I understand the Cummins turned up to 600hp that will last appr. 500 hour between rebuilds will be coupled to a NXT drive with a real tranny..Possibly by the end of the year.
that would be impossible for him to get his stories straight, he is on his 11 screen name. This guy (Jim Darr) could have a great conversation entertaining himself with all of his personalities back and forth. There is a saying, it's bad when you talk to yourself...but when you answer yourself...watch out....loose cannon.
I promised to post this a week of two ago, but things got busy for me. This is my take on diesel vs gasoline. Comments are welcome and encouraged.
Horsepower moves the boat. More horsepower can move a given boat faster. Gasoline or diesel, the prop and boat don’t care. The engine's job is to convert stored energy (fuel) to horsepower and the propeller's job is to convert the engine's horsepower to thrust horsepower. Different engines and different propellers do this with different efficiencies. Diesel engines run on fuel that contains more energy per pound than gasoline as well as converting more of the fuel's energy to useful horsepower.
If one were to take a gasoline powered boat and simply swap in a diesel engine that made the same amount of horsepower, the result would be unacceptable. The diesel engine will have a lower speed limit (rpm) and thus turn the prop slower, and have a much higher peak torque (which tends to break parts). In order for performance to be acceptable, other changes to the propulsion system are necessary.
With equal peak power, the diesel could push the boat to the same top speed, if the power could be transferred to the water with the same efficiency. This is difficult to actually accomplish in the real world. To make the swap to diesel successfully, I see four alternatives. To illustrate the points, an example boat with the following characteristics is used:
Gasoline engine: 500 hp @ 5500 rpm, peak torque 500 ft-lbs
Max speed: 74 mph
Propeller pitch: 26"
Sterndrive reduction ratio: 1.5:1
Diesel engine: 500 hp @ 3500 rpm, peak torque 950 ft-lbs
Sterndrive ratio: ??
Propeller pitch: ??
Alternative #1 (Change propeller pitch):
The propeller needs to deliver all available power to the water to reach 74mph. Changing the engine to a diesel doesn't change those requirements*. Since the diesel engine will have less rpm range than the gasoline engine, if we just swapped the 500 hp gasoline engine for a 500 hp diesel engine and left everything else the same we'd couldn't turn the prop fast enough to reach the previous top speed. Since, at planning speed, the power that the prop transfers to the water is reduced exponentially with rpm (x^2.7) the 500 hp diesel would actually be making far less than its rated output, even at rated speed. This could be overcome by changing the pitch of the prop.
The engine speed was reduced by 36% when we changed to diesel (5500-3500=2000; 2000/5500=.363). A 36% increase in pitch would "theoretically" correct this [ignoring the impact of slip, rake, cup, etc]. So, we replace the 26" prop with a 35" prop. This may not be practical on it's own due to the other variables involved in propeller dynamics that would make a 35" pitch prop difficult to live with (idle speed, planning ability, etc). There are other changes to the propeller could be made to allow the prop to absorb all of the power made available by the engine (diameter, cup, progressive pitch), but the reality here is that this solution is not a very good one on its own.
Alternative #2 (Change drive gear ratio):
Changing the drive ratio could allow you to maintain prop speed when changing engines. In this example, the gasoline engine turns the prop 3667 rpm at rated speed (5500/1.5= 3667). The new diesel engine will only turn it 2333 rpm(3500/1.5=2333). To get the prop to turn the required 3666 rpm, the drive ratio would need to change to .95 : 1 (3500/3667=.95). This would result in no loss of top speed or propeller efficiency, but may not be possible with available drive ratios. Another challenge here is that the power curve of the engine becomes a critical factor. This gear change puts a very high demand on the engine at much lower RPM than the original gasoline engine. If the engine can't deliver this, the boat performance will suffer (difficult to get on plane, difficult to stay on plane, etc).
Alternative #3 (Multi-speed transmission):
Not unlike the 2nd alternative, a multi-speed transmission could help solve the problem of not enough engine speed and too much torque available. The transmission would allow you to trade torque for shaft speed. Multi-speed could be as few as 2 or many as practical. Two is probably adequate for most applications. Heavier boats might benefit from more. The work here is balancing the gear ratios to make it work. The ratios would be set based on the desired prop speed.
Prop speed target: 3667 rpm
Max engine rpm: 3500
Drive reduction ratio 1.5:1
Low gear ratio: 1:1 (max prop speed =2444 rpm)
High gear ratio: .64:1 (max prop speed =3667 rpm)
Drive reduction ratio 1.15:1
Low gear ratio: 1.2:1 (max prop speed = 2444 rpm)
High gear ratio: .82:1 (max prop speed =3667 rpm)
This option is likely the most expensive to implement, but perhaps with some benefits not realizable with the other options, especially with heavy boats or those that require an unusually high amount of power to get on plane.
Alternative #4 (Overdrive gearbox):
Ideally, a performance diesel engine for a boat would turn the same rpm and make the same power (or more) as its gasoline counterpart. With today's engine technology, that's not realistic or cost effective. However, with a clever overdrive gearbox design, you could fake it. The overdrive gearbox could consist of coaxial input and output shafts with an overdrive ratio between them. The output shaft would be connected to the transmission or drive just like the gasoline application For this example, the input to output ratio would need to be 1:1.57 (5500/3500=1.57), more commonly written .64:1 (1/1.57). This overdrive gear ratio would reduce the torque seen by the driveline by the same ratio. From the drive's perspective, it would appear that the engine made a peak torque of only 605 ft-lbs (950/1.57=573). Maximum power output would be mostly unaffected (small % loss is overdrive gearbox efficiency). This solution results in the best overall solution to powering a sterndrive with a diesel while maintaining performance characteristics expected from gasoline engines. Low speed maneuverability might be compromised due to a higher prop speed at idle.
In practice, this is far from a trivial task and requires an extremely robust and, ideally, nearly maintenance free gearbox. Executing this properly would improve the chances of success in a diesel repower project. In theory, this could be easier and cheaper than a multi-speed transmission and still give satisfactory results.
There is no perfect solution for every application. The various factors (prop selection, drive ratios, boat weight, top speed, cruising speed, acceleration, handling, etc) must be weighed and a path chosen to minimize any negative impacts. Attempts at using diesel engines in performance boats have had varying degrees of success. As more people try today, perhaps the availability of modern components will improve the success rate.
Last edited by mthill; 04-11-2008 at 01:37 PM. Reason: bad prop change math, pointed out by Njawb
Nope. Look at your own math in Alternative #2: 2333 is still just 36% less than 3667, not 54%.This is magnified by the drive gear ratio
Yup, that part is true. It is also true that changing to a more efficient prop can be a good idea, especially when using a multi-speed transmission. Unless your goal is only fuel efficiency, you probably don't want to go completely to the big, low-slip route, but you probably can afford (depending a lot on gearing) to go somewhat more in that direction than with a gasoline engine.
Essentially, each choice is a compromise in one way or another. A big low slip prop would be good for long distance cruising, but hurt responsiveness.
I think most boat builders know what prop works for a given boat. You could power with diesel, gear (or regear) appropriately, and keep basically the same driving characteristics, while picking up some economy and durability.
It won't be exactly the same, hopefully better at most operating points.
There are of course other differences in the way diesel engines are controlled. Not having a throttle changes things. Traditionally, marine diesels run a variable speed governor where the "throttle" controls the engine speed setpoint. The governor adjusts fuel to maintain that engine speed as much as possible.
Modern full-authority diesels don't have to be governed this way, but they can be. They can also be governed based on a torque demand from the operator. From the operators perspective, this would more closely resemble a throttled gasoline engine. In a performance application, maybe this is preferred.
I don't know how the Cummins QSB/QSC, Cat C7 or Yanmar 6LY3 are governed. Maybe firefox or Joe know...In fact, I don't think the Yanmar is fully electronic at all.
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