Displacement vs. Semi Displacement, Plaining

[Rustybarge]

he you are Fleming 55 curves

Wow, 188ltr @ wot producing 24hp/gal.

Impressive !

...and 10ltrs (2.2 gal)/ 6kts to give 2.7mpg, when I seem to remember a similar sized Nordy single got only 1.3mpg over 6000 miles.

 

[eyschulman]

My take on the issue of twins vs single re; down side of twins. Cost more to buy and install twins.--crowds the motor compartment--is heavier--there is more maintenance cost and effort--harder to protect running gear--usually need two rudders--plumbing wiring cooling and filtration much more complex. The major up side is maneuverability and redundancy. I do not see the fuel burn as a significant issue.

 

[Seevee]

My take on the issue of twins vs single re; down side of twins. Cost more to buy and install twins.--crowds the motor compartment--is heavier--there is more maintenance cost and effort--harder to protect running gear--usually need two rudders--plumbing wiring cooling and filtration much more complex. The major up side is maneuverability and redundancy. I do not see the fuel burn as a significant issue.

Eyschulman,

Totally agree and makes sense. I could go either as long as there's a fast and slow speed.

I do like the maneuverability, but don't have enough experience to know that a single with good thrusters is almost as good.

As for reliability, all diesels are very reliable, and would bet that if the parts are maintained well and fuel is properly filtered, the chance of failure is extremely small.

 

[Nomad Willy]

I'd respond eyeshulman but I don't want to be the one thrown under the bus for being off topic. The topic is hull design and few posts have addressed that.

Re the question .. something to throw out here is that IMO most don't realize the wide range of SD hull design variations there is. Most of the people here that claim to have FD hulls (probably 75%) actually have SD hulls. That relates heavily to the question since the resistance of a SD hull that is very close to FD is going to have way less resistance than one that is almost a planing hull. For anyone really interested there is much posted on this on boatdesign.net. The hull shape of the stern mostly indicates what classification a boat should fall into. And there are a lot more planing hulls here than FD hulls IMO.

Again for scope the Willard 40 requires 23hp to make 7 knots.

 

[bayview]

Why all the fuel burn discussion? I thought the purpose of the loop was to burn as much fuel as possible in one boating season without spending much time anywhere.

 

[Bruce B]

I'd respond eyeshulman but I don't want to be the one thrown under the bus for being off topic. The topic is hull design and few posts have addressed that.

Re the question .. something to throw out here is that IMO most don't realize the wide range of SD hull design variations there is. Most of the people here that claim to have FD hulls (probably 75%) actually have SD hulls. That relates heavily to the question since the resistance of a SD hull that is very close to FD is going to have way less resistance than one that is almost a planing hull. For anyone really interested there is much posted on this on boatdesign.net. The hull shape of the stern mostly indicates what classification a boat should fall into. And there are a lot more planing hulls here than FD hulls IMO.

Again for scope the Willard 40 requires 23hp to make 7 knots.

So my in laws are not kidding when they claim that their Willard 40 uses no fuel!
I thought they were simply guilty of being enthusiastic about their boat...that and that at 84 years old perhaps they were showing signs of ...???
Bruce

 

[sunchaser]

So my in laws are not kidding when they claim that their Willard 40 uses no fuel!
I thought they were simply guilty of being enthusiastic about their boat...that and that at 84 years old perhaps they were showing signs of ...???
Bruce

OPEC hates Willards.

 

[Ken E.]

Eyeshulman......Do you ever run your boat as a single, ie turn one engine off? Most Twin Disc gears will tolerate being freewheeled, although many other gear brands won't, in which case you'd have to immobilize the shaft somehow (PITA) . Running at 7+ knots hull speed, I use less fuel, run up less total engine hours, and give the on-engine more load, which is beneficial. I ran on one side the entire trip to SE AK and back this year without a hitch. The autopilot has to crank in counter rudder, but that's a non-event while traveling. Of course, while docking, anchoring or going through high current sections, I've got them both on. Have you tried doing this?

 

[eyschulman]

Eyeshulman......Do you ever run your boat as a single, ie turn one engine off? Most Twin Disc gears will tolerate being freewheeled, although many other gear brands won't, in which case you'd have to immobilize the shaft somehow (PITA) . Running at 7+ knots hull speed, I use less fuel, run up less total engine hours, and give the on-engine more load, which is beneficial. I ran on one side the entire trip to SE AK and back this year without a hitch. The autopilot has to crank in counter rudder, but that's a non-event while traveling. Of course, while docking, anchoring or going through high current sections, I've got them both on. Have you tried doing this?

I have done it as test to see how it works. It works but I have no fuel #s. The boat is controllable and the autopilot adjusts for the off center thrust. When the boat was new 14K was attained on one motor.

 

[Nomad Willy]

Ken I think that works fair for some boats depending on keel, length to beam ratio, distance between prop shafts, size of props and gear ratios ect ect.

Just by doing that Ken you're a grand example of displaying that one does not need much power to go slow. But you'd be better off if you just took one engine out and converted to single. Less weight, less fuel required, less asymetrical thrust ect ect.

 

[Ken E.]

Not so, Eric. Many of us Double Guys like the redundancy of twins, and are willing pay a premium for the weight, complexity and cost of twins. I always chuckle when I hear single engine owners talk about the inherent reliability of diesels, and how they don't need a spare. That doesn't jive with the years I spent on salmon seiners out of Juneau years ago. Several times we were towed home when that one engine or drive train failed. At the wrong time, having a single-engine could be catastrophic. But, this is an argument much akin to politics and religion. I'm certainly not going to convince you about twins, or visa versa me on singles.

 

[BruceK]

Not so sure the rudder required to counteract the prop walk when operating a twin using one is so insignificant. A real world test is best, but anecdotally, a sailboat is faster with the rudder neutral, boat balanced on the sails.
Rudder when applied has to have a braking effect. We have run on one, not by choice,it needed a deal of rudder to track straight.

 

[Seevee]

Re: Running one of your engines......

Are there feathering props that will work?

And if running one, how does it work if you run one to push the boat and run the other just enough so it prop doesn't drag. So you one powered up a bit and the other at a lower power. Does it work?

 

[Ken E.]

Seevee.......Feathering props are available but are complex and expensive. I've seen them installed on North Sea type trawlers and motorsailers. They are rare here in the States as far as I know.

As to running one engine for speed and the other driven slowly to reduce drag, it sort of defeats the purpose of the concept. The slow one is burning fuel and idling for long periods, even though in gear. Long term idling isn't good for the engine. Better to leave the off engine cold until its next duty cycle.

 

[kmacdonald]

Reliability of an engine is somewhat determined by configuration and type. A naturally aspirated diesel is certainly more reliable than a turbo but suffers somewhat in efficiency. Loading of the engine and duty rating also contribute to reliability.

 

[North Baltic sea]

article which examines the topic of twins vs. single engine.


Single Or Twin Engines- Which Is Best? | | PassageMaker

 

[Fletcher500]

Is this your personal opinion? What facts do you have to say that a normally aspirated engine is more reliable than a turbo? There are hundreds of of thousands of turbo diesel engines out there, so I am curious to hear your thoughts on this? IE, if its your opinion based on personal experience, but lets keep the fact checker in place por favor.

Reliability of an engine is somewhat determined by configuration and type. A naturally aspirated diesel is certainly more reliable than a turbo but suffers somewhat in efficiency. Loading of the engine and duty rating also contribute to reliability.

 

[Fletcher500]

Edit.

 

[sunchaser]

A naturally aspirated diesel is certainly more reliable than a turbo .

i wasn't aware of that. I can tell you that in the off highway world that turbos are not a reliability concern. It is stuff like overheats, starters, fuel systems, electrical systems and hydraulics that accounts for most of the unplanned downtime.

BTW, what builder today makes a plus 150 hp non turbo Diesel engine?

 

[North Baltic sea]

If the turbo does not work, I imagine the diesel engine still fair, yes output falls and combustion is not optimal. Loads the machine just a little bit is turbo anyway do the job. As an example of the old boat twins Volvo KAD 44, 1600 under the RMP machines 100% naturally aspirated, 1650, the compressor starts, 2600 RMP KOMRESSORI stop and Turbo enough laps to continue the pressure is always 3900 rpm.

 

[kmacdonald]

Quotes from the article linked above:
Brian Smith of American Diesel, “It is important to size the engine correctly. American Diesel sticks with the naturally aspirated diesel design, and stays away from turbocharging for the majority of these applications.”

Bob Tokarczyk of Bell Power, “Buy a good reliable engine, an engine that is rated properly. Don’t go out and buy the smallest, highest horsepower engine-it just won’t be as reliable or as durable. Buy an engine that is designed for running longer periods of time, not just for high performance.”

 

[ksanders]

On my SD boat, at displacement speeds the turbochargers could fail and I would never know it.

Only when we get after it do we get to a point where the turbochargers are producing boost.

so...

From a reliability standpoint we would gain nothing by having a naturally aperated engine for 90%+ of our cruising.

Besides guys, this thread is about SD or FD, single or twin engines as they relate to cruising the great loop.

It's pretty hard to make a single/twin turbo/natural argument when you are cruising inside waters where there are parts houses close by, and if not FEDEX everywhere.

There are places where FD hulls and fuel economy shine. Long passages in the open ocean come to mind. It seems to me that inside waters like the loop might benefit from the speed advantage of a SD hull and a little horsepower to get moving to make that bridge opening, or fight that current, or make that unknown marina before dark.

If I get to a point financially where my budget is affected by fuel economy, then honestly I will not be in a position to even own a cruising size boat. It'll be time to sell for this mariner.

 

[Seevee]

I can relate to that. I'm not a fan of turbos. I'll take a bigger engine without turbos any time... but that's just me.

I've never understood the logic of turbocharging a small engine instead of just making a bigger one. If it's to save money, I'll spend the extra. And if operated at sea level, like most are, the only think is does is boost a small engine, and is just one more thing to break. Also creates a lot of heat and has a turbine that spins like crazy.

But a lot of people like them....

 

[kmacdonald]

When you get over 50 HP per liter displacement you are asking for reduced life and reliability issues. 20 HP per liter and it will run a lifetime.
Yes, most larger engines have turbos. A necessary evil to meet EPA emissions and get cheaper horses. Add aftercoolers and you add even more HP and maintenance.

 

[kmacdonald]

On my SD boat, at displacement speeds the turbochargers could fail and I would never know it.

Don't kid yourself. You may know it in a most unforgettable manner.

 

[North Baltic sea]

Turbo diesel engines have been already on the market for about 70 years. So there is no question of any fears to meet new invention.


I would argue almost all marine engines are turbocharged. Why pleasure craft, therefore, would not be? change the oil and filters regularly, you ditch run the engine warm before strain and engine cooler times before shutdown, so the turbo can outlive the rest of the machine lasts!

 

[markpierce]

Someone explain to me how adding a device like a turbocharger to increase power from a given engine doesn't make the engine work harder and thus increase maintenance costs and decrease engine life? Had the option to have a turbocharged JD4045 but elected the naturally-aspirated. (Current EPA requirements eliminates that option.) The NA engine provides plenty of power for my FD boat.

 

[North Baltic sea]

Someone explain to me how adding a device like a turbocharger to increase power from a given engine doesn't make the engine work harder and thus increase maintenance costs and decrease engine life? Had the option to have a turbocharged JD4045 but elected the naturally-aspirated. (Current EPA requirements eliminates that option.) The NA engine provides plenty of power for my FD boat.

Wärtsilä 31 the most efficient engine in the world

how many hours do you operate the engine in a year? how much longer think about the non-turbocharged engine sustainable? Why do nearly all commercial ships are supercharged and machines usually last longer than a steel hull, which are being scrapped and sold the engine?

 

[koliver]

Someone explain to me how adding a device like a turbocharger to increase power from a given engine doesn't make the engine work harder and thus increase maintenance costs and decrease engine life? Had the option to have a turbocharged JD4045 but elected the naturally-aspirated. (Current EPA requirements eliminates that option.) The NA engine provides plenty of power for my FD boat.

What is it that you have to do to maintain a turbo? I have had one on each of my twin engines for 22 years, yet have never had any added maintenance.
A friend had to replace a turbo this year. His engines are Cats (yes, twins) on his boat that is a FD hull built in 1912 and repowered in 1957 after 45 years on non-turbo engines. It has since getting turbos, run a further 59 years with turbos. So far that turbo replacement is the only "extra maintenance" required.

 

[Bay Pelican]

BTW, what builder today makes a plus 150 hp non turbo Diesel engine?

150 hp is the EPA cut off for naturally aspirated diesel engines.