Fuel economy

[motion30]

Just thinking. (I know often dangerous) has anybody pulled one propeller and just ran on one motor and what were the effects of fuel economy?

 

[Comodave]

Just thinking. (I know often dangerous) has anybody pulled one propeller and just ran on one motor and what were the effects of fuel economy?

PassageMaker Magazine had a story where a guy did that on a transit from Hawaii to the west coast. Don’t remember what the details were but you might be able to find it online somewhere. I believe it was maybe a GB 42.

 

[motion30]

I have been unable to find that article if anyone has a link I would greatly appreciate it

 

[RedRascal]

I've heard of a couple with a Bayliner 38 that I believe pulled a prop to get more range for making longer jumps in the Caribbean. The 38 was what they had and running 1 engine gave them the range to get where they wanted to go. Maybe one of the guys from the Bayliner forum will have more details. It was a great tale of making what you have work.

 

[Pete Meisinger]

I think it would make little or no difference.

pete

 

[O C Diver]

Just thinking. (I know often dangerous) has anybody pulled one propeller and just ran on one motor and what were the effects of fuel economy?

While I think it would improve your economy slightly, typically most twin engine boats have smaller rudders. Before I tried that, I would try maneuvering and docking with one engine to see how that worked out.

Ted

 

[Comodave]

The guy that did the Hawaii trip stopped midway and put the prop on the other engine and pulled the original prop…

 

[rslifkin]

Unless the rudder drag penalty is severe, running on one with the other prop removed should save a little fuel on most boats. Chances are, it won't be enough to justify the reduced maneuverability and loss of redundancy. Plus, depending on how the boat is propped and its top speed, you might be significantly speed restricted to avoid overloading the running engine.

 

[Comodave]

I have been unable to find that article if anyone has a link I would greatly appreciate it

I don’t have any of the old PMMs anymore and the new ones aren’t that worthwhile IMO.

 

[tiltrider1]

The Hawaii transit guy did it so that he could run the engine more loaded. He could have made it running both engines but the engines would have been just above idle and not making the appropriate heat.

One gallon of diesel makes about 20HP for one hour in most diesel engines. It takes a fixed amount of HP to move a boat through the water. Doesn’t mater if you get all the HP out of one engine or Half out of two.

Now some HP does get absorbed by the internals of the engine. By using only one engine you will gain some marginal economy, you unfortunately will give this back with rudder drag when you compensate for a crabbing boat. In almost all twin engine boats there is no economy gain by running just one engine, prop or no prop.

Now some one is going to pipe up about how they shut one engine down and got 15% inprovment in fuel economy. What they will fail to mention is that they slowed down on one engine. The act of slowing down is what gave them the economy. It goes something like this. “I normaly cruise at 1500 RPMs. I found when I ran just one engine at 1500 RPMs I saved a bunch of fuel.” This is true but what else is true is that they traveled at 7.5 knots on two engines and 5.9 knots on one engine. Had they throttled both engines down to 5.9 knots they would have saved the same amount of fuel.

 

[Delta Riverat]

If I was worried about saving 5% on fuel cost I wouldn't have bought a boat.

Prop on, prop off. Rinse and repeat. What's that worth in time and effort?

Hire a diver? Haul and remove? Put it back on?

Be my guest, I'm not doing it.

 

[RedRascal]

I think I found the couple that tried using just 1 prop on their Bayliner 38. They wrote a few books about their travels which are on Amazon. They must have worked something out because I think a 38 has about a 300 mile range and it's ways to Honduras and Guatemala.

https://www.amazon.com/Diamond-Lil-.../ref=tmm_kin_swatch_0?_encoding=UTF8&qid=&sr=

Blog here but I didn't surface anything quickly that spoke about the range of their boat. Diamond Lil's Adventures Maybe someone that is better at search can get the story if running one prop gave them materially better range.

 

[North Baltic sea]

Hi,

Litle bit this topic "articel."

https://www.passagemaker.com/lifestyle/strategies-for-long-range-cruising-with-twin-engines-2

I think best way saveing consuptions is low cruising speed 2-3 knots under you hull speed.

NBs

 

[markpierce]

Run the boat like it was designed.

Lots of previous posts.

Reduce speed to increase efficiency, At six knots, my boat's fuel consumption is half that at seven knots.

 

[mvweebles]

PassageMaker Magazine had a story where a guy did that on a transit from Hawaii to the west coast. Don’t remember what the details were but you might be able to find it online somewhere. I believe it was maybe a GB 42.

It was many years ago, but I remember that article. Was indeed a GB42. It seems unbelievable, but my memory is that he removed one prop and loosened the other. To balance engine hours, he dove on his boat and swapped props. I believe he was a single-hander.

The reduced drag of removing a fallow prop would be significant. Would definitely make enough of a difference on a trans-Pacific trip.

Even though I read the story with jaw on floor, I find my recounting above incredulous.

OP - why do you ask?

Peter

 

[rslifkin]

If you were planning to run like this regularly, folding or feathering props might be worthwhile. Much easier then removing a prop, plus it leaves the second engine able to be used if needed.

 

[Portage_Bay]

To find out if a given twin engine boat will burn less fuel by running on one you just have to try and see if it works. Some people claim it does. Tiltrider1 explains very clearly why it probably won't.

My very small data set of two boats says it's not worth the trouble. One had modern instrumentation that indicated % load and fuel consumption. The savings were not enough to matter and the % load on the running engine was far too high at anything more than minimum speed. The other boat is older with no such instrumentation but was clear within minutes the boat was all but unmanageable. Extreme rudder required to keep a good heading and crabbing through the water.

In both cases we did not pull the idle prop. It was a first effort to see if running on one was worth any theoretical fuel savings.

On the boater who swapped props mid transit to Hawaii. I recall reading the article and couldn't believe he did that. The risk / benefit calculation on that decision was in my opinion far out of balance. Apparently his goal was to run the single harder for proper engine loading at low speeds. I think there are better ways to address that concern. Either run on one engine to Hawaii and the other on another long passage or find a way to carry more fuel. It would not require a lot of extra fuel to run the boat at minimum speed and two or three times a day run her hard for 15 - 20 min.

Regarding economy. If one is looking to have a less expensive boat to run the fuel burn difference between twin and single is not a major savings. It's maintenance and repairs. For example I currently have twin CAT 3208s. It's time to replace the exhaust risers. I've got 4 to buy and install where a single inline 6 would have 1 riser.

 

[AlanT]

I recently had to run my boat on one engine for a day. One thing to bear in mind is that the offset rudder angle required to compensate for offset propulsion constantly changes with changes in engine speed and, I suppose, current. At low idle my small rudders were unable to compensate for offset and I was only able to turn in one direction and when trying to drive straight the boat would crab somewhat. With increased engine speed rudder has more bite and therefore less angle required and it was possible to turn in both directions.
~A

 

[wrm95]

U S navy would routinely on long cruises.

 

[tiltrider1]

U S navy would routinely on long cruises.

Yes but why, I doubt it was for fuel consumption. More likely it was to do maintenance.

 

[kapnd]

To run effectively on one engine, the propeller and probably the transmission and rudder specs would need to be re-engineered.
That’s an interesting exercise for an evening with the prop calculator.

 

[DennisB1]

If you are running at displacement speeds at least on my boat a GB49/49ft waterline. 8 knots is about 2 miles/gal,9 1.5mpg and 10 1 mpg. I have tried for short distances running between 7-8 knots at 1350 rpm with a single engine. The rpm is up about 150 rpm over the same speed with two. The big difference is the rudder requires between 10 to 15 degrees of angle to hold a straight course.
The prop covers the whole of the rudder so it is mostly not use fully on the engine off side. It the prop was off or there was a feathering prop I would expect the rudder angle to drop to the <5 degree range.
With both engines 2mpg @ 8 knots is about 80 hp total or 40 hp/engine on a 300+hp Cat. With one engine is a little more so I would expect the miles per gallon to increase. Between 7 knots the wake is very flat. At 9 it has a small breaking curl. At 10 it is quite large. I typically run about 9 1400 rpm. Its quiet and very smooth at that RPM.
Without investing too much money it would be interesting to pull a prop and run the boat to see what happens in the 5 to 9 knot range. If you were planning an ocean crossing feathering props could be interesting so you could run a single engine, switch to the other on a daily basis.
My experience with feather prop on a 40ft sailboat was they feather prop made a large difference while sailing and also on less rudder angle in a aperture in rudder prop mounting.
Negative of feathering is that they do need greasing on regular basis and also a costly. On the maxi web page they suggest that the 3 blade is about 96% efficient compared to a fixed 3 blade. On my sailboat the feathering seemed like it was more efficient (3 fixed vs 4 feathering).

 

[Nomad Willy]

The best set-up for running one engine for economy is to remove one prop and do it w a boat that has bigger rudders and the two props close to the keel or C/L. I’ve seen one or two boats w props close to the C/L. That would be the worst possible boat for maneuvering in close.

One could rig a rudder on the bow kinda like a vertical oar. The Vikings did mostly that to steer their boats. An oar-like thing w a tiller like thing on top at 90 degrees. Would need far less “rudder” angle to keep the boat on course.

If the props were optimized for high load changing the pitch and perhaps dia. to make them more optimized for the running single boat.

Just my thoughts …

 

[Jeff F]

I still think there's easy pickings to be had by just going slow. So with twins, how low are you willing to run them?

The only way I'd consider shutting one down is if I wasn't going slow enough with both at slow speed, maybe 900 rpm or less. I'm assuming diesel

There's a member here who has looped a few times in his PT 41 with twin Lehman's. I think he runs both at about 850 rpm, 6.5 knots.

 

[Hippocampus]

First if efficiency is the concern put on variable pitch props CPPs. Then measure exhaust temperatures and adjust the props accordingly. As you increase the pitch of the props you increase load. So the engines are placed in their optimal loading and operating temperatures. Of course loading is less of an issue with common rail but having adequate oil temp and pressure is still important.

Second twins with props quite close together can still be easy to maneuver at low speed. With twins we usually think of SD hulls so rudder size and details are designed to be efficient at speeds above hull speed and adequate at very low speeds. Now switch to a FD full keel hull and place the screws right next to the aft end of the keel. Even if you had a balanced rudder as long as it was close enough to the aft end of the keel and part (or all of it) was directly in the flow off the prop that prop would produce a vey strong steering force to the opposite side of the prop. Adding a smidgeon of reverse on the other prop you could move the stern around as well or better than a stern thruster.

 

[rslifkin]

I still think there's easy pickings to be had by just going slow. So with twins, how low are you willing to run them?

The only way I'd consider shutting one down is if I wasn't going slow enough with both at slow speed, maybe 900 rpm or less. I'm assuming diesel

There's a member here who has looped a few times in his PT 41 with twin Lehman's. I think he runs both at about 850 rpm, 6.5 knots.

That's about my logic on it. With my gassers, I consider a few minutes at idle fine (~4.3 kts), but I don't like running below 900 rpm (just over 5 kts on both engines) for any length of time while and if we're talking about 20+ minutes, I try to consider 1000 rpm as "minimum continuous" to ensure good oiling to the cams, a little more heat in the cylinders, etc. So if I had to run below 5.5 kts for a long period, I'd probably shut one engine down.



My boat has plenty of rudder authority, so turning into the live engine isn't an issue at any speed that's reasonable to run on 1 engine. The biggest issue comes in close quarters maneuvering, as reverse is questionably usable on 1 engine depending on the situation.

 

[Nomad Willy]

How hot could one run using the thermostat to raise the coolant temp? Eventually you’d need to increase cooling system pressure.

I agree just running a twin slow would be more efficient. But running two engines means you’re loosing double the heat. Lost heat = lost efficiency. Would raising the coolant temp reduce heat loss?

Running two small engines would work but you’d have an underpowered boat. Are people still buying over powered boats? If so putting underpowered boats on the market would no-doubt flop.

Looks like if you want to burn less fuel you’re gonna halfta buy smaller boats.

But lighter boats would burn less fuel.

But boats w FD hulls could be heavy and more efficient at 5 knots.

So it looks like you’re gonna halfta sacrifice speed or weight or size.
One can’t go slower than slow w a 67 Cadillac and get better gas milage.

 

[rslifkin]

Hotter coolant temp will help efficiency a little. How high you could go safely and how much you'll gain will depend on the engine in question.

 

[heysteve]

The guy that did the Hawaii trip stopped midway and put the prop on the other engine and pulled the original prop…

That is nuts! But I must admit at some level I really dig the commitment of the guy. You go with the boat you have, not the one you want.

 

[motion30]

Before I purchased my boat it done the loop. When it came time to replace the propellers I found that they were much bigger than recommended. I think they were 26-in propellers and everything on the chart told me 20 in. I did not understand why at the time. I've come to realize that the larger pitch propellers loaded the motors more at low RPM and allowed the motors to get to a normal operating temperature. Running my Motors at 1300 RPM now with the 21-inch propellers I think it is barely getting to proper operating temperature. At this point I am thinking about buying some larger pitch propellers as I never run on plane