Single vs Twin: It's Baaaaack!

[psneeld]

I'm going to toss a new wrinkle in the subject. The often stated and presumed assumption is that twins mean more friction and mechanical loss due to the second set of running gear, etc. I'll buy that, it's a no brainer, but WAIT!!

What about the fact that most of our boats have the single engine running gear mounted behind a big old keel. Disturbed water!! Singles are running in clean water well away from the keel. Aha an efficiency gain. Do they cancel each other out? They may well.

Much of the reading that discusses flow suggests that below 10 knots there is so little disturbance created that either twins or singles suffer little from the drag standpoint but running gear losses are greater than some would believe (depending on setups of course).

If drag at displacement speeds was such a big deal...I doubt you would see so many keel coolers that aren't all that streamlined. Sure approaching 10 knots and above streamlining helps....so a little is done but they are still sticking out to sme degree.

 

[Nomad Willy]

Not only that Daddy but the rudder does not need to be constantly deployed to counteract the prop walk.

With a twin engine boat w the same power as a single the shafts and struts will be much smaller and the drag much smaller too.

Twin engine boats aren't really less efficient but boats (like most of our trawlers) that have twice as much power as their single engine counterparts definitely are.

Re drag at displacement speeds I'd not want to paddle a kayak w a big wide submerged transom.

 

[Art]

Greetings,

So Long & Thanks For All The Fish! - YouTube

Damn, RT - That dolphin video brought tears to me eyes! Specially knowing what I know about the current money grubbing leaders of Big Govt and the Big Fossil Fuel Corps... being that I'm deeply into the clean-fuel industry.

 

[Art]

Originally Posted by BruceK
Good Grief! Thread resuscitated!

Clearly auto makers are in error. If I had 2 engines I could walk the car sideways into tight parking spots, or enter my garage without doing violence to the power steering on full lock (Peugeots need a football field like space for a U turn).
Auto makers will have to invent "wheel wash",typical maneuvering speeds are not fast enough for four wheel drifting.
Long live the thread!

3 point turn on an 8 lane highway ay Bruce? Haha :-D

Heck Guys: With the original single 430 cid / 360 hp V8 engine in my 1967 Buick Wildcat Luxo-Muscle-Car I can already spin 360 degree donuts on her own length... long as I cut her two front rudders hard to either side and let her big/bad engine's power lose! - LOL

That mean that a single screw boat should have three (3) rudders??? - One large in da middle and two smaller on da sides. Now there's a thought. Anyone ever try that? Just think of the possibilities...

1. All three rudders could be rigged to steer in unison
2. Outside rudders could be rigged in unison for steering while center rudder works independently and could be left in straight mode for more representative keel area at various speeds and for various needs
3. Outside rudders hydraulically can be pulled up for completely reduced drag at faster speeds or re-deployed downward for better slow speed steering control and improved docking maneuvers!

OMG - We could have had a V8! Tri-rudder, single screw trawlers, what will we come up with next?? LMAO

But – you know what... properly engineered this single screw trawler hydraulic deployment “tri-rudder design” just might grow legs!

 

[hollywood8118]

It appears that from the views of the shipping industry that most ships of recent origin seem to be all singles... one engine designed to operate at peak speed vs. hp with little excessive hp for bad weather etc. They don't use two smaller engines driving two exposed props. If you are talking peak efficiency it must be the most economical solution. Ships designed for speed.. aircraft carriers, cruise ships, ferries run multi engines.
HOLLYWOOD

 

[bfloyd4445]

I don't disagree. But then, at least half the time twins come limping home on one engine, the single-engined boat didn't experience an engine failure.

.......actually i think that the failure rate would increase expoentially so with two engines the failure rate would be somewhat greater than twice as much. Gonna have to see if i can't dig into the insurance companys data banks to see what there records show. However i would think that many of the twin failures would only result in a slight incovieneence while they clear the props and will not be reflected in insurance records. Wonder if we could set up a poll at this forum?

 

[bfloyd4445]

Originally Posted by BruceK
Good Grief! Thread resuscitated!

Clearly auto makers are in error. If I had 2 engines I could walk the car sideways into tight parking spots, or enter my garage without doing violence to the power steering on full lock (Peugeots need a football field like space for a U turn).
Auto makers will have to invent "wheel wash",typical maneuvering speeds are not fast enough for four wheel drifting.
Long live the thread!



Heck Guys: With the original single 430 cid / 360 hp V8 engine in my 1967 Buick Wildcat Luxo-Muscle-Car I can already spin 360 degree donuts on her own length... long as I cut her two front rudders hard to either side and let her big/bad engine's power lose! - LOL

That mean that a single screw boat should have three (3) rudders??? - One large in da middle and two smaller on da sides. Now there's a thought. Anyone ever try that? Just think of the possibilities...

1. All three rudders could be rigged to steer in unison
2. Outside rudders could be rigged in unison for steering while center rudder works independently and could be left in straight mode for more representative keel area at various speeds and for various needs
3. Outside rudders hydraulically can be pulled up for completely reduced drag at faster speeds or re-deployed downward for better slow speed steering control and improved docking maneuvers!

OMG - We could have had a V8! Tri-rudder, single screw trawlers, what will we come up with next?? LMAO

But – you know what... properly engineered this single screw trawler hydraulic deployment “tri-rudder design” just might grow legs!

all kidding aside I wonder if it would work? Any marine engineers out there that could anser this question?

 

[bfloyd4445]

It appears that from the views of the shipping industry that most ships of recent origin seem to be all singles... one engine designed to operate at peak speed vs. hp with little excessive hp for bad weather etc. They don't use two smaller engines driving two exposed props. If you are talking peak efficiency it must be the most economical solution. Ships designed for speed.. aircraft carriers, cruise ships, ferries run multi engines.
HOLLYWOOD

I'm a dummy when it comes to boats but from what i have learned on my DeFever quest the singles save almost a third of the fuel costs of twins. Then of course their is the hugh maintenence cost savings of the single over the twin. But on the other hand a single really could use a bow thruster if used in deltas and other tricky areas with many currents and hazards

 

[Nomad Willy]

"the singles save almost a third of the fuel costs of twins."

Sure because they have twice as much heat loss cause their engines are the same size. It's not that they are twins but that their engine size is doubled. See post 722.

 

[BruceK]

The inescapable logic: if one engine is better than 2, none is better than one. Imagine the reduction in water disturbance with the boat stationary.
I think we finally solved it.
Some sailboat owners claim this knowledge for thousands of years. Ignore them. It`s our discovery.

 

[RickB]

I have a bit of a hydraulic steering leak.

Geez, RT, that leak must be older than a 7-11 donut! I thought you were going to fix that years ago.

 

[bfloyd4445]

"the singles save almost a third of the fuel costs of twins."

Sure because they have twice as much heat loss cause their engines are the same size. It's not that they are twins but that their engine size is doubled. See post 722.

takes energy to generate heat and that means fuel. To bad we couldnt utilize that lost heat energy to propel our boats?

 

[bfloyd4445]

The inescapable logic: if one engine is better than 2, none is better than one. Imagine the reduction in water disturbance with the boat stationary.
I think we finally solved it.
Some sailboat owners claim this knowledge for thousands of years. Ignore them. It`s our discovery.

......that disturbance you mention is simply the effect of drag so lets factor that out of the equation, we will use even less fuel which means we will have greater resources available to make beer!!....beer is never a drag

 

[Art]

Been out cutten deals today... I'm back now. I do like this forum!

Sooo... I recommend we designate the next benchmark for this marathon thread as post # 888 (seeing as big/bad 666 fizzled-out in a whimper under the heat and energy of TF contributors – LOL). 888 is supposed to be the $$$ money $$$ number. Maybe it will bring all posters here good financial luck! lmao

To reach # 888 or well beyond I propose discussion on three items (other item suggestions wholly sought too!):

1. Triple rudders for single screw boats (for base postulation/description read my post 724)
2. Inset bottom of hull cavern/channel for single or twin screw with elongated (3’ to 5’ or longer) cork screw blade that is supported under hull by bearings at each end and hydraulically rotated by motor(s). If cavern is large enough the elongated corkscrew blade could be made to change angle-to-hull-length and therefore act as a course altering propulsion thrust, in forward or in reverse.
3. Pod Drives – They were becoming quite the propulsion and steering topic bout a year ago. Much of the discussions on them seem to have petered out.

Some other items that might be good to hash over are new configurations for hard-rudder or malleable-rudder designs. Also, hull-bottom/keel-side/keel-bottom inflatable/deflatable add-ons could provide hull configuration alternatives that might save fuel, increase speed, improve tracking, or create better stabilization while in various sea conditions. There are high-tech pliable fabrics today that could be used for many shape altering alternatives by simple changes of their internally contained air or water pressures.

Let’s let our minds wander, wonder, and postulate! There’s nothing better to help improve designs of any vehicle than the persons who have years/decades experience with continuous use of said vehicle.

No doubt about it - - > Boats-B-Us! And, We-B-Boats!

Happy Marine Design Daze! - Art

 

[bfloyd4445]

Been out cutten deals today... I'm back now. I do like this forum!

Sooo... I recommend we designate the next benchmark for this marathon thread as post # 888 (seeing as big/bad 666 fizzled-out in a whimper under the heat and energy of TF contributors – LOL). 888 is supposed to be the $$$ money $$$ number. Maybe it will bring all posters here good financial luck! lmao

To reach # 888 or well beyond I propose discussion on three items (other item suggestions wholly sought too!):

1. Triple rudders for single screw boats (for base postulation/description read my post 724)
2. Inset bottom of hull cavern/channel for single or twin screw with elongated (3’ to 5’ or longer) cork screw blade that is supported under hull by bearings at each end and hydraulically rotated by motor(s). If cavern is large enough the elongated corkscrew blade could be made to change angle-to-hull-length and therefore act as a course altering propulsion thrust, in forward or in reverse.
3. Pod Drives – They were becoming quite the propulsion and steering topic bout a year ago. Much of the discussions on them seem to have petered out.

Some other items that might be good to hash over are new configurations for hard-rudder or malleable-rudder designs. Also, hull-bottom/keel-side/keel-bottom inflatable/deflatable add-ons could provide hull configuration alternatives that might save fuel, increase speed, improve tracking, or create better stabilization while in various sea conditions. There are high-tech pliable fabrics today that could be used for many shape altering alternatives by simple changes of their internally contained air or water pressures.

Let’s let our minds wander, wonder, and postulate! There’s nothing better to help improve designs of any vehicle than the persons who have years/decades experience with continuous use of said vehicle.

No doubt about it - - > Boats-B-Us! And, We-B-Boats!

Happy Marine Design Daze! - Art

I love this idea but this is a diferent topic......

 

[Nomad Willy]

Floyd wrote;

"takes energy to generate heat and that means fuel. To bad we couldnt utilize that lost heat energy to propel our boats?"

That's easy. Just use an engine or engines that are of a reasonable size and it's done.

 

[Art]

I love this idea but this is a diferent topic......

Single and/or twin screw boat design is mentioned within my post! After well over 700 posts, rehashing the same input too often... I recommend we broaden the text. Enough competent TF contributors are into this thread to handle a morphing, and each person will likely continue posting.

 

[bfloyd4445]

Floyd wrote;

"takes energy to generate heat and that means fuel. To bad we couldnt utilize that lost heat energy to propel our boats?"

That's easy. Just use an engine or engines that are of a reasonable size and it's done.

If my memory serves me correctly over 20% of the fuel the engine burns is turned into heat that is removed via sea water and wasted. Currently diesel engines have about 38% total efficiency in converting fuel to prop hp. Gas engines are even less efficient by about 10% the rest being used up in friction losses, drag, heat, pumps, alternator.
So, of the total energy in a gallon of D you can expect about 38% to actually be used to power your boat, 28% if gas.

 

[Nomad Willy]

So if you have a typical twin engine trawler the amount of wasted heat/fuel is not 38% but more like 25%.

Twenty % heat loss sounds very small. Are you sure it's that small. These internal combustion engines are called "heat engines" as they basically turn heat energy into mechanical energy.

And not to be forgotten is the fact that gas engines and diesel engines are very close to equal in efficiency at WOT.

 

[BruceK]

Art, Firstly, Congratulations are due. Marine designers worldwide, take note.

A brief quibble, is there good reason to limit the corkscrew propeller to 3-5 ft length? Can it be full length, and articulated, so the fwd and aft sections can counter-rotate, at the helmsman's whim, for improved maneuvering. It would also function as effective fishing line cutter, and fish gutting device.

 

[RickB]

If my memory serves me correctly over 20% of the fuel the engine burns is turned into heat that is removed via sea water and wasted.

Pretty close. Here is a CAT heat balance for a modern marine engine.

 

[RT Firefly]

Greetings,
CAT heat balance?

 

[RickB]

Here's a real cat heat balance ...

 

[bfloyd4445]

So if you have a typical twin engine trawler the amount of wasted heat/fuel is not 38% but more like 25%.

Twenty % heat loss sounds very small. Are you sure it's that small. These internal combustion engines are called "heat engines" as they basically turn heat energy into mechanical energy.

And not to be forgotten is the fact that gas engines and diesel engines are very close to equal in efficiency at WOT.

That's what i said. Diesel engines were about 38% efficient in converting fuel to usable hp, approximately 20% more is lost in the form of heat. That's 58% leaving 42% for other losses such as friction, blow by, engine efficiency due to design and age, and of course the stoichiometric ratio is 14.5:1 and since fuel air mixture must be slightly richer than what is actually needed there is a very small % that goes out the exhaust. If the mixture is lean the result is less hp and more engine wear due to lack of lubrication. Diesel fuel is an oil and engine designs must take this into consideration if they are to meet EPA standards and consumer demands.

So, 38+20+42= 100% of fuel

So, the best we can expect is that 38% or 38 cents out of every fuel dollar spent will end up pushing our boat. I do not claim to be an expert in this area but i have been involved in some engine testing in the past and these are numbers from memory, perhaps some of our readers would have more precise numbers? I think the society of mechanical engineers in 1977 published an abstract in which they claim about 4% friction loss just from the engine. Since today's designs are basically the same i think that figure is likely still accurate still leaving another 38% loss for all other things like belts pumps transmission etc.

It sure would be nice if we could eliminate some of these losses. There is one engine design i read about that is an 2 cylinder but four pistons. I think this is a link to the company planning on marketing this more efficient diesel engine design http://www.ecomotors.com/ecomotors-internationals-opposed-piston-opposed-cylinder-engine-promises-revolutionize-commercial-ve

here is a link to another design planned for use in helicopters and a good video.

http://www.engineeringtv.com/video/Opposed-Piston-Opposed-Cylinder

 

[RickB]

... and of course the stoichiometric ratio is 14.5:1 and since fuel air mixture must be slightly richer than what is actually needed there is a very small % that goes out the exhaust. If the mixture is lean the result is less hp and more engine wear due to lack of lubrication.

WTF?

You are joking, right?

 

[Nomad Willy]

Floyd,

Diesel engines fuel mixture is about 60-1 at idle, 14-1 at WOT and anywhere in-between in-between. I think you're thinking of gas engines.

I reread your post and see where I went astray. I thought that only about 30% of the fuel went to moving the boat but it looks like I'm mistaken.

For people w old 35 to 44' boats w twin 380 cu in engines the road to economy and less heat loss is smaller engines or "a" smaller engine. And of course better hull shapes aft.

In your example Rick it looks like there has been no account for piston friction, bearing friction, gear friction ect. It would seem to me "mechanical work energy" would be output energy .... the power to turn the propeller shaft. Surely they need to separate the mechanical energy of frictional parts and what's left to power the boat .. or whatever.

 

[markpierce]

The captain of the cruise ship Norwegian Star told me 70% of the ship's power went toward propulsion while 30% went toward "life support" such as lighting, heating/cooling, cooking, sewage processing, water making, etcetera.

 

[bfloyd4445]

WTF?

You are joking, right?

Do you think that engine efficiency in regards to your pocket book is a joke? I'm not rich so i like to keep my engines operating at maximim efficiency as much as possible. If the air fuel ratio is not correct it can lead engine damage and if rich a waste of expensive fuel and possible engine damage. Neither of these is a joke. Both result in the expendature of $$ that would be better spent on beer

 

[RT Firefly]

Greetings,
Mr. markpierce. OK, 70% to power and 30% to life support. That's all well and good but what about the 346.7 Kw (aprox.) generated by the 3467 souls on board assuming aprox. 100w per body and given off as heat? Just look at all that steam in front of Miguel! Did Capt. Harstrom take THAT into account? And what about the excess methane generated on Taco night in the La Cucina restaurant? An awful lot to consider...

 

[bfloyd4445]

Floyd,

Diesel engines fuel mixture is about 60-1 at idle, 14-1 at WOT and anywhere in-between in-between. I think you're thinking of gas engines.

I reread your post and see where I went astray. I thought that only about 30% of the fuel went to moving the boat but it looks like I'm mistaken.

For people w old 35 to 44' boats w twin 380 cu in engines the road to economy and less heat loss is smaller engines or "a" smaller engine. And of course better hull shapes aft.

In your example Rick it looks like there has been no account for piston friction, bearing friction, gear friction ect. It would seem to me "mechanical work energy" would be output energy .... the power to turn the propeller shaft. Surely they need to separate the mechanical energy of frictional parts and what's left to power the boat .. or whatever.

I think i may have forgotten to define what stoichiometric is.
Stoichiometric is the ratio of one reactent to another reactant in a chemical reaction. The optimum ratio of fuel to air is called the stochiometric ratio and for gas its 14.7:1 and diesel it is 14.5:1.Any other ratio inside the cylinder will result in less than optimum results.