Cruising On One Engine

[Comodave]

I lost an engine once 6 hours from home. Since the engine was in gear at shut down I just left it that way. Dragging a stationary prop might not be the most efficient but it ended any concern about a freewheeling transmission.

I think that I read somewhere that a stationary prop is more efficient than a freewheeling one. They compared it to a helo auto rotating vs with the rotor stopped. Auto rotating has some lift stopped rotor has none. Maybe...

 

[tiltrider1]

I can tell you all about auto-rotation but I can’t give you any feed back on landing with a fix position rotor. I suspect I never will be able either.

 

[Ken E.]

Most Twin Disc gears can be freewheeled without damage. The TD website and the owners' manuals specify the models. I've run a one side quite a bit between here and Alaska, tracking fuel consumed between fill-ups and comparing it to twin engine running. To cover distance, i use about 15% less fuel at 7.5 kts. Despite that, I usually run on both....easier and more traction especially if its rough.

 

[Comodave]

I can tell you all about auto-rotation but I can’t give you any feed back on landing with a fix position rotor. I suspect I never will be able either.

You hope not anyway...

 

[catalinajack]

I think that I read somewhere that a stationary prop is more efficient than a freewheeling one. They compared it to a helo auto rotating vs with the rotor stopped. Auto rotating has some lift stopped rotor has none. Maybe...

I am far from an expert but here is what I know from personal experience. I recenty had my cutless bearings replaced. They were so swollen that, when running on one engine, the other shaft would not turn. With new bearings, the shaft in neutral spins freely and happily at the same speed as the driven shaft.

Now, consider this. Before the replacement, running on one engine required 20 degrees of rudder correction to keep a straight course. After, only 5 degrees of correction. Take from that what you wish but that tells me that a stationary prop and shaft causes a LOT more drag. Also, on one engine at the same RPM, speed thru the water was higher.

 

[psneeld]

When in the USCG, the engineers once told me on some of the ships a locked prop was less drag than a freewheeling one....explaining the science behind it.


My subsequent research shows "it depends".... on many variables...


So like many topics on TF, one must further research the question and see if it applies in their case rather than rely on "dock talk" or the "experiences of others"...


In fact the whole running on one engine topic is silly unless you set fairly tight parameters for what you have and plan to do...otherwise most suggestions are not applicable....

 

[Rufus]

Be also aware that many boats have a cross-connect hose between the two shaft seals. So running one engine flows water to the other. On one Viking with water lift mufflers, this filled the engine with water and forced an overhaul.

Right. There needs to be a check valve (or manual shutoff) to preclude back flow to the dead engine.

 

[Rufus]

The whole drag question of spinning versus fixed was hashed out on the forum years ago. I believe the consensus was that the flat plate area of a fixed pitch prop added up to more drag (power required from the running engine), than the power required/consumed to spin the prop (geartrain drag, shaft bearings, and drag due to lift on the prop). I can spin the prop shaft with the boat on the hard quite easily, and that's without lube on the cutless bearings. But it does vary. If the barn door prop could be feathered like on some airplanes, then the reverse might be true.

By the way, I believe the power lever on a Detroit Diesel, and some old Cummins sets the rack position, and the governor adjust the rack around the set point. (It's a power lever on a diesel, not a throttle).

 

[diver dave]

...

By the way, I believe the power lever on a Detroit Diesel, and some old Cummins sets the rack position, and the governor adjust the rack around the set point. (It's a power lever on a diesel, not a throttle).

Power lever if you are in gear. If not in gear, a pure rpm lever. If overpropped a smoke lever.

In a gasser, closer to a power lever.

 

[bayview]

I ran a single vs twin test on my 44' some years ago and the fuel was similar to what I would use with two engines at that same reduced speed. Thee was no appreciable fuel savings but it did feel better having one engine working rather than two in very low load condition for hours on end.

I alternated engines every hour and just let the prop spin. I had stuffing boxes and twin independent engine set ups so no cross cooling etc issues. I checked trans temps from time to time but no heating occurred with ZF IRM 301s

The single speed I chose was what I could run and still maintain reasonable steering control. This was done offshore and as a test for a planned longish passage.


PS many years ago practical sailor ran some tests and found a freewheeling prop had more drag than a locked prop because of more area interfering with water flow. YMMV

 

[psneeld]

The whole drag question of spinning versus fixed was hashed out on the forum years ago. I believe the consensus was that the flat plate area of a fixed pitch prop added up to more drag (power required from the running engine), than the power required/consumed to spin the prop (geartrain drag, shaft bearings, and drag due to lift on the prop). I can spin the prop shaft with the boat on the hard quite easily, and that's without lube on the cutless bearings. But it does vary. If the barn door prop could be feathered like on some airplanes, then the reverse might be true.

By the way, I believe the power lever on a Detroit Diesel, and some old Cummins sets the rack position, and the governor adjust the rack around the set point. (It's a power lever on a diesel, not a throttle).

The consensus of the forum is hardly proof of anything....At best it suggests something or a highly experience person reveals a pathway or links to much more of the story.

 

[syjos]

There was an article in a boat magazine in the 90's about a guy who cruised a 37 Diesel Tollycraft to Hawaii solo from the the mainland, I forgot the departure city.

He had extra fuel in bladders and cruised at 5 to 6 knots.

He ran on one engine half way across the Pacific with the unused propeller removed. At the half way point, he dove under the boat to remove the prop he had been using and put a propeller on the shaft of the second engine. He had some kind of rigging to retrieve and lower the propeller. He cruised the rest of the way on the second engine and arrived in Hawaii with fuel to spare.

He received heavy criticism for his diving alone under water and taking a small boat across the ocean alone. I recall that he was moving to Hawaii so it was a one time trip.

 

[psneeld]

One story from one guy ....hardly anything believable enough to bet your life or boat on..... or scientific about it.


It's like advice here from a guy who is a recreational boater, maybe 200 hrs a year, on his 4th boat of a lifetime...his first twin and first diesel.


Sure his/her advice might be spot on...but based on trusting what amount of experience?


Some people here can have their reputations checked with a simple google and a few phone calls...others, not so much or not at all...


So yes, there is abundant info available here...but you have to know what is even remotely worth checking on or do quite a bit of research on your own.


PS...that story was posted here at least once on TF and the boat was a Grand Banks in the last rendition I read...so there ya go.....

 

[Rufus]

The consensus of the forum is hardly proof of anything....At best it suggests something or a highly experience person reveals a pathway or links to much more of the story.

I believe it was RickB, a well regarded engineering type, who made the case for freewheeling being less draggy, although it depends a great deal on speed/turbulence with the prop locked down.

Regarding actual numbers for running on one, TimB (??) and I both ran fairly well documented tests on our boats ten years ago(free wheeling props). We both came up with about 10-15% fuel savings. In my case, I compared prop chart data and actual fuel burn using a sight gage on the tank. Long haul across Lake Michigan, no wind. I set power for 8.5 knots using both engines on one crossing, and pushed the power lever up to achieve the same speed using one engine on the next crossing. Prop chart fuel burn and that recorded on the sight gage compared favorably. We both came up with 10-15% fuel saving running on one. I guess I could lock down one shaft and do another comparison to settle the debate....a least for our boat...big rudders. Regardless, data tell the story for each boat. Easy enough to test if it's important to an individual. I believe Tim runs his boat on one fairly regularly, I don't. Better for the engines on semi or planning boats run at low speed, though.

 

[Ski in NC]

I think this issue could be settled quite easily by someone here with modern computer engines, like Cummins QSx, Volvo Dx, Cat Cx, etc. Those engines show real time fuel burn that in experience have shown quite accurate.

Go desired speed on two engines: Read gph on both and add. Pull one engine into neutral and add power to other to get the same speed. Read gph on the one still powering. If you want to really complete the task, repeat with windmilling prop shaft choked.

The reports of 10-15% reduction of burn rate per mile do seem to make sense, depending on the boat and seas, etc. To me it would not be worth all the fuss. But I have a single anyway!!

 

[psneeld]

The info I have gleaned is many boats running on one vs two must slow down to see the benefit. If one runs on two at 8 knots and tries to do it on one...depending on many factors...the required increase of rpm on one to continue at 8 knots may/probably will require a substantial increase in fuel burn....usually near what 2 burn.

So in many cases if you are willing to take the speed penalty, running on one can be a significant fuel savings....but trying to maintain results in a similar fuel burn...again depending on a lot of variables.

 

[Tomas]

Ski, I don't think the original poster was maintaining speed. He maintained the same RPM and accepted the reduction in speed as his new cruise speed. Under that condition I would think he would have a significant reduction in fuel burn.

 

[Ski in NC]

I gathered that as well. My above test is still valid, just do it at that lower speed. RPM will just be lower when running on twins.

 

[Rufus]

The info I have gleaned is many boats running on one vs two must slow down to see the benefit. If one runs on two at 8 knots and tries to do it on one...depending on many factors...the required increase of rpm on one to continue at 8 knots may/probably will require a substantial increase in fuel burn....usually near what 2 burn.

So in many cases if you are willing to take the speed penalty, running on one can be a significant fuel savings....but trying to maintain results in a similar fuel burn...again depending on a lot of variables.

Depends primarily on the fuel specifics of the engine. For a boat with big engines (a lot of excess power) slobbering along at slow speeds, the engine fuel specifics are likely a much bigger factor than drag (up to a point). A pair of 120 Lehmans are a different story as the engines are likely running at fairly decent place on the fuel burn curve to begin with. By the way, the primary reason you shut down one engine on a twin helicopter to get best range is to move the engine (with a lot of excess power) to a better place on the fuel specifics curve. The remaining engine is run at near maximum continuous power, and the airspeed for that power typically gives best range. Same thing for patrol aircraft. A little different for a boat because of the sideslip issue, but with big efficient rudders, the problem is minimized.

 

[Rufus]

I gathered that as well. My above test is still valid, just do it at that lower speed. RPM will just be lower when running on twins.

Your above test methodology is exactly what I described in the post before yours. And no I did not leave the power lever untouched for the test. I pushed it up to maintain speed. For a large engine running at very low power, the main objective is to move the remaining engine to better place on the fuel burn curve. Best range is about miles per gallon, not gallons per hour.

 

[sbman]

The whole drag question of spinning versus fixed was hashed out on the forum years ago. I believe the consensus was that the flat plate area of a fixed pitch prop added up to more drag (power required from the running engine), than the power required/consumed to spin the prop (geartrain drag, shaft bearings, and drag due to lift on the prop). I can spin the prop shaft with the boat on the hard quite easily, and that's without lube on the cutless bearings. But it does vary. If the barn door prop could be feathered like on some airplanes, then the reverse might be true.

By the way, I believe the power lever on a Detroit Diesel, and some old Cummins sets the rack position, and the governor adjust the rack around the set point. (It's a power lever on a diesel, not a throttle).

On Detroits, the throttle lever sets a desired RPM, the user has no direct control over the rack. The governor controls the rack and adjust the fuel to hit the commanded RPM. The governor uses a spinning fly-weight setup that is RPM based.

You can be at 1/2 throttle and the rack wide open using max fuel for that RPM or at minimum fuel to spin the motor over at that RPM if it's not in gear. Without a fuel flow gauge or EGT gauge you have no idea about fuel burn.

If you have twin detroits and shut one down while cruising, the other will add fuel as much as necessary to keep the same RPM. The boat may slow down a bit but the remaining engine will give no indication that it's using more fuel without some kind of instrumentation.

 

[diver dave]

On Detroits, the throttle lever sets a desired RPM, the user has no direct control over the rack. The governor controls the rack and adjust the fuel to hit the commanded RPM. The governor uses a spinning fly-weight setup that is RPM based.

You can be at 1/2 throttle and the rack wide open using max fuel for that RPM or at minimum fuel to spin the motor over at that RPM if it's not in gear. Without a fuel flow gauge or EGT gauge you have no idea about fuel burn.

If you have twin detroits and shut one down while cruising, the other will add fuel as much as necessary to keep the same RPM. The boat may slow down a bit but the remaining engine will give no indication that it's using more fuel without some kind of instrumentation.

Yes, someone gets it.
Also, great when running boats in heavy swells, or running greyhound buses up and down hills.

 

[sbman]

Yes, someone gets it.
Also, great when running boats in heavy swells, or running greyhound buses up and down hills.

It helps to hold the parts in your hands. Once you see the internals of the governor and rack control, it's plenty obvious how it works.

 

[Ski in NC]

My Cummins 450C has a governor much like the Detroit. I can be running in gear at 1400rpm, probably making about 100hp. Pull it back to neutral without touching the throttle, it just goes to about 1430-1450.

There are governors that try to simulate the way a gasoline engine runs. Called a "speed limiting" or "road load" governor. Those are common in road vehicles as the rpm holding governor can behave strangely on the road. Touch the accel pedal a little and it will jump to full fuel. The road load gov, push it quarter of the way down and rack goes quarter travel. Only if at idle or near rated rpm will gov actively control.

Road load govs are rare on boats. The 8.2 Detroit four stroke is one. Not aware of any others. One Italian boat with 12-71's had them, I found it weird. Cap't said the Euros kinda like them (???).

 

[Rufus]

My Cummins 450C has a governor much like the Detroit. I can be running in gear at 1400rpm, probably making about 100hp. Pull it back to neutral without touching the throttle, it just goes to about 1430-1450.

There are governors that try to simulate the way a gasoline engine runs. Called a "speed limiting" or "road load" governor. Those are common in road vehicles as the rpm holding governor can behave strangely on the road. Touch the accel pedal a little and it will jump to full fuel. The road load gov, push it quarter of the way down and rack goes quarter travel. Only if at idle or near rated rpm will gov actively control.

Road load govs are rare on boats. The 8.2 Detroit four stroke is one. Not aware of any others. One Italian boat with 12-71's had them, I found it weird. Cap't said the Euros kinda like them (???).

I had a boat with 8.2's (great engines in a marine application, by the way). Nothing weird at all about the governor operation. The load on the propeller is in effect the mid range governor. Rock solid unless it's really rough, and then the rpm hunts a little with changing load. Take one engine off line and the rpm on the running engine will shift a bit as the hull slows, but all very predictable. They're actually more intuitive than an RPM holding governor. The operator is in control, not the governor.

By the way, the one engine inop test I performed was done with Detroit 8.2s.

 

[Ski in NC]

If the 8.2 has a road gov on it, you can't rev it to like 1200 in neutral. Get above 1200 or so and rpms take off. That is the hallmark of that gov. Maybe yours had a different gov.

 

[syjos]

One story from one guy ....hardly anything believable enough to bet your life or boat on..... or scientific about it.


It's like advice here from a guy who is a recreational boater, maybe 200 hrs a year, on his 4th boat of a lifetime...his first twin and first diesel.


Sure his/her advice might be spot on...but based on trusting what amount of experience?


Some people here can have their reputations checked with a simple google and a few phone calls...others, not so much or not at all...


So yes, there is abundant info available here...but you have to know what is even remotely worth checking on or do quite a bit of research on your own.


PS...that story was posted here at least once on TF and the boat was a Grand Banks in the last rendition I read...so there ya go.....

The magazine article I read about the Pacific crossing was a Tolly 37.

I shared the story since it was interesting and related to the thread. And an example of a foolhardy stunt. I would not have related the story if I thought someone would get their panties in a wad about it.

 

[Rufus]

Ski, Yes, that was a characteristic of the engine. Can be surprising if you're not aware of it, but never an issue. It will hit the high speed limiter if you don't snatch it back quickly enough. You only do it once! I never had the need to idle at high rpm in neutral. And it was a built-in reminder to make sure the power lever was back at idle before shifting the gearbox from forward through neutral to reverse...when maneuvering into a slip for example. Safety alert to keep from crunching the transmission. Nice little engines with a great V8 rumble. Very fuel efficient and leak free. Never had to touch the rack....just kept on keepin' on. I'm thinking the lack of constant independent adjustments by the governors kept wear to a minimum and adjustments long lasting. The engines I had used the larger head bolts.

 

[markpierce]

Discussion a bit academic if the generator/alternative works off just one of the engines.

 

[Steve91T]

My Lehman’s aren’t setting an rpm, so shutting one down and running the other at the the same RPM and allowing the boat to run 1kt slower will save fuel. My last trip I believe I averaged about 4.0 gph at 8.5 kts with the gen running all day and 1 night (plugged in the other two nights). This was a 120 mile trip. We actually averaged 7.3 kts including no wake zones and the locks. That’s 2.1 mpg. If I shut one down, maybe I’d be burning 2.5 gph at 7.5 kts. That’s 3 mpg. Even if I’m burning 3 gph that’s still 2.5 mpg.