Discussion in 'Engines' started by chesapeake46, Dec 3, 2015.
I have witnessed the reversing buckets get ripped off when done a speed.
That makes perfect sense, thank you. Yes, I am familiar with the squeeze bang valves/ports of the DD.
EGT declines more evenly when the engines are brought back from cruise slowly. EGT is component of load and fuel......in a rich condition with same load, lower EGT temp. The slow deceleration causes the turbo EGT temperature to come down much more slowly, instead of spiking down and metal parts within the turbo contracting much quicker. If you ever run a set of MAN common rails and look at the EGT temperatures when coming off cruise.....they go from over 1200F to less than 400F pretty darn quick if you chop the throttles.
If you're running a twin turbo prop airplane, at cruise speed you can lean them out to where the turbo's are glowing red and can see it from inside the plane (at least on a King Air), and richen the fuel mixture and no more glowing red turbo's.
Uh J ... a King Air is a turboprop, always has been. If any part of the exhaust is glowing I can assure you it isn't because the pilot found a mixture control to play with.
Do you know what a turboprop is?
Assuming that post #23 is not just a troll, I have some more news for you ...
Regarding fuel air mixture ... I hope it doesn't come as a surprise that diesel engines do not work the same way a gas engine does.
All the builders trials & classification trials that I've been on had a crash stop performed and its a little unsettling the first few times but you get used to it. All classification society's require a crash stop be performed for vessel certification. This involves the machinery being set at max power ahead and then a quick transition to max power astern & the course kept and stopping distance are then measured and duly noted.
Engines and gear box's observed and measured to ensure the units are solid on their mounts during crash stops. Most, if not all surveyors will ask for a crash stop to be performed when conducting sales surveys (or the good ones will). I find the order of "helm hard over" at max speed test much more unsettling than the crash stop test but there are other running tests & trials that would make you think twice about doing them under normal circumstances.
I always got a kick out the crash stop tests on boats with EMDs and Falk air clutches. The smoke and sparks had to be experienced to believe.
No turbo failures noted though.
Yes, that can happen. Even that needs to be done in some moderation. At 35 mph it generally works ok and I've seen people try it at 60 mph with bad results. We do not do it with our RIB's. Don't even want to think what might happen.
I assume these crash stops are for newer Yachts/ships and equipment. I can understand the want for these things to work. Got to be one heck of a strain on the equipment.
I've run many surveys on smaller boats (less than 60' and old) where the surveyor request idle (some times fast idle) speed full shifts and while he observes the engine mounts. When they yell up faster and quicker, I'm glad I have the forms signed by the owner.
High speed full reverse shifts? That is a lot of faith in the systems that somebody else (currently in another part of the world) designed.
But that's a one time thing. We were a bit shocked our first time as part of a survey. My wife thought it was fun. I did not, but I was glad to know it was possible. But we've never done it again to a boat. One time for each is what they've gotten. Now, the helm hard over I'd done before. Frankly, I consider it a very important aspect of a boat's safety and handling. I see a very realistic possibility that in the lifetime of a boat you'll have to do a helm hard over at some point to avoid an accident. You have the same issue in cars. Can you make an extreme move or hard turn to avoid an accident without flipping. Many SUV's were horrible in that respect before it became a big issue. The propensity of Jeeps to flip are what brought the attention. I guess the crash stop too is the equivalent of slamming on breaks as hard as you can. I want to know if I have to do it, I can, but I hope never to have to. I would hope it's led to improvements in boats over the years. I know it has in cars. I've stopped to avoid an accident and thought that the cars when I started to drive never could have done it.
I remember when they started testing bass boats on their equivalent of helm hard over. At least what I saw happened shortly after a couple of bass fishermen had their steering come loose at 50+ mph. One just flipped around and was fine. The other was thrown out and drowned while the boat continued to circle since he was not using his kill switch.
We are on all of this talking however about one time deals to know your capabilities, not something you want to do on a daily basis.
The subject is (was) rapid reduction of power from max to idle. It is exactly what those same engines and turbos do a thousand times a day in garbage truck applications and in "dozer boats" and ship assist tugs.
While going from full speed to idle may create handling problems for some boats in some sea conditions for some people, it is not, repeat not, an issue as far as the propulsion system or its component parts.
It is not, repeat not, a problem for small turbochargers and by small I mean even the size of those on a 35 or 3600 series CAT, not just a nano turbo like on a little Yanmar.
Are you kidding? Going from full speed to idle speed instantly by chopping the throttles, stresses a lot of things on boats. Drive couplings, to name one when the props are dragging the boat to idle speed from 20+ knots because they're barely turning and are now creating drag instead of propulsion on planing hulls. Engine mounts get stressed. Transoms get stressed as the wake plows into the stern and puts excessive back pressure on the exhaust which stresses the exhaust hoses and clamps on most boats with aft exit exhaust. On Detroits it stresses the blower gaskets and can sometimes cause them to leak, just like pulling the air dams on them at high rpms. Yes, you can get away with it 99.9% of the time, but it does indeed stress various components more than during normal safe operation and most engine manufacturers will recommend not doing it unless you have to.
Having flown a turbo prop (ATR-42 with PT-6's) I can assure you there was no mixture adjustment in there...... IIRC it did have a FADEC. (Kinda like a DDEC but different).
I think you take these position just to get a response? You're offering evidence of a 'test' as to how you should run your engines on a regular basis? Do you regularly slow your boat with a "throttle" chop ( I love that phrase). This question arose because someone on this thread or another said they did it on a regular basis, and we all responded we didn't think that was good. Are you saying he should continue to do that with out regard to his engine longevity/health. Direct answer would be great -I have never operated a garbage truck. Just trying to learn from you guys.
No, and if that is all you have taken away from the discussion then you should try reading posts #25 and #30 again. Then go back and re-read the whole thread.
Just for fun let's do another debunk session:
"Drive couplings, to name one when the props are dragging the boat to idle speed from 20+ knots because they're barely turning and are now creating drag instead of propulsion on planing hulls."
So those boats should never use reverse power? Do you really believe that prop drag is greater than applying reverse power?
"Engine mounts get stressed."
Stressed more than if reverse power is used? Do you really believe that prop drag is greater than applying reverse power? Are there a lot of sportsfish boats falling apart because they didn't wait until the boat stopped before backing down?
"Transoms get stressed as the wake plows into the stern ..."
Do you have evidence that the "stress" imposed on the transom by a wake is any greater than any other load for which the boat is designed? Don't forget, the boat is still moving forward when the wave hits the stern and is probably oblique to the wave front so a great deal of that load is reduced. Show me the data and you might have something other than another myth. I am open minded, defend your thesis.
"...and puts excessive back pressure on the exhaust which stresses the exhaust hoses and clamps on most boats with aft exit exhaust."
Do you think the exhaust flow is the same at idle as it is as full power? Where does this "excessive back pressure" come from? How much backpressure is not excessive and what pressure can the exhaust system withstand before the hoses and clamps are over stressed?
"On Detroits it stresses the blower gaskets and can sometimes cause them to leak ..."
When the engine speed is reduced to idle the flow through the blower is also reduced to its lowest level. The air moved by the blower is at no more pressure than at idle, how can it be anything higher and how can idle output "stress the blower gaskets"?
" ...just like pulling the air dams on them at high rpms."
The emergency shutdown air flap is used to block air flow when the engine is running under power. If the engine is running at high speed then it is receiving more than idle fuel flow. The blower output of an engine reduced to idle is not blocked by the air flap and cannot increase above normal operating pressure. Bottom line, it is absolutely nothing "like pulling the air dams on them at high rpms."
If you think it is a bad idea to chop the throttles, just don't do it. No one is telling you to do that.
No one is telling you or anyone else how to operate a boat. But, don't spend a lot of time trying to tell people that chopping the throttle will cause all kinds of problems and damage.
I am stating very clearly that most of the stuff posted here about the threat of damage to propulsion machinery is pure myth and fantasy and much of it is absolute rubbish. People should read the posts very carefully before taking any feelings, beliefs, or imaginings as operational advice. Consider the source, consider the quality of the "facts" presented, check them personally, and consider their relevance to the issues.
I think the old saying of "If your good to your equipment it will be good to you" applies when operating any machinery. Pre lubing, warming up & applying power and reducing power slowly and smoothly would be prudent and will more than likely extend the life of your machinery. That being said, there's plenty of real time everyday testing and trials that show no data of couplings being stressed by drag, blower gaskets being blown nor transoms cracking or stressing from hydrodynamic loads from sudden deceleration nor delaminations of exhaust tubes or hose clamps reaching their failure points from sudden and substantial water ingress. If this were factual than there wouldn't be any classed vessels floating and the maritime industry would be riddled with casualty incidents and corrective steps would be taken within the industry from manufactures, classification societies & insurers. The majority of these quotes and statements I put right up there with the Green flash when it hits the horizon. I also don't feel that some members post replies to elicit controversy but rather want the author to provide documented proof or reasons for their statements & refrain from from propagating myths and armchair bloviation.
I appreciate the myth de-bunking, have to land in the camp of facts. Typically supported by engineering.
I am also reminded of a story about Bart Miller, legendary captain of the 42 Merritt, Black Bark out of Kona, Hawaii. I recall reading that he use to pull the stunt of running back to port at speed , aiming for the buoy marker and then grabbing full reverse on one throttle, pin wheeling her right around the buoy. I can assure you he new exactly what that Merritt propulsion system was capable of.
We (those who operate vessels) are usually the limiting factor, we tend to pull back while the boat can usually take much more. It is nice to have that margin in pleasure craft. Don't get me wrong, a conservative approach is fine, but sometimes its can be rather fun to push the limits. Safely, with everyone aware.
I was able to due most of the testing for a manufacturers' first Zeus model. It was a blast to be running wot and throw it into a full lock turn. Would be able to reverse course in what seemed like a diameter of less than two boat lengths and do it all over again. Chop throttles, hard accelerations, whatever we could do to find a weakness and break it. Never did break anything over a period of months and different models.
You contribute a tremendous amount of knowledge here. Most of it is way way over my head and pay grade. My point is that you leave the impression that you are a proponent of chop throttling as a regular operation. Direct question are you ? simple yes or no.
The problem with many readers is they want a simple yes or no to a question that has so many variables that the question itself is invalid. Any simple yes or no answer would be equally invalid and as useless as the myth on which the question is based.
What I personally prefer has nothing to do with the reality of the effect on the machinery but since it seems to matter to you for some reason, I prefer the easy does it technique.
That doesn't mean I have the slightest qualm or hesitation to (using a flying term) "chop and drop" when the situation or my mood calls for it. Any reader is free to use any technique they like, I am not one of those who says do it this way or invents some fantasy reason not to.
As a "regular operation" ? I don't have a boat in an operation that benefits from that sort of power management technique. But that doesn't mean there are not hundreds or thousands of boats that do that all day long as part of their normal operation. There are thousands of turbocharged diesel engines that operate from idle to max power and back within seconds, hence the garbage truck reference. You don't have to operate one to know how the engine is loaded and unloaded There are also thousands of industrial and mining vehicles that use the same engines as the typical boat found on this site, they also operate for thousands of hours in ways what some here would consider foolish.
The answer is yes. If you look at Sportfish that are fished hard in tournaments, they sooner or later have some of these failures that you're stating. Like the post where the muffler blew out when they started it and sunk in the slip (posted here). I've seen lots of failures on hard run Sportfish from backing down hard on fish and etc. Exhaust hoses ruptured or blown off. Shaft couplers broken. Shafts broken at the keyway. Motor mounts broken. Motors that ripped the motor mounts clear out of the stringer like the 63' Bertram SF hull #1 on it's first demonstration ride. And on and on. Chopping throttles and doing hard back downs and maneuvering does stress a boat more than usual.
And YES, I have done all kinds of crazy manuevering, like testing the first twin disc transmission where you can go from full fwd to full reverse, while twin disc adjusted computer settings for the gear, running a ZUES boat doing demo rides for CMD, Running the first MAN 900 common rails in the US with Man North America, so they could adjust the fuel mapping in the computer for power/throttle response/smoke. Wringing a Zues boat out to it's limits over and over. And yes, I have broken things in the process. On one of the first ZF pod boats, the carbon fiber driveshaft from the motor to the pod shredded at the engine coupler, etc. etc.