Single Engine... vs more

I have been wondering about this for awhile. In sailboat circles single engines are the norm for the auxiliary power propulsion... but not so much in motor yachts propulsion... particularly as they go over 20 meters or so in length.

It seems to me the use of a single engine would be the ideal. I really don't think in modern use there is ANY justification safety or handling for multiple engines. On the handling aspect bow and/or stern thrusters are more and more universal with larger yachts over 20 meter in particular... any single engine boat with the auxiliary thruster(s) can be docked in any situation better than a twin without or as well as if the twin had the thrusters too.

Reliability wise diesel engines are the most reliable power sources known. When I was in nuclear engineering (now a long time ago) statistically the diesel emergency generator was the most reliable piece of equipment and it was shown in meeting safety requirements ONE was all that was needed for emergency backup at a nuclear plant... shore or boat based. I can think of no more well studied or critical use situation than that period. In fact, I remember questioning why not two was not more reliable... and found out it had been studied and shown that ONE was more reliable than two... the surprising truth.

Most commercial ships are single screw... too!

Now if one goes to any marine diesel engine manufacturer they will state there is no reason as to reliability for one to use two engines rather than one... seems they would have vested interest in selling the two but in the real world of dollars and sense it makes none. So trying to sell you two is simply overselling.

So I thinking practically the only reason two engines are preferable in yachting is that people are ignorant OR some kind of marketing hype has become fact in the minds of everyone OR is like the mines bigger or more something malady. As to the last "bigger or more" you get over that real quick in the tonier marinas... particularly now days... there is always someone with a bigger better boat than yours... but do they enjoy it more?

I should have clarified or limited my post to displacement 20m+.

However, high speed craft can be single engine too. In fact, when you start going to high power diesels they get really too big and heavy... even in larger boats 30m+ and one should go just to a turbine: two 3600 hp diesels are not a lightweight proposition. Actually the turbine will burn less fuel because the boat can be so much lighter for the planing group.

Why are the water jet powered runabouts so popular... they are very light and sporty.... and usually single engined!

But going fast is fun for only awhile and I don't want or use a boat that way... know few that do. Usually the speedy planning boats set in the slip rusting the engine internals for lack of use.

A couple comments are that in the med and particularly in the US there are quite a few high speed boats... over 1.5 X square root of waterline length. However, other than sport fishermen... most do not speed a lot of time above displacement speed. Fuel is too expensive. They usually set in port except for a few flashes a year. You will find few ever even come close to bumping up against an "E" rating limitation.

Often one hears "oh I can do Monaco to Antibes or Sainte Maxime or Agay or something in around an hour or hour and a half or two or three. I don't like to be jostled around the boat that much and just walk or ride the cart over to the heliport and take the helicopter for a fraction of the fuel costs and at a much more reliable transit time... if the waves and wind are bad you are not going to be beat up when you get there... and its going to take longer. And, you can predict the time better just taking the helicopter. Of course, being a snail boater in the main what does it matter in my case. But even in general use I bet the twin engine is not better than a single even in planning with proper boat design... and lack of judging based on say so.

The following may seem off subject... but it seems as soon as I post something and forget a detail like limiting it to 20m+ displacement speed 1.34 x square root of LWL... it then gets sucked somewhere else not intended (not referring to Brain's or K1W1 posts in that).

Of course, I have done the same in other threads... so must be a malady of forums... <is that really a punctuation?

K1W1... So true. I like just like American iron better than German iron:

Part of the why is a company I owned a stock holdings and was on the board... worked in development of engine technology and did business with CAT and DD/ MTU. Occasionally, I would get involved with projects because the guy that ran the operations was personal friend and colleague from my engineering days and ex US Navy Engineering Officer. He knew I could help on certain limited issues and could talk me into it... . This was a now a long time ago. A project I remember from the later was simply nutty business wise to invest in... but it was simply gross dollars to the company doing the contract development work on it. I think... I was brought in this case because I had polite patience to listen to some intriguing and complex ideas and translate them into sense as to project planning... and my friend thought I had the time... and was doing nothing... or so he thought! Also, I had run a similar program to success some long time ago. Anyway, the European gentleman they had leading it was a very highly educated very smart but with no real experience man leading the technical direction... Herr Dr. type. I don't think he had ever done anything service work or mechanical work on an engine. That project went... guess where... nowhere. Made me wonder about the company's management... as it wasn't some Oxford educated Sheik funding it.

Was intriguing too as the Dr.'s wife was a very talented engineer and had run a the engineering department at a very prestigious technical institute before she retired... and her best friend was then President of another leading engine/power production equipment company... not associated with boating. At a party I asked the ladies why they did not work with Herr Dr. on the ideas... the wife said... "I am retired and I have to listen to it too much at home"... the other lady said... "he is a genius... but lives in an ivory tower." Also, found out he had dated both... before he was married! But the Herr Dr's ideas were valid and eventually they sold it all to United Technologies later... I suppose something to put in the technical portfolio for a sunny day. Not to say DD/MTU does not make fine engines...

As to high speed boats...
I really think you can do a high speed craft with one engine just as well as two or more. But due to convention and practical availability of power plants for boating it does not happen often, as example:

Last year I was looking at getting a 32m high speed boat... thinking it would be fun once in awhile. The boat l liked best had German iron in it (very low hours) about 2400 hp x 2... about 30 knots max. Well actually looked into replacing those with CAT C18 (4x) with the ZF Zeus drives... Now that is not what this thread is about but the dilemma one is presented with... unless you are willing to build it comes the way it comes. Why I was looking at a conversion... well just looking at those engines crammed in there and thinking oh... I know nothing about them [likely could not find out-- K1W1's comment applies as to confusion], really hate the service guys and.... those guys I would have to use would get me pissed off at waiting for them to eventually come around and the sticker shock bills compared to CAT service... and again not knowing much about those particular engines... just left me cold. But the boat was very well priced, had an motivated seller... and it was affordable to do some conversion work... or so I thought. So... I looked it as a hobby deal and excuse to hang around the yard for a couple months... but not for years the build process takes. Turned out the issue was the hull form was wrong... prop tunnels so the back 1/3rd of the boat would have had to been redone basically... you were stuck with two engines... meaning it was not feasible to really do anything. The builder who I know very well just said he would not do it and suggested a new build... that left me cold as I really don't want to go into the boat building business... which I could foresee might happen.

Then thinking on it... how often would I use the thing... the fight getting a berth nearby... and when I wanted to spend some time out I would just be happier laying on the deck watching the sails flap or napping after lunch on a heavy displacement motor boat gliding along towards dinner time... and a single engine chugging away would make not difference to me... might even lull me to sleep.

Well Oldboater and NYCAP123 are correct.

But now the issue really comes down to personal preference. It appears a larger displacement boat single is really fine... twins to those wanting them or needing them for their use.
Of course, if you are talking diesel electric... that is a different animal.

But the question of a faster than displacement speed boat... well that is a big question. And, I think that has some technical challenge because it is uncommon... and therefore not as much is known. Though I could make a very good case for a gas turbine water jet drive.

Comment on diesel electric later... but you could have one engine / generator driving several drives.

Also, no one has mentioned the benefit of protection that a single shaft protected by the keel has!

Thanks K1W1...

Yes, you are right on that NYCAP123... few really consider bottom strikes and full keels are rare on high speed planning boats.

Actually, have experience with bottom strike late at night off of St. Thomas, VI... we were anchored out and the large tender struck and took 2 blades off the (single 3 blade prop).... long slow bad ride back about 2 AM to 4:30-5:00AM from what I heard... was some of the crew coming back drunk after a few too many zombies... the driver only had three~~~
Could not get a prop right away and some other damage and the vibration even at idle would shake things pretty good too so ended up hiring tender services the rest of the trip as the ladies did not much like clambering in and out of the small tender. I could not stop laughing when I heard the horror story told me... knew it was all just a big sea story... hard to believe from bleary red eye hung over innocent victims... who knew nothing other than "not me". No way in hell would I go to a twin prop just to spare them that! That reminds me of the time taking an evening stroll on shore... in another tropical island port... and finding the pant less captain hanging off a hotel balcony with a couple half naked local girls helping him out of the predicament... !

Actually came on here to discuss the concept I am thinking about and its getting very late. And, this is my reason in thinking about this subject.

The idea... rolling around in my head is:

It seems to me having all these power sources suppling propulsion, hotel and various hydraulic systems are a waste and overly complex for a pleasure yacht. Sailboats are worse as there is less room for machinery but more need of it to supply all the power sail handling equipment as well as the rest. I had checked out the J-Class, Rainbow... and it had an interesting solution... a hybrid system. This got me to thinking maybe on could have ONE power plant for the whole boat... sail or motor.... lets say 35-60 meters size range (maybe should limit the up size to 50m). This has had me thinking...

So a hybrid of a form...

Have one engine driving a generator or twin generators if efficiency is critical. A large battery bank and power everything off a grid connected to the battery and generator. How the J-boat Rainbow works is a DC main bus with Cyco-phase-converters for each of the loads needing AC. Was on it this year... really impressive.

Therefore, under power and one could take power off the generator and for peak loads the batteries would cover it... under cruising conditions under power.

For top speed runs (seldom used) the two connected generators (remember one engine driving them) would provide the power. One could use one generator of the permeant magnet toothless type where the low load efficiency at 50% load would still be good. In port the generator system would only run to charge up the batteries every 8-16 hours... otherwise would remain off.

The batteries would provide emergency power at reduced speed if needed.

Multiple electric propulsion options are still available... if you want twin props.. no problem... as well as various trolling or maneuvering aids/thrusters.

This is much like an old fashioned diesel electric submarine system... so reliability issues are moot... as we are "on the water" not "under the water".

I think it makes perfect sense with some mods based on maybe the experience other forum members might provide.

Capstick...
Your comment as to efficiency is absolutely true... but at most in an extreme case it would be in the high 20s% improvement. This can be checked against engine spec. However, the large engine runs most of the time at lower output (well so do twins) and this reduces its fuel efficiency somewhat... see boats run mostly at a cruising speed not top speed and the power requirements are about doubled between cruise and top speed... generically.

Actually this makes the case for parallel twins... very poor.... but the case for what we could call series twins.... i.e.: if you had two engines powering generators which could supply two propulsion motors driving the props... then you could at cruise run on one engine at near its most efficient point of operation.... and have a back up. This solves much of the efficiency problem with twins.

Your comment as to redundant reliability of two does not usually apply to a boat. Why is they usually share fuel source and fuel supply system. So to achieve true redundancy you must have absolutely separate twin fuel supply systems... piping booster pumps, filters etc... two separate fuel systems. Also, you would have to have separated fuel tankage. Usually the failure mode on diesel power production on a boat is the FUEL SUPPLY... or the cooling system... so you must have totally separate cooling for the engines. They really are not separate.

But the probabilities mathematically do not work out that two is more reliable than one... one is higher reliability than two.

The comment on nuclear safety is incorrect. The emergency diesel power source is not for supply of power to the grid or the ship but... is for the plant itself... it is basically to provide emergency nuclear protection of the reactor any providing power to run safety critical systems. A series system of two generators as you suggest is less reliable. Strangely... the math works out a system of parallel generators sized to each handle the load is not better than one... hard as that is to intuitively comprehend... when all the factors are considered. But to give you a bone... on ships with two nuclear plants with each having one emergency generator... the numbers work out based on one plant operating slightly better but if both plants are running the numbers work out worse than one.

Ok my reasoning on the proposed single diesel power supply with a parallel battery system like an old fashioned diesel electric sub... seems very much the ideal as to reliability and practicality.

Single Power Source

Let me simplify my idea.

The idea is that a sail or motor yacht needs power to operate.
Sail yachts of a large enough size to use as a "yacht" that many would define as a super yacht need power for sail control, steering, hotel loads and auxiliary propulsion.
Motor yachts need power for steering, hotel loads and propulsion.
Lets call miscellaneous loads such a anchor operation; beach club hatches and boat handling; cranes; and all the other yacht operations are classified as 'hotel loads' for this discussion... really they are as they are convenience or necessity as per individuals thinking. Of course that beach club door not working can sink you just as easily as losing propulsion in dangerous conditions so everything needs to work well to be safe and happy. But the more you have the more that can go wrong.

Powering these load yachts have evolved complex engineering plants. Even sail yachts as their size has outgrown reasonable manning of the yacht causing intrusions on the owner / charters patience and space. I should say classic clipper ships sailed with small crews if one compared them to a man-of-war because of the commercial profit motive. Same on modern yachts both sail and power... owners prefer paying for service crew rather than operations crew... if it takes a large crew to operate a given yacht the space allotted to service crew becomes limited. This has driven sail yachts to evolve complex mechanical sailing plants and both motor yachts to evolve complex power systems to supply the needs of the yacht. Now the fewer grease monkeys and fewer sail handlers means one can have more crew devoted to providing comfort enmities. I for one do not want to be served dinner by crew with greasy or calloused hands and rough bearing from fighting with the machinery and or sails whatever the case.

Also, the space in whatever size yacht you care to consider has not only need to prove crew space but also machinery space. This makes the yacht bigger if you need more crew or machinery... and more expensive to build and operate. If it were practical to umbilical a tender yacht to supply crew space and machinery space this would not be a problem as much. Separate tender or service yacht support ships are common but you cannot easily power your yacht from one or transfer crew at sea between them without losing someone once in awhile.

So really my thread is approaching the space, complication and size issue from the source. The power plant. Having multiple propulsion engines and generator power production engines adds space and complication.. and manning needs. The ideal yacht would have a small magic power supply with infinite capacity which the helmsman could control and maintain by simply flipping a switch or pushing a button to turn it on or off... and it would require no maintenance or attention otherwise. This ideal is like a utopian society were no one works or produces anything but everyone has everything and everything is done for them... perhaps that is in Heaven but not here on land or sea. And, on sea one can sink which complicates matters somewhat. But practically on a yacht who cares if the power system has one engine driving it as long as it provides the needs of the boat and is reliable. If you need two propulsion engines, two generators powered by separate engines and a backup night generator powered by an engine... this all takes up space, requires maintenance and each piece is no more reliable than the next if based on the same technology. Each has the same probability of going out as the next one... so the chance of something breaking down is 5 times higher with the five units than if one unit was used. Now you may have back ups but you also have a broken down unit no matter. And, you have to have spares or hands available to fix it. Unless you are willing to go forward on one engine or one generator until you get to port and a repair. facility... and the attention diverted to the broken down unit is distracting so you have 5 times the possibility of being distracted.

Now herein everyone talks about well if one piece of equipment goes out and you have multiples for backup it is safer. That may be true to a limited extent but not true as each is based on the SAME TECHNOLOGY... look at the Yogi situation one engine went out and then the second went out. Lets say if you have fuel contamination... all will eventually go out unless each is supplied by its own totally independent system and tank. That may be the case on a twin engine outboard motor boat with a separate tank and line to each engine but not often on a larger and more complex yacht. Why there is usually sharing somewhere in the supply system. Also, cooling water even if supplied separately to each power source engine... can like in the Yogi's case be problematic due to sea conditions and operating conditions. It seems to be a law of nature or the sea that if one thing goes out it leads to another. It goes out because the conditions are ripe for it. Most every yacht lost it is one thing followed by another cascading to serious safety issues. And, the crew (unless highly trained) gets quickly confused and overwhelmed by the situation. This happens to highly trained naval crews, and yacht crews are maybe not so well prepared on all levels. One failure of even insignificant nature distracts and this leads to other failures. In bad sea conditions it is worse. Many boats are abandoned because the crew gives up... but are latter found still floating.

The less is more and therefore more safe is not to be underestimated. Each piece of equipment has likelihood of failure. But if fewer pieces of equipment are used the likelihood of something failing is reduced. Its that simple. Having an engine failure is statistically twice as likely with a twin engine boat a boat with one engine. Certainly, under ideal conditions you would have one to get home on with the two engine boat. But if one of the same type of unit fails the likelihood of the second or backup failing too is much high now. Several factors go into this but until the failure happens in the first the probability of losing the second is the same as losing the first. However, once one goes out the probability of the second going out is higher because a condition of failure exists. Beyond purely mechanical of physical situations the failure of one often leads to something else failing due to crew distraction particularly in serious conditions.

I could point famously to the Thresher submarine... it had a failure of a seawater connection due to poor quality of a brazed joint at near test depth... this lead to flooding in engine room... the first serious situation... this lead to blowing the emergency ballast... the emergency blow caused condensation freezing in the airlines suppling the ballast tanks... this lead to crew using emergency reactor power (with reduced cooling support due to the first failure) to power the boat to just below the surface on the planes... the reduced support systems for the reactor resulted in an automatic shutdown of the reactor plant... the shutdown resulted in the boat slipping back down to crush depth. Twin reactors would not have saved the boat. The eventual changes to the system would have... implemented in the sub safe program.

To show how easily things go wrong... and incident I was responsible for points out in any complex mechanical system operated by people things go wrong. We were on twin reactor powered ship. Operating normally... on two reactor plants. I was in charge of the electrical plant... we were doing routine maintenance... and of course plant training excises. The CPO doing the maintenance was only recently qualified on the plant and not perfectly familiar with the emergency reactor fill system. In doing a simple insulation resistance test on a reactor fill pump he shorted a terminal in a control box. This lead to the reactor fill system sequence starting from that point on in the hardwired system sequence. That next thing was a electrically controlled valve opening that because of the then existing operating conditions in both plants (existing for a short time due to the training operations) happened to cause the two reactor plants to have the primary cooling systems cross connected. As the two plants were at slightly different pressure one went low on primary coolant level and one went high. As the CPO worked for me and I knew what the was going on and I happened to be in the enclosed operating space of the first plant... went the reactor operator reported the level going high I also overhead on communications system the reactor operator reporting level going low on the other plant I knew instantly what had happened. I informing the watches... otherwise both reactors would have been emergency shutdown as per SOP. While this was going on the short cleared and but it was too late and the sequence started so it had to be stopped. This was very serious as the indications and procedures were to shutdown immediately and go to emergency reactor fill... which would have been a major disaster. This was an unimagined casualty caused by a simple mistake. If we had been in serious conditions or I had not realized what had happened right when it happened and been in the right place we would have been in deep merde. What happened... it was covered up by the Captain... I was against this as it was needed to be known for future accident prevention... but so are the perils of bureaucracy. But we would have lost both reactor plants for a simple oversight.

No yacht is as well designed or protected... no yacht crew is as well trained... do you think if twin engines were safer or more practically reliable than one would not the commercial fishing fleets of the world be running with two rather than one engine... and they often operate in conditions no yacht would want to be out in.

Also, the ability to maintain and operate is better with few units or less complications than more. The need for spares is less... and there is less diversity complexity. One must not forget if you are on a one engine boat and it goes out you fix it before going on. On a twin engine boat you might not. That means you are operating in a less than ideal conditions making it more likely of another problem and reducing you safety margins if you have a problem. With the addition problem of being distracted by the first issue.

What I am wondering if maybe we have over complicated systems and need to reconsider going back to what has been proven in the use of a single engine like most of the commercial world. Also, if we could come up with a system that relies on backups which are not just the same old thing. I think maybe a hybrid with an engine powered power supply with a back up of a storage capacity of energy might just be better than providing two engines or whatever.