Operating a twin on one engine and fuel economy

[timjet]

I just completed my first cruise on my newly purchased boat without a maintenance issue. Everything worked and I didn't have to crawl down into the engine compartment once. With the exception of fowling the prop with the dingy painter, those big props suck a lot of water, and a minor grounding everything went well. I envy you guys in the PNW with your majestic mountain views, but while you're putting your boats away for the winter, we're gearing up for good very pleasant weather cruising down here in Florida.

With no maintenance issues to contend with I experimented with running my engines at different rpm's and comparing the speeds with the engine manufactures fuel consumption curves. I came up with some interesting data.

At displacement speed on my twin engine planning hull boat which is approx 8.3 kts and 1300 rpm, my fuel consumption according to the fuel curves is 4.9 gal/hr 2 engine, which will give me about 1.69 Nautical mile/gal (Nmpg). At $3/gal that's $1.78/Nautical mile.

At 2000 rpm I'm getting approx 17 kts and burning about 13.4 g/h 2 engines yielding 1.26 Nmpg or $2.38/Nm. So by increasing the fuel burn by 34% I increase the speed by 204%.

But I found this even more interesting. Cruising at displacement hull speed as above at 8.3 kts getting 1.69 Nmpg, I shut one engine down left the remaining engine running at 1300 rpm and my speed dropped by 1.2 kts to 7.1 kts. Consulting the fuel curves gives a fuel burn of 2.45 g/h or 2.9 Nmpg or $1.03/Nm.
So operating single engine reduced my speed by 14.4% and increased my mileage by 58%.

What I didn't do and will do next time is to shut one engine down and increase the remaining engine rpm to remain at 8.3 knots and compare the fuel burn.

So I guess what's important is how you interpret this data. Seemingly it would make little sense to cruise at displacement speeds with two engines when one engine will only decrease speed 14.4% yet increase mileage by 58%. Note: my transmission manual allows this.

I also found that cruising at low planning hull speeds did not increase mileage significantly so in my case it would make more sense if planning is desired to plane at 2000 rpm rather than seeing a very small increase in mileage by cruising say at 1600 rpm. I did not cruise above 2000 rpm, not wanting to stress the engines. The fuel curves confirm however that at speeds above 2000 rpm, fuel flow rises almost exponentially.

Pehaps other can shed some light on this single engine cruising on a twin.

-- Edited by timjet on Thursday 21st of October 2010 12:50:50 PM

-- Edited by timjet on Thursday 21st of October 2010 02:35:06 PM

-- Edited by timjet on Thursday 21st of October 2010 02:41:22 PM

 

[Bendit]

*The fuel burn curves don't take account of the drag from the trailing prop (spinning has more drag than fixed) or the drag induced by the rudder correction to keep on course.
I experimented with my last boat, which had twins, and found that the economy running with just one engine was actually worse than with two

 

[Marin]

Make sure your transmissions can be freewheeled without damage if you haven't already. Some transmissions can be freewheeled, some can't and some can only be freewheeled at slow speeds (like the old BW Velvet Drive).

Also make sure freewheeling a prop will not overheat the shaft log and packing gland. We cannot freewheel a prop on our boat at any speed because our shaft log depends on a water feed from its associated engine to help cool the shaft log. If the engine has to be shut down the water feed to the shaft log stops and if the prop is allowed to freewheel the spinning shaft will overheat the shaft log and packing gland to the point of severe damage in very short order. This is why we need to tie off the shaft if we have to shut down an engine.

-- Edited by Marin on Thursday 21st of October 2010 01:18:18 PM

 

[Marin]

Bendit wrote:

*(spinning has more drag than fixed)

That's actually wrong.* MIT did a study on this that was posted several years ago on one of the GB owners forum.* A freewheeling prop generates less turbulence and thus less drag than a locked prop.

*

 

[timjet]

Jeff,
I'm no expert, but the fuel burn curves from the manufacturer simply state fuel burn at specific rpm. The engine doesn't care if the other prop is free wheeling or an anchor is being dragged. If the engine is turning at 1300 rpm then it is burning in my case 2.45 gal/hr per engine. Though the anchor analogy may not be a good one because a lot of turbulence would be created and that would effect the burn numbers, but not the drag a rudder would create or a spinning prop. The drag of the rudder and free wheeling prop is taken into consideration by the decrease in speed. But that is far off set by the fuel burn saved by the stopped engine.

You make a good point because your real world data indicates no advantage to running on one engine. Perhaps someone could explain it or otherwise refute the data I collected.

Marin,
My tranny's can free wheel without issues according to the manual. I'm going to verify this with the manufacturer. My shaft logs are not water cooled, so it would seem there would be no difference whether the shaft is turned by the engine or by the prop. Correct??

PS: Just changed my avitar

-- Edited by timjet on Thursday 21st of October 2010 02:04:38 PM

 

[RickB]

What's all this kilometer stuff? I guess it makes life easier for RTF but all that mixing of knots, miles, meters, and gallons is just plain weird.

 

[Marin]

timjet wrote:

Marin,
My tranny's can free wheel without issues according to the manual. I'm going to verify this with the manufacturer. My shaft logs are not water cooled, so it would seem there would be no difference whether the shaft is turned by the engine or by the prop. Correct??

I'm curious--- if your shaft logs/cutless bearings are not cooled and lubed by water, what are they cooled and lubed with?

However, if your shaft logs and the cutless bearings in them do not require a forced-flow water feed from the engine's raw water cooling system then they are getting whatever cooling and lubing they need from the water around the boat, so I would think that you are correct in that letting the shaft freewheel with its engine shut down would not be a problem.*

You can check this easy enough by monitoring how hot the shaft log gets or doesn't get while letting the shaft freewheel at speed.* On our boat, shutting down an engine and letting the shaft freewheel results in the shaft log (and shaft) getting too hot to touch pretty quickly at any kind of boat speed.* Hence the need to lock off the shaft.

*

 

[timjet]

Rick, You're right it's confusing. I corrected it.

 

[timjet]

Marin,
Correct, they are water cooled, not forced water cooled like yours. Thanks.

 

[Bendit]

I'm not an expert either but, I think the drag of the free wheel and rudder will be reflected in the*fuel pump delivery*required to maintain the revs. The fuel burn curve assumes a particular load, this will increase when running on one engine.

An engine turning* 2000rpm at the dock uses hardly any fuel, but when pushing a hull through the water at 2000rpm*the pump has to supply many times more fuel to maintain the revs you have selected*with the "throttle".*

Re spinning rather than fixed,*think of*a helicopter in auto rotate mode. If the blades are locked, it will drop like a stone, (not much drag) but if allowed to rotate, they provide sufficient lift (and drag) to reduce the rate of descent.


-- Edited by Bendit on Thursday 21st of October 2010 04:19:04 PM

 

[Marin]

Bendit wrote:
Re spinning rather than fixed,*think of*a helicopter in auto rotate mode. If the blades are locked, it will drop like a stone, (not much drag) but if allowed to rotate, they provide sufficient lift (and drag) to reduce the rate of descent.

That's an apples and oranges comparison.* The reason the helicopter "drops like a stone" with the rotor stationary is not because of drag or lack of it but because the stationary rotor blades are producing no lift whatsoever.* When the blades are turning, they can be used to develop lift at the end of the drop when pitch is applied to stop the drop.

A stationary boat propeller generates a huge amount of turbulence behind it, and turbulence is drag.* A freewheeling boat propeller does not generate as much turbulence behind it, thus less drag.* The MIT study made this very clear.



*

 

[JMYSS]

It seems to me that pushing a lighter aluminum hull with a single engine* and an absurd amount of fuel on board at 3gph and 8 kts is much easier then making all these calculations.

 

[Keith]

*...and I never have to worry about engine synchronization!

 

[koliver]

The amount of work done to push your boat through the water at 8.3 knots doesn't change, whether you are running one engine or two. What does change, is the efficiency of the hull being pushed from one corner instead of equally from two corners. Having a rudder cocked over to maintain a strait course will also cause more drag. Thus worse performance on one than on two.

 

[FF]

A stationary boat propeller generates a huge amount of turbulence behind it, and turbulence is drag. A freewheeling boat propeller does not generate as much turbulence behind it, thus less drag. The MIT study made this very clear.


What every study I have seen is that a really truel free spinning prop, (no tranny drag , no strut drag , no bearing drag) might have less drag than a locked prop.

BUT like the helocopter (at the end if its fall the drag from auto-rotation is huge) the slower than ideal speeds of the dragging prop is worse than the locked prop.

 

[Woodsong]

Having owned twin screw boats up til now, I really am liking the simplicity of a single screw set up and it's efficiency and yes....like already said, I don't have to ever worry about syncing the engines!!
That being said...I am pretty sure the first time I almost wreck while docking due to winds or currents or whatever that I will, at that precise moment, be strongly wishing I had twin screws!

 

[timjet]

There is no doubt the drag of the rudder corrected for the asymmetrical thrust and the wind milling propeller will slow the boat down and create inefficiencies.* This amount of drag will depend on hull design, the distance each prop is from the centerline of the boat and other characteristics unique to each boat. However any engine must produce a certain amount of power that is used to run the pumps, alternator (commonly referred to the accessories) and in the case of a diesel, the heavy fly wheel and crankshaft, power that is not delivered to the propeller, but uses fuel nonetheless. Another point brought up by Jeff is the fact that fuel flow will increase on an engine under load compared to an engine unloaded at the same rpm. The question of course is how much more of a load is produced by the asymmetrical thrust and the effort needed to counteract it.
My contention at least on my boat is the fuel consumed by an engine to run the accessories exceeds the greater fuel flow needed by the remaining engine to counteract asymmetrical thrust drag.

Another issue is the power to weight ratio of the boat. My boat unlike some trawlers has a higher power to weight ratio, allowing it to plane. When I shut an engine down and maintain displacement speed, the remaining engine is producing probably less than 50% of its available power. Perhaps as the power to weight ratio lowers, the argument shifts in favor of running on 2 engines.

The argument here is not 2 vs 1 engine as this is a personal preference issue that has no definitive answer that can be based on fact. There is a factual answer to the question of fuel performance at displacement hull speeds on a planning hull with one or two engines. Perhaps this question can only be answered by installing fuel flow indicators and further testing.* But my guess is this testing has already been done, I just need to find where the answer is.


-- Edited by timjet on Friday 22nd of October 2010 07:44:57 AM

 

[Baker]

Marin, what were the conditions of the MIT experiment? Because I can tell you right now there are not any emergency procedures on twin engine airplanes that tell you to leave the dead engine propeller to windmill.

And Tim, when you shut one down, did you have to advance the power lever on the good engine to maintain 1300 RPM? If not, I am willing to bet that the fuel flow remained the same as it was before....

 

[Codger2]

Baker wrote:" I can tell you right now there are not any emergency procedures on twin engine airplanes that tell you to leave the dead engine propeller to windmill."

Amen, brother.....The difference being able to feather the prop on the dead engine of course.

*


-- Edited by SeaHorse II on Friday 22nd of October 2010 09:57:23 AM

 

[Marin]

Baker wrote:

Marin, what were the conditions of the MIT experiment?

First of all, trying make a comparison between a thin aircraft propeller in air and* wide marine propellers in water is an apples and oranges situation even if the theories are similar.* Plus the prop in a multi-engine plane is going to be feathered so it makes obvious sense that stopping it in this condition will generate less drag than locked in an unfeathered position or even free-wheeling.

It's been a number of years since I saw the MIT study, but typical for them they did all sorts of tests, most if not all in a test tank with instrumented props and shafts.* They tested at a variety of speeds and with a variety of prop configurations, and the conclusion was that a prop that's allowed to freewheel transmits less drag to what it's attached to (the boat) than a prop that is locked off.* The drag amounts varied a lot depending on the speed through the water, the friction applied to the freewheeling shaft, etc.

But the bottom line of the study was very clear.* All else being equal--- the prop diameters, number and configuration of blades, and the speed and angle*through the water, etc.--- the prop that was allowed to freewheel, even with a high drag on its shaft simulating a transmission or whatever, always produced less measured drag than the prop that was locked off.* The locked off prop produced the maximum turbulence every time.* A freewheeling prop could be slowed by shaft friction to the point where the turbulence from the blades was very high, but it was never quite*as high as the turbulence off the locked prop.

And obviously the less resistance that was applied to the freewheeling shaft, the less turbulence was generated, and the lower the drag as FF pointed out earlier.

That's about all I remember from my one-time reading of the test results.* Someone posted it on one of the GB sites during a similar discussion to this one.*


-- Edited by Marin on Friday 22nd of October 2010 12:58:03 PM

 

[Nomad Willy]

I remember from my flying days w ultralights that a stopped propeller produced far less drag than a rotating prop. Ask any motorized glider pilot. But as Marin says the the props on a boat are way different. The ratio of the disc area (circumference) or blade arc diameter compared to the blade area of all the blades is what makes it a bad comparison. The blade area of the aircraft prop is probably 10 or 15%. The blade area of a typical 3 blade boat prop is around 50% and a 4 blade is about 70%. Air being compressible and water not adds to the difference I'm sure. Something that may be favorable to running w one is the fact that the running engine won't be as underloaded as w both running. timjet, I would check w the engine manufacturer about proper loading * * *....or consider converting to single engine.

 

[timjet]

John,Shutting one engine down did not increase the rpm on the remaining engine. Remaining engine remained at 1300 rpm with no throttle adjustment, speed dropped 1.2 knots after rudder correction. However though rpm remained constant at 1300 I believe fuel flow did increase as Jeff mentioned due to the higher load. IOW for the engine to maintain a constant rpm the flow flow had to increase to compensate for the increased load otherwise I would have seem a slight decrease in rpm.


I don't believe the prop whether stopped or allowed to free wheel makes a big difference in this case. I think as I asserted above that the power to weight ratio has a much bigger impact on fuel efficiency especially when operating a twin engine planning hull boat single engine.


Consider the fact that a twin displacement hull boat's engines are selected to power the boat efficiently on both engines running at probably 75% power. Same for a planning hull boat but at planning speeds. If you attempt to operate the displacement hull boat single engine, the engine must produce close to it's maximum power to move the boat at displacement speeds if even it can. Now looking at fuel consumption curves will show a huge increase in fuel flow above 75% that probably in some cases would negate the advantage of operating single engine.
This is not so on the planning hull boat. My engines produce enough power to plane the boat, but can easily power the boat to displacement hull speeds on one engine. At 1300 rpm I'm guessing I'm asking only 50% of the engines available power realizing a*significant*fuel savings and achieving close to displacement hull speeds.

 

[Nomad Willy]

Why would shutting down one engine cause the other to increase rpm????It should DECREASE as it can't maintain the speed attained w both running because the boat will be powered w approximately half the power/thrust. With the boat slowing down the load will obviously be slightly increased on the remaining engine but only because the slipstream decreased. So * *...if you don't increase throttle fuel consumption on the remaining engine will decrease specifically because the throttle remained the same and the rpm reduced. I know you said the rpm stayed the same so the rpm lost was obviously less than what you can read on the tach. The only way you're going to get an increase in fuel fuel consumption is to increase throttle or increase rpm or both.Most displacement boats don't cruise at hull speed. They cruise at 85 - 90% of hull speed and if you found a twin engined full displacement boat that could achieve hull speed w onlt one engine it would be ridiculously over powered.


I just thought about what I just said and I don't think you wanted to hear any of it. What you want to know is will cruising at 8.3 knots w one engine be more fuel efficient than cruising at 8.3 knots w both engines. I'd lay money on both engines but as you indicated earlier only a good fuel flow meter will tell. Or as you also indicated earlier finding the results of someone else's experiment. Please post whatever you find.


-- Edited by nomadwilly on Friday 22nd of October 2010 09:18:48 PM

 

[Marin]

nomadwilly wrote:

Why would shutting down one engine cause the other to increase rpm????It should DECREASE as it can't maintain the speed attained w both running because the boat will be powered w approximately half the power/thrust.

Why would the rpm change at all (on its own)?* Last year when I inadvertently let an engine get a slug of air during a fuel transfer (due to my forgetting about the fuel system setup) the engine died when I was on the aft deck doing something.* My wife was running the boat and she sent one of our guests back to get me.

When I entered the cabin she had pulled the power lever of the dead engine back to idle*and put its shifter in neutral.* However she had not touched the controls of the engine that was still running.* I looked at the tachs and the engine that was running was at exactly the same rpm it had been when I turned the helm over to her earlier--- 1650 rpm.

The boat had*slowed down of course since some of the thrust was gone and there was drag from the unpowered prop.* But the engine that was still operating kept operating at the same rpm it had been at before.

If we'd wanted to maintain the same boat speed after the engine died we'd obviously have had to add power to make up for the loss of thrust and overcome the drag of the unpowered prop.* But when the one engine died all that happened was the boat slowed down to the speed it wanted to go with the thrust from the one prop at 1650 rpm.

*


-- Edited by Marin on Friday 22nd of October 2010 09:29:07 PM

 

[timjet]

nomadwilly wrote:

Why would shutting down one engine cause the other to increase rpm????

*

Please post whatever you find.

-- Edited by nomadwilly on Friday 22nd of October 2010 09:18:48 PM
*

Eric and Marin,


Thanks for sticking with me on this. I found this reply to a similar question on another forum. I can't speak to it's*accuracy*but I think you might be interested.


When you shut one engine down you did notice that the speed dropped but you could not know that the govenor on the remaining engine increased the fuel load to make up for the increased loading without your intervention. Diesels adjust their fuel use for loading without 'consulting' their owners through the efforts of the govenor.

My words: " I also found that cruising at low planning hull speeds did not improve economy significantly" -

again you are basing this on a chart but using your numbers here the difference is 1.69/1.26 = 34% - that is a sigificant number to many folks.

In reality I think you will find that the difference in fuel burn per mile between about 7 knots and 17 knots will be more like 2:1. The best advantage between one engine at 7 knots and 2 will be like another 25% gain on average dependent upon the exact speed and the exact boat. And at 17 knots that boat is utilizing much more then 241hp as the curves led you to believe (13.4 X 18 = 241)


What he is saying in the last part is simply confirming that at low planning hull speeds around 13 kts, there is a 34% savings in fuel use over higher speeds in the 19 to 20 kt range. And operating single engine at slightly below hull speeds will realize a 25% savings over 2 engines at the same speed. My*theoretical figures were more like 58%.


And this from a 38' Bayliner owner:
I have run our 38 at hull or lower speeds, mostly lower on one engine for 4 years now. 4 mpg is pretty constant over these years. Approx. but very close.
One other thing. Running on one engine gets better mileage and puts 1/2 the hours on the engines.




*



-- Edited by timjet on Saturday 23rd of October 2010 08:13:50 AM

 

[Nomad Willy]

timjet,I thought of another way of expressing this. So you (roughly) halved the power on your boat at 8.3 knots and the resulting speed was 7.1 knots. What would the speed be if you halfed the power at 7.1 knots? I'm sure it would be considerably more than 5.9 knots. So the question that is most enlightening is "how much additional power would be required to increase your speed to 8.3 knots"? The answer is obviously 100%. You would need to DOUBLE the power to increase to 8.3 knots * *..... nowhere near double your speed. So as you reduce power you loose very little speed. This is what makes trawlers work. This is what gives trawlers long range and a good fuel burn. It's true w cars and airplanes too but not to such a great extent. Back to your boat. Say you were cruising at 8.3 knots and a magical force moved your boat forward increasing speed. There would come a point where your boat was advancing as fast as the pitch would allow without any slippage in either direction. There would still be a load on the propeller but only to rotate the propeller blades through the water. The're would be a far less load on the engines. There would be no thrust produced in either direction (fwd or bwd). One can compute this easily. At that point there is no power producing thrust. Only parasitic drag. Slow the boat back down to 8.3 knots and there is a highly loaded propeller producing lots of thrust propelling the boat fwd at 8.3 knots. Now lets shut off the engines without changing the throttle setting. Tie the boat to a good stout float w spring lines and restart the engines still without touching the throttles. I'm going to say that the engine speed will be LESS than 1300 rpm where it would be if we were still going 8.3 knots. The load on the propellers will be slightly more tied to the float but fuel consumption will be less because the amount of fuel injected with each stroke will be the same (throttle not changed) but the number of injections will be fewer. On propulsion engines the governors don't come into play until over max hp engine speed has been reached * * * .....correct me if I'm wrong but I think not. But the most important thing is that one would need to half the power of a trawler at least several times to half the speed. Now back to timjet. If you start the other engine back up again (no throttle change) you speed back up to 8.3 knots. You will burn twice as much fuel but the question is * * ...will you burn half as much if you back off the throttles to 7.1 knots and keep both engines running. Obviously it'll take half as much power (consider previous discussion) so one would be led to believe it would take half as much fuel and without the inefficiencies of running on one engine * * * ...SO * * ... I'm going to guess it would take less fuel to run your boat slowly on both engines than on one. I think the difference will not be great but great enough to easily measure.

 

[Marin]

timjet wrote:When you shut one engine down you did notice that the speed dropped but you could not know that the govenor on the remaining engine increased the fuel load to make up for the increased loading without your intervention.
*

This is the part I have trouble believing.* Because if you shut one engine down and do nothing else, the loading on the remaining engine will not go up because the boat will slow down to "match" the power being produced by the engine..* For the statement you quoted above to be true, the boat would somehow have to continue to demand a similar loading that it was demanding when both engines are running.* But that load demand is reduced the moment the other engine stops producing power because the boat is free in the water to achieve whatever speed the power of the remaining engine--- which remains unchanged--- is producing.

The boat is the varable here, not the engine.

If two people are paddling a canoe and one of them stops, if the remaining paddler expends exactly the same amount of effort he was expending before the other paddler stopped, all that will happen is the canoe will slow down to "match" the effort being expended by the remaining paddler.* The load on the remaining paddler will only go up if he attempts to maintain the speed both paddlers had produced together. or at least a speed higher than the speed he can produce by continuing to paddle at the same rate and effort he was paddling before.

So it makes no sense to me whatsoever that the remaining engine is going to experience any sort of load increase because the boat in water is going to slow down to match the power being produced.* It's not going to demand more power.* The boat simply matches the power being produced.* It's incapable of demanding more power on its own.

At least that's what logic tells me.* If an engine in a twin was producing 1650 rpm before, it will continue to produce 1650 and burn the exact same amount of fuel as it was before the other engine was shut down.* But the power from one engine at 1650 rpm is not the same as the power from two engines at 1650 so the boat will slow down until it reaches the speed that one engine at 1650 and x-amount of fuel burn produces.* So how does the load on the engine change if the boat is reducing the load by slowing down to match the power being produced?

The boat responds to the power being applied to it.* The power being applied to it doesn't respond to what the boat does because the boat doesn't produce anything for the engine to respond to.* In a vehicle, yes, because they go uphill and downhill and so produce acceleration and deceleration forces that are independent of what the engine is doing.* But a boat doesn't.* I just does whatever the power being applied to it--- engine, sails, or oars--- dictates.* Take away part of the power, the boat matches the remaining power.* It doesn't start demanding more power because it can't. I can only respond.

That's my take on it.* But I'm no engine expert.* I'd be interested to hear what someone like RickB has to say on this subject.




-- Edited by Marin on Saturday 23rd of October 2010 12:41:01 PM

 

[Nomad Willy]

When one engine stops the boat slows and the infeed water to the remaining propeller decreases * * *...increasing the load on that engine. If you could increase the the infeed velocity of the water to that propeller eventually you'd get to the point where there would be no load on the prop blades at all. To address the real question timjet has we could throw out this issue I tried to address in the first sentence of this post. Fly stuff. Dos'nt really matter to timjet. The real question is "if TJ is running at 7.1 knots and wants to go 8.3 will he need more or less than twice as much power to attain 8.3 knots?" Because it took half as much power to go from 8.3 to 7.1 knots. So if he starts his other engine up and runs it at 1300 like the other, like before, would the load on both engines be half as much * * ....as it takes twice as much power to go 8.3, or more * ..or less????? * It looks to me like it would be somewhat close. The difference in efficiency would be influenced by the efficiency of the boat going a bit sideways and the rudders deployed to keep the boat going straight w the asymmetrical thrust and the drag of the powerless propeller and the drag of the extra shaft and strut(s) and possibly more. That's a lot to overcome * * ...maybe it would'nt be so close. Looks to me like running both engines would be more efficient.



I agree w Marin in that the quote of timjet is false. But Marin * *...when you go into a dive in an airplane you unload the propeller and when you pull up to a stall you increase the propeller loading. It's the velocity of the infeed air or water that causes the load change when there is no throttle change.


-- Edited by nomadwilly on Saturday 23rd of October 2010 01:32:27 PM

 

[Marin]

nomadwilly wrote:

1. When one engine stops the boat slows and the infeed water to the remaining propeller decreases * * *...increasing the load on that engine......

2. Looks to me like running both engines would be more efficient.

3. But Marin * *...when you go into a dive in an airplane you unload the propeller and when you pull up to a stall you increase the propeller loading. It's the velocity of the infeed air or water that causes the load change when there is no throttle change.

Eric----

1. I don't think I go along with your infeed water theory.

2. Bob Lowe, one of the true "gurus" on the Grand Banks owners forum, once ran a pretty exhaustive test regarding running one engine on a twin vs. both engines using his 45' Alaskan (a deFever-inspired wood boat line built by American Marine at the same time they were producing their Grand Banks line.)

While I don't recall any of the figures and comparisons he plotted, I do recall that in the end he calculated the same conclusion you have stated--- that running a twin, at least a semi-planing twin like his Alaskan, on both engines is more efficient than trying to run it on one engine.

There is at least one famous exception to this.* Years ago a fellow decided to run his Grand Banks 42 from Hawaii to California).* He calculated that if he removed the prop from one shaft and set up the prop on the other shaft to be easily removed by a diver in the water, he could make the first half of the trip on one engine, stop the boat, remove the prop he'd been running on,* install on the other shaft the prop he'd been carrying, and then run the second half of the trip on the second engine, he could make the trip without having to carry a whole bunch of extra fuel.* He did this quite successfully, in large part because the weather cooperated and he did not encounter any overly rough seas (for which a GB is inherently unsuited).

But removing the "dragging" prop altogether is a whole different deal than simply trailing a prop, which produces a lot of drag, or locking it off, which produces even more drag.* So based on Bob Lowe's experience with his "Dreamer," I would tend to say that your conclusion is correct.

3.* The airplane analogy doesn't work for the same reason the vehicle analogy doesn't work.* You're adding gravity into the equation.* The descending plane is accellerated by gravity and the ascending plane is decelerated by gravity.* So the plane itself is introducing a force that the engine and propeller see.* You could switch off the engine in a diving plane altogether and it would continue to dive and probably even accelerate.* A boat is not influenced by a gravitational force that is assisting or opposing its forward motion.* It only responds to the power being applied to move it level through the water.* If you switch off the engine in a boat it will simply drift to a stop.


-- Edited by Marin on Saturday 23rd of October 2010 01:52:04 PM

 

[Larry M]

We had a "SHAFT LOK"*on a sailboat.* We had to lock the prop because of the transmission.* We didn't like the product.* You had to be careful of the speed when you locked the shaft.* We*ended up changing to a feathering prop.*

The home page does make for some interesting*reading though.<a href="http://www.shaftlok.com/">

http://www.shaftlok.com/</a>

Larry/Lena
Hobo KK42
Muertos, BCS, MX

-- Edited by Larry M on Saturday 23rd of October 2010 02:17:48 PM