Limit on lugging a diesel....

"Overproping" such that 80-90% load occurs around the torque peak rpm provides better efficiency (BSFC). And, at least at that operation point, produces the least wear and doesn't lug or overload. Will such a prop cause problems at other operation points - maybe, although not if you have gears or a CPP."

 

"Will such a prop cause problems at other operation points - maybe, although not if you have gears or a CPP."


The simple solution is an EGT exhaust gas temperature gauge.

About $100 , and you have to ask the engine builder what the limit is for YOUR engine.

No big deal , usually a red stripe on the Tachometer limit RPM is installed as warning.

 

You think EGT will be high when lugging.

This is all nonsence untill the engine chosen is identified.

Turbo engines when running on song will have red hot turbo housings --they love it --thats right in thier zone.

A 1960 gardener 5 cyl straight errr no.

Whats RPM got to do with load?

My engines will run 3800 and the turbo boost meter is still on zero untill 4200

No thats not a typo---what engine are we talking here

 

This is exaclty what I was wondering, and I might just think further about a dt466 or dt369, they run forever and parts are cheap and available.

That would be perfect if I could run it all day around 800 RPM's, and cruise around using very little fuel. I imagine 20-30 hp would be ideal.

 

I am a big fan of the old heavy-duty slow speed diesels. However there are some serious disadvandages to these old lovelies. The size and weight per horsepower is a major factor that helps these run upwards of 30,000 hours...in some cases 50,000 hours. At the same time ya gotta find room for these at the expense of other things. There is also a misconception of how efficient these engines are. For instance the Listeroids are lucky to achieve a BSFC of 0.70 lbs/hp/hr under best conditions, a modern gasoline engine can be under 0.50... a modern turbo-diesel can flirt with 0.40's. So the fascination is the seriously quiet operation and unreal longevity versus finding room for the prine mover and fuel supply.

I have wondered about hulls with more than one sweet spot. Passage making at 1.1 times the square root of the waterline AND then again at some higher speed. Picture a trimaran with an engine in each pod, running on the center engine at night when you can't see, then all three during the day to make time. What mechanism would allow this? I don't know, but when travelling the world it would have to be very reliable.

N.A.'s...is there a hull design out there that has two different prismatic co-efficients...maybe one going forward and one going backward? A joke maybe... but at the same time...is it possible to have a hull that comes close to a vessel that comes close to an optimized displacement boat AND skip across wave tops at double or triple 1.34 times the square root of the LWL?

Kind regards

 

All you need is an efficent not too heavy(fast) semidisplacement hull with enough common rail hp to move it at 12-15k(for these speeds you need some significant hp) with about 1gal/nm fuel burn. When you run this rig at one k under hull speed you will find good fuel burn numbers and long life of motors. Now you have a boat that is economical and can still get up there and move out if needed. No matter how you cut it you get about 20hp/gal deisel fuel. It does not take a lot of hp to move a boat at or just below hull speed. The prop and gear issues are inportant in that the motor must attain full rated rpm under load(some extra for future dirty bottom etc). I think the original poster is getting himself all twisted up in his prop issues and loseing sight of the ball. This type of issue is well suited to the Boatdiesel.com site where all the ifs and buts have been ironed out and there are some good specific articles on these issues.

 

I was thinking more about the first post and found some BSFC charts. The idea of running an engine at 800rpm for cruising and winding it up for sprinting brings up some issues. Any machine (and hull) will have a sweet spot where it is making the most out of the least amount of fuel. An engines speed and load will change how many horsepower a gallon will produce in an hour.

To make the most out of a combination the hull could be tuned for a certain speed, and the engine(s) sized to provide the required power to move the vessel at the rpm peak torque is made, and propped to load the engine at whatever continuous rating the manufacturer reccommends. Weather reserves mess things up a bit.

My point is running an engine outside it's sweet spot of rpm and continuous load is a waste of fuel (no to mention maintenence/longevity issues). The closest I have come to ideal is a controllable pitch prop and two identical engines coupled in line and run at peak torque rpm. In flat calm weather one engine would push at just under hull speed, when it kicks up both engines could run between peak torque and max rpm and have plenty of reserve.

Note how ineffecint engines are when lightly loaded.

 

Which engine is that graph for? It must be very oldfashioned one, a gasoline engine and/or a very small one. It shows the peak efficiency of 0.43 lb/BHP, which is about 265 g/kWh. Below 2300 rpm 25% load curve is more than double that is 500+ g/kWh. If you compare that to e.g. these rather modern engines:
http://ecomodder.com/wiki/index.php/File:ALH_BSFC_map_with_power_hyperbolae.png
http://www.yanmarmarine.com/theme/y...gines/Technical-outline-Drawings/PDF/che3.pdf

The peak efficiencies are ~200 g/kWh and the first one shows ~260 g/kWh at 1400 rpm at 25% (10 hp vs. 40 hp max). In order to reach 500 g/kWh, you would have to go to ~10% loading. Even at 1000 rpm 25% loading this graph shows 280 g/kWh vs. you graph showing 710 g/kWh.

The second one shows cubic propeller curves and for the bigger engines never go above 260 g/kWh. These curves stop to 1400 rpm and then you are taking about 17% of the peak power and loading is 30-40%.

 

I don't recall the engine. I like your charts, the first one the most. The second one (Yanmar) I couldn't find which power curve the BSFC belonged to. I would wager the advertising department shows the better numbers belonging to the max power. I agree that new engines are much better than old, but the trend is still the same...lighter loads means higher BSFCs, and peak torque rpm (within a few hundred rpm) makes the most with the least when loaded.

This chart shows the curves for a Mercedes OM617A and though it is an old car, I think it shows the trend I'm describing.

 

The Yanmar 6CH-UTE illustrates the issue - at around 1200 rpm the output curves will meet and you would be operating at a low rpm with 100% load and a fairly poor BSFC of 280. Yanmar doesn't indicate whether this is OK and at 800 rpm - who knows.

Conclusion - a ~5:1 reduction in output is OK, beyond that, economy and wear are questionable and you may need to consider gears, CPP or two engines.

Second conclusion - if you don't know what you are going (knowledge and instrumentation wise), stick to the standard prop selection guidelines.

 

If that would happen, it would be impossibe to accelerate through 1200 rpm. If you are already loading 100% at 1200 rpm, your rpm is not going to go up whatever you do. Since almost all boats accelerate easily through low rpm, it is not common to load even close to 100% at low rpm. The only exception to this is some planing boats, which may be close to 100% at semiplaning range and even over 100%, which means they can't get up to planing speed. Getting less than 50% of max rpm would be very exceptional on a displacement or semi-displacement boat.

The curves shown for 6CH-UTE are a bit misleading. Probably due to turbo charging. At 1400 rpm 6CH-UTE is already very close to naturally aspirated 6CHE3. I would guess you still get close to 50 kW at 1200 rpm while the cubic propeller curve would be at 20 kW. At 800 rpm the cubic propeller curve would be at 6 kW, while the engine could probably still produce well over 20 kW.

 

Boat's that can't accelerate get a new prop or engine after the first trip out. But this doesn't mean that such behavior is a non-issue that doesn't need to be considered when you are designing things. High (but not 100%) load at low rpm is even more likely (and not so likely to be noticed and fixed).

50kW from a 6CH-UTE at 1200 rpm - no way. But no question that this engine can work with the right hull and prop (ie, different curves).

 

The ideal setup for a heavy cruiser/motorsailer is the CPP, as prev. mentioned. They solve the very concerns/problems the OP enquired about. I've been tearing up the country looking for a used unit that has feathering capability. Contrary to what I thought, not all do. I'm not famaliar enough with their capabilities/availabilities for use on high speed power boats. I do know they are much more popular in Europe than on this side of the pond. Many balk at the cost of a CPP system but when one considers it eliminates the need for a marine transmission they become very competitive.

A yacht is not defined by the vessel but by the care and love of her owner---

 

I know first hand how this works.

My nmpg at 14 knots and 30 knots is the same-about .5 nmpg..
Between 20 and 24 knots it's the same...but about 10-12% better than at 14 knots.
At 10.3 knots I get 1 nmpg.
At 7 knots about 2.5 nmpg.

53' LOA,22 tons and 1300hp. Fixed props.
98% of the time I'm at 9 knots/950 rpm and IIRC about 1.8 nmpg-been doing this for years and the engines are perfect.Every hour I'll give them a little boost for a couple minutes. Not sure it helps-makes me feel better.


Running my boat at it's torque peak and best high speed cruise at 24 knots is a waste of fuel,not 9 knots.
And has a risk of hitting a tree,whale,orca,dolphin.

 

Those are good numbers. I wonder what would happen if 9 knots happened at peak torque rpm of a smaller engine. On the other hand 1.8nmpg or 30 knots are two very nice options.