Prop size vs cruising speed

[Kent33]

Not sure what computer controlled engines you have experience with,

Everything up to and including intimate knowledge of developing tuning and mapping the latest race tuned 1000cc super bike engines, in depth technical knowledge of both Moto GP and Formula One ICU plus Hybrid. Marine diesel’s from single cylinder Bolinders right up to the latest multivalve multi injection turbocharged engines with fully programmable mapping and telemetry for pleasure rating and racing development.

All in all it’s not rocket science or black magic, it’s simply perfecting the mixture and combustion of hydrocarbons and oxygen.

Sadly, there is so much false information and confusion about how thing’s actually work that I just wanted to dispel some myths regarding this and the problem of overloading and over propping.

 

[O C Diver]

You’re getting close but not quite there yet. To put it very simply (without going into all the sensor inputs), in your case, the JD’s ecu has a preset map of optimal fueling and injection timing throughout the rpm range based on the test bed data from running the engine on the dynamometer by the manufacturer during development.

No, that's not optimal fueling, it's the maximum allowable amount.

This map should work almost perfectly at all loads up to max rated rpm but not exceeding the engine’s power curve which was measured on the dynamometer. Exceeding the power curve by overloading will cause the ecu to reduce fueling which subsequently reduces rpm. Fueling (supported by timing) is what controls rpm on a diesel. The throttle is just making a request for more power which can only be delivered by increased fueling and subsequently increased rpm which then delivers the power but ONLY if it is within the engine’s power curve.

No, the throttle is making a request for RPM, not power. Power is a function of load on the engine. Think of it this way, with the transmission in neutral, the throttle requests 2,600 RPM. But that's a small fraction or the power generated in gear at 1,400 RPM.

Ted

 

[Kent33]

A mechanically injected boat diesel (which has an rpm governor) has no way of knowing it is overloaded.

No, that is incorrect. The governor is driven by engine speed (rpm) and governs the fueling. If it’s overloaded the engine doesn’t have enough power to increase speed so the governor is unable to increase fueling.

 

[Kent33]

No, that's not optimal fueling, it's the maximum allowable amount.

You have everything back to front.
The ecu contains the map of optimal fueling and timing together with the engine’s power and torque curves. The ecu controls injections and timing within those parameters. It’s like the ecu is the brain but the maps are the knowledge

 

[O C Diver]

You have been seriously misled. You have paid for let’s say 200 bhp at max rated 2600 rpm but you only achieved let’s say 70% of that because WOT under sea trial could only reach 2200 rpm. You’re happy with that?

My engine is 136 HP at 2,600 RPM. I bought that one as the naturally aspired 80 HP version was no longer available in 2014. Wasn't worth buying a new computer to derate it to 104 HP M1.

Obviously you don't understand the concept of displacement versus planing. In displacement, for my 45' boat at 8 knots required about 75 HP. 10 knots reqired over 400 HP.

My plan was fuel efficiency not speed. Cruising 6 to 7 knots yeilded 3.5 to 5 MPG, not less than 1 MPG at 9 to 10 knots. In 8 years I cruised 39,000 miles. Didn't want to spend $200,000 in fuel or sit at the dock when fuel was $8 or $9 a gallon when I went to Canada.

The load reading is just smoke and mirrors because the ecu reduces fueling to stay within the power curve so it will never show more than +- 100%

HELLO, that's what I've been trying to convey in this thread. The computer is going to keep you from overloading the engine if the boat is over propped.

Ted

 

[O C Diver]

You have everything back to front.
The ecu contains the map of optimal fueling and timing together with the engine’s power and torque curves. The ecu controls injections and timing within those parameters. It’s like the ecu is the brain but the maps are the knowledge

Believe what you want to believe, the map is for the maximum allowable amount for each RPM. The optimal amount might more resemble the propeller absorption curve. For my engine rated at 136 HP at 2,600 RPM, the maximum allowable amount is 136 HP at 2,600 RPM.

Ted

 

[Kent33]

No, the throttle is making a request for RPM, not power. Power is a function of load on the engine. Think of it this way, with the transmission in neutral, the throttle requests 2,600 RPM. But that's a small fraction or the power generated in gear at 1,400 RPM.

That again is incorrect. Increasing the throttle is a command to the ecu to increase fueling. That’s all the throttle control does. The ecu will check if everything’s within the map’s parameters and if so it will then increase fueling which leads to increased rpm and increased power.

 

[Kent33]

Believe what you want to believe, the map is for the maximum allowable amount for each RPM. The optimal amount might more resemble the propeller absorption curve. For my engine rated at 136 HP at 2,600 RPM, the maximum allowable amount is 136 HP at 2,600 RPM.

No. You have everything back to front. The manufacturer designs the map’s for optimal engine operation. They have no idea what props people are going to use or what boat the engine might be installed in and they certainly don’t re map individual engines just to suit your random setup.

 

[O C Diver]

That again is incorrect. Increasing the throttle is a command to the ecu to increase fueling. That’s all the throttle control does. The ecu will check if everything’s within the map’s parameters and if so it will then increase fueling which leads to increased rpm and increased power.

Not true.

First, when doing the engine install or replacing the throttle potentiometer, you have to set the stops on the potentiometer to control minimum RPM and maximum RPM on the throttle. The minimum throttle doesn't change 1 RPM when you shift from neutral to forward, but the fuel consumption increases. Explain how throttle position doesn't change but fuel consumption increases.

Second, when you set the upper RPM limit of the potentiometer, you are doing it in neutral to correspond with maximum RPM. With the engine running at maximum RPM in neutral, the fuel consumption is below 1 GPH. Explain how that maximum position only yields less than a 1 GPH but a lower throttle position can yield 3.7 GPH in gear going forward.

The answer is the throttle potentiometer controls engine RPM, not anything else.

Ted

 

[Kent33]

HELLO, that's what I've been trying to convey in this thread. The computer is going to keep you from overloading the engine if the boat is over propped.

Very inefficient though. Why not size the prop correctly to begin with rather than relying on the ecu derating the engine

 

[Kent33]

Not true.

First, when doing the engine install or replacing the throttle potentiometer, you have to set the stops on the potentiometer to control minimum RPM and maximum RPM on the throttle. The minimum throttle doesn't change 1 RPM when you shift from neutral to forward, but the fuel consumption increases. Explain how throttle position doesn't change but fuel consumption increases.

Second, when you set the upper RPM limit of the potentiometer, you are doing it in neutral to correspond with maximum RPM. With the engine running at maximum RPM in neutral, the fuel consumption is below 1 GPH. Explain how that maximum position only yields less than a 1 GPH but a lower throttle position can yield 3.7 GPH in gear going forward.

The answer is the throttle potentiometer controls engine RPM, not anything else.

Ted

First, you have to set the stops on the potentiometer to control minimum RPM and maximum RPM on the throttle. The minimum throttle doesn't change 1 RPM when you shift from neutral to forward, but the fuel consumption increases. Explain how throttle position doesn't change but fuel consumption increases.

Second, when you set the upper RPM limit of the potentiometer, you are doing it in neutral to correspond with maximum RPM. With the engine running at maximum RPM in neutral, the fuel consumption is below 1 GPH. Explain how that maximum position only yields less than a 1 GPH but a lower throttle position can yield 3.7 GPH in gear going forward.

The answer is the throttle potentiometer controls engine RPM, not anything else.

Nonesense. If that’s true then you’re saying every engine has unlimited power because regardless of load or fueling as soon as you increase throttle the rpm responds accordingly.

Wow, I really want that technology where can I buy an engine that works like that? Cloud cuckoo land no doubt?

 

[Kent33]

Explain how throttle position doesn't change but fuel consumption increases.

There is no rpm control on diesel engines and never has been, it’s always been done by fueling. The ecu responds to your min max idle speed settings by adjusting the fueling. When you engage a gear it instantly maintains this idle speed by increasing the fueling accordingly.

I strongly suspect this is all falling on deaf ears and it’s very tiring repeating the same facts so I’ll sign off on this subject and return to more productive matters. Goodnight.

 

[DDW]

Overloading or over propping is the same thing. It WILL cause over fueling (black smoking) in simple mechanical controlled injection and reduced fueling to avoid black smoking with electronically controlled injection as the rpm’s reduce due to load being above the engines power curve.

This is simply wrong. Overpropping is not overloading, unless it is also overloading. On a normal prop absorption curve there is a large distance between the torque (and power) absorbed by the prop and what the engine is rated for at reduced rpm. Running it in that region (within reason) is not overloading and will cause the engine no problem. This is the way these same engines are run in trucks, construction equipment, generators, etc., with full manufacturer approval. It is what they were designed to do.

If you are so overpropped that at the chosen rpm the torque required exceeds the engine's rating, then it is overloaded. So pick an rpm, overloaded implies overpropped, but overpropped does not necessarily imply overloaded overloaded. This is the whole theory (and practice) behind variable pitch props. Pick an rpm, then pitch the prop so that the engine is loaded at say 70% available torque. It isn't overloaded, that would be 100% torque. An engine rated at 3000 rpm being run at 1300 is using about 30% of the available torque. A lot of people would say that is damaging to the engine (though I would not agree).

 

[O C Diver]

Very inefficient though. Why not size the prop correctly to begin with rather than relying on the ecu derating the engine

I had the tallest transmission (2.5:1) available with a down angle and the propeller was 28 x 20, which was the least pitch available without a custom propeller. While you could cut the diameter down, the boat wouldn't perform as well in head seas and high winds. The boat was originally setup with a 28 x 28 and a 450 HP Cummins. 28 was the right diameter.

As far as efficiency, you will find very few 45' boats with 40,000 pound displacement that cruise as efficiently at 6 to 7 knots.

Ted

 

[DDW]

No, that is incorrect. The governor is driven by engine speed (rpm) and governs the fueling. If it’s overloaded the engine doesn’t have enough power to increase speed so the governor is unable to increase fueling.

This is also incorrect. Yes, on a boat diesel the governor is driven by engine speed (not true on the same diesel in a truck however). An overloaded engine will increase fueling to try and maintain rpm, which causes overfueling. The governor controls the fuel rack and doesn't really know what the torque curve is. It is a bit different on an electronic engine, which has multiple feedback mechanisms to control fueling, not just rpm and rpm request.