Max RPM for Cummins 150 hp motor

[randyyoung]

Hi all buying a ranger 2008 tug with a Cummins 150 hp Diesel engine. What can I expect to get for max Rpm? I believe its 4000. and I've heard that the prop needs to be re pitched to achieve that. if so what do you tell the prop adjuster

 

[Bruce Moore]

Bumping post delayed in moderation queue.

 

[BB marine]

Mercruisers recommended WOT RPM for the 150 hp is 4000 rpm. This rpm is not only recommended but Mercuiser/Cumins state that it should be capable of turning 4000 rpm to maintain the longevity of the engine. I personally like to see any Diesel engine reaching a minimum of 100 rpm above the manufactures recommendation. This gives the engine the ability to operate at its designed perimeters during changing conditions, wind, sea conditions and additional passengers on board. There is a misconception of many boat owners. ( As long as I don't run at WOT it doesn't matter if the WOT recommended rpm is not met) Or (I run the engine most of the time at 2000 so I am not loading it down to much even though I can only turn 3800 rpm at WOT) Both are not correct! If the engine can not turn the prop at the recommended WOT rpm it is over propped for all RPM ranges.

When you go to the prop shop. ( ALL PROP SHOPS ARE NOT EQUAL ) First go to a shop that can scan what you have. Many times the prop may have a stamped diameter and pitch sometimes cup. This may or may not be what you have. Make sure the Prop shop is knowledgable in calculating prop slip. You can change pitch or cup and get the rpm up but loose a lot of efficiency. Some cases increasing the diameter and reducing pitch and adjusting cup will give you the recommended rpm and maintain the efficiency. A good prop shop will adjust the prop correctly or recommend a different prop for the boat you are operating with the engine that is pushing it.

The information you need when you go to the prop shop.

It's a Ranger Tug that weighs approximately ______ the way I have it equipped.

My WOT RPM is _______
The Manufactures recommended rpm is 4000rpm and I would like the engine to turn 100 rpm over 4000rpm
My speed at my WOT RPM now is ______
The engine to the best of my knowledge is performing properly. I believe it is over propped because my RPM is under manufactures recommendation.
Know what your total fuel burn is at various RPM ranges

Fuel burn GPH @ 1500 rpm and boat speed MPH or KTS
@ 2000 rpm GPH MPH
@ 2200 rpm GPH MPH
@3000 rpm GPH MPH
@ 3500 rpm GPH MPH
@ WOT. GPH MPH
Record all this information before have the prop adjusted. Give this information to the Prop shop so they can determine prop slip at different rpm ranges this will give them information as to what would be the best place to adjust Diameter and pitch, just pitch, Pitch and cup, just cup, or all of the above. After the new prop or adjusted prop is installed compare the performance numbers that you recorded before the adjustment. The numbers will not be the same bit they should not be extreme. If they are go back to your prop shop and show the recored numbers. They probably missed the mark. A good shop will not miss the mark.

Sometimes reducing one dimension of the prop achieves one component but reduces the efficiency.

I can't stress enough find a GOOD PROP SHOP. Call Acme Prop and discuss with them your objectives. They will recommend the shop to go to or a prop for you to use.

 

[Tugdungeonmonkey]

Mercruisers recommended WOT RPM for the 150 hp is 4000 rpm. This rpm is not only recommended but Mercuiser/Cumins state that it should be capable of turning 4000 rpm to maintain the longevity of the engine. I personally like to see any Diesel engine reaching a minimum of 100 rpm above the manufactures recommendation. This gives the engine the ability to operate at its designed perimeters during changing conditions, wind, sea conditions and additional passengers on board. There is a misconception of many boat owners. ( As long as I don't run at WOT it doesn't matter if the WOT recommended rpm is not met) Or (I run the engine most of the time at 2000 so I am not loading it down to much even though I can only turn 3800 rpm at WOT) Both are not correct! If the engine can not turn the prop at the recommended WOT rpm it is over propped for all RPM ranges.

When you go to the prop shop. ( ALL PROP SHOPS ARE NOT EQUAL ) First go to a shop that can scan what you have. Many times the prop may have a stamped diameter and pitch sometimes cup. This may or may not be what you have. Make sure the Prop shop is knowledgable in calculating prop slip. You can change pitch or cup and get the rpm up but loose a lot of efficiency. Some cases increasing the diameter and reducing pitch and adjusting cup will give you the recommended rpm and maintain the efficiency. A good prop shop will adjust the prop correctly or recommend a different prop for the boat you are operating with the engine that is pushing it.

The information you need when you go to the prop shop.

It's a Ranger Tug that weighs approximately ______ the way I have it equipped.

My WOT RPM is _______
The Manufactures recommended rpm is 4000rpm and I would like the engine to turn 100 rpm over 4000rpm
My speed at my WOT RPM now is ______
The engine to the best of my knowledge is performing properly. I believe it is over propped because my RPM is under manufactures recommendation.
Know what your total fuel burn is at various RPM ranges

Fuel burn GPH @ 1500 rpm and boat speed MPH or KTS
@ 2000 rpm GPH MPH
@ 2200 rpm GPH MPH
@3000 rpm GPH MPH
@ 3500 rpm GPH MPH
@ WOT. GPH MPH
Record all this information before have the prop adjusted. Give this information to the Prop shop so they can determine prop slip at different rpm ranges this will give them information as to what would be the best place to adjust Diameter and pitch, just pitch, Pitch and cup, just cup, or all of the above. After the new prop or adjusted prop is installed compare the performance numbers that you recorded before the adjustment. The numbers will not be the same bit they should not be extreme. If they are go back to your prop shop and show the recored numbers. They probably missed the mark. A good shop will not miss the mark.

Sometimes reducing one dimension of the prop achieves one component but reduces the efficiency.

I can't stress enough find a GOOD PROP SHOP. Call Acme Prop and discuss with them your objectives. They will recommend the shop to go to or a prop for you to use.

I know you have discussed this topic ad nauseam. I am still amazed at how overlooked this topic is. This is right below change your fluids and filters regularly to get the most from your engine.

 

[BB marine]

I know you have discussed this topic ad nauseam. I am still amazed at how overlooked this topic is. This is right below change your fluids and filters regularly to get the most from your engine.

I agree! I have seen equipment prematurely fail, engines, gearboxes, electric motors, turbines..... loaded outside manufactures specifications. The over load does not cause a short term failure in most cases. It is long term! Your analogy of changing oil is spot on. Same thing the boat owner that does maintain the engine oil and filter changes on a regular basis is not going to realize the reduction in longevity short term. It is long term 2000 to 3000 hours later the engine is "worn out". The increased wear particles in the oil (dirty oil) increases wear to the lubricated parts. The same goes for overloading the engine. This increases wear. No problem when it has 300 hr, 800 hr, 1000hr or even 2000 hrs. The engines pushing the Rangers and Cutwaters, small displacement 2.0 to 2.4L DIESELS, are capable of reaching 3000 to 4000 hrs if maintained and loaded properly. If you operate these small displacement high rev engines over propped for hours on end you will not see the longevity they were designed for. Same has not changing the oil! Just my opinion!



I

 

[NorthernFocus]

Hi all buying a ranger 2008 tug with a Cummins 150 hp Diesel engine. What can I expect to get for max Rpm? I believe its 4000. and I've heard that the prop needs to be re pitched to achieve that. if so what do you tell the prop adjuster

Welcome aboard, Randy. "Rated" RPM on the engine is 4000 which is also target WOT. If I'm remembering correctly the ECM limits max RPM to 4100.

Any discussion about sizing the propeller before you operate the boat is a little premature. The prop needs to be sized for your particular operating conditions i.e. typical load, sea conditions, etc. Basically get out and run the boat a few times and collect some data as Brian suggested. Do you know what size prop is on the boat?

Fortunately the SmartCraft panel helps take some of the mystery out of the equation by calculating and displaying engine load based on RPM and fuel injection rate. Engine load is the best parameter to use to figure out the operating limit on the engine for load/sea conditions at any given time. If you operate the engine above 80 percent load you can expect to have problems. The first point of failure on these engines when they are run too hard seems to be the turbo. That's about a $5k bill.

There's a lot of word of mouth mythology out there about the "Cummins" engines. Much of it has no factual basis. But my first piece of advise is to stop calling it a Cummins engine. When looking for parts, service etc it will save you a lot of headache/frustration if you refer to it as a Mercruiser QSD 2.0 liter.

 

[NorthernFocus]

...If the engine can not turn the prop at the recommended WOT rpm it is over propped for all RPM ranges.

I'd be very interested in expanding my understanding by having access to the technical references that are the basis for this statement. Please share. While there's no argument that sizing the wheel for design RPM at WOT is best practice I'd like to understand how not doing so means that the engine is always operating outside it's design window/intended duty cycle.

 

[BB marine]

It is quite simple and the reason for proper propeller sizing. Most recreational boats and work boats have a single speed reverse gear. Engine manufactures specify a recommended max rpm specification. example the D3 220 hp 4000 rpm. When the manufacture specifies a max rpm for this engine it is providing this information to keep the engine operating at its designed loads through out the full rpm spectrum. Example: 4000 rpm = 220 hp @ 100 present load. Max Fuel burn specifications for this engine will be at 4000 rpm 12.2 gph. This is the designed engine operation illustrated in the torque curve. So at 4000 rpm the engine is at 100% load delivering 212 hp to the prop burning 12.2 gph. At 2000 rpm if prop correctly the engine is operating at manufactures design and generating about 37 hp burning 2.1 gph of fuel operating at about 20% load and just starting to hit the upper end of the torque curve. At 3600 rpm the engine is generating 167 hp burning 8.7 gph and operating at 75% load ( high cruise speed). If properly propped this is what the numbers would be and the numbers that Volvo designed the engine to be operated in. If the engine is over propped and the engine will not turn 4000 rpm then it is not being loaded properly to the manufactures design. If 3800 rpm is max the load is increased on the engine at 3800 rpm 100 percent load @ 3800 rpm burning 12.2 gph. The load on the engine at 3600 rpm is greater burning 10 gph generating 180 hp. The load on the engine at 2000 rpm is greater burning 2.5 gph. generating 45 hp. The engine is designed by the manufacture to provide X amount of power At Y rpm based on the manufactures specified Max rpm. If the Max rpm is lower than the specified amount the engine is being loaded incorrectly at any given RPM.

To operate an engine at 80% load at a rpm that should be 70% load is over loading the engine based on the engine manufacturers design.

There are no gears in a marine transmission to increase rpm to reduce the load on the engine when conditions change. The only adjustment that can be made is propeller change to maintain designed max rpm and properly load the engine. When most boats are built and delivered the boat manufactures specs a prop for the condition the boat is delivered. Example: My C26 would turn 4130 rpm when we took delivery. After we loaded all of our equipment and supplies it would turn 3850 rpm. This meant that when I was WOT I was at 100% load at 3850. I was at 85% load at 3600 rpm. I was at 65% load at 3200 rpm all are above the manufactures designed loads for those specific rpms. At 3200 rpm the engine was at a higher load than the manufacture designed it to be at for that specific rpm.

Running an engine at 80% load needs to be at the engine manufactures specified 80% load RPM.

If you want technical references call any engine or machinery manufacture and you will get references . This is basic propeller 101. When a boat is over-propped it means that the engine is not working at its optimal torque range.

 

[NorthernFocus]

You're a smart guy, Brian, but you connect dots with squiggly lines.

...When the manufacture specifies a max rpm for this engine it is providing this information to keep the engine operating at its designed loads through out the full rpm spectrum...

This is incorrect. On production boats the propeller is sized to allow the engine to achieve the rpm that allows the engine to produce "name plate" horsepower. Period. Particularly in the case of engines used in pleasure boats there is absolutely no correlation between engine design and propeller hp requirement other than that one point where the two curves come together. The engine isn't designed for this service it is adapted to it.

... 2000 rpm if prop correctly the engine is operating at manufactures design.....If properly propped this is what the numbers would be and the numbers that Volvo designed the engine to be operated in.

Per above Volvo didn't design the engine around a prop curve. Nor do they specify that a prop with a particular coefficient be used. All they specify is a prop that will match the HP required curve to the engine power curve at the name plate capacity of the engine.

..The engine is designed by the manufacture to provide X amount of power At Y rpm based on the manufactures specified Max rpm....

Again this is true only for the one point on the curve. The engine manufacturers design curve is the engine curve not the propeller curve.

...To operate an engine at 80% load at a rpm that should be 70% load is over loading the engine based on the engine manufacturers design...

You are well familiar with engine load/duty ratings. They are written around percent load with no mention of rpm for a reason. There is no "should be 70 percent load" at a given rpm specified by the manufacturer. I assume that what you are referring to the the engine power vs rpm curve overlaid with a theoretical hp curve. But there is nothing in the manufacturers specifications/requirements/limitations that refer to that curve other than the one point where the supply/demand curves cross(i.e.WOT). There is no specification requiring the use of a propeller than matches the demand curve on those graphs. Why not? Because it doesn't matter.

Running an engine at 80% load needs to be at the engine manufactures specified 80% load RPM.

Again I ask you to produce your reference material that supports this claim. There are significant differences in HP curves between propeller designs with different coefficients. If matching propeller hp demand curve to engine output curve was that critical isn't is logical that the engine manufacturer require more specifics about propeller design than simply one point on the curve?

There are no gears in a marine transmission to increase rpm to reduce the load on the engine when conditions change...

There is no gearing in any type of machinery that will reduce load on an engine and still do the required work. Load is load typically (in the USA) expressed in HP. When you change gears you simply trade torque for speed according the the equation HP=torque x rpm/5252. Also with internal combustion engines there is a relationship between potential power output and rpm because they have a fixed cylinder volume. HP output is a function of fuel burn/mass flow which is limited by fuel/air ration and the volumetric capacity of the engine. It's easy to dump in more fuel but air throughput it a function of rpm. If you want more power out of a given cylinder size you have to turn it faster. And/or turbocharge it but that's another discussion. Bottom line is that you don't change gears/increase rpm to decrease load but so that you can increase load/produce more power.

...This is basic propeller 101. When a boat is over-propped it means that the engine is not working at its optimal torque range.

This is incorrect. When a boat is over propped it means that you can't extract the available HP out of the engine. Per above the prop isn't sized to match a point on the torque curve it is matched to achieve nameplate horsepower output of the engine.

OK, so by this point I suspect you have steam coming out of your ears. Plus this SEEMS to go against all of your years of experience. Specifically when boats are over propped the engines tend to get burned up. Which is true. But it's not because the engine is running at higher loads across the entire rpm range. It's because when the boat is over propped the engine is torque limited below nameplate rpm therefore can't produce nameplate hp therefore can't go as fast as people want to go. So they run the engine above the load rating. If people continued to comply with the requirements but limited rpm based on ACTUAL WOT versus nameplate WOT they wouldn't burn up engines. But no, if nameplate WOT is 4000 people run at 3600 regardless of the fact that actual WOT is 3650 and they shouldn't be running above 3285. So basically they run at 95 percent load.

Bottom line is that if the engine is operated within it design load (aka duty cycle/rating) it doesn't make any difference what prop it has on it. It just requires running the engine based on load not black and white old school rpm limits. Or boat speed which is what most people do. For the engine that is the subject of the OP it's quite simple because the panel tells you exactly what engine load is. And it doesn't care what size wheel is on the boat 😉

Propellers don't kill engines people do.

 

[dgiles]

Regardless of how you spin it, lugging a diesel engine isn't good for it. Low RPM and high fuel burn will take shorten the life of the engine. Period.

 

[NorthernFocus]

Regardless of how you spin it, lugging a diesel engine isn't good for it. Low RPM and high fuel burn will take shorten the life of the engine. Period.

Absolutely "lugging" and engine is bad for it. Lugging an engine means that it is torque limited. IOW it is producing the maximum power that it can at that rpm on its power curve. Pushing the accelerator/throttle farther down doesn't increase speed/rpm. There is only one time that a boat engine is "lugging" which is at WOT. No argument that it is bad news if a boat engine is operated continuously such that the engine is "lugging" because that would mean that it is operating continuously at WOT.

These discussions about propeller sizing, what rpm should I cruise at, etc, demonstrate how poorly understood engines are in general but particularly in marine service. Even by professionals in the industry. No matter how a propeller is sized if the engine is operated within it's design window/duty cycle it will reach its design life.

Propellers don't kill engines people do.

 

[BB marine]

Dan, Theories are great and in many cases hold some water. Actual troubleshooting and surveying an engine under operation is the real thing. Bottom line an engine manufacture designs an engine to be operated at a specific rpm at WOT. This is not to just for WOT.( there is not one manufacture that recommends to operate a 5 rated duty ( recreational engine) at WOT. It is to confirm that the engine is operated at its rated HP and load through out the full rpm spectrum. This is what I was told during my training and I have used this theory in other applications in my career. I'm not an engineer. I was the guy that fixed the stuff that engineers designed.

This is my hypothesis that I use. This is based my findings while working on engines. Check the exhaust temperature ( I use a I-gun check at each cylinder rail going to the manifold, simple test any one can do) of a diesel that is over propped. Check it at WOT, high cruise, and slow cruise. I use WOT, 3200 and 2200 has my rpm ranges ( small displacement high rev diesel) and WOT, 2000 and 1500 rpm on larger displacement diesels. Next check the boost pressure at each of the RPM ranges along with the fuel burn. Record these numbers along side each RPM range WOT ( below manufactures recommended value because the engine is over propped) Now prop the engine based on the manufactures recommendation. With the engine able to operate at full rated RPM as recommended by the manufacture check exhaust temperature at the same places, boost pressure and fuel burn. Record. Now compare your findings between the over propped condition and the correctly propped condition.

My findings were the Exhaust temperature, boost pressure and fuel burn were all higher in all rpm ranges (over propped) The largest difference being the WOT, then the 3200 then 2200 as expected. My Hypothesis is the full rpm spectrum is influenced by the over prop condition.

There is always this misconception to over prop the engine so the boat gets better fuel economy at slow cruise. Many see their speed increase at the 2200 rpm range with a larger prop that does not allow the engine to turn manufactures max rpm. The thought process is "I'm not running the engine hard so it is not going to hurt anything".

Here is the analogy that was presented to me in a Cummins class. Similar analogies we presented at Mercruiser, OMC, Volvo and Crusader. Truck is hauling a load on the highway. Cruising at 2100 rpm the truck is in high gear. If the throttle is pushed to the floor the engine makes full rpm to governor. Now the truck is operating on an incline the rpm maintains the same but the throttle is pushed more to maintain the same rpm. The fuel burn is greater, exhaust temp is increasing, the turbo is getting warmer,the engine is loaded. The throttle is pushed to the floor. The rpm increases but can not hit the governor. The engine is lugging. If the truck driver maintains this type of operation that engine's longevity will be decreased considerably. The driver drops a gear and unloads the engine the engine is easily turning 2100 rpm again but the trucks speed has decreased slightly from 2200 rpm in high gear. The engine is now operating to be able to hit governor as the truck climbs the hill. The truck has gears to operate the engine at the recommended max rpm based on load. A boat engine does not. The only option available is changing the prop based on the conditions that the boat is operated. A light boat can have a different prop installed compared to a heavy boat.

A boat engine is always climbing a hill. (Dan I know you think this is ridiculous.It is true) Example: Truck is operating on a flat highway. Driver lets off the throttle the truck keeps coasting. The same truck is climbing a hill driver lets off the throttle. The truck comes to a stop quickly. Boat is operating across the water at 15 kts, Captain lets off the throttle boat comes to almost a complete stop immediately. Not scientific and not an example an engineer would use. But apply physics to the example and it will be proven to be fact.

I am not an engineer. I have worked with many intelligent engineers. The Engineers that I respected the most are the ones that can relate their theories to what is reality. I may connect dots with squiggly lines but I don't jump to conclusions without proving them and researching them. My goal is to connect the dot from A to B. I enjoy proving a Hypothesis even if it is not agreed upon. We all have our opinions based on applications, theories and reality.

Dan, I enjoy the discussion and apologies the the original thread poster for elaborating on his question.

Bottom line, "My opinion" it is important to Tug and Cut owners,Prop your boats based on what the Manufacture recommends , give it a bit of breathing room on diesels go 100 rpm above. The outboards have the factor built into it by giving a larger RPM spectrum I still recommend staying on the high side of the spectrum with the outboards especially the 300 hp.

 

[NorthernFocus]

...This is my hypothesis that I use. This is based my findings while working on engines. Check the exhaust temperature ( I use a I-gun check at each cylinder rail going to the manifold, simple test any one can do) of a diesel that is over propped. Check it at WOT, high cruise, and slow cruise. I use WOT, 3200 and 2200 has my rpm ranges ( small displacement high rev diesel) and WOT, 2000 and 1500 rpm on larger displacement diesels. Next check the boost pressure at each of the RPM ranges along with the fuel burn. Record these numbers along side each RPM range WOT ( below manufactures recommended value because the engine is over propped) Now prop the engine based on the manufactures recommendation. With the engine able to operate at full rated RPM as recommended by the manufacture check exhaust temperature at the same places, boost pressure and fuel burn. Record. Now compare your findings between the over propped condition and the correctly propped condition.

My findings were the Exhaust temperature, boost pressure and fuel burn were all higher in all rpm ranges (over propped) The largest difference being the WOT, then the 3200 then 2200 as expected. My Hypothesis is the full rpm spectrum is influenced by the over prop condition...

Thanks for making my point. Your test procedure is reflective of how most people operate boats. They are fixated on a given rpm and run there no matter what. Of course if you run at a fixed rpm the engine will run harder/hotter if it is "over propped" relative to optimal WOT sizing. No argument. Never has been. But run your same test at percent of WOT for the two props and you'll get different results. Or ignore rpm completely compare exhaust temperatures and boost pressure at the same fuel burn rate with different props.

I'm not arguing with your prop sizing philosophy. It is as close as you can get to idiot proofing a boat. But you make is sound like if a boat is over wheeled it's going to kill then engine if you leave the dock. And that's not at all accurate. If the boat is operated based on percent of actual WOT rather than percent of theoretical WOT the engine will be comparably loaded and will run just fine. Run the engine according to load(fuel burn if no load reading is available) rather than rpm and live with whatever speed you get and the engine will outlive the owner.

I didn't spend my career sitting at a desk. I've spent hundreds of hours standing beside rotating machinery analyzing operating problems. And guess what. Equipment actually operates according to theory(aka the laws of physics). Just like with boats 90 percent of the problems are due to equipment being operated outside its design window. For example pumping a higher sg crude oil than a system was designed for and not understanding why the motor is tripping out. But the design window of a boat engine isn't the propeller size. It's the load cycle.

BTW most of the techs that I've worked with over the years connect data points with squiggly lines 😉

Propellers don't kill engines people do.

 

[dgiles]

Regardless of how you spin it, lugging a diesel engine isn't good for it. Low RPM and high fuel burn will take shorten the life of the engine. Period.

Absolutely "lugging" and engine is bad for it. Lugging an engine means that it is torque limited. IOW it is producing the maximum power that it can at that rpm on its power curve. Pushing the accelerator/throttle farther down doesn't increase speed/rpm. There is only one time that a boat engine is "lugging" which is at WOT. No argument that it is bad news if a boat engine is operated continuously such that the engine is "lugging" because that would mean that it is operating continuously at WOT.

These discussions about propeller sizing, what rpm should I cruise at, etc, demonstrate how poorly understood engines are in general but particularly in marine service. Even by professionals in the industry. No matter how a propeller is sized if the engine is operated within it's design window/duty cycle it will reach its design life.

Propellers don't kill engines people do.

You obviously think what you say is correct. The fact you say you cannot lug an engine below WOT is well, incorrect. All engines at all RPM's can be lugged when at a lower RPM than the engine fuel consumption design. Take a manual transmission vehicle, grab a too high gear and go for a trip. You use waayy more fuel, boost is higher than needed and it is hard on the engine.
Trawlers are a bit different because they generally have more HP than required at low speeds and yes overpropping may help in better m/gal at lower RPMS.

https://www.sbmar.com/articles/is-my-boat-over-propped/

 

[NorthernFocus]

You obviously think what you say is correct. The fact you say you cannot lug an engine below WOT is well, incorrect. All engines at all RPM's can be lugged when at a lower RPM than the engine fuel consumption design. Take a manual transmission vehicle, grab a too high gear and go for a trip. You use waayy more fuel, boost is higher than needed and it is hard on the engine.

Flawed comparison. Yes what you describe is bad for the engine. You're describing running at a given highway speed regardless of gear. Comparable to running a boat at a given rpm regardless of throttle position/fuel burn. If that's how you run your boat then by all means follow BBs advice and under wheel it.

The whole point is that if a boat isn't wheeled properly and you can live with cruising at 12 rather than 13 knots at 80 percent load it's not hurting the engine. Though you may have to figure out what rpm that equates to under those conditions rather than just running at XXXX rpm because that's where the boat ran during sea trial or you read it on an internet forum. Load is load. It's not rpm. Nor is it burn rate though that's as close as you can get in lieu of a load reading. Which apparently is an advantage that us Mercruiser owners have.

Trying to inform here not argue.

Propellers don't kill engines people do.

 

[Tugdungeonmonkey]

You're a smart guy, Brian, but you connect dots with squiggly lines. For the engine that is the subject of the OP it's quite simple because the panel tells you exactly what engine load is. And it doesn't care what size wheel is on the boat 😉

Propellers don't kill engines people do.

Dear sir, I openly admit I am not extremely familiar with these tiny boats. I am currently in the process of purchasing one as I sit here in hotel in Canada. However I would assure you if you put the wheel from my boat on any of these vessels you would most assuredly not make it very far. Nor would the engine function in any capacity when it sits on the seabed with a 35k lb wheel on it. Granted in some scenario yes it would turn it somewhere. But not at load unless one hell of a red gear was used. Only having a few dozen thousand miles and a few million gallons of fuel burned as an operational marine engineer I would have to greatly disagree with your aforementioned hypothesis.

1.) would I be incorrect in saying that being over “propped” wheeled would lead to increased egts? And would you not concur that heat is a killer of engines though “props” may not be?
2.) what EXACTLY is calculating your load on your read out? This I am not familiar with as I use analog gauges and calculations? Fuel burn/egt/maf/boost or just one or a combination of those? I ask out of ignorance not arrogance but would this not be important to what the calculation is based on.
3.) the uphill method of explanation and the grabbing a lower gear would coincide similarly to over propping. If you have a 5spd why not just use 5th gear and skip one through four?

Being new to the forum I have no desire to make a riff however it seems to me that bb has made some very valid points and you have relayed some information as well. I did not see where he said you’ll kill the motor if you leave the dock bc it’s over wheeled but that you would increase wear and tear and decrease longevity which by all calculations and empirical evidence proves true. I.e. increase temps at any rpm. I may have overlooked something but I believe the proper wheel on the boat will provide and extended life in any replicated scenario vs a boat that is over propped. Would you disagree?

 

[Tugdungeonmonkey]

You're a smart guy, Brian, but you connect dots with squiggly lines. For the engine that is the subject of the OP it's quite simple because the panel tells you exactly what engine load is. And it doesn't care what size wheel is on the boat 😉

Propellers don't kill engines people do.

Dear sir, I openly admit I am not extremely familiar with these tiny boats. I am currently in the process of purchasing one as I sit here in hotel in Canada. However I would assure you if you put the wheel from my boat on any of these vessels you would most assuredly not make it very far. Nor would the engine function in any capacity when it sits on the seabed with a 35k lb wheel on it. Granted in some scenario yes it would turn it somewhere. But not at load unless one hell of a red gear was used. Only having a few dozen thousand miles and a few million gallons of fuel burned as an operational marine engineer I would have to greatly disagree with your aforementioned hypothesis.

1.) would I be incorrect in saying that being over “propped” wheeled would lead to increased egts? And would you not concur that heat is a killer of engines though “props” may not be?
2.) what EXACTLY is calculating your load on your read out? This I am not familiar with as I use analog gauges and calculations? Fuel burn/egt/maf/boost or just one or a combination of those? I ask out of ignorance not arrogance but would this not be important to what the calculation is based on.
3.) the uphill method of explanation and the grabbing a lower gear would coincide similarly to over propping. If you have a 5spd why not just use 5th gear and skip one through four?

Being new to the forum I have no desire to make a riff however it seems to me that bb has made some very valid points and you have relayed some information as well. I did not see where he said you’ll kill the motor if you leave the dock bc it’s over wheeled but that you would increase wear and tear and decrease longevity which by all calculations and empirical evidence proves true. I.e. increase temps at any rpm. I may have overlooked something but I believe the proper wheel on the boat will provide and extended life in any replicated scenario vs a boat that is over propped. Would you disagree?

 

[bob daily]

So why not just change out the transmission to a different gear ratio?
Just a old farm kid asking! Bob

 

[BB marine]

So why not just change out the transmission to a different gear ratio?
Just a old farm kid asking! Bob

Bob the easier solution is prop the boat correctly. Rule of thumb 2" of pitch gives or takes away 300 rpm. Example WOT 3800 rpm. reduce 2" pitch WOT 4100 rpm. EASY PEASY for the average boater!

 

[NorthernFocus]

Only having a few dozen thousand miles and a few million gallons of fuel burned as an operational marine engineer I would have to greatly disagree with your aforementioned hypothesis.

First off, lets get a couple of things out of the way that are different between you commercial experience and production pleasure craft(i.e. not custom yachts).
1) Engines in commercial vessels are purpose built. Also depending on the size, type, and age of the vessels you've served on the engine and wheel may have been specifically designed to match the hull and service. Engines and wheels on pleasure vessels are essentially off the shelf products selected to fit as closely as possible. The wheel(s) may later be tweaked if/as desired by the owner.
2) Loaded vs unloaded displacement of commercial vessels(for this argument let's consider tug/tow as one vessel) varies a lot more than pleasure vessels

1.) would I be incorrect in saying that being over “propped” wheeled would lead to increased egts? And would you not concur that heat is a killer of engines though “props” may not be?

You are correct for a given displacement and at a given rpm. But for a given engine load egts will only vary slightly due to differences in efficiency at the different rpm needed to achieve the same engine load. And yes, excessive heat is the enemy of all rotating machinery not just engines.

2.) what EXACTLY is calculating your load on your read out? This I am not familiar with as I use analog gauges and calculations? Fuel burn/egt/maf/boost or just one or a combination of those? I ask out of ignorance not arrogance but would this not be important to what the calculation is based on.

Well I guess that's indicative of the age of the equipment you've been operating. Or the trade union that has successfully resisted change 🙂 I can only speak for the (dated) Mercruiser controls which calculate fuel mass flow rate and air mass flow rate and relate it to engine rpm and throttle position. Based on my observations it appears that the engine performance curve is programmed into the controller as it displays load as a percentage. Unfortunately there is no egt sensor but there are sensors for map, iat, and fuel rail t/p.

3.) the uphill method of explanation and the grabbing a lower gear would coincide similarly to over propping. If you have a 5spd why not just use 5th gear and skip one through four?

Now that's disappointing. With your training and experience I'd think you'd recognize the fundamentally different load dynamics between road vehicles and marine vessels. Vehicle gearing is necessary due to the widely varying loads required for acceleration, changes in grade, significant changes in load, etc. On marine vessels power requirement is a very predictable exponential function of speed through the water. Nearly all of the power is used to overcome friction. Which is also why hull fouling and sea state can affect performance more than changes in displacement.

Being new to the forum I have no desire to make a riff however it seems to me that bb has made some very valid points and you have relayed some information as well. I did not see where he said you’ll kill the motor if you leave the dock bc it’s over wheeled but that you would increase wear and tear and decrease longevity which by all calculations and empirical evidence proves true. I.e. increase temps at any rpm. I may have overlooked something but I believe the proper wheel on the boat will provide and extended life in any replicated scenario vs a boat that is over propped. Would you disagree?

I agree if the operator doggedly persists in operating at a set rpm and/or boat speed which results in elevated temperatures at/above the high end of the engine design. I do not agree if the operator intelligently operates the vessel in a manner that keeps engine load within the design window/duty cycle. Which is the whole point of the discussion.

And people, PLEASE, stop comparing how a boat engine runs to cars/trucks. And if you can't figure out why then ask yourself why cars are sold with 200hp engines when it takes 20hp or so to cruise down the highway at 60mph. And why don't electric cars have gearboxes?

As much as I'm enjoying this little discussion I've got a plane to catch.