Running single on a twin to save fuel

If I had a 40' planing hull boat w twin diesels and wanted to save fuel would running only one engine at displacement speeds burn less fuel than running both?.

Easy Rider

 

you would have to try it and compare figures to be sure

 

I've spent several hours at different speeds trying to answer that question. For my 42ft planing boat there is virtually no speed where running on one engine is more reliable, not even close.

If you want to save fuel you do two things: Prep the bottom like a racing sailboat (600 grit finish) and keep the speed less than S/L = 1.1.

On my boat if I want the best speed with good range a combination of 25-26 knots @ .8 miles per gallon and 9.4 knots @ 1.9 miles per gallon gives a 14-15 knot average speed and 1.2 mpg. (Running the boat at 14-15 knots is .7 mpg).

 

Running on one engine is definitely saving fuel: friction losses in one engine, gearbox and auxiliary equipment is reduced by 50%. The actual gain is a bit less because prop slip increases due to the higher blade load and there is some extra rudder resistance, but is still significant.

I made a habit of using just one engine on long trips because it lowered the fuel bills and added a second power steering pump because the port engine didn't have one.

 

You'll definitely burn less fuel but you wont go as far/fast...

Overall savings, you'd have to try and see.

-Tom

 

You may have to lock your shaft on the engine that is not running. I have know several people who did not keep their shafts from turning and it burned up their gear!

 

Thanks guys for the input.
It would seem to me that if you could rebuild the boat as a single screw, the single screw (using the same engine gear ect as in the twin) on center line as usual ....it would burn considerably less fuel than the twin. But with the trailing prop, the asymmetrical thrust and all the other undesirable aspects of running a twin on one would it still be (fuel burn and range wise) better running on one? CDK is VERY knowledgable and I respect his opinion but there are so many variables. What if one boat had deep re drives w very large and skewed props and it was a short wide boat and the prop shafts were very far apart. But perhaps we can get a consensus of opinion for the average 45' planing diesel boat ...running at disp speeds.

Carteret,
Some do, some don't but it's not part of the question.
Sub Tom, The question assumes you run both single and twin at the same speed ....about 7 or 8 knots.
RHough, "running on one engine is more reliable, not even close." Did you mean more "economical" ? This is not a question of reliability.

 

Yes, I meant economical.

I have run fuel curves for my boat for speeds from about 7 knots to 10 knots. I tried running on one engine over the same speed range. There was no point where the fuel mileage was higher running on one engine. I ran the range with one engine stopped and with one engine idling in forward (to simulate a failsafe condition on one engine). The mileage was worse with one engine stopped than with one at idle in forward.

The power requirement at a given speed is the same. When running on one engine the added drag from steering to hold course burns more fuel at the same speed on my boat.

My experience is not the same as CDK's. I'm sure each boat and power package are different.

My boat is a Sabre 42 Express. Two Cummins QSB 5.9L 425HP engines and Zeus pod drives. 1.5 miles/gallon at 8.7 knots/1200 RPM and 1.4 miles/gallon at 9.2 knots/1300 RPM. Above 10.5 knots mileage is .7-.8 miles/gallon to 25 knots.

R

 

Easy, If you know the charter boats made by Delta Marine, the semi-displacement 43 and 50 both have guys that have seasonally commuted to Alaska and locked one shaft or removed one prop for the ride. They felt that it was of value. With fuel at the price level it is now, tho, I doubt that there are many commuters. Certainly here (Kenai Peninsula) there are no longer commuters.

 

I'm confused. Are you proposing to run one engine at planing RPM and the other at displacement RPM?

If so, I wouldn't recommend it for any period of time. The slower will be very lightly loaded and we all know how diesels don't like that.

I can appreciate the academic side of this question but the answer is going to be in the pudding (sea trials).

I would look more closely at your hulls planing dynamics and your engines performance/consumption curve.

I suspect your biggest fuel savings is going to come from throttling back both engines equally while staying on plane.

-Tom

 

That I cannot deny.
With the props far apart and a considerable rudder angle to hold course, the fuel savings effect is reduced.

My own experience is from the time I had twin Mercruisers close together. There wasn't much drag increase running on one engine in that case.
I had fuel flow sensors connected to a Navman fishfinder showing a reduction of approx. 30% if the speed was kept below hull speed. With increasing speed the savings were decreasing rapidly.
How it turned out at planing speed I do not know: maintaining planing speed on one engine was not possible.

The ultimate solution for saving fuel at displacement speed is of course the variable pitch prop because only with that you can obtain the proper engine load at low rpm. But that is quite another topic.

 

Similar deal with me, twin 610 hp Cummins QSM 11s.

 

Some shaft seal systems use water pressure from the engine as a lubricant and have limited capacity to run dry. Same 'run dry' issue with some gearboxes whose lubrication is driven from the input shaft. Once you lock the shaft (as may be required to protect the shaft seal and/or gearbox) your savings are less. Personally I just run my twins a bit slower for fuel economy, but probably at no less speed than I would run on a single with the 2nd shaft locked causing significant drag.

 

"but probably at no less speed than I would run on a single with the 2nd shaft locked causing significant drag."

The prop drag argument , lock it or let it spin , has been covered by rag baggers for decades, with lab and test tanks results supporting BOTH sides.

On a new build a pair of CPP would certainly be the answer.

On an existing boat , IF the tranny could free wheel with no harm , I would contemplate belting a DC motor to the shaft to give it enough power to overcome the shaft bearing and seal and tranny drag.

IF the prop freewheeled at 100% of the boats speed of advance , the drag would be on the alternator of the operating engine, not the prop.

This should lower the drag a good deal so the rudder angle could be lower.

A larger boat would use its Hyd setup to help freewheel the prop.

FF

 

this is a very good post
But there are sea conditions to weigh up too
In rough a lot of planing hulls cannot jump waves, they simply land badly, because of hull form in the land zone
Other hull can maintain fast planing speed and land well and softly The former are down to wl hull speed or less. then running on one will be and option The deep vee good entry boat will stay put on the plane at a set governer speed, the bad hull form will have the pilot playing with his throttles all day and so on