Twin engine skippers - single engine ops possible?

[scoutabout]

In my more delerious moments I imagine someday moving up to something the family could weekend on - between 30 and 36 feet I'm thinking.

Many boats in this range have twin inboards or I/Os. My question is - can you realistically run with one engine at slow speeds? There are a lot of inland waterways in my area where you are limited to 10 knots or so transiting between locks and such.

At this speed, could you realistically leave one engine off and in neutral and use the other? I liken it to the single engine taxiing you see pilots do to save fuel on larger prop commuter aircraft.

I realize this example is apples and oranges with respect to drag and asymmetrical thrust conditions but I do wonder.

Have any of you with twins tried this and did it make an appreciable difference in fuel consumption? Are there other issues to consider like damage to a freewheeling gearcase, etc - assuming the other prop would windmill and not act like a huge sea anchor...

 

[scoutabout]

Re: Twin engine skippers - single engine ops possible?

Good thoughts, Bubba - I hadn't considered the pitch issue. I know this idea sounds rather pathetic and anyone looking to save operating costs in this way probably shouldn't be looking at boats this size but it did occur to my in my particular application there would be lots of slow speed cruising and alternating engine use might be an idea at these speeds. Maybe a single diesel is the way to go.

I like the idea of twins for redundancy but perhaps a large single is a better bet. Anyway - still interested in all thoughts and experiences.

 

[trendsetter240]

Re: Twin engine skippers - single engine ops possible?

There was a guy at my marina last summer who had similar thoughts. He had a 32' cabin cruiser with a pair of big blocks. He wanted to save fuel as he spent about 50% of his time at no wake speeds or trolling for salmon. He ended up mounting a new 25hp Yamaha 4 stroke outboard on the swim platform in the center of the stern.

This outboard was plumbed into his hydraulic steering and he could control everything from his cockpit. He said it would pay for itself in 2 seasons in fuel consuption savings, lower hours on his main motors and cheaper maintenance. Also it was much quiter putting around on that 25hp, almost silent compared to the rumble of 2x454 IOs.

He would leave his drive legs down while cruising on the outboad, not sure if that ever gave him any problems as I haven't seen him yet this season.

Cheers.

 

[JoLin]

Re: Twin engine skippers - single engine ops possible?

I don't see it as a fuel saving measure. At idle, my engines each use 1 gallon per hour, as verified by a scan tool. Anything higher than idle speed, I need both motors to properly move the boat.

Now, I have run on a single engine (the other in neutral) to cut my speed when I'm moving through extremely close quarters or I think I'm headed into skinny water. That move knocks about 1.5 mph off my idle speed. Could be handy for trolling too, I suppose.

My .02

 

[Fireman431]

Re: Twin engine skippers - single engine ops possible?

There are bigger issues with this than fuel savings. Here is the lowdown on running on one engine:

Hulls are designed for twin power plants for a reason. The drag on the hull has to be overcome for the vessel to move according to design. When operating on both engines (with the correct prop/pitch), the engines are operating at peak efficiency. When shutting one engine down, the engine has to pick up the additional load, and now is operating at less than 50% efficiency. What this boils down to is burning greater than 50% more fuel in one engine than you did in using two. This also equates to slower cruising speeds (which means more fuel burned because you're running longer), less maneuverability, and the boat trying to "walk" at an angle because it's being pushed from one side.

In addition, when you're talking about inboards, in the even of an engine failure, the prop shaft has to be secured with a drag strap and 2x4 in the bilge to keep the prop from turning. Even with the engine off, the water moving past the stationary prop will cause it to turn. This means the gears in the transmission are turning as well, but not the pump thats pumping the lubricating gear lube. So, this has the potential to destroy tranny parts. I'm not sure if an I/O does the same thing, but I know it's like that on a straight shaft.

All-in-all, it's actually cheaper to run both engines, tuned and propped correctly, than it is to try to skimp by with one. Easier handling, better maneuverability, better use of fuel, and the ability to control your vessel should problems arise.

 

[Thalasso]

Re: Twin engine skippers - single engine ops possible?

With an I/O you will need both engines running. It will be difficult to manuver with only one. It will be hard to steer. You cannot raise the I/O high enough to get it out of the water.In no wake areas the boat will be slow enough at idle speed. I have twin I/O's on a 30ftr. Mine use 18gph each at 3000rpms.I need this speed to maintain plane. If i drop it to 1700-1900 i can get 8gph but it takes twice as long to get where you want to go.

 

[scoutabout]

Re: Twin engine skippers - single engine ops possible?

Thanks for all your input folks. This is exactly the kind of perspective I was looking for. I think if the time ever comes for a vessel of this size I should expect to run both engines all the time. Unless of course I went with something like a Grand Banks 36 with a single diesel. I'm quite partial to those boats although I gather one can't be in much of a hurry to get to your destination.

Thanks again.

Cheers,

Scout

 

[sschefer]

Re: Twin engine skippers - single engine ops possible?

Every boat is different but for the size boat you're talking about they're pretty much all the same. Your shift and throttle are on the same stick but separate for each engine. You just disengage it while in neutral and drag it. Yes the prop/wheel/screw will spin but at the speed your planning on going it won't hurt a thing. I drug one all last summer on a the companies 60 footer and it was no worse for wear when the engine was replaced this winter.

What does bother me is that you'll loose your manueverability unless you have a bow thruster. I had one and used it so much I went through three sets of batteries. I wasn't trying to save on fuel but if I was I would have lost any savings in running the Genset trying to keep the batteries up not to mention the cost of replacing them.

On that note, unless you have a lot of hours at the helm and know well how to handle a larger boat don't do it. It's not a bad idea to practice it though because there will be a time when you need to run on a single and it may not be the best time to learn.

 

[Fireman431]

Re: Twin engine skippers - single engine ops possible?

Yes the prop/wheel/screw will spin but at the speed your planning on going it won't hurt a thing. I drug one all last summer on a the companies 60 footer and it was no worse for wear when the engine was replaced this winter.

I have to respectfully disagree. I have read many places in manuals that say not to allow this to happen for periods greater than 30 minutes or you run the risk of permanent damage. However, after some additional research, I did find a site that indicated that it was OK to do this with a "Fully Mechanical" transmission. Most transmissions, like I mentioned earlier, have to have the engine running for lubrication. The boat you skippered may have been the exception to the rule.

I also have to agree with the other post that says to occasionally practice with docking with a single engine in case of failure. I had to do it once and with the wind and current, it's amazing how a larger boat is so slow to react with with just one prop.

 

[Mikeyboy]

Re: Twin engine skippers - single engine ops possible?

Sschefer when you were dragging the prop was the engine running and in neutral or shut down completely? Just curious.

 

[QC]

Re: Twin engine skippers - single engine ops possible?

operating on both engines (with the correct prop/pitch), the engines are operating at peak efficiency. When shutting one engine down, the engine has to pick up the additional load, and now is operating at less than 50% efficiency. What this boils down to is burning greater than 50% more fuel in one engine than you did in using two.

And I respectfully disagree with this . . .

This is a very complicated discussion, but speaking of engines only, gasoline engines, this is backwards. If "efficiency" is the use of fuel to produce output, they are most "efficient" with more load at a given RPM, than less. So the 50% thing is kind of off.

The biggest issue I have always heard of is the trans operating with a prop spinning in neutral. I do not know which trans models are affected most or to what degree.

 

[scoutabout]

Re: Twin engine skippers - single engine ops possible?

Thanks again guys for the continued debate. To be clear, I would never operate a twin screw vessel in close quarters without both engines running. I was more imagining slow speed transit between locks (depending on congestion of course), trolling, leisurely sunset idle about the lake, etc. In other words, speeds at which the asymmetrical thrust would hopefully not be a huge issue.

I had wondered about the advisability of a freewheeling gearcase for extended periods or even the prolonged effect of drag on propshaft and seals if they were not designed for it.

 

[jayhanig]

Re: Twin engine skippers - single engine ops possible?

At this speed, could you realistically leave one engine off and in neutral and use the other? I liken it to the single engine taxiing you see pilots do to save fuel on larger prop commuter aircraft.

I can't answer to the marine aspect of your question but I have landed a twin engined aircraft with one fan out and found it difficult to taxi once the speed bled off. That would not be my choice for operation under any circumstance I can conceive of.

You might consider the turning moment the single engine is likely to contribute... a turning moment you will need to counter with opposite rudder. All of that increases drag. Plus it would be really sporty around a dock.

 

[JoLin]

Re: Twin engine skippers - single engine ops possible?

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[25thmustang]

Re: Twin engine skippers - single engine ops possible?

Every boat is different but for the size boat you're talking about they're pretty much all the same. Your shift and throttle are on the same stick but separate for each engine. You just disengage it while in neutral and drag it. Yes the prop/wheel/screw will spin but at the speed your planning on going it won't hurt a thing. I drug one all last summer on a the companies 60 footer and it was no worse for wear when the engine was replaced this winter.

In honor of the thread, I have to disagree. The shift and throttle on the same lever but separate for each motor? Not on the two boats in my signature. I have 4 levers, throttles are separate from shifters on each engine.

 

[Thalasso]

Re: Twin engine skippers - single engine ops possible?

In honor of the thread, I have to disagree. The shift and throttle on the same lever but separate for each motor? Not on the two boats in my signature. I have 4 levers, throttles are separate from shifters on each engine.

And you probably have v drives? Never seen 4 controls for I/O's.Never

 

[QC]

Re: Twin engine skippers - single engine ops possible?

Then you've never seen a go fast with Twins. They all have four controls and I/Os where I boat

Yes more common above 28 ft. with Inboards, but not exclusive.

 

[CaptainKickback]

Re: Twin engine skippers - single engine ops possible?

My 1981 Sea Ray (rebuild project) had separate shifter and throttle controls with twin i/os. The Cruisers 2870 I had was twin I'll
/o with combined throttles/shifters. It seems a question of personal preference, or just what came on the boat. Neither configuration has any effect on the OP's question.

For that, I have to go with the fact that one engine, not driving the boat from the center, creates side torque that has to be overcome by steering back, which costs fuel. My wallet doesn't care about gph. It cares about mpg. That's how I measure fuel efficiency.

 

[rbh]

Re: Twin engine skippers - single engine ops possible?

My .02c is yup on just one engine, keep the second drive down for the extra steering you need.
what ever engine you run will want to kick your boats bow in the opposite direction, so you will be doing lots of course corrections.
If we fish/troll, we just use the one engine, but in the middle of the lake it does'nt matter if you look like a spawning salmon going up a river.

OH, and about the prop spinning thingy, how many boats have you followed down the HWY with the prop spinning in neutral??
Not seeing an issue.

 

[DuckHunterJon]

Re: Twin engine skippers - single engine ops possible?

OH, and about the prop spinning thingy, how many boats have you followed down the HWY with the prop spinning in neutral??
Not seeing an issue.

That's a whole 'nother debate - BTDT!