Twin Engines/Twin Rudders on a Mono?

[Snowpetrel]

My folks boat has a single offset prop to port, as Herreshoff often designed. It works very well on a long keel hull eliminating the need to a big aperture ahead of the rudder, significantly improving steering. The offset prop is definately more inclined to catch ropes and weed, and in certian condition when motorsailing to windward on the port tack the prop gets pretty close to the surface and can occasionally come out of the water. This is pretty hard on the engine and drivetrain.

I'd imagine this could be a problem with a light wide stern hull shape in similar conditions. I guess you have the advantage of being able to wind back the revs on the windward engine and load up the lee engine.

Having sailed a twin rudder boat to antarctica 3 times I would reccommend avoiding twin rudders for ice work on a narrower hull. The offset rudders were very vulnerable and clouted every bigger chuck of ice that we pushed past with a sickening thud. The boat I sailed had a single centreline prop, and it frequently ate ice. Twin props out wide would be worse in this regard. The twin rudders certainly didn't help the boats handling at sea. She was very difficult to steer downwind and the windward rudder alternately ventilated and lost grip then bit in hard as the boat rolled in a quartering sea.

A sensible way to incorporate these ideas was used by Amer Klink on Parrati 2. He had twin engines and shallow rudders protected by shallow bilge keels. For motoring and shallow water work. On the centreline was a deep swing keel and a deep swing rudder for deepwater sailing. But this has lots of drag.

For me the ultimate would be single strong centreline rudder with a good cassette style emergency rudder. One big diesel with a Hundestat VP prop and a smaller genset aft of the main engine with an inline folding prop just in front of the rudder. Alternately this engine could be electric or hydraulic. I'd love to try a small electric motor for this purpose. Being instantly ready for use on battey power and saving having to start the big diesel for short runs. But avoiding the need for massive battery banks. The e-motion rudder drive system looks like it would be another great way to go for a backup system.

Fitting an outboard bracket to the stern makes sense as your final if it all goes to custard propulsion option.

[Dockhead]

Originally Posted by Snowpetrel

My folks boat has a single offset prop to port, as Herreshoff often designed. It works very well on a long keel hull eliminating the need to a big aperture ahead of the rudder, significantly improving steering. The offset prop is definately more inclined to catch ropes and weed, and in certian condition when motorsailing to windward on the port tack the prop gets pretty close to the surface and can occasionally come out of the water. This is pretty hard on the engine and drivetrain.

I'd imagine this could be a problem with a light wide stern hull shape in similar conditions. I guess you have the advantage of being able to wind back the revs on the windward engine and load up the lee engine.

Having sailed a twin rudder boat to antarctica 3 times I would reccommend avoiding twin rudders for ice work on a narrower hull. The offset rudders were very vulnerable and clouted every bigger chuck of ice that we pushed past with a sickening thud. The boat I sailed had a single centreline prop, and it frequently ate ice. Twin props out wide would be worse in this regard. The twin rudders certainly didn't help the boats handling at sea. She was very difficult to steer downwind and the windward rudder alternately ventilated and lost grip then bit in hard as the boat rolled in a quartering sea.

A sensible way to incorporate these ideas was used by Amer Klink on Parrati 2. He had twin engines and shallow rudders protected by shallow bilge keels. For motoring and shallow water work. On the centreline was a deep swing keel and a deep swing rudder for deepwater sailing. But this has lots of drag.

For me the ultimate would be single strong centreline rudder with a good cassette style emergency rudder. One big diesel with a Hundestat VP prop and a smaller genset aft of the main engine with an inline folding prop just in front of the rudder. Alternately this engine could be electric or hydraulic. I'd love to try a small electric motor for this purpose. Being instantly ready for use on battey power and saving having to start the big diesel for short runs. But avoiding the need for massive battery banks. The e-motion rudder drive system looks like it would be another great way to go for a backup system.

Fitting an outboard bracket to the stern makes sense as your final if it all goes to custard propulsion option.

I think between this very convincing argument, and what Evans has written, and some of the other extremely interesting comments, I have finished thinking about twin engines and shafts and have rejected the idea. Thanks very much for taking the time to write all of this.

[Dockhead]

Originally Posted by estarzinger

mmmm . . . . I am trying (and failing occasionally, like now) to avoid commenting on your 'dream vessel'. It is very personal, and my perspective is so very different.

But I can't resist today

If your objective is either/both (1) a good sailing boat, and (2) a high latitude boat . . . . just dont think about two props, two struts, two shafts, two stuffing boxes. In order of priority . . . That is a huge amount of underwater drag. It is extra weight (down low yes, but still sinking the boat). And it is extra complexity (for haul outs, for annual maintenance, for the design and build). In the incident investigation team, I saw an interesting comparison of the Bounty in original configuration (as a pure sailing vessel), and in 'modern configuration' with two props. It was a massive hit to sailing.

Diesel engines are enormously reliable AND they are super easy for even an idiot to work on. Don't waste even a single neuron to thinking about 'engine redundancy'. This all true also of the electronic common rail systems - although there part of the process can be the idiot simply swapping in a spare black box.

Following that up - personally I believe in (a) simplicity, (b) best gear, (c) correct and super accessible installation, (d) spare parts and tools . . . . rather than redundancy. Eg do it right.

That broker boat morning star - I look at it and very honestly my initial gut reaction is 'what a nightmare'. That hard Bimini in a storm at sea . Trying to short tack in light winds . Trying to get upwind in a gale . Trying to troubleshoot a problem, when there is most probably a forest of black boxes, pumps and motors hidden in pretty much ever corner of the boat . I doubt she sails on her design lines . It may well be someone else's dream boat, but not mine (and obviously also not the guy who commissioned her).

I really dislike the 60-65' size range. Owners think they are getting a 'big' boat and cram way too much stuff into it. And they become too big for the owner and too small for pro crew. Dashew's personal boats were 80' for a reason, and it was not big crews (their layouts were really designed for two people), that 20' buys you a lot in terms of space to lay out mechanical equipment. The above boat might work at 80' but would be better at 100' - honestly it looks to me like the guy chartered a bunch of big boats and made long lists of clever things (or things that looked clever sitting in an anchorage) and then hired a NA with the mission to 'cram all this in the smallest possible boat'.

I have rarely seen a 'motor sailor' that sailed well. If they get big enough a sailing focused boat can motor well, but, at least in my experience, that is more like in the +72' range (the oyster 72 motors pretty well . . . but as an aside their machinery space is way way too small).

edit - oh, btw, there are pretty good reasons for integral tanks on aluminum boats (except black tanks). For water tanks you do need to be a bit careful with coatings. And while most NA's prefer bare aluminum in the diesel tanks, I prefer coatings (different than for water) in there also. For both you want to design good access, which includes easy access thru the interior that is placed on top (for some reason, the NA's so often put good aluminum access ports/plates, but forget about all the interior built on top of the tanks).

And yes, it is your dream . . . .so you can file this in the circular file - lol

I always pay a great deal of attention to what you write, particularly on subjects like this -- you have actually, practically done a lot of the stuff I'm planning to do over the next 10 years, and the benefit if your experience and of your thought is incalculably great.

As always, I've learned a lot from this post, and as I wrote above, I've now abandoned the idea of twin engines and shafts.

I get what you write about simplicity, but there is a tension between simplicity and redundancy which has to be balanced with the greatest of care, don't you agree? Redundancy has saved by azz innumerable times with various systems, but also tools, skills, and spares -- as you wrote. Both are required in various combinations, with various systems. I've never had a propulsion engine just fail on me while under way (knocking on wood here), and I understand that diesel engines are among the most reliable devices we have on board provided we have the fuel system sorted.

But wouldn't a wing engine still be a good thing to have, if it does not mean an extra diesel engine?

My idea, which I keep coming back to, is to radically simplify the generator by using a propulsion engine driving a high output alternator (EcoTec etc.), instead of a packaged, complex, and hard to repair AC generator. If the alternator is the same as the one on your main engine, then you can carry a spare covering both and you have a huge guaranty of ability to generate power.

I guess you could do away with the second diesel engine altogether and just use the main engine driving a big enough alternator (or gang of them) to give it a reasonable load -- if you're charging LiFePo batteries, you can do your charging in short, intense runs which are much better for the diesel.

That's probably worth thinking about.

But the alternative is to use the second diesel engine as a wing engine. So how about a simple retractable gear like used on Open 60's? Adds a small amount of complexity, costs some money, but weighs very little (all carbon), causes almost zero drag, and won't snag anything when retracted.

What do you think about that? I guess that whole unit will be similar in weight and complexity (or maybe rather less complex) to a regular heavy duty AC generator like what I have now, yet it is much easier to repair and serves an additional completely different function -- backup mechanical propulsion.

What do you think about that?


Concerning size -- your comments are very useful. Having been on some 60 to 65' boats, including a Sundeer, I don't agree that this is an inherently bad size. But I do agree that at this size, we still do have a lot of tension between adequate mechanical space and desired accommodation space. Sundeer is basically a 45' boat with hugely enhanced mechanical space and deck storage (a brilliant approach IMHO). It may be that I'm chasing something I can't catch by hoping I can have accommodation not so much worse than what I have now (54' boat optimized for accommodation space, like most modern cruisers).

I do need space for 4 or 5 people on board -- I don't plan to cruise as a couple -- I'll either be single handed or with a full crew.

I do plan to have professional crew. Not the usual captain and steward, but an engineer/mate, conceivably a couple. I had someone like this my first couple of years on this boat, and it was fantastic, and I plan to go back to this as soon as finances allow. So I need at least three sleeping spaces (I have four presently).

Can I do this, and still have an excellent mechanical space (a la Sundeer), in a 65' boat? I don't know -- I guess I will need to hire the NA.

Could I go to 70' or 80'? Well, if it were necessary. I spent time on a friend's 90' Swan, and I don't like that size -- it's definitely too much for two or three people, very expensive to run, impossible to berth in most recreational vessel marinas. 65' will also exclude me from many normal marinas, but it's definitely not too much to single hand, especially if it's a ketch or in any case, if it has a modestly sized rig like I plan to have. It's only about 10 feet more than what I have now, and I find 54' to be no problem at all and actually very lovely when single handing as the platform is so much more stable than a smaller boat. 75' would be just a little too much, I think, and I do need to be able to single hand the boat. So I'm going to try to keep it to about 65'.

As to custom building -- this will obviously cost me double or more, compared to just buying something, someone else has made. So my mind is open to buying an existing boat, if I can find something which ticks at least most of my boxes. So I keep looking and looking.

[a64pilot]

I have commented several times mostly as a lark, sort of if wishes were horses kind of thing.
However if your really going to do this, the one thing I leaned from designing and manufacturing simple single turbine engine aircraft for 15 years is something I already knew as a mechanic, its KISS.
I say it differently though, I say if it's simple and works, it's not stupid.
You need to be very aware of two things.
1. What I call creeping elegance, that is in the design and prototype phase you will often think of a better, more elegant way to accomplish something. However it all adds time and money, and often weight and complexity. It's tough to resist though, and often is what kills projects.
2. What I call the good idea cut off point, AKA the time to shoot the Engineers and just build the thing. Cost continue to escalate during the planning phase and often great projects die simply to running out of money.

Then lastly, of something has always been done one way, but you think you know a better way. You really need to try to consider why your way has not been adopted long before. There usually is a very good reason, you just haven't discovered it yet.
The joke in aircraft design is the Engineer says it looks great on paper, the answer is of course too bad we don't fly paper airplanes

I suspect you can substitute boats for airplanes

[TJ D]

Hi Dock,

I think that for truly remote sailing, especially if you're going to be headed to polar regions where you REALLY depend on the engine, some kind of a redundant propulsion setup would be high on my list, if you can swing it. A wing engine like on Dashew's FPB's, even better if you can do a recessed prop.

Yes, diesels are spectacularly reliable, for sure, but they do still break (or the reduction gear, or the shaft, or the prop, etc).

In my working life, I've had occasion to tow in several vessels with broken propulsion systems. These are big, commercial boats with exceptionally good maintenance, and they still wind up disabled from time to time. Mostly, it's gearboxes and shafts, as it's possible on most of these big engines to disconnect a failed cylinder to limp home.

You might be interested to learn that every single boat that I've been involved in towing due to a breakdown (or for that matter every boat that I can remember being towed by anyone) was a single screw vessel. I'm a big fan of twins.

Twin rudders? Probably not worth it. But, I certainly don't object to RS's arrangement. Yes, they're spade rudders, and vulnerable to get hit, but their skins are nearly an inch thick, solid carbon. The rudder tubes are beefier still. I think that they'd survive a pretty big hit. But, I wouldn't dream of taking RS into pack ice, either.

TJ

[Dockhead]

Originally Posted by a64pilot

I have commented several times mostly as a lark, sort of if wishes were horses kind of thing.
However if your really going to do this, the one thing I leaned from designing and manufacturing simple single turbine engine aircraft for 15 years is something I already knew as a mechanic, its KISS.
I say it differently though, I say if it's simple and works, it's not stupid.
You need to be very aware of two things.
1. What I call creeping elegance, that is in the design and prototype phase you will often think of a better, more elegant way to accomplish something. However it all adds time and money, and often weight and complexity. It's tough to resist though, and often is what kills projects.
2. What I call the good idea cut off point, AKA the time to shoot the Engineers and just build the thing. Cost continue to escalate during the planning phase and often great projects die simply to running out of money.

Then lastly, of something has always been done one way, but you think you know a better way. You really need to try to consider why your way has not been adopted long before. There usually is a very good reason, you just haven't discovered it yet.
The joke in aircraft design is the Engineer says it looks great on paper, the answer is of course too bad we don't fly paper airplanes

I suspect you can substitute boats for airplanes

This post is jam-packed with wisdom, and I hope others will read it and gain from it also. Thanks for taking the time to write this.

In my opinion, everything in here is absolutely right. I've learned some of it the hard way -- building stuff, working with architects. I had a really bad experience designing a lake house which was supposed to be my ultimate dream house, on an almost unlimited budget at a time some years ago when I had a lot more money than I have now, and when I still cared about houses and land life (that passed ).

It was kind of a disaster. The lack of budget limitation made "creeping elegance" (I love your phrase) turn into "racing elegance". I interfered so much with the architects that I finally had to throw the first design away and start all over again with other architects.

This boat which I'm designing so far in my head is still just a dream, as I don't have the money to build her (yet). So I'm trying to take advantage of having a little time (while I'm earning the money ) to think through all of these issues, to consider and dismiss things like twin propulsion, which I have now dismissed with the help of all you guys.

To work KISS back into the design brief after considering and dismissing different elements.

It might even be, with this particular question, that I will come all the way around to just eliminating the second diesel engine altogether and just go with massive alternators on the main engine. That would eliminate a redundancy that I have concretely needed in the past -- pretty scary. But imagine how that will unclutter the engine room and simplify different systems. And save weight.


And the last thing you wrote, might be the wisest of all -- be modest and pay a lot of attention to what other people have done -- don't assume you know better. Words to live by, A64

[Dockhead]

Originally Posted by TJ D

Hi Dock,

I think that for truly remote sailing, especially if you're going to be headed to polar regions where you REALLY depend on the engine, some kind of a redundant propulsion setup would be high on my list, if you can swing it. A wing engine like on Dashew's FPB's, even better if you can do a recessed prop.

Yes, diesels are spectacularly reliable, for sure, but they do still break (or the reduction gear, or the shaft, or the prop, etc).

In my working life, I've had occasion to tow in several vessels with broken propulsion systems. These are big, commercial boats with exceptionally good maintenance, and they still wind up disabled from time to time. Mostly, it's gearboxes and shafts, as it's possible on most of these big engines to disconnect a failed cylinder to limp home.

You might be interested to learn that every single boat that I've been involved in towing due to a breakdown (or for that matter every boat that I can remember being towed by anyone) was a single screw vessel. I'm a big fan of twins.

Twin rudders? Probably not worth it. But, I certainly don't object to RS's arrangement. Yes, they're spade rudders, and vulnerable to get hit, but their skins are nearly an inch thick, solid carbon. The rudder tubes are beefier still. I think that they'd survive a pretty big hit. But, I wouldn't dream of taking RS into pack ice, either.

TJ

Thanks, TJ, that's really useful.

A good sailing boat already has redundant propulsion -- the sails. The problem is that in polar regions you may have dead calms for a month at a time. That was what originally inspired me to think about this.

After all the exceptionally good advice I've gotten in this thread, I think where I'm at is here:

Either:

1. Take KISS to its logical extreme and have only one diesel engine. No point in having the second diesel if it can't do propulsion work.

OR

2. Have the second engine drive a retractable shaft and prop, like on the Open 60's, besides a pair of EcoTec jumbo alternators.

I first saw this on an Open 60 laid up here in Cowes:






It's all carbon and I'm sure pretty expensive, but would seem to be the ideal wing engine drive for a sailboat.


Sturdy carbon short twin spade rudders like you have on Rocket Science have a lot of advantages. But I think with a single screw, the drawback in maneuvering would be a killer for me. I don't know how you dock RS in tight places -- you've got bigger balls by far than I do. And the possible vulnerability to fishing gear and ice, although I think we have to keep in mind that the fact that twin rudders are shorter and stronger might compensate the fact that they are not behind the keel.

[Snowpetrel]

^^ neat, but it violates the KISS principle pretty serously. I quite like the idea of an independant backup propulsion system, but less underwater moving parts would seem less problematic long term. Prehaps a hydralic drive with a folding prop in the back of the keel or a saildrive behind the main prop?

[TJ D]

Originally Posted by Dockhead

Thanks, TJ, that's really useful.

A good sailing boat already has redundant propulsion -- the sails. The problem is that in polar regions you may have dead calms for a month at a time. That was what originally inspired me to think about this.

After all the exceptionally good advice I've gotten in this thread, I think where I'm at is here:

Either:

1. Take KISS to its logical extreme and have only one diesel engine. No point in having the second diesel if it can't do propulsion work.

OR

2. Have the second engine drive a retractable shaft and prop, like on the Open 60's, besides a pair of EcoTec jumbo alternators.

I first saw this on an Open 60 laid up here in Cowes:

Attachment 157034

Attachment 157035


It's all carbon and I'm sure pretty expensive, but would seem to be the ideal wing engine drive for a sailboat.


Sturdy carbon short twin spade rudders like you have on Rocket Science have a lot of advantages. But I think with a single screw, the drawback in maneuvering would be a killer for me. I don't know how you dock RS in tight places -- you've got bigger balls by far than I do. And the possible vulnerability to fishing gear and ice, although I think we have to keep in mind that the fact that twin rudders are shorter and stronger might compensate the fact that they are not behind the keel.

Hi Dock,

We're on the same page on the sails being propulsion-when there's wind. I was talking specifically about polar cruising when I called engine propulsion REALLY important. If I were heading for the NWP or someplace like it, I would spec 2 mechanical sources of propulsion (I would go with diesel-electric for the wing engine) if I could. I don't think that sails should be considered as useful for the most part.

I don't have a lot of interest in cruising in ice (I do enough ice at work to not make me want to vacation in it, thank you very much...), but the occasional accounting I read about NWP transits- there's little sailing being done. I think that a broken propulsion setup would be an existential threat to the yacht on this passage.

So, we agree, I think.

My boat's pretty much a nightmare to maneuver in a tight spot-I have to come in very fast to have any control. No thruster, no prop wash on the rudders... She's made me look dumb more than once. Fortunately, because of her size, we wind up in berths with decent approaches most of the time. Definitely go BIG on the thruster(s?) if you do happen to end up going the twin rudder route.

Jenny and I had the budget a few years ago to either re-Awlgrip the boat or install a retractable thruster. We (Ok, I did, over objections) decided on new paint. So far, we've put in 2 small dings and 3 small scratches while docking. So, I guess I've gotten used to handling RS. I can tell you that the speed we're carrying into docks does cause some horror on the dock. If you've ever seen 'Captain Ron', where he stuffs the boat at high speed into a slip in a big cloud of smoke while onlookers flee for their lives... That's pretty much us! You get used to it...

TJ

[Dockhead]

Originally Posted by Snowpetrel

^^ neat, but it violates the KISS principle pretty serously. I quite like the idea of an independant backup propulsion system, but less underwater moving parts would seem less problematic long term. Prehaps a hydralic drive with a folding prop in the back of the keel or a saildrive behind the main prop?

Aren't both of those more complex, than the rather straightforward retracting drive? And more draggy? The retractable drive even has a simple fixed prop.

[a64pilot]

Being somewhat new to sailing myself I don't know much.
But I believe the first steamships with props way back when were often built with retractable props to reduce drag as they were primarily sailing vessels. I say that as if it could be done way back then, then likely it can still be done
Personally I believe that the drag of a folding prop isn't enough to warrant making it retractable.
I know nothing at all about ice, but my gut says build Hell for strong and accept the drag associated with it and forego fancy expensive carbon fiber etc.
I think ice pretty much rules out folding props though? Assumption is anything has to be strong enough to chew through ice occasionally?

[Dockhead]

Originally Posted by a64pilot

Being somewhat new to sailing myself I don't know much.
But I believe the first steamships with props way back when were often built with retractable props to reduce drag as they were primarily sailing vessels. I say that as if it could be done way back then, then likely it can still be done
Personally I believe that the drag of a folding prop isn't enough to warrant making it retractable.
I know nothing at all about ice, but my gut says build Hell for strong and accept the drag associated with it and forego fancy expensive carbon fiber etc.
I think ice pretty much rules out folding props though? Assumption is anything has to be strong enough to chew through ice occasionally?

I don't know. I want a Hundested variable pitch prop, which unlike my Autoprop, is pitched manually. It looks hell for strong to me, but I don't know whether it's as strong as it looks. The smallest one they make requires 150 horsepower, so that will be the main engine size, which will make the boat slightly overpowered (like it want her to be) if she comes in at 25 tonnes or so light ship.

I do want the prop behind the keel, for some protection.

[Snowpetrel]

Originally Posted by Dockhead

Aren't both of those more complex, than the rather straightforward retracting drive? And more draggy? The retractable drive even has a simple fixed prop.

Last year a Round North Island race yacht had a system like this jam and cause issues.

The way I figure it, if the racing boys can't make a retractable drive reliable with there regular bottom scubs and frequent use, I don't see how a cruising boat thats slipped only every year or two is going to manage. Things like weed growing inside the case and corrosion on the external universal joint are issues.

The retractable drive system is unproven, cutting edge technology that even most of the highest tech racing boats don't yet use. Just the combination of carbon fibre and aluminium underwater raises Galvanic corrosion concerns. And how would a system like this cope with freezing?

Folding props are well known and proven. Hydraulic drives (not a hydralic gearbox, a proper hydralic system driven of the engine) are also well proven. A mate has one coupled to a perlins 4108, it has worked flawlessly for 35 years or so, and on a bigger boat the hydraulic system could also run a bow thruster and or anchor winch if thats the way you choose to go.

Another get home system that may work is something like one of the new diesel outboards. I think they are 30 hp or so, so they would drive a decent sized dinghy and do backup as a reserve engine om a suitable mount. Of course you could just use your regular OB and carry extra petrol.

The problem is that in heavy ice a real mistake like getting caught in heavy pack moving or driven on to shore by ice and wind will quite likely take out rudders and propeller even if they are strongly built. Hence the benefits of things like outboards and emergency rudders that can be stored inside the boat.

[Panope]

I just occurred to me that a wing/get-home engine installed on a motor yacht should be capable of driving the boat against fairly strong wind. Conversely, a wing/get-home engine installed on a SAILING yacht needs to be capable of driving the boat in calm wind ONLY.

That makes Snowpetrel's outboard motor idea that much more compelling.

Steve

[Sailmonkey]

What about a custom built hybrid like this?

https://www.westmarine.com/buy/maste...stem--11419801

This allows you to keep a single shaft, the. Coupled with a purpose built/sized generator you would have back-up motive force indirectly from a second Diesel engine.