RPM minimum

What is the correct RpM standgas or minimum, 900 RpM are to much?
The engine are Ford Lehmann SP 135 natural.
_______________
Norbert

Please explain your question. At idle my Lehman runs at 600 rpm and still moves the boat. I believe the idle rpms can be set, don't know how that is done.

According to the operators manual for the FL90 and FL135, the idle rpm for both engines should be 825-875 rpm. The idle adjustment is usually found on the fuel injection pump.

The manual for the FL80/120/150 engines specify an idle speed of 600-700 rpm.

(Source: Lehman operators manuals on the Grand Banks owners site)

I had my 135s set at 700rpm. I think you'll find 800+ a bit hard on your transmissions for no reason.

Capt.Bill11 said:

I had my 135s set at 700rpm. I think you'll find 800+ a bit hard on your transmissions for no reason.

Click to expand...

I would agree with that, and in fact I found it surprising the idle rpm was called out so high in the operators manual.

Our FL120s idle at about 650. I've never run FL135s so I don't know how they respond to a low idle setting, but I would think that 700 would be a better choice than 825-875. At least if you have BW Velvet Drive transmissions.

in my service book I could find a indication, my idle speed is almost 900 RpM but the reverse gear is a bit hard in the pure noise. I change the speed with the adjusting screw or do I have to adjust the Bowden cables too?
______________
Norbert

Capt.Bill11 said:

I had my 135s set at 700rpm. I think you'll find 800+ a bit hard on your transmissions for no reason.

Click to expand...

Capt. Bill, could you elaporate your position for me. As this subject is not my forte, I'm not sure what you mean. If you run the engine at the manufacturers specified RPM, why would that be "hard" on the transmission? I would have thought that at some point and time during the engineering process the two manufacturers would have come together, or maybe not, and discussed these things. I don't work on systems of this size or type so your logic is interesting but unclear. Just trying to gain insight as I hope to be getting into a trawler soon. Thanks

It's not hard on the transmission its hard on the damper plate.

I have a friend who says you should set your idle as low as you can without it stalling in gear. Much better for maneuvering, especially with a single.

My Cummins book says to not idle the engine for more than 5 minutes, but it doesn't specify if it's in gear or not.

I don't think slow idle speeds are good for the engine and I know it's bad for damper plates. I set my engine at a bit over 1000rpm when warm and it idles dependably at 900 when it's only been run for several minutes. If your'e wondering I set my idle speed just high enough so when I back out of my slip and shift into fwd gear through neutral the engine shows absolutely no sign of quitting. The resulting 1000+rpm idle when warm has no downside whatsoever .. that I know of.

There's no good reason to idle excessively slow unless you're over propped. It is in fact one of the reasons not to over prop.

Anthonyd said:

Capt. Bill, could you elaporate your position for me. As this subject is not my forte, I'm not sure what you mean. If you run the engine at the manufacturers specified RPM, why would that be "hard" on the transmission? I would have thought that at some point and time during the engineering process the two manufacturers would have come together, or maybe not, and discussed these things. I don't work on systems of this size or type so your logic is interesting but unclear. Just trying to gain insight as I hope to be getting into a trawler soon. Thanks

Click to expand...

It's just causes your transmission to "bang" into gear needlessly IMO. Sure the initial force may be on the drive plate but it gets carried to the tranny and drive train at some point one would think. Just like dumping the clutch on a car.


Plus it can make for a noticeably higher idle speed through the water that some may be uncomfortable with.

manyboats said:

I don't think slow idle speeds are good for the engine and I know it's bad for damper plates..

Click to expand...

It's the other way round. The damper plate is what takes the "shock" of the transmission being connected to the engine by the shift lever.

Whenever you are connecting (pushing) two things together that are rotating, the closer the two rotation speeds are to each other the less shock and wear to the clutch or damper plate will be at the moment of connection.

In the case of a running engine being connected to a non-rotating transmission in neutral, the only way to lessen the shock and wear is to rotate the engine's crankshaft as slowly as possible. And the only way to do that is to set the engine's idle as low as possible without stalling the engine or causing it to run roughly.

The FL120 manual I have says to shift into gear at the engine's idle speed (600-700 rpm) and NEVER shift the engine into gear at an rpm over 1,000.

As an interesting side note, I wanted to know how long a prop shaft would continue to rotate after its transmission was shifted from forward or reverse into neutral. So we opened the engine hatch and timed it. With the boat moving at idle speed (about 3 knots for us), the prop shafts continued to rotate after the shifter was moved to neutral for about 2.5 seconds. This was the case with both engines.

What that told us is that when maneuvering and shifting from forward to reverse, we should pause for a minimum of 3 seconds in neutral before shifting to the opposite gear to give the prop shaft(s) a chance to stop. This reduces the shock of the shift itself, and enhances the longevity of the damper plates.

Obviously the freewheel rotation time will vary with the type and condition of the transmission, the condition of the cutless bearings, and so on. But with our boat, it's 2.5 seconds.

We also tried this with the boat at cruise speed, pulling then engines to idle and then shifting into neutral. The shafts continued to rotate until the boat had slowed to idle speed. This takes a surprisingly long time.

Marin,
The inertia issue when shifting into gear is another thing but what I was referring to is the effects of torsional vibration. Shifting only involves one shock in one direction but torsional vibration is back and forth thousands and of times even in a minute much less hours.

Shifting into gear varies tremendously from boat to boat. Direct drive and a light prop could probably be safely shifted at 1500rpm whereas a heavy flywheel and crankshaft w a deep reduction would require shifting at a proper idle speed w a pause through neutral. Some gear problems are the result of shifting too slow. One should shift quickly and to the stop without hesitation ... IMO.

Speaking of that I think your operators manual idle rpm of 850 is probably best for typical trawler applications. The lower idle speeds may be for non-marine applications.

Damper plate springs can only handle so much and for so long. A slow idle may cut the lifetime of a set of damper plate springs in half IMO. Long periods of idling like trolling for fish is bad too. Trollers (fish boats) have a lot of trouble w damper plates.

Capt Bill,
I think dumping the clutch in a car is different than quickly shifting a BW gear on a boat. I think the hydraulics control how fast the clutches engage the gears as long as one dosn't dally with the shift lever. So slamming the lever home causes no more shock than shifting normally.

manyboats said:

Speaking of that I think your operators manual idle rpm of 850 is probably best for typical trawler applications..

Click to expand...

Actually, that is the manual's description of the idle speed for the FL135. The manual for our FL120s specifies an idle speed of 600-700 rpm. Our engines idle at 650, and there is nary a discernable bump when the transmissions are shifted into forward or reverse. It's been that way for the 16 years we've owned the boat.

Interestingly, 600 rpm (or thereabouts) is also the idle speed of every one of our vehicles, including the Range Rover (small V-8), the Subaru (Flat 4), 1973 Land Rover (straight 4) and our new F-150 (big V-8).

Most straight sixes can idle well and shift well at 600-700. Don't leave it that low any longer than you need to, if not shifting kick it up to maybe 800. Each boat is different, so set it where it feels good.

Most here seem obsessed w low rpm operation of trawler boat engines.

What is the point of it all? Why is higher rpm as found in some Volvo's and Yanmars considered to be some kind of plague. The rpm designed into engines and used in operation should be dictated by engineers that know what engine speeds are best.

Actually I think my little Mitsubishi will idle slower than 600rpm once warmed up but what would I gain from it if I did that? Nothing that I can see.

Marin most of my cars idle at 800 but idling a Lehman at 600 may be just fine. Who wrote the operators manual for the Lehman? Lehman? Why do you suppose it specifies 850rpm?

Ski "need to" ... Don't see any need to.

Most engine manufacturers specify an idle speed. I'm w Tom White on this one ... follow da book. FF too.

manyboats said:

Who wrote the operators manual for the Lehman? Lehman? Why do you suppose it specifies 850rpm?.

Click to expand...

The engine specs were written by Ford of England, who designed and built the engines. The manuals that boaters have were written by Lehman of New Jersey, which is the company that took the Ford diesels and designed and had fabricated the marinization kits for them. Thus "Ford Lehman." The engine specs in the Lehman manuals were simply lifted from the original Ford manuals.

But we're talking two different engines here, Eric.

The original poster has Ford Lehman 135s, which is a marinized engine based on the Ford of England 135hp Dover diesel engine. This is the engine whose manual calls out an idle rpm of 825-875.

The engines I have in my boat are the earlier Ford Lehman 120s, which are marinized versions of the Ford of England 120 hp Dorset diesel engine. The manual for this engine specifies an idle rpm of 600-700.

While the Dorset and Dover diesels are similar in basic layout, they are not the same engine, hence the different horsepower ratings. The 120 hp Dorset engine was designed in the late 1950s. I'm not sure when the 135 hp Dover engine was designed, but my guess would be the late 1960s or even early 1970s.

The 120 hp Dorset was marinized by Lehman (and a lot of other companies around the world) in the mid-1960s. I don't know when Lehman marinized the 135 hp Dover engine, but my guess it was sometime in the later 1970s. Bob Smith at American Diesel would certainly know, as he worked for Lehman during this period.

So it's easy to confuse the two, but they are distinctly different engines from different time periods.

Trivia of the day: The British like to name things. They're the ones who came up with the name "Mustang" for the P-51, not us. Rolls Royce names their turbofan aircraft engines after British rivers. Ford was not so creative with their diesel engines. As I understand it, the 120 hp Dorset and the 135 hp Dover were named for the Ford of England engine plants where the engines were designed and manufactured.

Manyboats- Idle rpm is often an issue. On my personal boat, it has a Cummins 450 that per spec should idle at 600. At 600 the boat is doing 5kts and dragging a pretty good wake. So it is adjusted down to 550 in neutral, about 500 in gear. That gives a bit over 4kts dead slow, ok for no wake zones. For hull speed travel, it is 950rpm, about 7.5kts. Most of my travel is at 950 at 1.9gph as I am too cheap to run fast. At 1900, it is 18kts and 9gph, 2100 is 23kts and 13gph, full load is 2700 and 29kts and 24gph. Too expensive to run there unless in a hurry or if someone else is paying the fuel bill!!

So the book may say 600, but for my needs it needs to be slower. Slower it is, then. Any harm to the engine? Maybe, maybe not. I suspect not. In 3000hrs, all is still well. I doubt there will be any harm to a six, but some governors are unstable down there. Luckily, the P7100 is super stable down low.

manyboats said:

Capt Bill,
I think dumping the clutch in a car is different than quickly shifting a BW gear on a boat. I think the hydraulics control how fast the clutches engage the gears as long as one dosn't dally with the shift lever. So slamming the lever home causes no more shock than shifting normally.

Click to expand...

I was not referring to shifting fast as much as shifting at a higher RPM.

Just based on what I've felt through my feet and heard with my ears over the years, I'll stick with 600-700 RPM.

When our boat was new to us I occasionally got doing things too fast and shifted into gear at the too-high rpm of about 1000. The difference between that and proper shifting at 650 or so is pretty significant. At 1000 rpm there is a definite "shock" that can be felt throughout the boat.

At 650 the only way one knows the shift into gear has occurred is the boat starts moving. There is no bump, no clunk, no shock, nothing at all.

That tells me that all those whirly bits down in the engine room are a whole lot happier with me shifting at a nice low rpm than at a higher one.

The practical issue for me is when maneuvering at low speed. You want to be in gear, and out, and back in, and you don`t want the crockery coming out of the cupboard to see what the heck is going on when you shift into gear. Result is I tend to set higher "idle"rpm during close maneuvering, and anchor raising. Anchor raising, the windlass mfr says to run at an even higher 1000 rpm, to feed the 1200w motor. I haven`t had any bad effects, but I am conscious of not abusing the machinery.

If you are idling too high, your maneuvering speed is too high, so you end up shifting in and out of gear in order to keep the speed slow. Also, your rudder is nearly ineffective when you are in neutral so you have to keep shifting into gear to steer.

Also, your transmission type matters too. Velvet drives are all or nothing when you shift. Twin Discs can be eased into gear to ease the shock and this can be done at higher engine speeds.

Set up your boat where you like it. Like Ski says, doing 5 knots at idle is pointless and just wears everything out faster.

RPM by it self does not communicate much info without knowing the gear and prop specs when discussing maneuvering.
I too am not a fan of high rpm shifting. The prop spinning while being shifted to the opposite direction is one reason.
Ski is correct that a six will idle smoothly down to lower rpm than a four. I wonder if high idle numbers were simply a copy of the idle speeds of the 4 cyl model if there was one.
The noticeable jerky slow idle of 4 cyl can't have good effect on dampers and transmissions. Slow a six to 500 rpm and it too gets jerky. The jerking can't make bearings happy either.
Modern engine have a bumper spring in the IP to reduce the need for high rpm when shifting.

Many thanks to all for the interesting discussion

I have now found a operators manual from July 1983 of Lehmann Power:


" When properly serviced and after the initial " break in " period your engine should idle within a general range of 600 to 700 RPM; when new, idle may be somewhat higher. "


__________________
Norbert

Interesting. If that's a manual for the FL135 specifically, it seems they recommend an idle rpm that's lower than the idle rpm called out in the table of engine specs from Ford. 600-700 rpm seems to me to be preferable to the factory's 825-875.

I never seen the 825-875 numbers before this. Only the 6-700 numbers.

OK I may be off base on the slow idle. I had a Sumnercraft Express 29 that had a 120hp Sabre and it was supposed to be a "bluprinted" Ford 380 cu in engine that I was told was the same block or base engine as the 120 Lehman Ford. Never did confirm it and I wish I could remember the engine speeds I used. I do remember it topped out at 2500 (20 knots) .. right on specs but I don't recall the idle. If I had to guess I'd say 700. And if it had a flywheel and crankshaft weight with a compression ratio that resulted in smooth operation even 4or 500rpm may be appropriate. Like a diesel electric a very low idle speed is nice in tight quarters.

Something interesting about this conversation is that there's lots of talk about the BW gear suffering or sending "shocks" through the boat when put in gear. I have the same gear most of you do (BW) but w an 18" prop and the flywheel and crankshaft of a modern 107 cu in Mitsu. I have shifted into gear in the vicinity of 1200rpm w complete smoothness. But my gear was rebuilt and fitted w soft clutches and a hydraulic gear pump gear i/3 the standard size. So my shifting may be slow as a result. When I get the boat back in the water (week or two) I may try a 1500rpm shift and report back.

I certianly wouldn't regularly shift a boat into gear whereas there was a big clunk or shudder resulting. Nor would I idle at X amount rpm when 1 or 200 higher the engine was twice as smooth. Gears and damper plates take too much of a beating when the idle shudders significantly and a slight (1 or 200rpm) increase should be made. How many here have an idle of about 600 whose engine is much smoother at 800?

Thanks for the clarification on the two engines Marin.

And for an over propped boat idling as low as possible is preferable until or if the problem is rectified.

If one has weak clips holding your universals on the propeller shaft, it will come apart ...

"If one has weak clips holding your universals on the propeller shaft, it will come apart ..."


Or one or more of those clips may not have been well seated in the groove.

Capt.Bill11 said:

I never seen the 825-875 numbers before this. Only the 6-700 numbers.

Click to expand...

Here you go. Straight from the horse's (Ford"s) mouth. Ninth line down, "Idling Speed."

Marin said:

Here you go. Straight from the horse's (Ford"s) mouth. Ninth line down, "Idling Speed."

Click to expand...

Thanks. Never seen that. But of course I never thought to look. As all the ones I've ever seen seemed to be set below that.