Discussion in 'Hatteras Yacht' started by Captain Dufy, Nov 8, 2015.
Same head, Same messed up back. When younger, I did it my self, Now near crip, I hire it out.
So the Army has different criteria, really just a set power for a limited amount of time, then rebuild and start all over. EPA emissions get waived. Their ground vehicle fleet is huge, old, and all over the globe with foreign military sales. Do you think the EPA is a factor in US military vehicles operating in the Middle East? I don't think so, just deliver the power required for the defense application for the duration specified and you are in business. Armed Force lives are on the line , and the EPA is a non-existent blip on their operational radar screen.
Even the Cummins VTA 903 is still alive and kicking at ungodly ratings from 900 - 1000hp in land vehicles. In marine form, the 903 only hit 450hp.
Ok now we have sound opinions about 8 71s and 12 71s. But can they efficently power a boat as heavy as an Hatteras without costing a fortune in fuel.I wonder if it would not be simpler to buy an older Hatteras ( cheaper) and repower with cats or cums?
And can somebody explain to me what the added TA or TI means and what do they add to the power and what is their consequences to reliability ? Then i can go shopping at least knowing what to look for. Thanks
Horse power moves a boat.
1/10 of a gallon per hour per horse power / 2 for a turbo diesel (each).
The modern fuel savings of the newer engines are the h p to weight, and the reduction of over fueled emissions when not making max horse power.
You want to move a Hatteras, this is where you start. Save weight, smarter fuel burn with no over fueling. These things cost bux.
Or spend the extra dollor$ and run what you have.
It may be a life time before the new equipment pays it's self off in the fuel savings (if ever). Then you still have an old boat.
I'm on the old front. My Detroit's ran well today from Jax to StAugustine. 100 gallons off shore against a south wind and current. Even tried some fishing off PV (No Joy).
A QSM11 would have consumed the same fuel with todays weather.
A calmer day on flat water, a QSM11 would of out run me on less fuel. It would take near 15 years of this for a (rebuilt) set of QSM11 to start saving money after the investment. The credit card interest alone would kill this on a south GA budget.
Even if one of my 12V71 blocks really cracks, it's still cheaper to stay with what I have. Heck, it's a 40 year old boat.
All Detroits have supper chargers, shortly called blowers and without any turbo charger, the term is naturaly asperated or NA.
TI - Turbo/Inter cooled. After the first air compression by the turbos, this hot air is cooled thru a raw water heat exchanger before it enters the blowers, then the air box, then the cylinder.
TA - Turbo/After cooled. The hot air from the turbos travels thru the blowers, then in the air box (V valley) is the heat exchanger that cools the air just before entering the cylinder. This offers the coolest/compressed air needed for extra horse power.
TA have a bit more plumbing and an interesting (may complicated) heat exchanger system.
It DEPENDS how you run them. At cruise they don't use terribly much more fuel than say a modern cat or man. Maybe 10% more fuel. Under the turbo's they're actually about 20% more efficient than new engines. I ran a Hatteras 75' MY with 12v71 TI's, at 1000 rpms we did 10.3 knots at 12 gph for both engines. A set of C 18's would probably be around 16-18 gph. At cruise a different story.......If you do the 1000 rpm crawl, you should run them up to cruise for 30 minutes every 6 hours to clean them out.......
Capt. J, I am looking at a Hatteras 80 with DD 12v71 TIs ...my question is this. I am moving over from a large sail cat with obviously much smaller 100hp turbo yanmars. I nearly always cruised under power on one engine at about 8 kts. to conserve fuel and engine time. Is that a practical alternative on the Hatteras to cruise under one engine, alternating engines obviously but hoping to achieve a burn rate of perhaps 8 or 9 gph.? Your thoughts appreciated. SailorVee
I wouldn't run them on one engine. The engine is operating in an overloaded situation (being it's propped based on both engines). 12v71TIs will burn 12 gph for the pair (both) and you should get around 10.5 knots at that RPM. That's pretty darn efficient for a yacht that size.
Thank you for your prompt response. I must understand this much less than I thought I did. I "get" engine overload in attempting to achieve speed and exceeding rpm, boost and engine design temperature parameters, but I am having trouble getting my head around why applying single engine low rpms while trying to achieve 8 to 10 knots on a displacement hull that normally cruises at +20 kts would create an engine overload condition. What am I missing?
The engines are propped so that both achieve 2300 rpms at WOT. If this vessel was built with 1 12v71, it would have a lot less propeller pitch. They have a propeller curve and at a certain lower RPM they should see a certain load. If you are only running 1 engine instead of both, you will then see DOUBLE the amount of load at a given RPM....... Sort of like driving a manual car at 20 MPH in 4th or 5th gear, The engine will be really lugging to make that speed. It's not good for it and you won't save any fuel by doing it. DD's are VERY fuel efficient at 1000 rpms or under. You want to save fuel, do 800-900 rpms with both instead of 1000 rpms. You're also going to have to run them at cruise for 30 minutes every 6 hours to clean the airboxes and turbos out.
Don't forget that you can't windmill-drive a gearbox for long before causing damage. Big engine gearboxes don't usually have an oil pump driven off the output shaft, so locking down the shaft and dragging a prop will work against fuel economy. For what it's worth, I used to run a 68' Chris Roamer with 12v71TI's, and traveling up and down the West Coast at around 1300 RPM averaged 1.2 gallons per mile with 20kw Northern Lights running. The boat was extended with a fuel tank swimstep, so picked up a performance and fuel economy boost from increased waterline, or so I was told by the previous Captain.
Your biggest issue in running on a single engine is what to do with the trailing prop. You can not just leave it in neutral with the engine off as you will fry your gear as it rotates without cooling to the unused neutral gear. You can not used the gear as a brake by putting it in gear. So you have to have either a shaft brake apparatus on each shaft to lock it in gear or have an external cooling/heat exchanger system to sufficiently cool the gear if it is left in neutral. Leaving it in neutral and spinning the unused prop is a bit more efficient than locking the prop shaft and just "dragging" it.
I don't understand any of the comments of overloading the single engine underway if you are just running on one screw at 8 knots or displacement speeds.
It is nonsense, as the single screw is running a bit better loaded at 8 knots as compared to having both screws online at 8 knots, so you are in a much less "under" load condition. You can alternate use of screws on multiple legs/trips and keep the engine hours balanced.
An engine has a propeller curve where it will and should see certain loads at a certain RPM based upon it achieving WOT RPM. The newer electronic cats will not go over 1100 RPMs if they see to much load at that RPM point because CAT feels it's an issue. At 1000 rpms, a typical Detroit diesel 12v71 should see a load factor of somewhere around 30-35%. Well run one engine when the boat has 2 engines AND propped for both engines and it's seeing 60% (guestimates) at 1000 rpms if only one is in gear and pushy a big heavy 80' boat. You are LUGGING the engine and overloading it for that RPM point. You will burn more fuel because you're also dragging a propeller and crabbing and the engine is working a hell of a lot harder than it should be at that propeller point. You will also be running much higher EGT's at that RPM point.
It's no different than running the pair of engines in the same boat and being grossly overpropped where they only make 1900 rpms at WOT than 2300. It is MUCH harder on the engines throughout the RPM range and they will live a much much shorter lifespan.
Geez, where to respond?
There is no pre-defined load assigned to "certain" rpms, especially for a mechanically injected engines. Your guesstimates of load have no basis, an engine is not in a lug down condition until it actually starts to lug down and see the propeller demand exceed its ability to provide the needed horsepower, try it, run your twin screw on a single engine all the way to as much rpm that it can deliver and you will find the lug down rpm for your boat. And I gaurentee you it will not be at 8 knots.
At displacement speeds it is not as aggredious as you claim, it can be done in a safe and non-damaging way and could be healthier for the single engine. The load is not a 2:1 ratio as your simple math wants to convince you, it is about what horsepower is demanded from the propeller. Your understanding of LUGGING an engine is flawed as it does not follow the entire propeller curve and is creating false conclusions.
Any marine engine should be propped to achieve WOT RPMs fully loaded. Running an engine in a manner where WOT does not achieve WOT rpm's it is overloaded throughout it's ENTIRE RPM range, plain and simple. So if you run a twin engine boat, propped to achieve 2300 rpms utilizing both engines, with 1 engine, it is in an overloaded state at ALL rpm's because the load does not match it's propeller or HP/torque curve. AND, yes those load factors are very close to what I posted. CATs engine computers will not even let you run them over 1100 rpms in this manner and will generally throw codes. You're logic is like saying it's ok to prop a 12v71TI to make 1700 rpms WOT and as long as you run it at 1000 rpms it's perfectly ok for it and doesn't hurt it's longevity.......BS. Lugging is anytime you ask the engine for more HP than it is capable or meant to create at a certain RPM point. Taking a tractor trailer fully loaded up a hill in top gear at 1200 rpms would lug it.
Another example is Arneson boats. They're grossly overloaded anytime they're at hull speed with both engines. Arnesons are propped to achieve WOT rpm's utilizing 1/2 the propeller. So at hull speeds you're running twice the pitch you should be. I have one where the governors with DDEC's cannot even get the motors up from 440 rpms when you put them in gear to where they're supposed to idle at (680rpms) until the boat gets moving forward 150 yards and the drives are in the totally up position.
Your logic is so far off the mark I won't address it anymore, you have no technical background to understand why you are so wrong.
Back to the question asked earlier - yes, you can run your vessel on a single screw at 8 knots, as long as you address the trailing prop issue of the off-line engine . That is an undisputed fact.
Run this scenario by the Detroit Diesel dealer and they will tell you the exact same thing. Actually they'll say detroits were designed to be run at cruise.......at 8 knots the single engine is much more heavily loaded than it should be and you'll burn more fuel doing so and run it at a higher RPM than running both at a lower RPM to achieve 8 knots. You don't have the technical background that the boat was propped based on using BOTH ENGINES. Running any engine in an overpropped situation is BAD for it. Running a twin engine boat on one engine and it instantly becomes overpropped. Not to mention the higher propeller blade load on the propellers, the more load on the one that's running cutlass bearings, more wear and tear on the steering, the higher load on the bottom end of the engine, plus you have to lock a shaft and you're dragging around a propeller. Do you realize the HP requirement for a given speed is the same????? So how are you going to save fuel forcing 1 engine to make twice as much HP and drag a dead propeller around?
We probably should of started a new thread on this. Admin, with their magic keyboards may make it happen.
We run a 1978, 58 x 18 Bertram MY with twin 650hp, 12V71TIs. Original TD 514 clutches. 2.5" shafts, 32x32 wheels. We have a nice keel. Rudder surface area has been increased by roughly 15%. Trim tabs have very little effect less than 15 kts. Non Stabilized. FlowScan installed on everything (including gen-sets). Air compressor and dual alternators (32 & 24Vdc) direct drives off the engines.
I don't want to spread any extra organic solutes on anybodys parade here but real life results may help all camps on this single vs twin 12V71 load/fuel thingie that has developed.
Offshore on LDS (Longrange Displacment low Speed) runs. I still try to turn for 10 kts (not so slow maybe). Against the Gulf stream it may take up to 1450+ rpm. With the current as low as 1100 rpm.
10 Kts just makes it easy to DR on long rides that may be 80 to 200 miles. Last long leg was from Strangers Kay Bahamas direct to Jax Fl.
Lets say 1250 rpm average to turn 10 with no current (close to factory specs).
Both engines sync'd up and running at 140 - 145 degrees (180 thermo stats), no turbo action.
No gen-set running (inverter system) we do rite at 2 gallons per mile. All easy math.
Years ago we tried one engine operation, before the (high speed) rudders were modified. Engine temp did come up (thought that was a good sign). I had to keep the outboard (leeward) engine running from the wind. With the windward engine running, the AP has a heck of a time driving a straight line. In the rivers, you will have a hard time turning towards the running engine (the improved rudders helped just a little later on).
Now near 170 degrees, the turbos still have no effect. 1450 to 1500 rpm to maintain 10 kts. Rudders are over 15 degrees (later 12+ degrees).
It is a micro amount, @ 10 kts, we burned (0.1 mpg) more fuel.
The added drag did require a micro amount more fuel. The added drag of the turned over rudder and propeller out weighed (drag) the parasitic hp loss in the second running engine/ clutch on line.
Even though the higher temps probably made a happier engine and clutch and included oils, for us @ 10 kts, it was not worth the hassle.
AND Worrying about even the mighty T D clutches without fresh oil on the down engine.
Remember, it takes a certain amount of H P to move a boat. Ad more drag (rudders over, dragging propeller), you will require more H P for the same speed.
As far as lugging the engine, Ah, ae, oh, ah,,,, I cant say for sure but I don't think so at low, aspirated engine speeds..
Now, turn lower rpm's; harder to turn towards the running engine, more rudder to drive straight if you can at all, more drag fighting even lower h p to drag. I can imagine better mpg, but on a single but I don't think so,,, unless you like driving in great circles.
You start revving higher on one of two (wanting more hull speed on single engine), black puke will start pouring from the exhaust. IMO, here is when you start damaging your equipment. Lugging the engine, no turbo assist, more fuel than air, yada. yada,, NOT good.
If the engine can not spin up to turbo speed, make compressible air, then your over propped. You will kill a possibly good engine.
These are our thoughts; Run both, maintain your engines and clutches well. Throttle down both if you really want more more MPG. To clear the air box and turbos from excessive slobber, Run them up before making port.
Usually, we open them up a bit before entering an inlet to ensure all is good (if needed) and either in the inlet or when in Jax, shortly before we enter the Ortega river range.
IMHO; Run Them Both.
Pacblue you have obviously never Captained an 80' Hatteras MY from that era. In any kind of sea above flat calm....running that boat on one engine at 8 knots in the ocean and you'll do 120 NM over ground to do a 100NM trip. Almost impossible to steer it straight at that speed on one engine. Not to mention on autopilot it will swing 45 degrees in either direction on 1 engine.......Not to mention an 80' Hatteras with 12v71's is an extended boat with the props and rudders further forward than where they should be.