Single engine / dual shafts?

[dreamer]

Title pretty much says it all. Would this configuration make any sense?

Advantage:

Maneuverability and weight distribution.

Disadvantage:

Only one engine! ;)

[philSweet]

If the engine block is an inline six and you lay it on its side and stick six pistons in from both sides with porting in the middle you can do pretty well.

[PAR]

There's a number of ways you could do this, but it's not practical. Frictional lose increases, additional running gear drag, power separation loses, etc. would kill any advantages. You could spin a couple of shafts off the same transmission, but you'd lose all the benefits of having twin wheels.

[apex1]

There is no advantage doing that. Spinning two props means cranking two engines.
And, (not to offend) the cheapest and the best way to increase a vessels maneuverability is exercise! Learn to handle a single prop boat like all the fishermen worldwide had to learn.
Regards
Richard

[dreamer]

There's a number of ways you could do this, but it's not practical. Frictional lose increases, additional running gear drag, power separation loses, etc. would kill any advantages. You could spin a couple of shafts off the same transmission, but you'd lose all the benefits of having twin wheels.

I was actually thinking two CV (constantly variable) transmissions. You could vary the thrust of each output shaft or reverse one for turning.

Agreed the frictional losses would be an issue. Perhaps there'd be a way to do this with some sort of torque converter. Maybe that would help reduce friction?I suppose such a setup would not save any weight over two engines either.

Probably the only advantage is that it might be a little cheaper.

Just thinking out loud. This place is good for that.

[apex1]

Nothing wrong with thinking loud. But that sort of arrangement has no advantage you would otherwise find it in one or the other commercial installation. And bringing more components into a system is usually counterproductive due to higher failure rate, higher service efforts and cost and the like. There is a reason why commercial ships run one engine only (not the cargo ships they do just to save money), the less stuff to service, the more likely it will be done, the more reliable the installation will run.
Regards
Richard

[CDK]

Going through all the trouble and cost of installing two engines is rewarding only in case of an engine failure. The added manoevrability is a nice extra, but there are cheaper ways to achieve that.

[apex1]

Going through all the trouble and cost of installing two engines is rewarding only in case of an engine failure. The added manoevrability is a nice extra, but there are cheaper ways to achieve that.

I mentioned the cheapest.;)

[StianM]

Two props and one engine would be quite complex with gears.
I think if this should be done it should be solved diesel electric.
1 generator and two frequency converters and two motors driving fixed pitch propellers.

A solution like that would make it quite easy to later expand it with a second diesel for reliability and adding AC driven bowtrusters.

[mydauphin]

I rather have two engines (one large, one small) with one shaft... Still hard to do..

[dreamer]

let me step back a little.

What I would like to do is keep as much of the weight near LCB as possible and as low as possible. This is easier to achieve with a single engine than two engines, however one can't deny the advantages of dual props in terms of maneuverability. A continuosly variable transmission on each shaft would not be too complex for this type of application and would allow for variability between the two props for tight maneuvering. The setup would be similar to a transfer case on a 4 wheel drive vehicle.

the best solution would be as Stian mentioned and have two motors, each attached to rotating prop housings (as seen on modern tugs), but that puts a lot of weight in the ends where I don't want it.

Most likely I'll fall back to dual electric motors in a traditional, shaft driven setup.

At this point, not so interested in 'cheapest' in terms of initial investment. More interested in forward-thinking means of propulsion. Diesel electric isn't it either, though diesel generator + batteries would work in the short term.

[mydauphin]

The number of shafts, props and engines have a much to do about the hull design and it uses. Not all hulls can be made single or dual. Single allows for narrower hulls, and greater efficiency in keel and rudder. But duals are better for shallower drafts and higher speeds. Form follows functions.

[apex1]

Due to the fact that there is no such gear available "off the shelve" and that there are a lot of good reasons why that is so, I would give up the idea.
The "rotating prop housings" are named "POD" drives or "Azipod", they must not be installed under the transom or at the aft end, but one of them is as much money as the whole conventional engine installation. And they add notecible draft.
The "forward thinking methods of propulsion" are interestingly the steps back to good old technology as a single Diesel and a CPP for example, combined with a articulated rudder, you have a very sophisticated and economical driven propulsion that lives forever. The Diesel Electric way is not the way to go at present and bringing batteries and inverters into the game makes it even worse.
Regards
Richard

[Guest625101138]

The majority of boats are draft constrained meaning the props are smaller diameter than that required to give best efficiency. The possible exception is tugs where the prop is the most important part and the hull is designed to accommodate whatever is required.

When prop diameters are constrained by draft there is some benefit in splitting the installed power between two props of similar or larger diameter than a single prop installed on the centreline.


Taking an example of a 26 x 16 prop running at 1500rpm and pushing 10000N at 10kts the efficiency is 60%. Splitting the thrust between two 26 X 16 props drops the required engine speed to 1200rpm and efficiency lifts to 70%. So the result gives a 16% improvement at the prop. This gives quite a lot of room to play with extra losses from the gearing and additional appendages. There is a reasonable likelihood of overall performance benefit in certain circumstances.

Rick W

[mydauphin]

Both Apex and Rick are right. After much consideration. I went with dual props, for draft reasons. Cost was similar between two smaller props/shafts and one single big one. Also get redundant engines, tranny and props. Unfortunately efficiency drops. But for coastal cruiser I think it is better. For max efficiency go with single prop. Diesel/electric good for cruiseships casino ships and military that need lots of power.

On another note, I saw a Lazzara 80 something at Miami Boat Show. After a impressive maneuvering demonstration, I ask salesperson, "What happens if you hit something with pod".... He answered,"That is what insurance is for...." We are going to see many of these guys stuck on the rocks when they become plentiful.

[ben2go]

What about using hydraulic pump and motors?Never done this on a boat.

[apex1]

Because the noise will drive you nuts before you cross the port entrance. And hydraulics leak sooner or later (usually sooner).

[Boston]

the basic father and son rig running one screw was not uncommon on many of the old lobster boats we worked on
kinda the oposite of what your original question was but its fuel efficient
the maneuverability issue was handled beautifully at the start by someone when they basically said
learn how to drive

if you had one huge engine and enough torque and extra power to run twin screws you would be wasteful enough already
but adding two transmissions and a transfer case and your going to be selling the children to pay for fuel

on the other hand
to each his own
if its really what you want go for it
if you placed the screws far enough apart you just might be able to do a spin

cheers
B

[ben2go]

Because the noise will drive you nuts before you cross the port entrance. And hydraulics leak sooner or later (usually sooner).

I can agree to that.

[Jim_Hbar]

Dreamer:

I've pondered the advantages of the drive system you have proposed, and although the hardware might not exist at this time, adaptable components may be available in the near future. Google "toroidal cvt".

I believe one large potential benefit not mentioned in the above posts, would be the possibility of eliminating rudders and the associated appendage drag. And the second would be the ability to load the engine into an efficient operating range. Both of these items would allow the vessel to operate at significantly increased efficiencies.

Will it be a system for everyone, and all applications? Not likely, unless it becomes fully developed by one of the majors. But if fuel becomes expensive enough, and the CVT technology matures in the automotive market, the economics may change sufficiently to warrant the capital and maintenance costs associated with such a complex mechanical system.

Just my 2 cents.

Jim

[apex1]

Dreamer:
I believe one large potential benefit not mentioned in the above posts, would be the possibility of eliminating rudders and the associated appendage drag. And the second would be the ability to load the engine into an efficient operating range. Both of these items would allow the vessel to operate at significantly increased efficiencies.
Jim

Well Jim, how would you eliminate the rudders? And why? A twin engine twin prop system does not eliminate them (for about 57,5 good reasons).
And if you want to load the engine sufficiently (a very good idea with a Diesel) you should use a common and very reliable CPP instead.
There is just NO need for a system with a single engine two props!
Regards
Richard

[Boston]

gotta go with Apex on that one
you would find yourself wanting a rudder in about the first trip out of port when you had one running full reverse and one full forward and you were still heading towards the breakwater

thing you are after if you want to eliminate the ruder is vectored thrust

[ben2go]

Just tossing out ideas,but how bout a single prop with a kitchen rudder.This will eliminate the need for a transmission.You'll still need a reduction box if your engine spins above a certain RPM.A kitchen rudder gives reverse,neutral,side thrust,and forward thrust without having to adjust engine speed.

http://www.proboat-digital.com/proboat/200608/?pg=63

http://www.boatdesign.net/forums/boat-design/kitchen-rudder-418.html

http://en.wikipedia.org/wiki/Kitchen_rudder

http://www.oldmarineengine.com/discus/messages/5/1010.html

[ben2go]

Forgot to add that one engine and two props will not give more thrust.The drag plus the loss of powering two props and gear will cut down the power at the props.It will kill effieciency also.The engine will have to work twice as hard to turn two props and over come the extra drag of the gear.

[Boston]

Kitchen ruder = vectored thrust
course I had never seen a kitchen ruder before other than on a jet turbine as a brake
but that kitchen ruder is exactly what vectored thrust is

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in this next video you can clearly see a type of Kitchen ruder being applied to a jet engine as a braking mechanism

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[apex1]

Thats all very nice mates, but we can discuss Azipods, Kort Nozzles, Voith Schneider propulsion and the like a several hundred times more. I do´nt see a benefit in a single engine two shafts installation, thats all! And thats all what originally was asked for.
Regards
Richard

[Jim_Hbar]

Richard:

One engine driving two final drives has been used for years, just not in mainstream marine propulsion applications..

And if one was to attempt that in a marine application, then CPP's would likely be the prime candidate, at least initially. They have the advantage of going from forward through neutral to reverse that is absent from the forms of CVTs that are used in the automotive world (and would likely be adapted to the marine world , if it ever occurred).

IIRC, Yellowfin proposed the two drive, surface piercing, CPP with rudderless control - and AFAIK, proved that it was not economically viable at this time also. Unfortunately, much of the content that was available on their web site is no longer up, but others on this forum are very familiar with their offering - perhaps they will chime in.

Dreamer, the thread starter, originally asked about CVTs. I was staying on topic, and my post was directed to him specifically. The point I was attempting to make was that such a system is likely not commercially viable at this time. But "Can" and "Should" are two different questions. I am attempting to provide input to the "CAN" question.

There are other threads on this forum that are discussing many things outside of the mainstream - ie: Boston's thread on external combustion engines, or the old DDWFTTW thread.. And in many, people jump in and say - "That's not the way we do it - it can't be done!"

And responding to your summary - There is just NO need for a system with a single engine two props! - I would add there is "just NO need" for twin engines with twin props, or boats that plane, or even recreational boats that use anything other than paddles or sails.

Jim

[apex1]

Richard:

One engine driving two final drives has been used for years, just not in mainstream marine propulsion applications..
That is just a plain lie[
And if one was to attempt that in a marine application, then CPP's would likely be the prime candidate, at least initially. They have the advantage of going from forward through neutral to reverse
I am shure , I am the last one here who must be told how CPP function.

There are other threads on this forum that are discussing many things outside of the mainstream - ie: Boston's thread on external combustion engines, or the old DDWFTTW thread.. And in many, people jump in and say - "That's not the way we do it - it can't be done!"
I regularely contribute to these threads, thank you!

And responding to your summary - - I would add there is "just NO need" for twin engines with twin props, or boats that plane, or even recreational boats that use anything other than paddles or sails.
words
Jim

Well, Jim one question was not answered in your post: how and why would you eliminate the rudders? Pod´s are out of the race already.
Regards
Richard

[Jim_Hbar]

Richard:

Rather than resorting to personal attacks and implying I am a liar, I suggest that you go look at a tracked machine, whether it be a crawler tractor (CAT), or a tank, for an example of a prime mover driving two final drives. One could even consider an automobile to normally have at least two outputs, although they typically do not control the torque distribution to the driving wheels, so are not a good example in this case. All have two (or more) power outputs from a single engine..

I believe this "boat" (http://www.youtube.com/watch?v=eaGLNcIkSm0) works without rudders also, two final drives, one engine..

I've only stated my opinion, that I believe the CVT approach is not commercially viable at this time, and mentioned the example of the Yellowfin, which I understand is/was a rudderless system. You have ignored that in your reply. Personally, I'm not interested in engaging in pissing matches on forums, I have much better uses for my time.

The "Why" for eliminating the rudders? Reducing the appendage losses of course.... :D Is it possible? In some applications, perhaps.

I gather you want your name taken off of the pre-order list??:(

Jim

[rasorinc]

Price out your transmission costs for 1 engine 2 props. Do that and you will desire 2 engines. You are insulting folks that build many yachts and you are going on an idea only---not experience. We try to help here and if you do not like the help then just go to other places. that is a simple solution and does not require put downs. I'm just a simple small wood boat builder and try to give good advise-- that's all.

[apex1]

Richard:

Rather than resorting to personal attacks and implying I am a liar, I suggest that you go look at a tracked machine, whether it be a crawler tractor (CAT), or a tank, for an example of a prime mover driving two final drives. One could even consider an automobile to normally have at least two outputs, although they typically do not control the torque distribution to the driving wheels, so are not a good example in this case. All have two (or more) power outputs from a single engine..
Sorry are you mad? This is the boat design forum and the question was to propel a boat, no tank, no helikopter. And my reply to your post was naturally related to that fact.


I gather you want your name taken off of the pre-order list??:(

Yes please

Jim

What are we talking about here?

[Jim_Hbar]

Richard:

I believe the initial point we were discussing were CVT's and and the possibility/merits of using them to drive two fixed pitch propellers.

BTW, I am not trying to be argumentative, but I respectfully suggest that you are not reading the intended meaning of my words. I'll try to get my point across again.

In my initial post, I stated, (with emphasis added):
I believe one large potential benefit not mentioned in the above posts, would be the possibility of eliminating rudders and the associated appendage drag. And the second would be the ability to load the engine into an efficient operating range. Both of these items would allow the vessel to operate at significantly increased efficiencies.

Will it be a system for everyone, and all applications? Not likely


In my second post I stated (emphasis added):
One engine driving two final drives has been used for years, just not in mainstream marine propulsion applications..

And if one was to attempt that in a marine application,

Furthermore, I've mentioned the Yellow Fin drive system, and provided a link to video of the FastTrack amphibian.. Both of which I believe to be rudderless, differential steering drive systems with one engine, and two final drives, in marine applications (which I believe we can agree on are not "mainstream"). I additionally provided other examples of one engine with two final drives, and you, a few more, which are not "mainstream marine propulsion". The point I'm attempting to make is that the technology exists, but is not currently used by the mainstream marine industry.

I would define "differential" steering to be where the thrust of the two outputs are varied in a precise, controlled fashion, to effect steering of the craft.

The one question you have not asked is where I believe there is a possible (note I used the word "POSSIBLE", not "probable", not "certain", not "future") application of such a drive system? I can see it having theoretical technical merit in a high speed surface-type drive system, in a relatively small light craft.

I would suggest that having precise control of the relative thrust from the two prop shafts is necessary for differential steering - this can be accomplished by several means, one of which could be twin CVTs, another is twin CPPs, a third way would be a twin engine set-up with closed loop control of the engine(s) speed. There are other ways to accomplish this also.

The FastTrack video shows differential steering - it could be done in a similar fashion if the final drives were propellers instead of tracks..

The benefits of the single engine, twin prop approach, as I see them, listed in approx. order:
1) The ability to swing two props, with twice the "disc area" of a single, with similar or reduced draft, and the resultant prop efficiency gains - see Rick's post #14 above.
2) With any variable drive system, it then becomes possible to load the engine for better engine efficiency in normal operating ranges.
3) With two CVTs, (or CPPs), it then becomes possible to differential steer the craft at speed. And with differential steering, rudders are possibly not required, particularly at speed.
4) With CVTs and fixed props (compared to CPPs), the propeller efficiency is potentially better across the full range of speed, and surfacing drives are a possibility. (I don't see surfacing CPPs as being currently economically viable, and I understand that CPP blade geometry is optimized for one particular pitch)
5) Possible overall reduction in drive weight, when compared to twin installations.

The disadvantages of twin CVTs:
1) Most designs being developed for automobile use do not use the mechanism to reverse - a separate gear change occurs. CPPs are superior in this regard.
2) The CVT technology is currently not mature, and has not been applied to marine applications.
3) Mechanical complexity, (and thus potential reliability issues), and cost.

The disadvantages of single engine, two props as a concept:
1) Mechanical complexity, (and thus potential reliability issues), and cost.

The advantages listed above would most likely result in a more "fuel efficient" craft, when cruising at speed, as compared to conventional twin engine, twin fixed pitch prop, rudder system. It is doubtful that it would be "capital efficient".

As an additional note, I suspect that the torque impulses from surfacing propellers would quickly destroy existing CVTs.

Are these drive schemes appropriate for displacement craft? I rather doubt it.

But, in the interest of moving the thread along, let's say the application has two Power-Vent like surface drives, counter-rotating props with a single engine, and a CVT driving each prop shaft. The objective of the craft is to plane with best possible fuel efficiency, and ultimate top speed is not a design objective. The craft would be in the 8 to 10 metre range, and weight is to be minimized.

I suspect that there may be challenges steering such craft when in displacement mode, without rudders, but could you please explain why such a "differential steering" drive system, as I've spelled out above, cannot work in such a craft at cruising speed?

Seriously, I do not see why it won't work at speed.

Jim

[apex1]

Sorry, now I see much clearer what you wanted to express! And I have to agree and excuse!

I suspect that there may be challenges steering such craft when in displacement mode, without rudders, but could you please explain why such a "differential steering" drive system, as I've spelled out above, cannot work in such a craft at cruising speed?

No, I cannot explain that, it should be possible. A boat that is not (or nearly not) steerable at displ. speed remains undesirable though.

But apart from all those, partly sophisticed, partly more theoretical solutions, if we brake it down to the first question, then I still see absolutely no benefit in such a installation. A twin engine CPP arrangement is for shure not more expensive but very efficient, reliable and off the shelve. The minor disadvantage of the wider weight distribution (dreamer made that a issue) one can almost forget, I think.

Regards
Richard

[mydauphin]

I seldom see a boat with a rudder that is too big. A intelligent computerized electric drive twin prop could work. But then the computer has to be constantly adjusting for yaw. People forget water is a constantly moving thing. Even in a heavy ship - it moves sideways and need constant rudder adjustment to keep bearing. It seems to me a lot of people in this board with great ideas have never been on a real boat...

[john.G]

Without reading the entire thread through consider this:

Have your main run a hydraulic pump and have this in turn driving a hydraulic motor on each shaft.

Advantages:
Engine can be located anywhere you like, and can be mounted at any angle to the longtitudinal plane.

Engine can be run under constant load at all times.. prop speed is governed by regulating the oil flow to the drive motors not by engine RPM. This makes the engine operate at best fuel efficiency.

Compact hydraulic drive motors can be fitted in just about anywhere... means short shafts can be set low down to provide flat shaft angles and increases prop efficiency. All you have to do is run the lines from the engine/pump unit to them... one in and one out.

The technology is already out there, is cheap, and is operating under severe service conditions on extended service intervals. Conpanies like Hydash, Cat and Hitachi build and fit these motors to excavators every day. The life of a walk motor between overhauls is about 6000 hours, and they aren't hard to overhaul.

Weight is light, particularly if you water cool your oil. You don't need 40 gallons of oil if you can cool it quick.

Reliability is very high. If you don't check your hoses and replace them when required and one blows.. you can shut it off and come home on the other because they are on seperate circuits of the same pump.

Disadvantage:

Noise... go stand next to a 20 t excavator when its walking.

[john.G]

oops.. I see someone has already raised this as an option.

Regarding hydraulics.... if you worry about them on a boat, never ever go aboard a working trawler. Or any vessel with a winch thats not electric... hydraulics only leak if you don't maintain them. Same as with engines and transmisssions.

[john.G]

Propellors push water, rudders only direct it a little.

Thats why on a game boat when you need manoverability you face backwards.. sit your ass on the wheel to hold it amidships, and drive her off the throttles.

BUT without a rudder on a long course you would have to constantly adjust heading via the throttles... tedious at best ( I brought a twin engine barge home 600 miles once after the rudders were damaged)... and you can't slave the throttles to an autopilot.

[peter radclyffe]

The majority of boats are draft constrained meaning the props are smaller diameter than that required to give best efficiency. The possible exception is tugs where the prop is the most important part and the hull is designed to accommodate whatever is required.

When prop diameters are constrained by draft there is some benefit in splitting the installed power between two props of similar or larger diameter than a single prop installed on the centreline.


Taking an example of a 26 x 16 prop running at 1500rpm and pushing 10000N at 10kts the efficiency is 60%. Splitting the thrust between two 26 X 16 props drops the required engine speed to 1200rpm and efficiency lifts to 70%. So the result gives a 16% improvement at the prop. This gives quite a lot of room to play with extra losses from the gearing and additional appendages. There is a reasonable likelihood of overall performance benefit in certain circumstances.

Rick W
your saying the majority of boats, dont have the right props,how many types of boats have you studied , i challenge you to take this up with prop makers,

[Boston]

your barking up the wrong tree on that one Pete

Rick and I may not have agreed on more than a few things in the past but one thing this guy does know is props

Ild bust out the eraser and modify that last if I were you

cheers
B

[PAR]

Actually Rick is quite correct. Not only are draft restrictions an issue but most boats aren't prop matched to their employment.

[Guest625101138]

your saying the majority of boats, dont have the right props,how many types of boats have you studied , i challenge you to take this up with prop makers,

Peter
Until I spent the effort to understand propeller physics I thought there were fundamental efficiency based reasons for the difference in shape between airplane props and boat props.

When you understand the physics you would always go for high aspect blades and low velocity ratios if you wanted efficiency and this was the only requirement. The only way you can do this in most boats is to go to much larger diameter than you actually see and much lower area ratios than you actually see.

You do not need to take my word for it. Just take a look at the Wageningen "B" series data as per attached:
http://www.boatdesign.net/forums/attachments/surface-drives/27701d1229556018-surface-propeller-low-speed-displacement-boat-picture-1.png
This is a standard design but only two blade and 0.3 area ratio. Not like most boat props but a tested design. So it is not unknown by prop designers, rather simply not widely known as your belief is not unusual.

I even have higher aspect, around 15% area ratio, and can get high eighties efficiency with low speed operation around 6 to 7 knots. At higher speed, say 10 to 20 knots you can get efficiencies in the low nineties.

From what I have seen there are not many people building boats versed in prop design from fundamental physics.

The pleasure boat market has not been overly concerned about efficiency up till a couple of years ago. Efficiencies of the order of 50 to 60% has been acceptable and most boat designers/builders believe this is just the way it is.

The practical problem with using such propellers is the draft required to swing the required prop diameter.

If you want to get a grasp of general prop design take a look at JavaProp. It is based on the fundamental physics, not extrapolated output from limited test data.

Rick W

[kapnD]

The concept of two shafts from one engine makes sense on boats where draft and manueveribility are issues. Mr. Willoughby has run the numbers on it for us, and it looks good, though one must understand the constraints of the system and match them to its intended use.
A west Florida builder (was it Dolphin?) turned out plenty of these on a nice 40' lobster hull. The single 500 hp diesel was low and midship, and the in-deck fishhold was cavernous. I talked to several owners and visited their shop, and was very impressed. Nobody was complaining about fuel hogging or rudder drag! They accomplised the "split" with a tooth belt drive in a box beam off the bobtail, running seperate transmissions on each shaft. It looked and felt stout, but was much lighter than twin 250's.
Kiko Villalon's IBEX Pro Boat 34 is set up this way, and I'm sure he's not just blowing smoke at us.
The article is in the Feb/Mar 2009 issue of Professional Boatbuilder if you'd like to see particluars about his design focus.

[apex1]

Interesting comment, thank you.

[kapnD]

OK, the coffee's kickin in, the builder's name was Dorado, not Dolphin.
They were really into "multi" configurations, and produced some very interesting combinations of engines/drives. One boat I saw in their shop had split the power of two big diesels into four Volvo sterndrives, presumably to absorb the massive torque related problems of matching diesels to outdrives, and to better match propellors.

[Boston]

well who would have thought

two propellers just might be better than one

[Guest625101138]

well who would have thought

two propellers just might be better than one

Yup - more than likely in certain circumstances. See post #14!!!

Rick W

[Ad Hoc]

"...Taking an example of a 26 x 16 prop running at 1500rpm and pushing 10000N at 10kts the efficiency is 60%...."

Is that 60% theoretical for the prop, or for whole PC for the design, since that is what it really is all about? The efficiency of the whole installation.

If somone is getting 60% as a PC on a standard vessel at 10knots??...i would love to know. In real world it is next to impossible owing to the flow into the prop and the losses in the whole system.

Perhaps some real world stats are in order, from real built vessels (well ones I've designed and worked on). A well designed Prop/hull in general have an efficiency (PC) ranging around 45~55% -in the boat measured on sea trials. The most I have ever had has been 72% and that was on a SWATH. Excellent flow into the prop, almost 'open water' characteristics. When i say 'boats', i mean more than 5m models running up and down rivers! Boats carrying 400 passengers, or other running at 35 knots chasing drug runners for days at sea with a crew of 14.

The numbers spited out of a program may say 60% or 70% if 2 props used versus one may sound "great", but you wont get that when the props are in the boat attached to the shafts and you take the boat out on sea trails for the first time. You'll get a lot of disappointed faces!

The flow of water into the prop is so varied and affects the efficiency significantly. Hence hull design aft is very important on prop boats. Not to mention almost all installations are angled shafts, often owing to lack of adequate flow into the prop and space in the engine room.

With angles shafts it has an even more major effect of the propulsive efficiency. In general the majority of designs the real thrust is more likely to be in the order of 5%~10% less than shaft line thrust. Just this alone affects the efficiency and so the downward spiral goes.

[Boston]

I was thinkin as I read along that if the trany eats power and the shaft causes so much resistance then an electric in an azipod would both allow for the maneuverability and the electric allow for the variability while at the same time eliminating the need for the shaft and its supporting structures being replaced by the pivoting system for the Azipod.
would also allow for a for and aft configuration rather than a side by side one, as long as they were far enough apart it should work.

the biggest slowest blade you can fit being the most efficient

kinda makes a good case for electrics

thing I really like about the Azipod pivoting system is that the rotating mechanism can easily be designed to be above water line allowing for very little chance of water infiltration through the shaft, stuffing tube area

[Ad Hoc]

Electric drives = heavy

Only really suitable for large ships.

[Frosty]

I was thinking about 2 engines and one shaft with hydraulic motors and drives.

The result should be simplicity with reliability using either or both to a single shaft. Hydraulics could drive windlasses etc.

Economy with one engine with the stand by being available for higher speeds.

[mydauphin]

I was thinking about 2 engines and one shaft with hydraulic motors and drives.

The result should be simplicity with reliability using either or both to a single shaft. Hydraulics could drive windlasses etc.

Economy with one engine with the stand by being available for higher speeds.

Everyone says hydraulics are cheap and easy. I understand how it works and how to make it work, but me tell where to buy the stuff to put it together. Large hydraulics pumps aren't cheap or easy to find. Any places anyone?

[Guest625101138]

Everyone says hydraulics are cheap and easy.......

Not everyone!

You must not have been through this thread:
http://www.boatdesign.net/forums/boat-design/distributed-hydraulic-systems-boats-why-not-25792.html

Rick W

[mydauphin]

Not everyone!

You must not have been through this thread:
http://www.boatdesign.net/forums/boat-design/distributed-hydraulic-systems-boats-why-not-25792.html

Rick W

No didn't see this one... thanks

[Frosty]

Heavy machinery, oil rigs, ships etc. Try a ship breakers yard or yellow pages.

Bolt it up and pipe it, no bleeding just get a tank of oil.

They don't need to be big , you would be surprised at how small they are.

Dirty oil is the only killer. Well filtered oil is good. Once tolerences are lost the pump or motor looses power.

[rasorinc]

Add to that all tractor companys even the ones that make small garden tractors. Don't bother to call CAT. Theirs would sinlk a small boat.

[fpjeepy05]

http://www.boattest.com/Resources/view_news.aspx?NewsID=3230

[dreamer]

http://www.boattest.com/Resources/view_news.aspx?NewsID=3230

well how about them apples!

This is exactly what I was thinking.

[Boston]

ya but that seemed like a lot of money for a couple of 90° gear boxes

[dreamer]

well, costs can be dealt with. The conclusions of the design are interesting.

[Lurvio]

For the thrust steering discussed above, there would be the setup used by John Deere to steer their rubbertracked tractors. It has an angle gear (after engine and gearbox) and on both sides planetary gears. Their ring gears are connected to a hydraulic motor, when the motor is stationary, the output rpm's on both sides are the same. When the motor starts turning the ring gears, it creates a difference in rpm, hence the vehicle turns. A steering wheel is used, a computer reads the position of the steering wheel, more the wheel is turned, faster the motor turns. The steering is accurate enough to work on the tractor up to speeds of 35 km/h on public roads(john deere 9030T).

I'm not going to go on the feasibility of such system on marine use, this is just an example on the though of taking the rudder of the equation.

[FAST FRED]

"I even have higher aspect, around 15% area ratio, and can get high eighties efficiency with low speed operation around 6 to 7 knots. At higher speed, say 10 to 20 knots you can get efficiencies in the low nineties. "

Given no structural hassles what size, shape prop would you chose for 120 HP and an 18K top speed?And a desired LRC of 12K?

FF

[flyinwall]

ok i just briefly scanned this and i didnt see (it might have been there but i didnt see it) what sort of hull this idea is going into the best way to get 2 props from 1 motor would have to be with hydrolics that way you could put the motor where you want and just run a couple of hydrolic lines to the motors and be done with it (i have seen this done with a 60' twin hull cat prawn trawler they mounted the motor (a big diesel v8 catapiller) under the sorting tray in the middle of the deck )

[Guest625101138]

"I even have higher aspect, around 15% area ratio, and can get high eighties efficiency with low speed operation around 6 to 7 knots. At higher speed, say 10 to 20 knots you can get efficiencies in the low nineties. "

Given no structural hassles what size, shape prop would you chose for 120 HP and an 18K top speed?And a desired LRC of 12K?

FF

Fred
It becomes purely academic if you remove structural aspects.

Also I assume your engine rating is based on a low efficiency prop. With a prop that was much more efficient you would use much less power.

I would design for the cruise and check structural and cavitation for higher speed.

Taking the 12kt cruise and assuming drag of 2000N you could get 86% efficiency with a 1.2m diameter prop spinning at 328rpm. The blades would have a maximum cord of 100mm. This is the sort of size needed to handle the bending loads. Power would be 18.6HP.

If the boat is more easily driven such that it takes 1000N at 12kts then a 1.2m prop will get 89%. The chord would be 80mm. Power is 9HP.

Now assuming the prop can handle say the 5000N at 18kts the power will be 68HP but cavitation is coming into play. Efficiency is 87%. So more care needs to be taken with the blade shape to suit the higher speed.

The root of the blades would need to be beefed up quite a lot to cater for the a 5000N thrust.

You need to consider the hull rather than some assumed power. I expect you can appreciate that a 4ft diameter prop absorbing 65HP is going to have a lot less slip than a 20" prop absorbing 120HP. A 4-bladed 20" prop will absorb around 88HP to produce the same thrust as a 4ft at 68HP so this is getting closer to your nominated power. Efficiency down to 69%.

So it can be done structurally for the prop but you have to decide if you like the idea of swinging a 4ft diameter prop under a small boat. Being two blade it would need a little over 2ft deep to float the boat or you could have some lifting system. Yachts often have much deeper keels than this of course.

Rick

[u4ea32]

On twin engine powerboats, its not all that unusual to leave the helm centered and only use differential thrust to steer, especially while docking.

On my boat, I sometimes do this. Usually, I find the vectored thrust of the I/O drives to be extremely useful. But when getting on the trailer, docking, or driving through mooring fields, its usually better to steer with the shifters than with the wheel.

The complaint I have heard about these splitters (take power from one engine and apply to two props) is NOISE, along with cost and weight when compared to single engine driving a single prop.

Nevertheless, I've also been intrigued by the same idea -- single engine, two props, separate transmissions (FNR) and separate CVTs -- for all the following reasons, in no particular order: (a) the potential for shallower draft for bigger props, and (b) the resultant potential for increased efficiency. And of course, (c) to obtain twin screw maneuverability with a single engine. While single engine boats can be amazingly maneuverable in one direction, they aren't in the other, and they generally can't back down with much directional control at all. And finally, (d) to be able to put sufficient load on the diesel to actually achieve the 20 hp/gallon/hr that they usually don't achieve due to light loading at cruise settings.

Now, I'm thinking about low power applications, where simple approaches like bicycle chains and the CVT transmissions used by millions of scooters would suffice. Low cost, low weight, cheap, and very available. The key problem with small diesels (10 to 50 hp) is that they really don't achieve, in practice, much better efficiency that gas powerplants: often only about 12 to 14 hp/gallon/hour, versus the pervasive 10 hp/gallon/hour for outboards. And outboards are just so advantageous in so many ways.

So far, my trade studies have always resulted in choosing one outboard over even two outboards, and well ahead of the single small diesel with a custom complex power train.

[fpjeepy05]

Honestly I think the only benefit is that they do not make an inexpensive stern drive that can reliably handle the amount of torque that a 480hp diesel puts out. That leaves Arneson and NXT. Both are very expensive. Two rebuilt Bravo ones and a gear... your looking at around 17k. Arneson your looking at around 25k and its not going to have nearly as good of handling.

[Rik]

Honestly I think the only benefit is that they do not make an inexpensive stern drive that can reliably handle the amount of torque that a 480hp diesel puts out. That leaves Arneson and NXT. Both are very expensive. Two rebuilt Bravo ones and a gear... your looking at around 17k. Arneson your looking at around 25k and its not going to have nearly as good of handling.

The ASD8 Drive for a diesel is only 16K. Handling will be better than the NXT. Fountain Powerboats found this out when they were comparing the performance characteristics between the two.

[BMcF]

We built a demonstration craft..a trimaran, long and slender main hull with all propulsion in the main hull..that had a single large V-8 turning a pair of 12JG Berkeley jets, each independently steered and, of course, with independent control of the reversing buckets.

The splitter transmission used two 3" 'Gilmer' belts to divide the engine output to the two output shafts. Each belt/pulley combination was rated to 300HP, so 600HP total engine input power.

The 'transmission' was not particularly noisy but it did radiate the belt noise always associated with 'blower drives'.

Since the propulsion was via waterjet, all the benefits for maneuvering that you would expect from a twin jet installation were realized. Obviously, that simple (reliable and fairly cheap too) splitter transmission had no forward/revers/neutral capability at all, and so would not suit for prop propulsion if the whole idea was to gain the manuevering benefit.

[dreamer]

Nevertheless, I've also been intrigued by the same idea -- single engine, two props, separate transmissions (FNR) and separate CVTs -- for all the following reasons, in no particular order: (a) the potential for shallower draft for bigger props, and (b) the resultant potential for increased efficiency. And of course, (c) to obtain twin screw maneuverability with a single engine. While single engine boats can be amazingly maneuverable in one direction, they aren't in the other, and they generally can't back down with much directional control at all. And finally, (d) to be able to put sufficient load on the diesel to actually achieve the 20 hp/gallon/hr that they usually don't achieve due to light loading at cruise settings..

These were all the things I was thinking of when starting the thread. Also, with this arrangement, it would be entirely possible to have the shafts be completely horizontal while having the engine down low in a boxed keel or deep v.

[rasorinc]

A while back a design of what you purpose was posted as a picture. I thought I saved it but I've looked to no avail. It was a drawing of an engine, transfer case, 2 drivelines etc. Maybe you can find here on the site.

[dreamer]

You're probably thinking of this picture:

http://www.boattest.com/images-gallery/News/blueprint.jpg

From this link:

http://www.boattest.com/Resources/view_news.aspx?NewsID=3230

[fpjeepy05]

The ASD8 Drive for a diesel is only 16K. Handling will be better than the NXT. Fountain Powerboats found this out when they were comparing the performance characteristics between the two.

This is accurate, that is what a ASD8 costs. However, if all you spend is $16k your not going to be going anywhere. If you want to actually drive the boat, Include about $4k for a prop, $5k for a transmission, $2,834 for a power steering kit if you don't already have one. $28k will likely be more accurate. A rebuilt Bravo has a transmission, prop, and power steering for $6k.

[Rik]

This is accurate, that is what a ASD8 costs. However, if all you spend is $16k your not going to be going anywhere. If you want to actually drive the boat, Include about $4k for a prop, $5k for a transmission, $2,834 for a power steering kit if you don't already have one. $28k will likely be more accurate. A rebuilt Bravo has a transmission, prop, and power steering for $6k.

I am sorry that things are not free and you forgot the cost of the engine, the boat and the fuel the rigging, etc.. Oh, with the twin drive single engine you have to factor in the cost of the splitter box, drive lines, couplers, etc. etc..

Put a Bravo on an engine that has over 200+ diesel and see what that cost long term. This is why they want to split the power off to multiple drives. I've done this in the past for different reasons and there is no cheap and easy way of doing things. Those two thoughts are mutually exclusive of one another.

Also, price out what a NEW Bravo complete, not just a leg cost and then compare apples to apples. Used items are obviously less expensive than new things. Your prices for the items are a bit high, but that will all come down to the actual application at hand. If the customer has a Cummins QSB480 and you want to use a Bravo, best of luck to you. Even Mercury/CMD will not do that even if you beg them.

The Power Steering system does not cost $2,834 of a single engine application, especially when the engines you are referring to come with a power steering pump on them. No need to purchase things twice.

Propellers will have to be purchased for a Bravo also so please do not forget to add their cost in also. Last I checked, nothing is free from Mercury either.

[fpjeepy05]

http://floridasportfishing.com/magazine/boats-reviews/dusky-s-geared-up-dusky-33-open-fisherman-3.html
The system works. I talked to owner of the company who has the system on his personal boat, and he says it works flawlessly. Has over 1000 hours on it and hasn't broken a drive yet.

As for price... Professional Boat Builder Issue #122. There is an article stating that the price for the three gear boxes are priced at around $5,000. Call Mastery and I'm sure they can give you a more accurate price.

If price is the major driving factor. I see it as a viable solution.

[Boston]

at some point, this price for just the gear boxes and as has been pointed out all the other associated stuff, begins to add up

I suppose my next question would be, would not the maneuverability gain vs cost be significantly less than say a geared Azipod system ?

just a thought I've been working on for my own build is a Azipod without an electric drive unit at the turn but instead a simple bevel gear along with another gear at the top set up exactly like a automotive pumpkin so it can still swivle 360° and leading to an engine that instead of tilting down along the typical shaft line tilts up so the line of the drive shaft meets the top of the Azipod'

I know its kinda junk yard wars but I'm working on a budget over here and frankly the diesel out of my truck is just right for the build.

I can see a maneuverability increase with two props but I can see a larger one for less money, less gear connections and less to go wrong with an Azipod using off the rack components

only thing custom in there is the Azipod housing and a few shafts and even then you could probably find shafts of the right length and diameter to go with standard bevel gears at the turn, that and a few bearings and grease seals
or even slice and dice the business end of a standard outboard rather than do much of anything requiring a custom casting

anyway

we now return you to your regularly scheduled programing

cheers
B

[fpjeepy05]

I can see a maneuverability increase with two props but I can see a larger one for less money, less gear connections and less to go wrong with an Azipod using off the rack components
B

I look at the increased maneuverability as a byproduct. The problems that the gear actually solves is an inexpensive drive for large diesel engines, and fits design constraints that limit building to a single engine.

[Bigfoot1]

Pacific Yachting recently did an article about a guy who had a system installed, pulled 2 diesels, put in a bigger and it worked fine, Less weight more engine room space.
Bracewell Marine in Vancouver did the work, The main reason that you would not want to run hydraulics is the horsepower lost in hydraulic pumps and motors as compared to helical gears. Dont hold me to it but you may lose up to 30% for hydraulics and say 8% for gears, Anyway and interesting article. though you would still need rudders.

[seeds]

It's done in the R/C world sometimes. They tend to use up to 5HP motors. I read an article back at christmas about a US company who was manufacturing a form of "splitter" drive. The idea was that it could use one engine to power two props independantly or two engines to pust four props.

[Easy Rider]

There is an off the shelf system availible. Philbrook's Boat Yard LTD. (Sidney BC Canada) installs them. philbrooks.com There is an article in Passage Maker Magazine .. Feb 05 pg 112.

Easy Rider

[andynoshoes]

but the better idea from all this I think is to dispose of the big main engineand seperate gen set and install two smaller engines driving one shaft with a cpp

[tugboat]

the only advanatage i see is that you reduce draft. and can get away with smaller props. better to go with two engines ...here is a youtube of the same system- twin props one engine...
http://www.youtube.com/watch?v=tX2PSot7PE4&feature=related

[tugboat]

a same boat here but longe vid http://www.youtube.com/watch?v=LhtB5fRDSV4&feature=related he explains somewhere how its done..

[seeds]

Add in the fuel economy of one engine and coupled with having that one engine sitting in the very bottom of the boat to lower the COG, and you'll see there are lots of advantages to this system. I have no first hand experience with it, but I'm sure there are people on here who do. No one system works for every boat, but for someone this design could be exactly what they need.

[apex1]

Some mad ideas will never die..............

[seeds]

So why is it a mad idea? Have you installed or used this system of one engine having two drives and know something the rest of us don't? If so please share...

[apex1]

So why is it a mad idea? Have you installed or used this system of one engine having two drives and know something the rest of us don't? If so please share...

Have you ever installed oars and sails on a tugboat? But you might agree that it is a mad idea?

Do some calculations and you will find, that the single engine combined with a CPP solves the problem of a very flexible propulsion, much better and cheaper. And it is proven in hundreds of thousands of vessels.

Regards
Richard

[Easy Rider]

Dick Carr owns Superior Gearbox Company of Stockton Missouri and he had a running prototype in a Mainship Pilot 34. The twin prop single engine boat was more efficient from 15 to 26 knots. There was a full feature article on the "geared up" prototype in PMM February 05. They definitely didn't consider it a mad idea. Read the article instead of talking about oars on a tug boat.

Easy Rider

[apex1]

Dick Carr owns Superior Gearbox Company of Stockton Missouri and he had a running prototype in a Mainship Pilot 34. The twin prop single engine boat was more efficient from 15 to 26 knots. There was a full feature article on the "geared up" prototype in PMM February 05. They definitely didn't consider it a mad idea. Read the article instead of talking about oars on a tug boat.
Easy Rider

It is nice to hear Mr. Carr has a good running business, thanks.

Instead of attacking those who build boats in noticeable numbers, i.e. me, you should ask yourself why the commercial world and the Navies have given up on that concept.

You may find after some studies, that the gearboxes did not live long. And so the much simpler and cheaper CPP was the replacement in the successors of such craft.

But one has to know the market to be aware of that. PMM is not the only source of information, and sure not the best either.

Regards
Richard

[Easy Rider]

To be sure you're correct and GM had all kinds of combinations of DDs and multiple shafts and engines that did'nt match. AND the "Geared Up" drive system WAS a proto type AND I haven't heard anything about them since.
I was going on memory saying it was an over the counter item. I was wrong and sorry I mentioned it however it seems as though they DID show that from a standpoint of efficiency the system works.

Easy Rider

[apex1]

To be sure you're correct and GM had all kinds of combinations of DDs and multiple shafts and engines that did'nt match. AND the "Geared Up" drive system WAS a proto type AND I haven't heard anything about them since.
I was going on memory saying it was an over the counter item. I was wrong and sorry I mentioned it however it seems as though they DID show that from a standpoint of efficiency the system works.

Easy Rider

When you study CODAD systems (which is the right term for these applictions) you soon will find more issues than advantages.

Sure, a installation providing more flexibility than the common ones has some benefit in efficiency, thats natural. But when it gets complex, the advantage quite often is gone. We see the same effect in hybrid applications.

Regards
Richard

[yellowcat]

I checked the volvopenta web site, if an outboard motor could have the duoprop forward arangement, then install the motor on a quick raisable / lowerable plateform, all that into a well vented enclosure amidship of a cat pontoon... AND diesel. Fuel economy, torque, low fumes, etc. This would also allow the possibility for water pressure from a water pillow squished slowly by the weight of the engine at mooring. Hence, have a watermaker run at night with no electrcity. Needs more explaining perhaps.
As for the hit something problem, having relatively small diam. props allows for a 18 inches water dip with directional like jet tubings. It could be called the "outboardpodtubine" . I guess i'll have to do with a std 4 strokes until ... could be an electric pod. "e-outboardpodtubine" wireless runned with an "Ipad" ...
Food for thoughts , sounds crazy, but trying to make heavy stuff on a boat work for something else from their original purpose makes sence to me, especially on a cat.

[keysdisease]

There are a couple of the Geared Up boats running around down here in South Florida. At least one Dusky 33 and I have seen the Mainship and a couple others.

Like anything else having to do with boats these set ups are compromises. There are some good features and like anything else this is not for everyone.

The concept is proven, the components proven reliable, the boats go from A to B with reasonable economy. There are some handling quirks like the speed is constant for both props so when you have one in forward and one in reverse you can't increase power to just one prop.

It's not for everyone but it is not a "mad idea" by any stretch.

Steve