Twin engines

[lionelz]

I'm after some knowledge if possible.

I currently have a single engine motor boat and in the next few years I was looking to upgrade it. I was originally going to go for a new boat but having looked at new boat at SBS I have decided I am unwillining to commit to a new boat. My current boat is 30ft and would like to go slightly bigger but get 2 separate cabins. The one boat that appears to fit this bill is a s34. I would have liked to keep to single engine but I understand that this limits my option s although there are still boats out there.

I believed (WRONGLY I think) that by having 2 engines you basically doubled the fuel burn. For example if at 3000 revs you burn 40ltrs an hour to achieve 20 knots then by adding a second engine you would now burn 40ltrs per engine. So 80lts an hours. At the boat show I was informed this was not the case. Can someone please explain why this is not so.

Thanks in advance.

Lionel

 

[Momac]

I changed from a s23 with single kad32 to a F33 with twin kad32. My fuel burn has doubled. But so has the weight of the boat.

 

[benchmark]

I'm after some knowledge if possible.

I currently have a single engine motor boat and in the next few years I was looking to upgrade it. I was originally going to go for a new boat but having looked at new boat at SBS I have decided I am unwillining to commit to a new boat. My current boat is 30ft and would like to go slightly bigger but get 2 separate cabins. The one boat that appears to fit this bill is a s34. I would have liked to keep to single engine but I understand that this limits my option s although there are still boats out there.

I believed (WRONGLY I think) that by having 2 engines you basically doubled the fuel burn. For example if at 3000 revs you burn 40ltrs an hour to achieve 20 knots then by adding a second engine you would now burn 40ltrs per engine. So 80lts an hours. At the boat show I was informed this was not the case. Can someone please explain why this is not so.

Thanks in advance.

Lionel

Probably because lets say your single engine does 20 kts burning 40lits per hr at 3400 rpm. A twin engine boat may do 20 kts with each engine only at 2800rpm while burning 30 lit per hr each. These are of course just hypothetical values but the point is that twin engine complement each other as you have double the amount of prop blade contact with the water and hence more propulsion at lower rpms.

 

[Irish Rover]

My FP Greenland 34 power cat has 2 x 75hp engines. In calm conditions it does 7 knots @2200rpm with just one engine running and burns 3.5 lph. With both engines running @ 2200rpm it does 9.5 knots. Different animal to the boats in question in this thread but interesting nonetheless I hope.

 

[scottie]

If a boat requires X hp to drive it at Y speed then other than the extra drag the number of engines doesn’t matter and the fuel required will be the same plus the amount required to overcome the extra drag

 

[scottie]

Assuming a planing craft increasing the hp normally increases the speed and the fuel consumption

 

[lionelz]

Thanks for all your replies, that's really helpfully so basically as I understand it because if the engines were same less revs would be needed to hit a certain speed so there for less fuel burn per engine, however there would be an overall increase of fuel because there are now 2 engines it's just not double

 

[blampied]

Double servicing costs,
Fuel consumption should be a penalty of around a third extra

 

[Dunadd]

If it’s any help our experience as first time owners of twin engines. We changed from a large single outboard rib to a 2x315 yanmar Antares this year. Ideally we wanted a single engine boat but couldn’t find anything without compromising on size vs cruising speed. I was expecting circa 60-80ltrs per hour burn but each fill up is returning low 40’s-50ltrs per hour @ 2800-3200 18-21knts which is just about bearable for us...... still to service them though.

 

[[2068]]

I think "a third extra" is too high in the context of a cruising boat with a top speed of 30kts or so.

For example:

Sealine S330 - available with Single D6-400 or Twin D3-220.
Both will hit 31/32 kts flat out.

Fuel consumption of a D6-400 at a fast cruise will be approx 50L/hr.
Fuel consumption of a D3-220 at a similar speed will be approx 28L/hr per engine, so 56L/hr for both.

That makes the twins something like 12% less efficient than the single, maybe 15% or so.
A different boat will produce a different result, but I'd be surprised if anything (not too quick) is more than 15-20% different with twins.

The real cost difference is not in fuel consumption, unless you are going to be cruising vast distances - it's in servicing costs.

.

 

[Assassin]

In reality there is no specific answer as there are so many variables involved, beginning at the beginning we have the design of the boat and its intended use and the general perceptions of the boat designer/design team as to what is the major priority/priorities are and where they focus their priorities in relation to that of their intended market.

In simple terms.

If you have a boat with a specific requirement for power output then how do you achieve it? do you go for twin 500HP engines or a single 1000HP engine, here is the first problem as people operate on horsepower which is actually a theoretical figure as the real figure is torque and it is the torque which powers a boat, and how do you deliver it? in reality a twin engine system is often lighter than a single engine until you add in the transmission, and then it becomes heavier as the additional weight of the transmission is double, and then we have the hull loadings. With a single engined boat you have the huge torque loading in a central position and this has to be spread over a larger area and this ads complexity to the hull design and increases the amount of materials needed to construct the hull to spread this large single point of torque across the hull, by comparison a twin set up reduces this by half and spreads this over a larger area of the hull by its very nature, and this reduces the coat of hull materials in its construction and actually simplifies it.

Intended use is a major factor as this throws another cap into the ring, is the boat used mainly for shorter inshore trips or sustained long distances as this throws up the safety issues!!!!!

Lose a single engine and you lose everything, drive, power for auxiliary systems and even battery power after a short time, not a major issue for an inshore boat where help is readily accessible in a short period of time, but on a longer trip it can become life threatening and twin engines ensure you have some form of propulsion and you have battery power being maintained to maintain your systems such as navigation and radio equipment where help may be many hours away.

Lots of factors to consider and lots of variables.

 

[jfm]

The description above about torque is wholly incorrect and misunderstands the physics. In particular, the statement that “torque powers a boat” is a fundamental misunderstanding of physics and engineering. Torque is dimensionally different from power because torque dimensionally does not include time whereas power is work done per unit of time. Chalk and cheese to an engineer. Your question is answered by considering power.

As said above, a given boat needs a given power to run at a given speed. Eg 500hp to do 25 knots. Ignoring drag and slight engine efficiency differences you can have one 500 hp engine or five 100hp engines to get the 25 knots and fuel burn will be the same.

But as said a few posts above in real world you can’t totally ignore drag and efficiency. Two engines will have a bit more friction and internal losses than one, and two shafts cause more drag than one. So a twin 250 hp boat will burn somewhat more fuel than a single 500 hp, but definitely nothing like double. More like 15-20% extra, ish.

 

[[2068]]

Substitute "thrust" for "torque" in the previous post, and it makes slightly more sense, although I'm fairly certain the transom on an S330 will be exactly the same thickness whether you opt for single or twins.

 

[Assassin]

The quote about torque is very correct as power is only torque x revs and it doesn't misunderstand the physical at all, perhaps you have misunderstood.

Without torque you cannot have power, without revs you cannot have power, and as power is torque X revs you have no time requirement, what you seem to be alluding to is hypothetical power which is something used during design for example and they only use torque and torque requirement for this.

 

[jfm]

The quote about torque is very correct as power is only torque x revs and it doesn't misunderstand the physical at all, perhaps you have misunderstood.

Without torque you cannot have power, without revs you cannot have power, and as power is torque X revs you have no time requirement, what you seem to be alluding to is hypothetical power which is something used during design for example and they only use torque and torque requirement for this.

I have not misunderstood and I'm not referring to "hypothetical power", whatever that phrase even means in this context. This is basic physics/engineering and I'm happy to agree to (very fundamentally) disagree. No point hijacking this thread with a daft argument about basic physics/engineering principles.

 

[lionelz]

The description above about torque is wholly incorrect and misunderstands the physics. In particular, the statement that “torque powers a boat” is a fundamental misunderstanding of physics and engineering. Torque is dimensionally different from power because torque dimensionally does not include time whereas power is work done per unit of time. Chalk and cheese to an engineer. Your question is answered by considering power.

As said above, a given boat needs a given power to run at a given speed. Eg 500hp to do 25 knots. Ignoring drag and slight engine efficiency differences you can have one 500 hp engine or five 100hp engines to get the 25 knots and fuel burn will be the same.

But as said a few posts above in real world you can’t totally ignore drag and efficiency. Two engines will have a bit more friction and internal losses than one, and two shafts cause more drag than one. So a twin 250 hp boat will burn somewhat more fuel than a single 500 hp, but definitely nothing like double. More like 15-20% extra, ish.

Thanks for all the replies. I'm not very technically minded and have only recently undestood what torque was:ambivalence:. But I get the general jist that it's a combination on many things but double fuel burn is way of the mark and 15 to 20% increase is much closer. I understand that fuel burn should not be the only thing that should affect my decision as having 2 engines includes extra cost and risk

 

[Assassin]

It also includes extra safety and this cannot be ignored and this needs tailoring to your requirements, for short trips in readily accessible areas a single engine may be fine but for longer trips in relatively inaccessible areas the additional safety of having two engines may be beneficial, your decision to make.

 

[John100156]

I tend to agree with JFM, torque may be associated with the turning force as a result of props/grip in the water, but essentially to move a boat of a certain mass from A-B requires a certain amount of energy, how this is delivered with similar performance props/grip should be similar. Of course, there are many variables, additional weight of two engines, how the boat sits, different props with twin and the like - I think 10-15% addition sounds quite reasonable to me... Additional cost of maintenance offset by easier handling

 

[benjenbav]

I ran two mobos in the 10 metre range over an aggregate period of just under 10 years. Very different hulls, but both twin turbodiesels (One with Volvos, one with Cummins). At the time I actively wanted two engines for the perceived resilience/redundancy benefits. If I was buying another boat of that size now, I would think much more about a single engine set up. Not to reduce fuel burn, particularly - although a 15% improvement would be a nice windfall; but for:

• Cost and ease of servicing - at this size of boat there always seems to be a service item on one of the engines that is in a difficult-to-access place. Plus - as mentioned - there are 50% of the number of items to service.

• Space - two engines inevitably take up a lot of the available space in boats of this size.

The issue of two engines providing additional resilience or not is often discussed on here particularly in the context of fuel, where a common contaminated source is likely to prejudice both engines. The nature of one's usage of the boat is relevant in this context. The theoretical and actual benefits of being able to lose one engine and limping home on the other give greater peace of mind to some than others.

 

[John100156]

I agree, always a compromises of one form or another....

I recall many years ago (Oct-2008) coming back from Soller, Majorca, to Sant Carles (around 98nm) in a two year old Targa 34. It was a rough crossing, predicted F2 for days but ended up F4/5 on the nose which for a 34ft boat was quite challenging. We saw some debris in the middle, probably shot from a container ship which we took great care to avoid, but unfortunately, some polythene sheet wrapped around one of the legs and caused an engine to overheat which was quickly stopped, we could not release it in those conditions with a boat hook and reverse didn't clear it. I was very glad I had two engines on that occasion! Fortunately, when we got back to our berth, it was easily cleared.

That said, we have just last weekend returned from a trip along the Caledonian Canal, Inverness-Fort William. We hired one of the Caley Cruisers (40ft) and when they showed us the engine room, I must say the extra space for servicing seemed enormous and cost and ease of servicing did come to mind. Mind you, travelling through Lock Ness at around 10knts was very enjoyable indeed, but boy did I miss the power of twin engines from time-to-time. The fuel bill at the end however was equally enjoyable, what with the 60/40 split.....

Horses for courses as always...