Using one engine on a twin engined boat (Part Two)

[Greg2]

We sometimes run our Broom 41 on one engine on the river. Whilst she has a planing hull there is also a small skeg keel to help with keeping her straight. I find that dropping one trim tab and with around ten to fifteen degrees of helm she tracks straight and can alter course etc easily. The gearboxes allow this as they have sufficient oil circulation when the engine isn't running and were probably chosen by Broom for that reason.

We do this for a couple of reasons. Firstly to work one engine a little harder, which is probably better for it and secondly to effectively halve the engine hours we would be adding when doing slow passages inland.

Our steering system is independent and doesn't require an engine to be running and by running half an hour at a time on each engine we avoid the cutlass bearings getting too warm (water cooled from the engine where they go through the hull) and we share the battery charging. It also helps with keeping to river speed limits.

 

[simples]

thats an interesting setup, it looks like the smaller engine is attached via chain to the propshaft below?
it must have been a struggle finding counter rotating gearboxes and gearboxes which dont need hydraulic / lube pumps from the attached prime mover.

 

[rustybarge]

thats an interesting setup, it looks like the smaller engine is attached via chain to the propshaft below?
it must have been a struggle finding counter rotating gearboxes and gearboxes which dont need hydraulic / lube pumps from the attached prime mover.

Most people estimate twins use about 25-30% more fuel, so twin in-line engines driving a single prop means you can have the security of two engines with the efficiency of a single: a big bonus on a passage maker which needs long range.

 

[dragoon]

Energy required to move the boat with 1 engine or 2 is the same, so the difference in running 2 engines versus 1 is going to be the additional cost of rotating the second engine, energy wise. But, given the efficiencies of symmetrical thrust versus asymmetric, my guess is there may be no saving. That was my conclusion when I tried this some years back and we didn't notice any fuel usage reduction over the season. Plus there are potential lubrication issues with having one gearbox windmilling, lack of manouverability, safety issues in case of engine failure etc.

 

[MapisM]

Energy required to move the boat with 1 engine or 2 is the same, so the difference in running 2 engines versus 1 is going to be the additional cost of rotating the second engine, energy wise.

That would be true if a monohull would be equally efficient with 1 or 2 screw, but it isn't.
For any given hull, the drag is fairly lower with a single engine/prop, the prop diameter can be larger, it sits deeper in the water, and it's aligned with the keel (and so is the rudder).
There's more than just saving the cost of one engine, behind the fact that most workboats (and also leisure trawlers) are driven by one screw only...

 

[dragoon]

That would be true if a monohull would be equally efficient with 1 or 2 screw, but it isn't.
For any given hull, the drag is fairly lower with a single engine/prop, the prop diameter can be larger, it sits deeper in the water, and it's aligned with the keel (and so is the rudder).
There's more than just saving the cost of one engine, behind the fact that most workboats (and also leisure trawlers) are driven by one screw only...

That might be true in the case of a true single screw boat, but in the case of a twin running on one engine, most twin engine boats will still have a second screw dragging through the water, and the thrust delivered off-centre causing the boat to yaw which is compensated with further drag from the rudder. Whilst I accept a true single engine setup is more efficient, I find it difficult to believe that there is anything much to be saved by turning off one engine on a twin engined boat. My own experiments seemed to back this up.

 

[MapisM]

All agreed - actually, that's exactly what I meant.
I just misunderstood your statement "Energy required to move the boat with 1 engine or 2 is the same" as generic, rather than meant to twin engine boats running on one engine.
In which case, if we should split hairs, the boat itself actually requires a tad MORE energy when running on one engine, for the reasons you mention. :encouragement:

 

[oldgit]

Having just changed from a twin screw semi D hull (Broom 37 ) with a small keel which was merely not very pleasant to handle with one engine and required some concentration to keep in straight line going ahead and difficult to steer astern,now have a boat with planing hull.
On a single engine, the planing boat constantly needs correction at displacement speeds and is well nigh impossible to maneuver with any accuracy astern.
The rudders are probably responsible with not much prop wash volume acting on very little rudder blade area.
Counter balanced rudders do not help?
As our boat is frequently used in both offshore and on non tidal waters, it is easy to decide if single engine running is worth the trouble.
A recent upper Thames trip confirmed that single engined running is not worth the effort.
With the short distances between locks and the occasional need to do an abrupt change of speed or direction between , constantly re- starting an engine when going onto the waiting laybye or prior to entering lock is just a pain in the bum.
..and if you are hanging around midstream waiting for the ancient crew of some old tin slug to wake up and realise the gates have actually been open for the last five minutes,you really do need to be on both engines.
Having said that, now with some experience with planing hull on the non tidal Thames, the much used wash excuse that a planing boat needs to be moving quickly to maintain control, does appear to indicate a lack of skill by the skipper rather than any shortcomings of the hull design.

 

[Piers]

To push a vessel through the water at a given speed requires a certain amount of horsepower, whether being propelled by one or two engines. On a two engined boat, driving on one engine will introduce an asymmetry which will need to be countered by an offset rudder bringing an increase in drag, requiring more fuel, plus a persistent strain on the rudder stock.

The only benefit I can see is fewer engine hours on the unused engine, thereby saving on servicing.

 

[oldgit]

To push a vessel through the water at a given speed requires a certain amount of horsepower, whether being propelled by one or two engines. On a two engined boat, driving on one engine will introduce an asymmetry which will need to be countered by an offset rudder bringing an increase in drag, requiring more fuel, plus a persistent strain on the rudder stock.

The only benefit I can see is fewer engine hours on the unused engine, thereby saving on servicing.

If you do your servicing annually, irrespective of hours run , no saving ?

 

[Piers]

If you do your servicing annually, irrespective of hours run , no saving ?

Depends if there are parts which need changing at a set number of hours run.

 

[Topic]

Interesting, except for the pointless spat! I have twin engine 13m displacement hull with 2 x 225 bhp Perkins Sabre M225TI diesels. The power is too much as the hull will not plane. Therefore, on long passages I always run on one engine and use two to manoeuvre into port. With one engine I get 2.77 ltrs/mile at 8 knots and with two engines its 3.71 ltrs at 9 knots. Only very small helm angle of say 4 deg necessary to counter offset thrust and autopilot copes fine. Engine hours are less and boat is quieter on one engine. Steering is independent and batteries are charged in either mode. I think I will continue running one engine only at sea.

 

[rustybarge]

Interesting, except for the pointless spat! I have twin engine 13m displacement hull with 2 x 225 bhp Perkins Sabre M225TI diesels. The power is too much as the hull will not plane. Therefore, on long passages I always run on one engine and use two to manoeuvre into port. With one engine I get 2.77 ltrs/mile at 8 knots and with two engines its 3.71 ltrs at 9 knots. Only very small helm angle of say 4 deg necessary to counter offset thrust and autopilot copes fine. Engine hours are less and boat is quieter on one engine. Steering is independent and batteries are charged in either mode. I think I will continue running one engine only at sea.

Those numbers make sense, and fit in with boats that make long sea passages where fuel consumtion is a vital part of calculating the available range.

Twin engines use about 30% more fuel...give or take.

 

[MapisM]

With one engine I get 2.77 ltrs/mile at 8 knots and with two engines its 3.71 ltrs at 9 knots.

Those numbers make sense

Ermm, actually no, they don't.
On a 13m boat at D speed, a 1 knot difference (particularly at a speed which is already borderline between D and SD) is more than enough to explain a 33% higher fuel burn, regardless of how many engines are running.
It's the difference between one and two engines at the same speed (if any) that would be meaningful.

 

[Portofino]

Interesting, except for the pointless spat! I have twin engine 13m displacement hull with 2 x 225 bhp Perkins Sabre M225TI diesels. The power is too much as the hull will not plane. Therefore, on long passages I always run on one engine and use two to manoeuvre into port. With one engine I get 2.77 ltrs/mile at 8 knots and with two engines its 3.71 ltrs at 9 knots. Only very small helm angle of say 4 deg necessary to counter offset thrust and autopilot copes fine. Engine hours are less and boat is quieter on one engine. Steering is independent and batteries are charged in either mode. I think I will continue running one engine only at sea.

"Pointless spat " was related to running a high speed P boat on one engine @ D speed .

Running a D hull @ below best D on one engine is different scenario .
To me as its all D and presumably purchased deliberately because it was D speeder ,then on a long passage or river speed restriction etc ,that's Ok.

 

[dragoon]

All agreed - actually, that's exactly what I meant.
I just misunderstood your statement "Energy required to move the boat with 1 engine or 2 is the same" as generic, rather than meant to twin engine boats running on one engine.
In which case, if we should split hairs, the boat itself actually requires a tad MORE energy when running on one engine, for the reasons you mention. :encouragement:

Yes, I think I can safely say we are in violent AGREEMENT

 

[Bouba]

In a river, keeping the bank side propeller from spinning and avoiding obstacles is probably the only reason to run on one engine

 

[QBhoy]

Admittedly I haven't read everyone's comments, but I suppose there are some things to think about;

Depending on the set up, some boats have stern gland cooling fed from the engine cooling system, so if the shut down engine shaft and prop (if true to the application) are free to rotate, is there a risk of melting the shaft packing (if applicable) after long term running ? The alternative is having the shut down engine in gear...this would cause a fair bit of drag surely ?

Second problem might be...I have a friend who has a Sealine sc29 with twin VP d3 and duo prop drives...I have asked him if he has considered doing just this. He has mentioned that he may have power steering problems..can't remember exact details, but I imagine that perhaps either each engine runs each drives power steering or on engine runs both and would need to be run as such.
There is also the obvious mathematics to be done regarding the efficiency and whether it would be worth while or not.
Calorifier hot water systems are also usually run off one of the engines...
Is there also a risk of staggered service intervals ?
Would you be forcing un even or undue wear on one engine ?

Overall perhaps a minefield of questions to get through, but I understand the reasoning behind this and have often pondered upon the question !

 

[Momac]

Is there also a risk of staggered service intervals ?

It would be unwise to run on one engine and run up lots of hours and therefore end up with a big difference in hours compared to the other engine.
I dont think anyone is suggesting that.


It doesn't sound like the fuel saving is as great as might be imagined or none at all if people expect to load up the one running engine and run at 8 or 9 knots. I think if the boat speed was under 5 knots it might be a more economical outcome.

 

[Piers]

Apart from the issues I list in my above post (29) you will have to ascertain the freewheeling limits on the gearbox. In addition, if your stuffing box is water fed by the engine, you should check what happens when the water supply is cut-off.