Low speed DD running

[MikeP]

This was referred to in another thread but I thought I post it in a new thread rather than hijack that one.

Yesterday on the dock we were talking about our DD motors and the fact that most of us now run at hull speed instead of the speed that the boats/engines were designed to run. Consequently, there has always been "common knowledge" that the engines need to be run up periodically during the day.

I heard an explanation for why this periodic runup should be done that makes a lot of sense to me, one that I had never thought about before. I had always looked at it totally from a temp-related aspect, cylinder washdown, etc...until yesterday. The explanation was:

When the engine is running at low speed, the distance the piston travels in the bore is a tiny bit less than it is at high speed/temp. (THis is, of course, totally correct with any engine though it is a non issue with a car engine).. Running at low speed/temp for long enough will eventually put a ridge on the cylinder wall (and carbon deposits) at the point where the upper edge of the top ring stops/reverses direction. If the engine spends a lot of time at low RPM, the ridge will be located at a point below where the top ring will be at high power/RPM/temp conditions. So if the boat has been run for a long time at low power settings, if high power is called for, the ring can strike the ridge, causing the ring to fracture and the engine to fail.

I do not have any sort of data re this on DD motors but it is entirely feasible from a mechanical point of view since all piston engines form that cyl wall ridge. How much low speed/temp operation it would take to create a ridge that could cause a failure at high power, I have no idea. I would assume it would take several hundred hours but that's a guess based on nothing at all...

 

[whaler23]

I've always been "taught" since the late 70's anyway by my father that low speed idling (trolling off Cape May in his case) should always be followed by high speed runs. It worked for him for the last 30 years so I'm inclined to believe it. That being said last Sunday we motored at 10.5 knots from Solomons all the way to Eastern Bay and then ran it at 22 knots up to the narrows to do exactly what you refer to in your post. Temps were around 165 all the way and hit 180 for the high speed run.

 

[bobk]

Interesting thought.

WaltP, if you still have my old 6V92 kits, can you check them for a ridge and report here? These have some 400 hours by the PO at 'normal' speeds' plus about 1500 mostly at 1000-1100 rpm with run ups to 2000 rpm once every 10-15 hours. Run up time would have been generally just a few minutes to once or twice 15-20 minutes while trying to make a bridge opening. I don't recall a ridge when I saw them.

One additional point, the rings were intact and would turn at room temperature but not real freely. I presume the rings would be a bit looser when hot.

When running at low rpm, one concern is wet stacking. What are the symptoms of this that one can see while under way?

Bobk

 

[Walter P]

Sure sounds logical. Bob I do have those kits back in my container. I will check them out within the next couple of days and report back. If in fact I can detect a ridge, how will we know if it is at the location of "high rpm use" or from the much greater time at "low rpm"? Thoughts guys?

By the way, if I do detect a ridge, I will slit one of the cylinders in half and check it close on an optical comparater which I have access to.

Walt

 

[SKYCHENEY]

I doubt this is the issue at all. Wet stacking is the main concern. Its the sticking rings that take down the engine prematurely, not the throw length of the piston.

I think this is just another example of "Don't believe everything you hear on the dock".

 

[bobk]

I doubt this is the issue at all. Wet stacking is the main concern. Its the sticking rings that take down the engine prematurely, not the throw length of the piston.

I think this is just another example of "Don't believe everything you hear on the dock".

I'd sure like to know how to determine if my practice of running 1000-1150 rpm is causing wet stacking. With the new 170 degree thermostats, there is no smoke as I run them up to 2000rpm/15 knots. What else should I look for? I remember Genesis saying above 165F there is no issue.

Bobk

 

[krush]

Sounnds like the wannabe scientists escaped from the kitchen. Who the hell comes up with these theorie?

How about a little engineering fact? It is claimed that the temp difference is 165F vs 180F or at most 140F. Look up how thermal expansion is calculated. The temp difference is small, and the length of the metal pieces (liners and piston) is small. Therefore, the "length change" will be damn near irrelevant.

10-20 story utility boilers I work on grow 4-8inches from hot to cold...yor cylinder not so much.

 

[SKYCHENEY]

I'd sure like to know how to determine if my practice of running 1000-1150 rpm is causing wet stacking. With the new 170 degree thermostats, there is no smoke as I run them up to 2000rpm/15 knots. What else should I look for? I remember Genesis saying above 165F there is no issue.

Bobk

Just run em up once and a while and you'll be fine. No theory or science needed, just what most have realized from practical experience. You can't run slow for too long without getting the temps and pressures up and cleaning everything out.

 

[rsmith]

Pull an exhaust manifold off and look at the valves if you that worried. This topic has been beat to death on here. I torched an exhaust valve which I believe was from excessive slow speeds. Half the valve was gone, melted. And it happened when I spooled the engines up after several days of trolling at 850-900 rpm. When I pulled the head all the valves stems and the exhaust chambers were covered with hard chunks of oily black crud. When you spool the engines up this is what you see burning. And its burning in places like the valve stems and seats that were never meant to see direct combustion. Introduce some excess oxygen and you have a cutting torch inside your engine,which is what my valve looked like.

 

[MikeP]

Rod stretch is real and has to be accounted for.

We allowed .030" for rod stretch with steel rods, .050 with AL rods. Either figure is enough to break a piston ring on a ridge if it develops at the low speed/low power point and the engine is subsequently run up to full power.

In a car engine it's almost impossible for this to be a problem due to the constant change in rpm and power loads but if a boat engine is run for several seasons at hull speed rpm/load and then run at full power it could be an issue.

 

[rsmith]

Rod stretch is real and has to be accounted for.

We allowed .030" for rod stretch with steel rods, .050 with AL rods. Either figure is enough to break a piston ring on a ridge if it develops at the low speed/low power point and the engine is subsequently run up to full power.

In a car engine it's almost impossible for this to be a problem due to the constant change in rpm and power loads but if a boat engine is run for several seasons at hull speed rpm/load and then run at full power it could be an issue.

I may be wrong but comparing rod strech in a 10,000 rpm lightweight race engine to a over built low rpm diesel where your talking only a little over 1000rpm between hull speed and wot might be a bit of a stretch in itself (pun intended)

 

[krush]

mike, provide some sort of evidence...even anecdotal

 

[MikeP]

R, That´s true of course and I have no idea if it could actually be a factor in the DD service. It´s logical that it could happen but, as you noted, there is not a lot of RPM difference though heat is also a major factor.

K, If you don´t believe me as far as providing the clearance, look up any text on performance engine rebuilding. There will be specific guidance on clearances needed to ensure the piston does not strike either the valves or the head itself depending on engine design. Cyl wall ridges always develop on any piston engine so the logic involved in the ¨"dock talk" makes sense though again, whether it could really ever happen on a normally used DDengine seems unlikely.

I have since been told that the mechanic that brought this up has worked for many years on DD motors at the Naval Academy. I did NOT hear it directly from him.

 

[rsmith]

I know itsa problem in aircraft engines because they have "choke"

 

[luckydave215]

Rod stretch and piston/crank/crankcase flexure is real, any experienced racing engine builder is very familiar with it.
Don't think just because our old fashioned Detroits only turn 2300 rpm that they won't experience it, as it's a function of piston speed (quite high in a long stroke engine, even at relatively low rpm).
The highest piston speed found in street motorcycle engines is near 4000fpm, the upper ceiling for reliability, and it's found not as you might suspect in 14,000 rpm sportbike engines, but in old fashioned long stroke design Harleys.
With an 80mm stroke and an 11,000 rpm rev limit, we need .050" piston to head clearance in the race engine we're currently running to avoid contact at peak rpm. At .040", I saw light contact between the pistons and the heads. The engine is a Rotax built Aprilia 990cc twin with a supercharger and nitrous that makes 380 rwhp, my two partners and I ride it at the Texas Mile, a standing start one mile event. Best pass so far 192.8 mph.

 

[jim rosenthal]

Dave, is this like a fuel dragster engine that you get a few passes out of and then rebuild it, or can you get some time on it?

 

[luckydave215]

This will be the fourth time we've ridden this bike, (the Texas Mile is a twice-a-year event), and each time we end up running roughly 40 gallons of fuel through it on the research grade eddy current dyno we use to set up the fuel, nitrous, water/methanol injection/quick shifter, etc. before it even sees the track.
Each event requires a fresh assembly.
See http://www.apriliaforum.com/forums/showthread.php?t=233574 for more details.

 

[Walter P]

Well, I just got back from my shop where I inspected Bob Kassal's old cyl. kits. As Bob stated earlier in this thread, they only had about 400 hrs prior to his ownership (unknown kind of use) and about 1500 hrs at low speed since. I cleaned a cylinder carefully and felt for a ridge, which I did notice....slightly. Upon further inspection, the ridge was accumulated carbon which came off with a popsicle stick. I know that Bob's old kits are not really represented as typical, but one would think there would be some ridge. I also agree with Krush that the "stretch" factor is very slim to non existent on our engines. (Unless I misunderstood Krush) A very high performance engine such as most modern motorcycles have or some hot rod cars will probably experience measurable stretch.

In any event, Bob Kassel's engines will probably never have an issue with stretch so to folks like Bob and many others, myself included it's a non issue.

Walt

 

[krush]

So the rod/bolt stretch ya'll refer to is due to mechanical stresses (piston speed) not thermal....

I never said clearences and stretch/growth don't exist. I know all about theories and all that jazz...I spent several years listening to "experts" talk about it. But at some point we must speak practical.

Also, the rods,bolts, etc on diesel are much "meatier" than the hi-perf gas engines. Mechanical stretch is probably less or non-existant. The last thing designers of commerical and industrial engines want is unknowns. The engines need to last many hours.

 

[luckydave215]

So the rod/bolt stretch ya'll refer to is due to mechanical stresses (piston speed) not thermal....

I never said clearences and stretch/growth don't exist. I know all about theories and all that jazz...I spent several years listening to "experts" talk about it. But at some point we must speak practical.

Also, the rods,bolts, etc on diesel are much "meatier" than the hi-perf gas engines. Mechanical stretch is probably less or non-existant. The last thing designers of commerical and industrial engines want is unknowns. The engines need to last many hours.

Krush, rod bolts are already stretched, they're springs clamping the cap to the rod. I'm surprised you would misunderstand this.

"Meatier" is no factor here, it's the weight of the rods themselves that provide the force to elastically deform them. So, although "meatier" rods are stronger, they're also heavier, in the end it's a wash. Only material with a better stiffness-to-weight ratio can change it. Such material is too expensive for production engines.

Reciprocating assembly/crankcase elasticity is not an unknown, it's well understood, and is compensated for in the engine design by providing adequate clearance.