Low Powered Cruiser

[Nomad Willy]

Tom,

"overpropped means you achieve more than full RPM."


No. Under proping will allow higher than max rated rpm. And I think a tiny bit of it is ideal. then you can run almost at will right up to max rated rpm. My engine man says one can run WOT for one hour at a time. If I was under proped a bit I could run even longer without overloading the engine and conversley if I was over proped I could'nt run anywhere near max without being (or being in danger of) overloading. Like a truck going up hill in too high a gear, lugging the engine. The thing that wears out engines is not rpm, noise or the speed of the piston but the high temperatures and most important the forces generated. At mid-stroke the piston is pushing down w the force of an explosion and pushing the piston sideways (lots of force) because the crank pin is off to one side. An engine makes more noise at high rpm but it's much easier for the engine. Under proped and sing'in along at 80% of rpm is easy for the engine if proped right or a tad under proped. That's like downshifting a truck when you CAN make it over the hill in 9th gear but it's much easier on the engine to downshift to 8th gear. If you ALWAYS have a very light load you can over prop like a truck going up a slight grade. If you have a go fast go slow trawler you can over prop but the option of go fast will be lost.
"You cannot go wrong by propping by the book, especially with a new TierII JD. In general, the book says to select a prop size*to allow full RPM to be achieved. "
Absolutely right Tom.

 

[RickB]

markpierce wrote:I like the sound of it.*

It might roll of the tongue nicely but at maximum torque your engine is only delivering about 5 hp to*a "well matched"*propeller.

Why not just install an old Briggs and Stratton and save a few tens of thousands of dollars if all you want is 5 hp?

 

[RickB]

nomadwilly wrote:My engine man says ...

I would start looking for another "engine man."

*

 

[markpierce]

RickB wrote:



It might roll of the tongue nicely but at maximum torque your engine is only delivering about 5 hp to*a "well matched"*propeller.

Why not just install an old Briggs and Stratton and save a few tens of thousands of dollars if all you want is 5 hp?

!600 rpm (about 10 hp)*will theoretically,*under ideal conditions, result in a speed of 1.2 times the S/L ratio which the Coot's*designer finds desirable.* More power/rpms will be needed for less-than-ideal circumstances.* Leastwise, the designer and builder believe it important the boat*has much*more than minimal power available.

*

 

[Marin]

sunchaser wrote:

Underpropped means you cannot achieve full RPM and overpropped means you achieve more than full RPM.

I have always understood it to be the other way round.* Overpropped means having a prop with "too much" pitch where underpropped means having a prop with "too little" pitch.* The folks on the GB forum (who could be wrong) always describe the older GBs as being "overpropped" by which they mean the prop or props have more pitch than the "proper" formula would suggest.* However with the engines they used at the time--- like the FL120--- this slightly higher prop pitch gave a bit more speed at the typical cruise settings used but did not overload the engine to the point where it was a problem.

*

 

[sunchaser]

Marin and Eric - yes, I was bass ackwards.

 

[Nomad Willy]

Rick,It's been a busy day today and I type w one finger so eventually I start cutting corners.
I meant "man" as short for manual. And I have no engine man. The last guy that worked on my boat removed the propeller and 8' from me threw the bronze prop onto the ground from waist height as if to say "there you are". The ground was very hard packed gravel. He will never touch my boat again. I told him how I wanted the prop removed and he apparently he was offended * *...told me he knew how to do it.

 

[RickB]

nomadwilly wrote:I'm mostly familiar w 20 to 55 hp engines and all that I can recall have 22 - 23 to one compression. Diesel may not be so but I think gas engines are more efficient w higher compression. Something I've wondered about that you may be able to enlighten me on is the pre-combustion (swirl chambers) on many of these engines. It seems my old Yanmar w direct injection started really easily and didn't have heaters/glow plugs. What do they have to do with tier2 regs? I would think the pre-chambered engine would start better but the reverse seems to be true from my experience.

Higher CR does equate to higher efficiency. But the increase is incremental and small, and this is the bigger but, it also equates to higher loading of parts which means heavier stronger parts and greater inertial loads that in turn require larger heavier bearings and other parts.

Small engines - very small engines - have very small cylinders that don't contain much charge air and have a large surface area in proportion to the volume compared to larger engines.

Your Mitsubishi conversion for example has about 1.8 square inches of metal for every cubic inch of swept volume, that is a lot of heat absorbing surface and that 20something to 1 CR is needed to make some heat. FF's dream engine has about 1.33 square inches for each cubic inch and a CR of around 18.7:1. A large (but far from the largest) engine that produces around 1600 hp per cylinder has about .33 square inches for each cubic inch and starts very well with a CR of 14 or 15:1.

To put that in workable terms, it means that a tiny charge of air has to be very highly compressed very quickly to retain enough heat to ignite the fuel charge. That requires a high compression ratio and means the engine can be very hard to start when it is cold. Which brings us back to the precombustion chamber.

A direct injection engine requires a fuel injector that uses very high pressure to create a perfectly atomized spray of fuel. But, it also has to deliver that fuel in a manner that can penetrate the very dense air charge so that the droplets are heated quickly and burn properly. Each drop has to find an oxygen molecule before it gets too hot and simply turns into a microscopic sphere of carbon and leaves the boat as soot.

One way to get around the low temperature of the charge air and the need to have high pressure pumps and expensive injectors is to provide the injector with a little room of its own. A cheap and clunky little low pressure injector can squirt a coarse stream of fuel into a little cavern where it can be ignited by a red hot glow plug and then squirt out at very high velocity into the clean, oxygen rich environment of the cylinder. As it squirts out it creates a great deal of turbulence that mixes the "rich" fuel charge better than its high class cousins.

When the engine has run for a bit the glow plug and the little house get toasty warm and the system works pretty well on its own. But back to the CR, you can see how the cold cylinder walls (even when the engine is up to operating temperature, the walls are "cold" compared to the combustion gas) soak the heat of compression out of the charge and create all kinds of other problems with ring lubrication and cylinder wear. That is one of the reasons those high strung little engines just don't last as long.

Looking at the "man" for your engine (an S4L2, is that correct?) it can be run continuously at 3000 rpm if it is propped so that the power produced at that rpm is limited to 35.1 hp. That one hour restriction applies to engines that deliver 37 hp at 3000 rpm. So, prop accordingly and run it on the stops all day long if you want.

To tell the truth, unless you install a hp meter on that thing you won't ever know when it is delivering 37 or 35 to the prop and 3000 rpm must sound like a can recycling facility at full capacity!


-- Edited by RickB on Saturday 16th of October 2010 09:38:44 AM

-- Edited by RickB on Saturday 16th of October 2010 09:44:26 AM

 

[koliver]

Rick:

Thanks for your class in diesel engine technology. I used to think I understood the old beast, but now, I realize I was only scratching the surface. Please continue to treat the rest of us as if we are in your grade one class, and we will continue to be a good audience for you.

Again thanks.

 

[RickB]

koliver wrote:Please continue to treat the rest of us as if we are in your grade one class ...

Thanks, but I only put it in terms and make comparisons I can understand myself or would like it explained to me.

*

 

[Nomad Willy]

Well Rick you gave me lots to chew on. I know longer stroke engines of the same displacement have less surface area (combustion chamber, piston crown and cylinder) than short stroke engines and this works to make them more efficient since there is less surface to dissipate heat resulting in less heat loss. A diesel engine being a heat engine * * ...any heat loss is efficiency lost. Using rectangles increasing and decreasing volume increases and decreases surface area to the same degree so I am led to believe that the cylindrical part of the cylinder causes the volume/surface area ratio to vary. Does that tally to you?Why do you call my engine a conversion? Are'nt all engines that would be suitable to go into a trawler conversions?
"To put that in workable terms, it means that a tiny charge of air has to be very highly compressed very quickly to retain enough heat to ignite the fuel charge. That requires a high compression ratio and means the engine can be very hard to start when it is cold". Is it that the very high pressure is delayed in the swirl chamber and thus the SC provides a lower pressure place for injection so lower pressure injection can be used? "One way to get around the low temperature of the charge air and the need to have high pressure pumps and expensive injectors is to provide the injector with a little room of its own. A cheap and clunky little low pressure injector can squirt a coarse stream of fuel into a little cavern where it can be ignited by a red hot glow plug and then squirt out at very high velocity into the clean, oxygen rich environment of the cylinder. As it squirts out it creates a great deal of turbulence that mixes the "rich" fuel charge better than its high class cousins.". This makes it sound like all diesels should have the wonderful swirl chambers but of course they don't. "One way to get around the low temperature of the charge air and the need to have high pressure pumps and expensive injectors is to provide the injector with a little room of its own. A cheap and clunky little low pressure injector can squirt a coarse stream of fuel into a little cavern where it can be ignited by a red hot glow plug and then squirt out at very high velocity into the clean, oxygen rich environment of the cylinder. As it squirts out it creates a great deal of turbulence that mixes the "rich" fuel charge better than its high class cousins." So it seems just like gas engines * *...higher CR and higher speed = less engine life. That seems like a reasonable assumption but I'm not concerned as I don't think I'll be able to run up enough hours on my "high speed" engine to be look'in at a rebuild. I'm 70 but that probably apply to just about everyone else on this forum as well. I run my S4L2 proped so I get 3000 rpm at WOT. I cruise at 2300 most of the time and 10 or 15% of the time I run at 2500. When it gets rough I ALWAYS reduce throttle to 2000. Willy will run all day through 6' seas at 2000 and dosn't require any more power to make reasonable progress. And I know that at 3000 rpm the little engine is developing 37 hp when I'm proped right.
" 3000 rpm must sound like a can recycling facility at full capacity!" No not at all. We have been running w the engine hatch covers closed lately.

 

[RickB]

nomadwilly wrote:
Why do you call my engine a conversion?

Because it is a Mitsubishi industrial engine that was marinized (converted) by Klassen for use in a boat. It isn't an insult.

 

[Nomad Willy]

Rick,Sounds like you just don't like small engines. I like them equally but I'm really fond of the fact that I can run mine really hard for a week on less than $200 in fuel.*
Are there any marine engines? What is the smallest one?

I remember the real marine engines like Lathrop and Hercules. They had oil pans of cast iron whereas the transmission was part of the engine and the oil pan extended to the aft end of the transmission. Everything was heavy with low compression, slow revs and updraft carbs w flame arrestors. Didn't have flexible engine mounts and probably didn't need them. By the way most of that last post of mine was quoting you.

 

[RickB]

nomadwilly wrote:Sounds like you just don't like small engines.

Color me confused but I haven't a clue what you are talking about. I don't "like" or "dislike" little engines any more than I like or dislike big engines. They are just machines that produce power and do (hopefully) useful work. I don't take any of them personally, not even my own and I sure as heck don't idolize or look down my nose at anyone else's engine, big or small.

If I missed something in your post or if you interepreted something I wrote as less than neutral, please let me know. I don't sell engines or boats and I could care less what other people do with theirs or how they do it.

 

[Nomad Willy]

Rick,Please don't concern yourself about the "small" stuff. Pun intended. I must have said the wrong thing.
If you have time please comment on the tech stuff especially the surface area thing. And any more about the swirl chamber would be welcome too.

 

[RickB]

nomadwilly wrote:
If you have time please comment on the tech stuff especially the surface area thing. And any more about the swirl chamber would be welcome too.

Cylinders only have one "side."* L = 2pi rh

The "swirl chamber" makes it possible to build a less expensive engine. It is less efficient though because of heat losses in the chamber (increased area for small volume surrounded by coolant) and it requires glow plugs for starting. It also reduces the need for high pressure, high tolerance fuel injection systems because fuel atomization is not as critical.

Good combustion depends on good mixing of fuel with air. The air charge in a direct injection cylinder is less turbulent than one that uses a precombustion chamber. Because it is less turbulent, it depends on high pressure and very small tip orifices to atomise the fuel. Because the finely atomised fuel cannot penetrate the very dense charge air, ignition delay is more important, and the simultaneous ignition of the fuel creates a sharp pressure rise which produces that "diesel knock" that some people don't much like. Diesel knock is greatly reduced in precombustion engines. That is why they were/are popular in high speed automotive and industrial engines.

The significantly lower efficiency of precombustion chamber engines works against them iin higher power applications and except for low cost high speed industrial engines the technique doesn't offer much anymore.

Newer automotive engines have gone to high pressure common rail injection and direct injection in the interest of fuel economy. Modern electronic fuel controls have eliminated the "diesel knock" issue as well.

 

[koliver]

The difference between glow plug in a separate pre combustion chamber and direct injection,

Case in point:

Volvo TMD40s, that my boat came with are a glow plug, pre-ignition, engine. 6 cyl, 145 hp
TAMD41s, that I put in, are direct injection, 200hp.
My fuel economy improved by at least 10% with the change, and that is running one knot faster, so if I slowed to my old cruising speed my fuel economy would be better still.
Same block, just a different head, same turbo and add the aftercooler.

 

[Marin]

koliver wrote:

Thanks for your class in diesel engine technology. I used to think I understood the old beast, but now, I realize I was only scratching the surface.

If you haven't already got it or read it Nigel Calder wrote a pretty good book on marine diesel operations, troubleshooting, and repair.* It may be that it is more basic than you need but for someone (like me) who isn't all that familiar with diesels and their systems it's a good intro if nothing else.

*