electic kW / diesel kW

Have not seen name calling here Matt.

 

Do you know what is the relation of hp and Nm - as in relation of power adn torque? By definition?

torque*multiplier(for correct units)*rpm=power

It really is that simple. That means BY DEFINITION that exact same torque can be geared for output from different sources regardless of their shaft torque if they have same power output.

The only aspects that benefit electric in the comparison:
- e-motors often rate for 24/7 output and if well cooled they can temporarily produce way over rated hp. Assuming the controller etc. can provide the juice.
- in some applications the current IC setup has very inefficient gearing so the motor works in a rpm range where it cannot provide even near its rated power.

 

Power curves for Internal Combustion Engines are not fully linearly related to the torque curves and vary dramatically over the rpm range of an ICE. This is why there is such a "sweet range" of ICEs where power and torque are optimal, but at lower revs the torque is severely impacted and the power decreases. At very low revs of below around 600rpm the motors stall.

Electric motors tend to have much flatter power and torque curves giving them the ability to turn a reasonable diameter prop with high pitch that a similar powered ICE would stall in trying to rotate. This rotation can be very controlled to hardly turning or full revs to reverse in very short tiimes that make docking with electric motors much more controlled. See an example power curve for a 5KW BLDC MOTOR http://goldenmotor.com/hubmotors/48V5KW-09052602-3800RPM%20Performance%20Curve.jpg where you will notice subtle curve in the power output but nowhere nere the mountain shape of an ICE curve.

 

of course - you are totally right - if you have a ICE rated for lets say 100hp/4500 rpm and you put it in application where you are going to run it at 800rpm you will not be very pleasantly surprised. The question is who would do that? It would be idiotic.

However using constant rated diesel engine of lets say 10hp will perform very well in an application where 5-10 hp is needed. And you cannot replace it with 3hp electric engine and expect same performance.

If the ICE stalls then the gearing is totally wrong. Show me a prop and tell me how fast it needs to spin and show me an electric engine that can do that and I will show you a diesel that can be geared to do exact same task - assuming rated powers are in line.

Also I hope you are aware that running e-motors at high torque at very low rpm is very inefficient and mostly creating heat.


Here is a power curve of a 1000cc v-twin sports bike. The flatter curve is torque and power curve is the one that keeps climbing. As its pretty high performance engine it needs relatively high RPM to be "in the range" - however note that despite being a 100hp/liter sports touring engine it has very flat torque curve any speed above 3500 rpm to the redline.

Show me an electric engine busting out 100hp on shaft and I will gear this engine to do the same exact task.

 

That statement is blatantly wrong. They are exactly related - on any chart that you can see a torque and rpm you can plot the power.

You CANNOT have two charts with same torque values but different power values. Unless you cheat of course. You fail to acknowledge that in its simplicity:

torque * RPM * 1/5250 = hp

or the other way round

hp = (torque * RPM) / 5250

(this is with lbft for torque - with Nm the multiplier is different)

 

Theory is fine but reality of power to a prop is different especially for slow reving diesels that we are talking about comparing to electric options, not gas powered sports engines.

Marine diesels have a very narrow power band and are pretty gutless below 1500rpm.

In yachts most users are replacing their old stink pots with 5KW to 20KW motors with considerable success. Easier docking under very fine control and plenty of power up to hull speed. For planing an ICE will always be better, but that was not the theme of this string.

 

Were these car related articles?
In which case there is an element of truth.
I got interested in converting a car to electric drive a couple of years ago.
One of the recommended books on the subject is "Build Your Own Electric Vehicle" by Seth Leitman and Bob Brant.

In table 4-2 they list options for conversion based on cost

OK: Van with 19 hp
Ideal: Pickup truck with 22 hp
Best: Car with 30 hp
Unlimited money: Custom built Starship Electrocruiser with 50 hp

The factors that make these vehicles practical compared to the power of the IC engines they replace appear to be:-

1. the existing vehicles are over powered for the way they are used.

2. Running an electric motor at its rated power is pleasant, an IC engine at max revs is not accepted in a car.

3. Batteries are the limiting factor so drivers are willing to forgo performance in favor of increased range.

4. the torque at low revs is so high that you can keep up with or go ahead of city traffic.
I was at a seminar on battery vehicles last year, one guy had a small pickup that he could spin his wheels in
third gear.

5. for short periods you can get much more than the rated horse power from the electric motor.

This does not transfer to boats.

Displacement boats are not generally overpowered. Running the engine at the upper end of its rev range is more accepted. Boats usually run at a constant speed so having the option of a short burst of extra power is of little value.

Having researched electric car conversion I decided against it due to living in a hilly area and would not use it enough.

 

You still need the stink pot to generate power to your electric engine. So what is the gain?

If you do the thought experiment below, maybe you could stop posting messages about torque being nonlinear.


Imagine that the engine in the example above (http://www.boatdesign.net/forums/attachments/hybrid/47140d1283498595-electic-kw-diesel-kw-new-1.jpg) has a crankshaft having a crank of 1 meter. At 2000 RMP the engine can produce up to 300 Nm. In SI units, power is measured in Nm/s, which is called Watt. So how many Nm/s can we produce? Well, if we crank on the crankshaft, we travel in a circle having a radius of 1 meter. How far is that? It is pi * 2 * 1 meter. How many meters do we travel per second? 2000 RPM is 2000/60 RPS = 33.333333 RPS. So now we crank on the crankshaft (using 300 Newton force). We travel 33.33333 * pi * 2 = 209 meters every second. So the power would be 209 * 300 = 62 700 Nm / s = 62 kW.

Now this power is available to the gearbox. Assume we have 10 % loss in the transmission. 56 kW is available to turn the propeller shaft. If you do the thought experiment backwards, you can calculate the torque the gearbox can apply to the propeller at a certain RPM of the propeller shaft. High power allows for high torque at the propeller.

In reality, gearboxes have limited gear ratios, for boats usually one. This complicates matters. Torque is still torque
and power is still power. It always will be.

 

Not to bother you, but that statement is wrong again. You are too broad in your way of looking at the topic. A medium speed marine Diesel for example has a max rpm of around 500 - 800rpm. and starts living at 120rpm.

Of course in a sailboat we don´t find them. But even the average sailboat engine has a relatively wide power band (more important a wide torque band), in comparison with the same engines setup for car use.

The El motor does not, and cannot replace a IC engine if not able to crank out the same power! The ONLY advantage of the El. motor is the fact, that it provides relatively high torque at low rpm, therefore allows for better maneuvering at slow speeds. Thats it

The obvious disadvantage is the fact that you have to carry several times the weight of a IC engine setup (incl. tanks) when going El. to the same properties. There is no way around it and there will be no way around for the next 15 -25 years, no matter how nice one cheats the comparison.

By so far there is not one single boat on earth propelled electric to the same weight, performance, endurance and speed as a IC propelled sibling.

The actual state of the art el. energy storage is still lightyears away from a diesel tank and there is not one, not a single one, system in discussion at present which would be able to change the situation substantially. We know that there are about 15 or more years between studying/discussing a new technology and mass production, right? Then there are another 5 - 10, sometimes 50 years from mass production to marine application.

So, where is your El. propulsion?

In dreams and cheating comparisons, thats all.

Regards
Richard

 

Fully agreed.

'Advantage' of using more torque at lower RPM requires huge diamter of propellers that are not likely to fit in any real boat, with limited draft. If we are talking about speeds of 3-4kts on 40' boat it is true and it works (but such speed is hardly acceptable outside of marina). Once we need higher speed the required propeller diameter will be over 1m

There is a lot of advertised fiction and brainwash about electrical propulsion. I used to say to my customers clearly: we do not design electrical propulsion on offshore boats. On lakes - yes, we do

 

Batteries

I haven't finished reading this thread yet, but I'm providing input on the question of a "diesel battery" vs deep cycle vs starting battery.

I have experience with all three, and all three do exist.

1) What people are calling a "diesel" battery is a battery that is somewhere between a starting battery and a deep cycle battery. The "diesel battery" is a starting battery with thicker plates, though not so thick as a true deep cycle battery. These "diesel batteries" can be discharged to 50% just like a house battery, but have less service life. They also start engines. I would know. My Sprinter came with one as standard equipment and it's still working well. The previous owner of my Sprinter had an inverter hooked to the thing.

2) House batteries can start diesel engines, if you have a good bank. I have a set of 4 Trojan T-105's I am using as a combination house and starting bank. Mostly, they are used as house, but they start a Mercedes Diesel Straight 5 engine without any trouble, even when they are at 50% discharge. (I've never taken them below 50%, so I can't comment as the charge is reduced)

 

No real Electric propulsion yet or for over a decade! It's here already.

You obviously have not heard of the "Planet Solar" Project where an electric cat has been built to sail non-stop around the World - Any diesel boats of comparable size able to do that???

PlanetSolar is a multlihull vessel topped by a large array of photovoltaic solar panels, constructed by Knierim Yacht Club, in Kiel, Germany. Built in 14 months, the biggest solar boat ever built has impressive dimensions and is yet both silent and clean. Also uses Li Ion batteries. The goal is to navigate around the world at an average speed of 7.5 knots — no mean feat for a solar-powered craft.


See project at http://www.planetsolar.org/bateau.en.php and actual photos and videos at http://www.planetsolar.org/multimedia-photos.en.php
Note the five bladed props/"wheels" at rear of each pontoon with a huge diameter and surface penetration (only half submerged) for efficiency.

 

I think no one on this forum can afford that boat.

 

True there are different "tastes" of starter batteries.

But as it is in all businesses we have a clear terminology in marine applications.

Starter batteries are not tagged as deep cycle, because they are not.

Of course a deep cycle house bank can crank a starter when sized appropriate, but that is not what they are designed for and it does not make sense to use them that way.

And then, we are a bit off topic discussing battery properties here.

Regards
Richard

 

Planet Solar

Further info on Planet solar boat and its

http://plugboat.com/2010/03/04/planet-solar-yacht-plans-to-circle-the-globe/ The props are clearly seen here.


Also Scientific Amerian website http://www.scientificamerican.com/article.cfm?id=worlds-mightiest-solar-boat

"On board, the world's largest lithium-ion battery, a 13-tonne monster capable of storing 1,300 kilowatt-hours of energy when fully charged, will allow the boat to slice through the water at an average speed of 13 kilometers per hour for three days straight in complete darkness before its charge is completely exhausted."

"The ship is designed, however, to operate at sea indefinitely, and under normal conditions its 38,000 solar cells average enough power production to muster the 20 kilowatts of power needed to keep the boat cruising along at its average speed. The maximum electrical output of all the cells together on a sunny day is about 100 kilowatts."

* Length: 31 m
* Width: 15 m
* Length with flaps: 35 m
* Width with flaps: 23 m
* Height: 6.1 m
* Weight: 85 t
* Surface of solar modules: 536 m2
* PV panel efficiency: 18.8 %
* PV installed power: 93.5 kW (127.0 HP)
* Average engine consumption: 20 kW (26.8 HP)
* Average speed: 7.5 kt (14 km/h)
* Maximum speed: 14 kt (25 km/h)
* Crew: 3 - 4 skippers
* People that can go onboard: 40
* Autonomy: Never-ending solar navigation


To me 20 KW of electric power to propel a 100' cat weiging 85 tons at over 7.5 knots continuosly up to max of 14knots indicates more efficiency than possible from Internal Combustion engines - could two 10KW (13.5HP) ICEs move this boat at any speed, let alone at cruising speed? I don't think so...

I rest my case with you narrow minded, fuel sniffing, ICE biggots. Open your minds and consider the potential instead of being so closed minded.