Running single on a twin to save fuel

Per* --* Quite the contrary. On newer boats, twins are often cheap in comparison to buy once you get above 45'. I recently made an offer on an expensive Brand X single engine boat. Fortunately they turned the offer down so I can*cruise with*my cheap twins for the rest of the year it appears. Until another expensive Brand X single comes on the market that is ----.

This discussion reminds me of the guys who buy a* Lexus, Infiniti*or Porsche hybrid*to save money on gas.

sunchaser wrote:
FF -* You forgot to add another way to save money is keep a navy launch in my Florida backyard with a 6-71 for power. I'm so envious, some of us have all the luck.

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*FF,*

Have you met the guy who has a navy launch he keeps at Rialto Harbor. We rafted up with him and his wife for dinner and the night 2 months ago. I believe he lives in Ft Myers. Very nice job of converting her.*

Very nice job of converting her.


For someone that wants a nice cruiser , has a couple of years , and the skills and background to do the work its a nice way to go.

There would need to be a special Desirement , that a stock cookie cant meet to make it worth the effort.

One advantage to a 50 ft hull is one could liveaboard very early in the conversion.

The low time hulls with running , mostly cummins power, are running about $35K in Boats and Harbors.

white vapor at the tail pipe (over fueling)

White smoke is underloading or poor compression or low cylinder temps.

BLACK smoke is too much fuel.

The trailing prop is a big drag , one cure is power feathering. A small 1 hp or so electric or hyd motor is installed to spin the prop, but not push the boat.

This will allow centered ahead rudders , and no prop drag.

Alternators and DC motors are not efficient,, but way better than a dragging prop.

boatk9 wrote:
*The corresponding fuel flow*was*3.1 gph (per the chart).* That's about a 23% improvement over twin operation...far more than I expected.* I attribute the improvement to bringing the turbo charger on line. *



-- Edited by boatk9 on Monday 6th of June 2011 12:52:43 PM.

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*My experience is very similar to yours. I have a planning hull with turbo charged after cooled engines. I believe the turbo's kick in around 1400 rpm. I set both engines at 1300 rpm and achieved 8.3 kts with a fuel flow of 4.9 gal/hr. For my boat this is displacement hull speed. I shut one down and left the remaining engine at 1300 rpm. Speed dropped to 7.1 kts and fuel flow was cut in half to 2.45 gal/hr. A whopping 41% improvement in fuel mileage.*

I attribute this improvement to several factors. The engines are huge compared to the boat displacement in order to achieve 25 kts. At displacement speeds only a small amount of power is distributed to the prop but a larger amount of power is used just to keep the engine running, ie alternator, raw and coolant pumps, etc. Shutting one engine down doesn't really cut the power to the props in half, but does eliminate all the other ancillory items from consuming fuel.*

My props are close together so assymetrical thrust is minimual or at least less than perhaps other boats. A 3/4 turn of the wheel into the stopped engine is all it takes.*

My guess is if you were to leave the throttle setting the same ie did not increase the remaining engine to maintain speed, your fuel mileage would be even better. I believe the tubo only increases performance at the top end, above 50% power thus the reason it kicks in at a higher rpm.*

The very existence of this conversation indicates someone has the wrong boat. Do you guys think this is a solution to your problem? I wonder what it would cost to convert to single engine? Weight is a huge part of a boats resistance (especially a planing hull) so you would be shedding not only the weight of the engine removed but half the fuel, perhaps a start battery, a prop and shaft and likely other stuff. Could be 2 tons with a boat that size. But resale value may go down*** .... it may go up but not likely as a prospective buyer could buy a regular single engine boat. Thinking out loud.

nomadwilly wrote:
The very existence of this conversation indicates someone has the wrong boat. Do you guys think this is a solution to your problem?*

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*Wrong boat?? *Solution to our problem??

Not sure what the problem is. Discussing the most economical way to operate a boat does not indicate someone has the wrong boat.*

Your boat is perfect, OK nearly perfect for you. You can take whatever those Alaska waters can throw at you cruising along at 7 kts.

For me in So FL cruising the ICW where waves rarely exceed 2 ft takes a different mind set. I like sitting up high in the breeze watching the sea life and shore go by, sometimes at 20 kts. I also like my wide beam, full size bed that I don't have to climb into, relaxing sundeck, wide salon with wide screen TV and stairs from the swim platform to the sundeck. Yea, I know, I've gotten soft and comfort trumps sea worthiness, but have you never had the desire to get somewhere quickly, to beat a storm or anchor before sunset, or just relax before it gets dark. If so then Eric, may I humbly suggest that perhaps you have the wrong boat.*

*

Attached below:

Bridge view of Florida Straits, 1/4" following seas, 20 kts, 70 mile run in 4 hrs, 70 gallons

boat9 and timjet,

I'm really sorry guys. I had the idea you were trying to operate your cruisers as trawlers. Yes the practice does save fuel and it looks like now and then you're taking advantage of that. Perhaps some winter I'll come down there and see what it's like. I did post on BoatDesign but I did'nt start the thread. I would never have thought of that. I just wanted to tap the knowledge and information that came from it and share on TF. Sometimes the same discussion is on TF and BoatDesign and some on TF (like me) are on BD as well. I like to think out of the box and I like to share my thinking with others and I know I sound rather uppity and come out like a know-it-all fairly often. I like to say what I want to say and share my opinions freely. My comments like changing to a single screw or converting a sailboat to a trawler are completely speculative also unless otherwise presented. When it comes to a good seaworthy small and economical trawler I do have a good boat but ther'es lots of features of the W30 I don't like. And I did or still do think I have the wrong boat and spent lots of time this winter shopping for another but the Willard is close enough considering that most likely I'd jump into a a project boat or nearly so so I decided to invest my time in the Willard and do more cruising. Speaking of cruising you'll get a break from me as I'm headed for Sitka in a few days. Anyway I'm sorry about the misunderstanding and I'll try hard not to do that again.

"So you're saying a smaller set of injectors wouldn't help reduce incomplete combustion at low power settings? "

YES , the load is so light the compression pressure (after ignition) is too low.

Much of your fuel use efficiency comes from getting the turbo to work , instead of being an intake restriction.

For most engines at least 1 pound of boost is required to get ant efficiency.

There are waste gate turbos , and dual spool units that solve this at big cost.

Ship engines today use 4 turbos which can be taken off line as required , now that "slow steaming" is in vogue for some portion of operation.

No disrespect but I do'nt see how a planing hull can be run w only one engine (with it's asymmetrical thrust and necessary rudder deflection and dragging prop) and be "quite competitive with full displacement." in fuel burn. 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 without any asymmetrical thrust or dragging propellers. Remember that FD boats don't run at hull speed. Hull speed is their TOP speed unless they are overpowered. It just dos'nt seem possible. Feel free to beat me up on this if I deserve it.

"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.

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

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.

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

"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.

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.

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*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.

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.

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.

"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?

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.

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.

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?

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*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.

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.

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.

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

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.

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*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

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

*

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.*

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*What model number is that?

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.*

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*What model number is that?

*ZF63

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

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*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).