement and planing hulls.
The graph shows that in the displacement cruising speeds range (Fn,L = 0.35), a planing type hull will have a resistance about 2.2-2.5 times higher than a displacement hull of same length and displacement. Which is, If I understand it well, what you've been seeking for in your initial post."
This was the post I started on BD to help answer the question on TF. I also assume the tank test hulls had no keels, struts or other appendages. This sounds like it was a very well controlled experiment. "Opinion's" differed widely on the BD thread but all agreed running on one engine was more economical at disp speeds*** ....disp speeds being less than hull speed. One poster (highly respected) told of his experience running a twin IO powered diesel boat w the engines rather close together. Seems to me to be about the best platform for doing this as could be found as the lower unit and prop can be raised out of the water and asmetrical thrust would be minimized. On this boat w high tech fuel flow sensors he achieved a 30% decrease in fuel consumption compared to running w both engines. Generally speaking it seems a planing hull boat should have at least twice as much drag at disp speeds as a full disp boat. However, there are many variables.









-- Edited by nomadwilly on Thursday 9th of June 2011 08:43:24 AM
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Old 06-09-2011, 08:55 AM   #43
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RE: Running single on a twin to save fuel

Speaking of the wrong boat, I am at chilly Shearwater Marina today. The Eisenglas topped saling cat behinid me had his diesel heat running all night and he*seldom raised his sails as he cruised 400 milies north from Friday Harbor. Nice to have a boat designed for the intended cruising grounds.*Proper selection, beyond price, *plays a role too.
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Old 06-10-2011, 09:23 AM   #44
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Running single on a twin to save fuel

The title of this thread "Running single on a twin to save fuel" has gone from a simple answer to an engineering question that seemingly has such a complicated answer that no non technically minded person could possibly understand.

The real question: Can I run my twin engine boat more economically by shutting one engine down?

Then answer is dead simple: Select whatever rpm you wish to make this comparison. Note the speed. Shut one engine down. Note the speed.

If the speed on one engine is greater than half the speed on two engines then it is more economical to run single engine at that rpm.

A planning hull with the power necessary to plane will probably always be more economical to run single engine below displacement speeds. The reason; less than half the power produced by the engine at less than displacement rpm is transfered to the prop. This is probably not true of a non-planning boat.




-- Edited by timjet on Friday 10th of June 2011 09:32:00 AM
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Old 06-11-2011, 03:49 AM   #45
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RE: Running single on a twin to save fuel

"Can I run my twin engine boat more economically by shutting one engine down?

Then answer is dead simple:"


Only if the transmission on the dead engine can be trailed , with out need to lock the shaft.

Operating the boat with a freewheeling prop (with the wrong tranny) could cost a tranny rebuild.

NOT a way to save money.
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Old 06-11-2011, 06:29 AM   #46
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RE: Running single on a twin to save fuel

Quote:
FF wrote:
Only if the transmission on the dead engine can be trailed , with out need to lock the shaft.

Operating the boat with a freewheeling prop (with the wrong tranny) could cost a tranny rebuild.

NOT a way to save money.
*FF is corrrect. Don't always trust the tranny manual either. Mine says it's OK to free wheel. A call to ZF Hurst disbuted that. The tech at ZF Hurst said to swap the engines every 30 minutes.
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Old 06-11-2011, 07:45 AM   #47
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RE: Running single on a twin to save fuel

I have been trying single engine runs on long offshore passages where there would be no need for maneuvering.

The best way I could find to secure the unused shaft was to put a wrench on one of the coupling bolt heads, and have the wrench lie in a nearby stringer.

The autopilot would deflect the rudders about 10 degrees to maintain a straight course.

I would do this at night when I wanted a slow speed in case I hit anything. About 4 knots. Very quiet.

I never figured how much this helped the fuel burn.
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Old 06-11-2011, 08:06 AM   #48
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RE: Running single on a twin to save fuel

I took a 2x4 cut it in half down the middle, clamped it back together again and used a hole saw to drill a 1 1/2 inch hole centered on the seam. I then glued rubber obtained from a tire tube on the cut out hole surfaces. Two bolts thru bolded at the ends completed the block. I have yet to use it.
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Old 06-12-2011, 05:54 AM   #49
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RE: Running single on a twin to save fuel

"I have yet to use it."

For an emergency .

The wrench can be disconnected in an instant by starting the engine , touching reverse , the wrench will drop, and then engaging FWD

What will you do?
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Old 06-12-2011, 08:08 AM   #50
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RE: Running single on a twin to save fuel

Good point FF.

Also, when single engine with the other shaft stopped by a wrench, I only operate at near idle to prevent the bolt head from becoming to tight or actually shearing off.
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Old 06-12-2011, 10:05 AM   #51
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RE: Running single on a twin to save fuel

timjet,

You asked "The real question: Can I run my twin engine boat more economically by shutting one engine down"*

For sure but the benefit's will vary depending on how far apart your screws are, how big your props are, if you can trail your shut down gear ect ect. But most to everyone that has done it reports a fuel savings.

"If the speed on one engine is greater than half the speed on two engines then it is more economical to run single engine at that rpm" That would seem to be a given. How could it be otherwise? The propeller loading curve and propeller slippage should guarantee that. Planing boats have smaller propellers and that should help you do the single engine dance.
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Old 06-13-2011, 08:50 AM   #52
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RE: Running single on a twin to save fuel

Quote:
nomadwilly wrote:"If the speed on one engine is greater than half the speed on two engines then it is more economical to run single engine at that rpm" That would seem to be a given. How could it be otherwise?
*Yes, that very simple exercise/formula can't really be disputed, though I've had many people tell me that running single on a twin is not more economical. But as you mentioned the amount of savings depends on the boat. My boat does 8.3 kts at 1300 rpm but drops to 7.1 kts on one engine at 1300 rpm. Seems like a no brainer if I'm going to go this slow. I'm having a little trouble convincing the admiral to cruise at this speed, she wants to get there.

Concerning blocking the shaft. I would never do that unless the engine was disabled. I allow the stopped engine to freewheel for 20 minutes and then switch engines, with the blessing of the tranny manufractuer.
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Old 06-19-2011, 08:06 PM   #53
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RE: Running single on a twin to save fuel

I have twin 6.354 Perkins driving Borg Warner Velvet drives with "wet shaft logs". *I checked with Borg Warner many years ago and they said free wheeling the shafts was fine, but they recommended swapping engines every two hours to keep the bearings and seals lubed. *

We have been cruizing on one engine off and on for 30 years now and have had zero problems with the transmissions (knock-on-wood). *Yes, there is a very apparent fuel savings and the boat is much quieter running on one diesel. *We can make 8.5 knots on one engine at 2400. *But when we are running on one, it's usually because we are not in a hurry and enjoy a much quieter boat running at 7.5 k at 2000-2100 rpm.

"Wet Shaft Logs" Make sure your shaft logs aren't pressure water lubricated/cooled. *Some dripless shaft logs have a pressurized water line from the engine to the log and rolling the shaft without water pressure might not be good for them.
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Old 06-19-2011, 08:24 PM   #54
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RE: Running single on a twin to save fuel

Read the Marine Propulsion Systems manual for its transmissions*and it says it's fine if the propeller/shaft freewheels, and that it doesn't matter if the transmission is in gear or not.

By the way, I must change the transmission oil and filter for*my MPS ZF63 transmission.* The manual doesn't specify the filter needed, but it's the kind that fits in a recess and the dipstick fits*through the center of it.* Are all transmission filters the same or is a special one needed, and would it be available at an auto-parts store or a marine supplier?* Thanks.
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Old 06-20-2011, 04:20 AM   #55
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Running single on a twin to save fuel

Sadly it may have to come from ZF.

If it does , buy a case and seal them with a vaccume sealer.

Oil filters are NOT built for years of storage in a damp locker before use, they can rust rapidly.


-- Edited by FF on Monday 20th of June 2011 04:21:02 AM
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Old 06-20-2011, 06:54 AM   #56
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Running single on a twin to save fuel

Quote:
markpierce wrote:*Are all transmission filters the same or is a special one needed, and would it be available at an auto-parts store or a marine supplier?* Thanks.
*My ZF Hurst Transmission filter is made specifically for my model tranny. It may fit others, I don't know. I don't think you can buy this special purpose filter at an auto parts place or NAPA dealer though it would be worth a try. I got mine from a diesel engine parts supplier locally. Call ZF Hurst if that's the manufacturer, and they will tell you the correct filter and I would ask about running single engine while free wheeling the other. As I mentioned even though the tranny manual indicated unlimited freewheeling was OK, the tech said not to freewheel more than 30 minutes.*

Just a heads up. Good luck.

*

Oopps, forgot you are single engine, never mind.



-- Edited by timjet on Monday 20th of June 2011 06:56:44 AM


-- Edited by timjet on Monday 20th of June 2011 07:00:58 AM
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Old 06-20-2011, 07:17 AM   #57
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RE: Running single on a twin to save fuel

I had a Hurth 630, which was the old ZF63. The filter was a steel mesh unit that could be cleaned and re-used.

*
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Old 06-20-2011, 07:46 AM   #58
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RE: Running single on a twin to save fuel

Quote:
markpierce wrote:The manual doesn't specify the filter needed, but it's the kind that fits in a recess and the dipstick fits*through the center of it.*
*What model number is that?
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Old 06-20-2011, 10:51 AM   #59
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RE: Running single on a twin to save fuel

Quote:
RickB wrote:markpierce wrote:The manual doesn't specify the filter needed, but it's the kind that fits in a recess and the dipstick fits*through the center of it.*
*What model number is that?

*ZF63
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Old 06-20-2011, 04:31 PM   #60
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RE: Running single on a twin to save fuel

Quote:
FF wrote:
If it does , buy a case and seal them with a vaccume sealer.

*Well, they cost $50 or $100 each, depending on manufacturer.* So rather than a case, I bought just two (for the 30-hour and 300-hour changes).
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