jleonard
Guru
- Joined
- Jun 25, 2008
- Messages
- 5,064
What ghost said. I cant add to that.
I wonder how accurately a good prop man can remove just the right amount of metal to up 100rpm?
I would not want to reduce a props diameter. With repitching, you can undue the change if the new pitch is unsatisfactory.
On my repitch from 22 to 20, the recommendation was to reduce diameter instead of repitching. More tip clearance to hull was a consideration but the tip clearance was already more than adequate and I wanted the undoing option, which I ended up needing. I also did not want to reduce prop walk with a smaller wheel. And, would'nt the tips be thicker after shortening the blades?
A clarification on running over-propped. It's not just an overload at high rpm's, its technically an overload at all rpm's on a boat with a fixed transmission ratio. The difference is that at low rpm's your engine likely has some extra cooling capacity that you can take advantage of. Being overloaded on most boats near cruise to wide open is a good recipe for a heat related failure. One of the most non intuitive methods of cooling an engine is not the raw water/antifreeze circuit, in many ways that's a backup. The primary way an engine cools itself is in the very combustion process in which the engine exhausts its spent exhaust and inhales a fresh and cool supply of air/fuel. The water cooling system simply cools the secondary heat remaining in the block. If you run the same HP and fewer rpm's, then you are operating at a higher heat load per cycle. Which is why "load" on an engine is so critical. The engine may be more "efficient" at this higher load, but it is less forgiving. A well running engine has some built in margin to deal with this. But keep in mind that over time, your heat exchanger gets a bit more gummed up, your oil loses some of its lubricating potential at the end of the season, your valve train may drift a little out of spec, and your fuel may lose a bit of its cetane rating, your old impeller moves just a bit less coolant. All these things taken together, put you a little closer towards the margins as well. I've had my current boat for over a dozen years now.
Hey Ted, I was trying to think through where we fundamentally disagree, but the mechanic of your description met pretty closely with what I was trying to represent. Namely, you were comparatively normalizing by rpm, in your case horsepower for the same rpm. I see I used the term “overload” which is biased as a conclusion on whether things are good/bad. I should just say “load”, since without direct measurement there is still opinion as to whether it’s under or over. So maybe if I restate to increasing load at all rpms, that would be a better representation.
So you follow all your engine manufacturer's recommendations including his brand of engine oil and only OEM parts?But re overpropping is not recommended by any engine manufacturer.
So you follow all your engine manufacturer's recommendations including his brand of engine oil and only OEM parts?
Ted
I have found over the years that this calculator to be VERY accurate.
https://www.vicprop.com/displacement_size.php
Just insert the REAL numbers
From an expert....
"A diesel is overloaded when the load imposed limits the rpm demanded by the governor. This is seen as the point where the governor calls for more fuel delivery but the speed can not increase. This creates combustion conditions that produce black smoke and without other instrumentation, is our first indication of an overload condition.
As long as the engine can reach or exceed the selected rpm, it is not overloaded. Period. If an engine is propped so that at a given rpm the prop absorbs less horsepower than the engine can produce on a brake (the device used to create the power curve that is shown above the prop demand curve on all those performance charts no one seems to be able to read) there is no overload, there is a power reserve. Only when the propeller demand exceeds the power available at a given rpm is there an overload.
With regard to cooling, look up "Sanky diagram" or "heat balance" to see where the heat available in a unit of fuel goes. The amount of heat lost to the exhaust is normally around the same as that lost to coolant. That does not mean the exhaust cools the engine, it just means the BTUs in the exhaust stream are about the same as that carried away by the coolant. If you consider the very large mass (around 10 pounds per second for a little 150HP engine) of the exhaust and its very high temperature, it should surprise no one that a large amount of heat goes out the exhaust. Again that does not mean it cools the engine, it is hot gas that leaves the cylinders. The amount of heat removed by the small amount of coolant in the closed loop carries just as much heat away. Also, lube oil and radiation from the block removes a considerable amount of heat. Bottom line, the claim that exhaust cools the engine is rubbish.
A turbocharged engine uses valve overlap (exhaust and intake open at the same time) to provide complete cylinder scavenging and to help cool the exhaust valves but this is in no way a major heat removal device. The very short period of overlap compared to the period when the exhaust valves are closed and transferring heat to the coolant means relatively little heat is removed during this event."
The terms "overpropped" and "underpropped" are a bit sloppy as technical terms in trawler type boats. But useful. Very useful and important for planing boats.
On a trawler type boat, most operate with engines significantly more powerful than needed for cruising. Lots of trawlers run their engines at like 30-50% of what they are capable of. Like a Lehman 120 burning 3gph when at full power it is like 7gph. On a sportfish, engines typically run around 80% of max. Also those are turbo and charge air cooled. On a sporty you have to have the propping right.
On a sporty, if you can make a little over rated rpm at full power, then at cruise you can be pretty certain the engine is at a happy load spot.
On a trawler, it is different. Most are not capable of planing, so as you increase power up to full power, the shaft hp curve takes a steep upward curve. Running at a power setting above hull speed is really an economic no-go zone. Digging a big hole and wasting fuel.
On a trawler using the sporty criteria for correct propping is not always the best approach. And on most trawlers the engines are not super high output TA versions (some are!!) and thus the engines are not super sensitive to overload.
Each engine has a performance map, often called a BMEP vs RPM map, with BSFC curves present. The point of propping is to put the engine into a happy spot on the curve for all expected vessel speeds. Unfortunately, engine MFR's don't like to publish the charts and us engine guys end up having to snoop around for bits and crumbs to get any idea of them. Some MFR's are better than others (Cat, Cummins).
On a trawler you can never go wrong propping it for rated rpm + a bit at full power, even though full power usually is a silly place.
The problem is when power is backed down to a sane cruise, the engine is no longer in a happy spot on the performance chart. Rpm can be too high and load too low.
So it makes sense in some boats to add pitch and/or diameter to increase specific load and drop rpm. Can be a good bit quieter. Fuel burn likely a little better. A little.
But you don't want to increase prop too much. Any black smoke and engine is not happy.
And you do then need to avoid running at high power as engine will then be overloaded. And in an emergency you may need to do exactly that.
Double edged sword.
I run into this frequently with boaters, they brag about how their boat is over propped not realizing the damage that can be done to a motor that functions at lower RPM pushing more weight.
So why do people over prop?
Most of my life I had over propped engines except in (real) tugs. And they were all singles. I don't normally use the top 20% of hp in turbo engines in order to extend the time between rebuilds. In naturals it usually doesn't matter because they don't get the high exhaust temps. If you're always running at below peak hp, then you get a really good mileage benefit by over propping and still keep some of the former wot speed. For me it's been more than 10% better mileage. But obviously you can't run at wot for anything more than short periods and you will probably have black smoke. But they do maneuver better (the way I maneuver). Because the prop throws more water, I rarely did more than just shift in and out of gear at idle.
I had at least 6 vessels that had 2 or more sets of props. All were twins. Two of those I knew the later history of had the stock props reinstalled and then after a time went back to the larger props, but I didn't hear the details of why.
Well said Lepke, and several others in this thread. The reason this subject gets rehashed over the years is because there is a different answer for every boat, every captain and those change from day to day based on wind, weather, currents and how a captain feels like running his boat today. This is Trawler Forum and I personally drive a slow boat that falls into a category somewhere between full displacement and semi and this in my mind is a totally different demand on an engine than, say a sport fishing boat trying to get the impatient clients 50 miles out to the fishing grounds catch their limit, and be back before happy hour. I have a 6.5 liter NA 6D14 Mitsubishi rated at 115 to 126 hp ?? on a 33,000 pound 40' boat. I ran the Vic prop calculator (thanks for posting that!) which shows displacement hull speed at 8 kts which I have trouble reaching most days. So I cruise at about 6-7 kts and burn about 2 gal/hr. I have had to do a complete redesign of my cooling system due to chronic overheating problems from an under designed cooling system at the time of retrofit to a two cabin cruiser from a fish boat.It seems that if the boat stayed at a slow cruise in 50 degree water it would not overheat but that put it right on the edge in summer. The biggest and most well known marina locally told me I was overpropped. That is why I was overheating. It took me a couple of years to totally understand all the dynamics of my cooling system. One of the simple calculations that I found useful to understand how much heat I was making was fuel burn. This also links to the topic of this thread - overprop situations or in other words overload. If I only ran at 6 kts burning 2 gallons per hour the boat did not overheat most days. To start to understand all aspects of cooling you need to put some numbers to several things related to cooling like BTU generated. GPM = BTU divided by 500 (water) X Delta T. Gallons per minute of coolant flow converted to pounds per hour to calculate BTU which is per hour. Delta T or the degree drop in temperature of your cooling water as it returns from the keel cooler to the engine is another big one. When one gallon US of diesel gets burned there is about 135,000 to 140,000 BTU released that has to go somewhere. A very general rule of thumb is 1/3 goes up the exhaust, 1/3 goes into the cooling system, and 1/3 gets turned into rotary energy which starts coming back into the engine room as heat from the gear and friction from anything that turns. 5-10% of heat generated is also radiated off the engine block and exhaust system which greatly effects the heat inside the engine if the air in the engine room is above 110 degrees F as it gets drawn into the combustion chambers. My setup also has a dry stack and water cooled exhaust manifold that seemed to be forgotten about in design of total cooling system heat load and was part of the overheating problem. Most of the keel cooler design guys refused to talk about that extra heat that has to be dealt with in the gallons per minute of flow of the engine cooling system. It seems that possibly 40 - 50% of the 1/3 of heat that normally goes out the exhaust was ending up in the engine heat load. Plus the 5% of the 1/3 going to the transmission etc. etc. Bottom line was I installed a new Fernstrum keel cooler that takes care of just engine water jacket cooling and use the old cooling loop for exhaust, transmission and hydraulic cooling. The new Fernstrum was up graded to be able to handle the heat load of a 6BT 210 that might be somewhere in the future. I now have about 3 times the original cooling and can not make the engine overheat. All of these heat loads require every component to be working at maybe 100% on some high performance boats running at WOT for extended periods. What I am getting at is in a system that is designed right on the edge of being too small, everything has to work at 100% or you overheat and do damage to engine components. This is where the overpropping problem comes into play if this is how your boat is used. But if your boat is like mine and just chugs along at 1500 rpm burning 2 gallons per hour you are only making about 34 hp. Based on fuel burn specs of about 16-17 hp hrs/gallon for most engines. Not an overload at all on my engine and not creating enough heat to be harmful to any components. If I run WOT I can only reach 2400 rpm which the "experts" say is below the 26-2800 the book says but I don't care because I know that and would only ever run it there for a few minutes in an emergency. It is nice to not be on the edge of overheating if you have to use that full power and that is one of the reasons you need to be aware of where your engine is at. This seems to be where the confusion starts. It is simple as several here have stated, if your engine will not reach full rpm, then back off the throttle to a lower rpm, lower speed and lower power requirement until it can gain rpm when you increase the throttle. Perfect if this is how your prop is selected and where you want to run your boat for lower rpm, fuel burn, noise and longevity of engine. Last weekend in my other life as a heavy equipment and truck owner/operator I made a 300 mile trip to pick up a 30,000 pound load on my 9400 IH hiway tractor and tridem hiboy trailer. This truck has a C15 Cat 500 Hp and a 18 speed transmission and is geared pretty fast. Overpropped. On the way down south empty with a tail wind I could run in the top gear (18th) at about 70 mph and 1300 rpm with the boost gauge showing only 4-6 lbs most of the time. Fuel burn was under 1/4 tank or 8-9 gal/hr. Just idling along for economy. When I was loaded and heading back north into a storm with 60-70 mph winds, the engine could nowhere near pull the load in that gear so I simply select a different gear until the engine was running free and could gain rpm when I step on the throttle. My choice of ratio as an operator. Some of the time I was down to 3rd over (16th gear) and running 55-60 mph at 16-1700 rpm and the boost gauge ranged from 20-30 pounds. 33 is WOT. So my engine was close to full power maybe 10% of the time but working pretty hard most of the way home. Fuel burn was in the 20-25 gal/hr range. Still well within the power range of that truck but if I had only had 250 hp as a maximum, I would simply have had to gear down down to where the engine had the power to maintain less than 100% power output and extend the time of the trip by maybe double. Or wait out the storm and drive home later that night. Getting home for supper cost me 200-$250 in extra fuel!! My point is it is not nearly as easy to change props as it is to change gears on my truck but it is a situation every driver deals with every single day, trip, hill they come to. So on a trawler style boat, just back off on the throttle, reduce the hp required, which reduces the heat produced, and everything will be just fine. You can change the prop if you want or you can just adjust the throttle back a bit and sit back and enjoy the trip!!
Several excellent insights,Transaxial, particularly that HP is directly related to fuel burn. Actually, HP is calculated as torque -- itself a product of energy released during fuel burn and the length of the piston stroke -- times RPM divided by a constant which depends on whether the engine is a 4-stroker or 2-stroker. But here's an important point:
You can still overload your engine at RPM below its maximum rated RPM. It all depends on the amount of throttle you need to reach the RPM in question. For example, suppose your engine is rated at max 2100 RPM and you run at 1500, but require full throttle to reach that engine speed. In such circumstances, you are likely overloading the engine... even if the engine is not (nominally) overheating.
BTW, to get an accurate measure of engine heat levels, you need a cylinder head sensor, coolant temperature is rarely sufficiently accurate and usually lags behind anyway.
If you can accurately measure fuel burn (net of fuel flow that is used to cool the injectors, etc. and returned to the tank), then by comparing the fuel burn to the specified fuel burn for your engine on the maximum rated HP curve at the RPM (in this case 1500 RPM). If you're burning more fuel than specified on the max rate HP curve, you're overloading the engine.
However, I suggest that it's not just a matter of overheating. Overloading an engine places abnormal stresses and strains on the parts involved in power production, especially the crank, connecting rods, and pistons. Granted, some older engines were so overbuilt and detuned, it might not matter. But newer engines, with their significantly higher HP-to-weight ratios, will not respond as well to being overloaded. At best the MTBO will be reduced, at worst an unexpected catastrophic failure could ensue.
That is why engine manufacturers like to see a boat propped so that the engine's can reach slightly above max rate RPM at WOT. Because if they do, then it is generally guaranteed they are not overloaded at any RPM below max rated. Cheers!