Cigatoo
Guru
Does Yanmar advise on what EGT to rpm values you should see?
11 or 1200 max.....From my memory
Does Yanmar advise on what EGT to rpm values you should see?
11 or 1200 max.....From my memory
I am not an engineer. In my overly simplistic mind the more RPM that you turn to produce a given amount of HP the easier it is on your engine.
If that were true, then fast turning Yanmars would last longer than slow turning Luggers, but that doesn't appear to be the case.
Does Yanmar advise on what EGT to rpm values you should see?
Bingo. Which is why we always want to err on the side of under-propping!I am not an engineer. In my overly simplistic mind the more RPM that you turn to produce a given amount of HP the easier it is on your engine.
11 or 1200 max.....From my memory
"But that tells us nothing about whether the soot on the OP's boat is caused by over-propping; something the builder of the boat puts somewhere down the list of the possible answers."
If one examines the theoretical prop curve it cam be seen that a slight 10% reduction in RPM will lessen the prop load a good deal.
So the simple answer to the OP might be to operate at full throttle , see if there is black smoke from overload and if not , pull back 10% or more to cruise..
Then you would be out of the overload zone, so any smoke would not be from prop overloading.
As the OP runs 600-800 rpm below max RPM , an overload caused by the prop is not possible. The transom smoke is engine related.
Perhaps a better comparison is a constant speed prop on a plane. When I would take off in mine, I'd firewall the throttle, and flatten the prop. This gives you max power to get off the ground in the shortest runway - low gear, so to speak....
So I get that a sport fishing boat needs to dig itself out of the hole to get up on a plane, and that the engine will be heavily loaded at less than full RPM while doing so. So during that transition time I think we can all accept that the engine will be loaded somewhere above the nominal prop curve.
But the real questions is how much, and more specifically, does it exceed the max power rating, which would constitute overloading.
The nominal prop load curve is well publicized and well understood. What does the get-on-plane load curve look like? There must be good data on it. Anyone seen anything credible? Inquiring minds want to know....
Don’t sell the Yanmar’s short. They will go many hours if propped correctly. (Most are not) it’s not RPM that kills diesels. It’s heat and therefore load that does them in.
Obviously, the first and most glaring problem with this analogy is that air is a >>highly<< compressible medium and water is a totally >>incompressible<< medium. Accordingly, aircraft propellers and marine propellers are completely different from physics, design and engineering perspectives.
then an airplane engine with a trimmed prop for cruise must be overloaded since it is incorrectly pitched for take off. Since that clearly is not the case, over-propping within reason will not, as you suggest, take an adverse toll on an engine at all speeds.A boat that is overpropped will be overloading the engine throuout the entire powerband. Overpropped is overloaded, regardless of speed.
So, when I read CAT's statement, I read that where a planing or semi-displacement hull will land on a power curve at a given rpm is dependent on the hull. Big whoop. You read this as vindication to your repeated statement that any over-propping will result in overloading the engine at all speeds, which is nonsense. As TT and I have pointed out to you, unless you make a power demand that is greater than the design max power output of the engine as shown by the power curve, the engine is not being over loaded. It is operating within the power band it was designed for. The fact that displacement hulls operate at the low end of the band is a function of hull design. The fact that other designs operate above that is a function of their hull design, but neither is over loading the engine as long as it is somewhere between the power and prop curves.Twistedtree,
The idea that these load curves are >>impossible<< to predict (unless you are a PURE displacement hull) is exactly the point I have been trying to make.
It is not just applicable to a sport fishing boat, it is ANY boat that is designed to operate above displacement speed!
Re: "There must be good data on it."
Sadly, there is not! Every non-displacement hull is different, and that is EXACTLY the problem. The only solution (for non-displacement hulls) is to make sure you can >>easily<< get above the minimum WOT RPM number, and even this does not guarantee a good result at transition speeds!
Below is from Caterpillar's Marine propulsion engineering manual. Note THEY choose the word "impossible".
The only solution is to keep reducing pitch until the semi-displacement or planing hull can reliably climb out of it's hole under the worst conditions that can be expected.
Yes...this is an entirely unsatisfying answer to most of us, but it is reality.
Fortunately the over load, if any, on a diesel can be fairly easily determined. No guess work or interpretation is needed. Boost and EGT measurement provides key data on older engines with newer electronic engines providing spot on data. I've seen larger diesels where data for each cylinder is monitored to control fuel flow and cylinder head temperatures.
Amazing stuff is going on with tugs and ships utilizing constant speed Diesel engines and variable pitch props, not unlike the airplane engine mentioned by Delfin. Hate to see us get too hung up on the nuances of predicting over loading with so many direct measurement or newer engine algorithms available.
I too would rather have and engine making hp at a higher rpm than lower, if lower meant high specific load. Lots of bad can come from too high of a load, not much bad can come from higher rpm.
Amazing stuff is going on with tugs and ships utilizing constant speed Diesel engines and variable pitch props, not unlike the airplane engine mentioned by Delfin. Hate to see us get too hung up on the nuances of predicting over loading with so many direct measurement or newer engine algorithms available.
That seems like the critical qualification. Just going off my own experience, I took 1.5" of pitch out, and now cruise 100 rpm higher, but at the same EGT, speed, and fuel consumption as before. It doesn't seem like there are any markers of higher specific load before detuning, but am I missing something?
Re:
Consider this example. I run my semi-displacement hull up on 'plane'. It takes a LOT of throttle to get me over the hump and up on top of the water. Once I'm on plane, even a very slight reduction in throttle can cause me to 'fall off' of plane, and now I'm plowing water!
In this very common case,
Many boats will pick up speed after getting out of the water , but usually not semi displacement boats as they frequently are not up on a true plane , as say a sport fish with 2x the installed power.
To plane the boat needs to be up on top of the bow wave , and have the boat hull run at about 4 deg or so.
If the bow doesn't come all the way down , your still in SD and need every bit of power you have to stagger along bow high.