Limit on lugging a diesel....

She's designed to cruise at 20-24 knots,she gets better nmpg at 24 than at 14.
People never believe me when I tell them that.
Engine is in peak,props are tuned for that.

So yes, a couple small Yanmars with props tuned for 9-10 knots- I've wondered that. Maybe 2.5 to 3 nmpg?

If I put in CPP it'd be better,but at the low hours I do, to make up the cost it would take me decades to pay them off.
But if I nailed both props on a reef or blew up both (never happen) transmissions I'd be on the phone ordering CPP the next day.
If I blew up one,I'd need to run the costs....

 

It is clearly stated "cubic curve" in both the power output and specific consumption. Thus it is not for the maximum output.

If you look at big engine at low load vs. small one at high load, note that bigger ones typically have better efficiency, even at low load. E.g. 6CHE-DTE3/UTE (154/188 kW) are below 260 g/kWh all the way to 1400 rpm along cubic curve while 4JHYE (35 kW) has its peak efficiency at 270 g/kWh (200 g/hph).
http://www.yanmarmarine.com/theme/y...gines/Technical-outline-Drawings/PDF/che3.pdf
http://www.yanmarmarine.com/theme/y...ines/Technical-outline-Drawings/PDF/4jhye.pdf

 

I have seen big one-lunger lobster boats in Maine -circa 1962 - gasoline, but similar to the 6-1 listeroid. These were about 25 - 35 feet and essentially of open construction. Fitting Petter air-cooled one cylinder engine in native fishing boats is documented online - long url - withal, it seems to me that a discarded sailboat hull and a 6-1 lister can be mated nicely, though the result would be something like a slow old "dragger", and there would be many opportunities for the intrepid builder to make grave errors. Still, done well, the old "'loid" would give something like 6 kts and burn about 1 quart per hour - something to consider. Are people really in a hurry? Of course, strong wind and currents can be a real problem in slow boats.

 

I wish you could get a large "listeroid inspired" v twin diesel engine.

60 hp would be ideal, 2 giant pistons and heavy construction, 1200 rpm's redline type of thing.

it would sound awesome and probably be great on fuel.

 

Your off topic --lugging a diesel is a term used for running a loaded diesel under its operating range and not as in a slow revving engine.

 

I think of "lugging" as over fueling the engine for any given speed.

You COULD run an engine close to idle as long as the load was light enough;
And you could still lug down the engine at higher RPM with a good enough load....

 

Also I wanted to add this:

As I posted before, I rented a boat out of a fleet of boats.
Some boats, which are used for fishing, have "trolling valves".
That is because guys that were fishing, wanted a slower speed than idle.

I heard about this, and I assumed maybe it was a plate or something that partially covered the propeller... imagine my surprise when I learn it is a transmission pressure reducer, and from all I can understand, rides the clutch all day long ?!?!

 

Unless you can provide a link for that I dont believe it. Trolling speed is 6 knots anyway.

 

3.6
A.
Operation with Optional Trolling Valve
General
Trolling valves are available as optional equipment for most Twin
Disc marine transmissions. The trolling valve allows reducing and
controlling propeller speed below that normally attained by operat-
ing the engine at low idle. Examples of operations requiring such
slow speeds are trolling or movement through “no wake” restricted
areas.
Most trolling valve can be used with the selector valve in either the
forward or reverse positions. Actuating the trolling valve reduces
clutch apply pressure, which reduces propeller speed.



That is from a twin disc manual

 

Here is the pdf of the manual, it is section 3.5 that has the trolling valve info...

 

Good post,-- thank you I had not heard of that before.

Just shows how robust a marine box can be to reduce pressure to the clutches. A procedure that would destroy a car tranny in minutes. Mind you and quite understandably there are limitations to its use.

 

If you read, there is a limit to using the trolling valve,

I have a feeling the clutch doesn't actually come into contact, but creates a fluid coupling.

Any clutch ever that was wearing like that would not last for thousands of hours like these do, so again I think the pressure is "just right" to make them run as a fluid coupling.

I know the boat I had the guy rigged up an alarm so that if you used too much throttle with the trolling valve on, an alarm would sound.

 

If that was the case then no need for RPM restriction would be necessary.

TD seem to make it quite clear that use of it has to be careful and that heat build up could be a consiquence.

It appears that electric valves are available and kits can be bought. However TD also says that de selecting gear and re selecting for trolling speed can be continuous without damage.

I have 2x ZF 630 on my boat and I wont be fitting trolling valves , no way.

 

I wouldn't put a trolling valve on my boat either (if I had one, haha), but the guys I met that run them say they work great, and they haven't had any issues with the transmissions, or trolling valves.... and those are rental boats !

 

recip engines have minimum speed requirements. the minimum safe rotation is lower when in a no-load condition, but it's going to be, even on big slow engines, fairly fast, something like 250 revs for journals in the ballpark of 2 inch diameter. this requirement has mostly to do with oil film thickness. fitted with correct prop and operating in the more or less linear region associated with very low hull speed, the two minimums should be very close to identical, ie it's not really possible to lug if things are set up properly and load stays at more or less 75-80% of potential max output for the engine speed in question. severe wind can, obviously, change that and create lugging conditions on otherwise proper design. withal, look for black smoke and avoid...