One engine or two?

[Nealo]

I'm sure this issue must have been debated on the forum but as I'm new and can't find anything specific on the search facility, at the risk of provoking a collective "oh not that one again" can I ask members for their opinions.

Most of my boating experience has been on inland waterways but I'm going to be buying an offshore cruiser next year and the one issue I can't quite lay to rest is whether to consider single engine boats. I put it this way because logic seems to favour the twin engine option, and having handled such a boat it really was a revelation in terms of manoeverability. Added to which is the security of two engines at sea.
But, one engine is more economical, (as I understand it) reduces the capital cost, saves on maintenance costs, and takes up less room.

Would anyone consider the pro's of single engine installation outweigh the obvious advantages of two?

 

[MapisM]

Would anyone consider the pro's of single engine installation outweigh the obvious advantages of two?

Absolutely.
Some of the most serious, ocean going vessels run on one engine.
The inherently higher security of two engines is, per se, just a myth.

 

[cataldo]

one engine or two?

we had 5 boats in fourteen years,
the first a single outboard and auxiliary

then all twins , and we had to limp home once

on the auxiliary and once on on one of the twin diesels

also I think lifeboats are usually twin engined.

 

[Rum_Pirate]

I have two x 85hp outboard YAMAHA engines.

If I have an engine failure (with a single engine installation) the next stop is central America or similar ie several hundred miles away.

We do not have the luxury of the RNLI or similar out here.

 

[Ripster]

One (diesel) is fine for local coastal cruising. Just make sure its maintained correctly and well looked after. Maybe carry a spare prop and the usual sets of filters and belts, but for longer offshore ventures, the security of two gives peace of mind and more options in the event of failure. One indeed will burn less fuel and obviously be cheaper to service etc.

My first two boats were only single diesels and were usually fine. But on one venture my prop bush went and we had to crawl back at 6 knots. On another, the drive clutch went and we had the same crawl back again. Then we had the oil cooler spring a leak. We got back fine every time, but were reliant on the fault not developing. You can also carry an aux OB engine too as a further option.

Close quarters is easier on twins -its true. But an OD and bow thruster is nearly as good.

But our latest boat has twins and shafts as we want to travel further.

 

[Kawasaki]

I have two x 85hp outboard YAMAHA engines.

If I have an engine failure (with a single engine installation) the next stop is central America or similar ie several hundred miles away.

We do not have the luxury of the RNLI or similar out here.

But
Didn't you recently post, both beggars wouldn't start?

 

[ADLS]

Lazy days is fitted with twin diesels and I've often had to rely on one to get me to port. First time was on the upper Thames when a throttle cable snapped, the gear change still worked so was able the manover into/out of locks on tick over, the motored on one engine, this was from Shepperton to Wraysbury late at night.

Another time we blew a head gasket coming into the Thames estuary and made it up the Medway to Cuxton, I did let the CC know and they sent out the soufend boat to escort across the shipping lane.

2008 got a builders sand sack wrapped around a prop just approaching Lime House and managed to get into the lock. But had to use the incoming tide to help turn around as though we have twin engines the boat only has a centre rudder so steering is difficult to say the least.

I would always go for twin engines for the added safety and manoverabilty, but do try and keep the total packeage seperate so fuel problems etc wont effect both engines and have twin rudders fitted.

 

[ian38_39]

And there lies the problem, if you pull the fuel from the same source for both engines and then find the fuel is dirty both engines will stop.

It is a dedicated boater that fills one tank from one source and another from somewhere else.

Also need to check that the return doesn't feed back to the opposit tank as is often the case.

That said it is 2 engines for me and I wouldn't want it any other way, my boat is 15 years old and to me that is too old to rely on 1 engine, maybe different if I was buying new.

Ian

 

[rosssavage]

Don't wish to 'lakesailor' but as mentioned above, both fuel systems must be seperate. Yes, filled from a common source perhaps, but the tank could leak, be contaminated by a leaky deck filler etc etc... Further, it not only engine reliability that is the issue - transmission is also vital. My only problem (so far...) at sea was a stbd gearbox coupling failure and bent prop following a heavy prop strike at speed. Luckily I had another complete propulsion system on the port side so limped home without having to bother the RNLI. Yes, two is more expensive, but not half as scary as being adrift near a shipping lane. Worth EVERY penny in my book.
MapisM, many of those 'single' engine ocean cruisers also have a wing engine, and some a hydraulic backup driven by a generator in case of main engine failure. They also tend to be displacement craft so the engine / transmission systems are not overly stressed.

 

[ian38_39]

Sorry can't pay out on the Lakesailor, was just expanding a point you had already made, started the post by agreeing with you.

Will allow a partial one on the return feed but it was intended to show that separate systems are not as simple as 2 tanks 2 engines. Maybe hold a token credit.

 

[Nauti Fox]

For me I would always go for twin rather than single. Mainly just for the reassurance, if you have an engine/starter fault or foul a prop you can still get home.
By the way, big ships with single engines do break down, I know, I've been on several that have, and remember the resources they have on board, fully equipped workshops with lathes etc. Tons of spares and fully qualified engineers to sort it out, not really the same for us being rolled all over the place while trying to reassure the Mrs.

 

[rafiki_]

The other thing to consideris residuals and saleability. It would also be easier to sell a twin diesel engined boat.

 

[late-night-lochin]

Well, most commercial boats are single engined and they're out every day.

Twins are great if you can afford not only the cost but the space.

My boat is single engined and you can get all round the engine easily. With the twin engined varients, you not only loose space but there's one side of each engine that's almost impossible to get to. This could also mean that it doesn't get the level of maintenance tht it should.

The weight of a big single is positioned deep down in the centre of a boat which should be more stable than the weight of one on each side.

Close quarters manoeverability with a single engine is poor but a bow thruster cures that.

 

[oldsaltoz]

And there lies the problem, if you pull the fuel from the same source for both engines and then find the fuel is dirty both engines will stop.

It is a dedicated boater that fills one tank from one source and another from somewhere else.
Ian

Our cat has twin engines and separate fuel tanks that can hold close to 200 litres each. When refuelling fuel is added to the lowest tank only, we also have a fuel transfer pump and change over valves on the fuel return lines.

Fuel is only transferred from one tank to other after several hour running and even then via the return fuel lines when possible ensuring it has passed through the filters first.

Only had to run on one engine once, when the fuel filters on the port engine needed changing; now have twin filter set up on both engines so it's just the turn of a valve and no priming to restart. The old filters can be changed any time later, like at the end of the day or when back under sail.

The main reason for twins is the ease of manouvering into and out of the marina berth.

Avagoodweekend and all the best for the festive season and new year to all.

 

[Deleted User YDKXO]

Absolutely.
Some of the most serious, ocean going vessels run on one engine.
The inherently higher security of two engines is, per se, just a myth.

Sorry to disagree but I think that comparison isn't valid. Yes many commercial vessels do have only one engine but it's usually a very different type of engine, a low revving, large capacity, understressed unit with a design service life of many tens of thousands of hours compared to the high revving, small capacity, overstressed units with a design service life of a couple of thousand hours normally fitted in leisure boats. In addition, commercial vessels have engineering staff and a mountain of spares on board to keep that single engine running.
I can understand fitting a displacement boat with a single engine mainly because builders of such boats can make a conservative choice of engine because power is not an issue. But for a semi-d or planing boat which needs a powerful engine to attain it's design speed, the builder is going to fit one of those overstressed turbocharged, supercharged, aftercooled engines that don't last 1000hrs.
Even then, twin engines have other safety features such as separate fuel systems (sometimes) and the ability to foul one prop and still keep going. The type of drive has a bearing too. If you can lift the prop to clear it, as with a sterndrive or outboard, a single engine is relatively safe but if the boat has shaftdrive or IPS, you're stuffed as you can't get to the prop unless the boat has a hatch in the hull.
The cruising area you plan on doing also has a bearing. For coastal or inshore cruising, there's usually going to be someone else about to help or you are within VHF range of the coastguard so a single engine is acceptably safe but for offshore cruising out of radio range, it's got to be twins

 

[Nealo]

Many thanks for those contributions - I think all the points made are valid, though maybe MapisM might wish to defend his comments??

I appreciate what many have said about being detached from Coastguard support, VHF range etc. but it's interesting that the Beneteau trawler yacht recently reviewed in MBM only has one engine - whoever buys one of these surely isn't thinking about never losing sight of land?

I may be displaying my ignorance of sea going craft here, (in fact boats in general) but why are offshore prop driven craft not fitted with weed hatches like many inland boats are? Also, it always seems to me that twin screw boats are more prone to getting stuff wrapped round the props - am I wrong, if not, is there some physical reason for this?

One factor that has been mentioned which I forgot about is the truly awful access on many twin engine boats. In many you seem to have to virtually dismantle the saloon to get at them, even to do routine checks. I guess this boils down to modern boats, like modern cars, needing to have showroom appeal rather than staying power.

 

[Deleted User YDKXO]

Many thanks for those contributions - I think all the points made are valid, though maybe MapisM might wish to defend his comments??

I appreciate what many have said about being detached from Coastguard support, VHF range etc. but it's interesting that the Beneteau trawler yacht recently reviewed in MBM only has one engine - whoever buys one of these surely isn't thinking about never losing sight of land?

I may be displaying my ignorance of sea going craft here, (in fact boats in general) but why are offshore prop driven craft not fitted with weed hatches like many inland boats are? Also, it always seems to me that twin screw boats are more prone to getting stuff wrapped round the props - am I wrong, if not, is there some physical reason for this?

One factor that has been mentioned which I forgot about is the truly awful access on many twin engine boats. In many you seem to have to virtually dismantle the saloon to get at them, even to do routine checks. I guess this boils down to modern boats, like modern cars, needing to have showroom appeal rather than staying power.

Yes you're right, the new Beneteau St34 does have a single engine but I think that is as much about keeping the price down as anything else. Its a v nice boat for the money but the vast majority of other boats of this size have twin engines. The St34 does have a keel and that might help keep debris away from the prop but I wouldn't count on it.
Good question about weed hatches. I don't really know the answer other than most offshore boats probably don't have the space for them and there may be issues about structural integrity as offshore hulls are much more stressed than river boat hulls. Another good point about whether twin props get more fouled than a single. For sure there is more metal in the water but I still see the fouling issue as an advantage for twins. I've lost an engine or fouled a prop quite a few times but never lost both at the same time.
Yes, access can be a problem with twins but the answer is if you can't get easy access to the service points, don't buy the boat, especially if it's used, as you can bet that previous owners have had similar difficulty and the engines may have been neglected as a result. It's certainly not a given with twin engined boats that engine access is difficult but obviously a single should be better but even that is not always the case. I happened to look at a Botnia Targa 25 in the summer and the access to the single engine was dreadful.

 

[BartW]

for me,
twin engines is just an extra fun factor,
the extra possibility's you get for manouvring
gives more pleasure to the boating

 

[oldgit]

Purely down to money,if you afford boat + two engines you will buy two,if cannot,buy one with one.Simples.

 

[Greg2]

I think the comments on semi-displacement and planing hulls are probably relevant as the stresses involved are greater but on a displacement boat I think single is fine. Nordhavn build boats designed for blue water cruising and have a single, albeit with a wing engine for backup.

Personally I would be happy with a single on a displacement or even a semi-displacement boat but not a planing boat.

It is probably worth saying that the most likely breakdown on a diesel boat is bad fuel. Our boat draws from one tank so I don't feel at all reassured by having twin engines.