electic kW / diesel kW

hy
i am looking forward for an hybrid drive for an sailboat. i have seen on many articles from electric motors, that for example an electric motor of 10 kW can easely replace an diesel up to 40 kW.

I just can't find the right answer for that... ?

i just think maybe if the electric motor is forced and overheated, otherwise i can not see how an electric motor of 10kW to give the power of an diesel of 40 kW..

 

Use the 'search button' and look up "diesel electric" in quotes. It will surprise you with the amount of material.

For instances
http://www.boatdesign.net/forums/hybrid/diesel-electric-propulsion-sailboats-9310.html

Welcome to the forum

 

Kw is a unit of power. It doesn't matter if the propulsion unit is electric or diesel. The power applied to the propeller is the only calculation that is important to calculate the size of the motor. The rating of the motor is another variable to consider. That is, if you want to have reserve power or run it at its maximum.

 

Hi horry_bv, welcome aboard

The simple answer is that a kilowatt is a kilowatt. The thing on one end of the shaft (a propeller) has absolutely no idea what is on the other end of the shaft, nor does it care. All it cares about is that the shaft is delivering the proper torque at the proper RPM.

This perception that an electric drive can have a lower power rating than a diesel or gas engine exists for several reasons. One is that most people won't run an engine at full throttle, and most designers and engineers will choose an engine so that, at cruise speed, it's running well below full throttle. 1/2 to 3/4 of WOT is where most people tend to run their boat engines. (Note that if you size the electric motor for full power at your cruise speed, you won't have the reserve power that the final 1/4 of the throttle arc gives you when you need to claw off a lee shore or whatever.) Now, that engine is also turning auxiliary gear- alternators, maybe a hydraulic pump- that can sap a fair bit more power. In the electric boat, these loads draw directly from the battery (which must be sized accordingly).

The types of electric motors used in hybrid drive systems tend to produce very high torque at low RPM. Thus, in manoeuvring situations, an electric can 'feel' like it produces as much thrust as a diesel with a higher peak power rating. Of course, if you push the throttles of both the 10 kW electric and the 30-40 kW diesel boat fully forward, the difference will become quite apparent.

A rule of thumb sometimes used to determine whether a hybrid system is worth considering is:

Calculate the total energy demand of all your onboard systems, based on their power demand and how much you use them: lights, instruments, radar, watermaker, A/C, whatever. You'll get a number probably in kWh per day, or per week.

Now calculate the energy demand of your propulsion system. How many kilowatts of power, for how many hours. Use the power setting you actually run at (it'll probably be an educated guess), not the peak power of your installed engine. Consider how long you run under engine, and get a figure in kWh/day or kWh/week.

Now, if these two figures- house (or hotel) load and propulsion load- are somewhat comparable, or if house load is higher, then it's possible- BUT NOT FOR SURE- that diesel-electric or some other hybrid system could work for you. Cruise ships, for instance, tend to fall in this camp, and reap very substantial cost savings from their multi-diesel-generator, multi-electric-drive-pod systems.

If, as is the case for many leisure craft, the propulsion load outstrips the house load by more than an order of magnitude, it is highly unlikely that electrifying your drivetrain would yield efficiency improvements commensurate with the cost and weight of the equipment. Indeed, a hybrid drive in such a boat could very well be noticeably less efficient than a straight shaft with a good gearbox and prop.

There are, of course, exceptions- folks who want silent running, or travel on restricted waterways that don't allow engines, will obviously go electric even if it is heavier and/or more expensive.

 

Also remember these Murphy's law,
A diesel is lot easier to keep dry than a electric motor...
A diesel fuel tank never loses charge...
The bad weather will invariably happen when your batteries are low, and you will wish the extra speed a diesel would have given you...
A diesel engine boat will always find a buyer and home port...

 

A diesel engine rated at 40kW will produce that power at its shaft at the rated rpm. This will not align with its peak torque. The peak torque will be at roughly 80% of rated rpm. The torque available at lower rpm will be considerably lower. Also some IC engine ratings are given without allowance for auxiliary loads such as water pump, oil pump, fuel pump and generator. The power available at the shaft is less than the rating. All IC engines start wearing at day one and, apart from a brief period of freeing up, they are on a downhill slide until they are overhauled. So in operation they only delivered factory rating in the first few hundred hours. After a few thousand they will be down by maybe 10% and fuel consumption up by even more.

In matching a propeller to a diesel engine it is usual to target peak power to be delivered close to peak rpm in calm conditions. The engine will not be able to deliver peak power once the prop loads up in heavy weather. It could even be that the engine will not get to peak torque but this would be unusual and likely the boat overpropped to get efficiency.

Typical electric motors are able to deliver 2 to 3 times rated torque through their rev range. Some are built to get over 5 times rated torque for certain applications but they will be heavy. Electrics motors maintain their power output throughout their life. The rating is based on the ability to keep the frame cool. Usually this is based on 40C ambient in still air. In cooler locations with forced ventillation the available power increases. The power rating is what will be delivered; no auxiliary loads rob power.

So electric motors have some advantages in terms of comparison with IC engines from the nameplate rating. Saying 40kW IC is equivalent to 10kW electric cannot be a blanket statement. It has to consider the basis of comparison.

As an example you will see electric dragsters with a fraction of the rated power of IC dragsters doing better times as this sort of application brings out the huge torque advantage of an electric motor. You only need a motor with enough torque to spin the wheels, set the current limit to produce that torque value, have enough voltage to get the required top speed and you can have the wheels on the verge of spinning throughout the time it takes to cover 1/4 mile. The IC has to be revved, clutch dropped in the most uncontrollable fashion and possibly a gear change at some point in the 1/4 mile.

Rick W

 

If I may add:

and home!

Sasha

 

Due to the constant power of an electric, larger diameter props with higher pitch are used. These are more efficient, with less slip so power is delivered with much les losses (including the ones listed above).

If you put the same prop on a diesel, it would not get up to its reasonable torque band efficient range. Hence performance is much better KW for KW due to the contant high torque.

With IC motors, same Hp motors may develop different torque curves that show that torque and power are not directly linked.

Go with non smelly electric and your yacht will not have that distinctive aroma of diesel fuel!

Colin

 

This leads to the question, How easy is it to find propellers that are optimally designed for electric drive ?

 

Assuming you're talking inboards, and not outboard motors like the Torqeedo: As easy as it is to find propellers for any other inboard engine. If you have a particular boat that needs __ kN of thrust at a boat speed of __ knots, with a maximum diameter of __ cm and a prop shaft that spins at __ rpm, it doesn't matter whether that shaft is spun by an electric motor, or by a gear reduction from a diesel, or by thousands of hamsters in little wire wheels. The prop will be the same.

What can be hard to find, at times, is large diameter, low RPM, low power props. There are some efficiency gains to be had by going to large diameter and high-aspect-ratio blades in some low power applications, but since there are so few low-power boats around that can swing such large props, they are not necessarily easy to find. Rick Willoughby has built a few such props for his own experimental boats.

 

"multi-electric-drive-pod systems."

Also figured into the ships DE "savings" is the lack of need for tugs to assist berthing and departing every day with the az-pods.

"Due to the constant power of an electric, larger diameter props with higher pitch are used. These are more efficient, with less slip so power is delivered with much les losses (including the ones listed above)."

IF a vessel is designed to be fitted with large diameter props , the diesels are easy to fit a 6-1 or deeper reduction gear.

Big problem is on a sail boat there are not folding props , so the more expensive CPP would be required.

While "co generation" can be done the drag from a 36 to 60 inch prop would be similar ( But worse) to sailing with a drogue that diameter.

FF

 

There are no opimal props, apart perhaps from the Torqueedo prop, which is much larger diameter than other 'trolling motor props" they are up to 12" diameter with two to three blades with considerable surface area and pitch. The only way is trial and comparison as the rules are new and too many variables like hull speed and force required to reach close to hull speed.

As you'll see on Electric Boats Forum, you want to turn as big a prop as you can, dependant on gearing, motor and drive, with a high pitch, sometimes pitch close to diameter.

It depends upon many factors such as boat length, displacement etc, but an IC and their clutched gearboxes are much less efficient at turning a KW of energy input into a %KW output. Like a car, it is power ar the wheels that counts. A larger diameter, surface area & pitch prop has less slip in the water, hence better efficiency by far. An IC engine could not get a prop that large turning at its low power low revs, whereas an electric with basically a flat power curve can - hence why a smaller rated electric can equal a larger output IC.

Note though, this is for up to hull speed only, for planing speeds, an IC Wins!

 

Thats quite not the full truth!
The CPP is the (close to) perfect prop when DE doesn´t make sense. (and it never does in a yacht)

Regards
Richard

 

The more I look into this subject , the more it appears that trial and error is the rule. Even an issue as basic as batteries is mind boggling. I looked up the performance of a battery described as 1225 Amp and found that the 1225 Amps is for only 5 seconds. When I buy batteries at my favorite automotive shop a 400 Amp battery will deliver 400 Amps for 20 minutes. If I buy a 225 Amp battery at my favorite industrial supply it will deliver 225 Amps for 1 hour. According to a convincing report I read this morning a lead acid battery delivers a maximum 20 watts per kilo for 1 hour while the latest state of the art battery delivers up to 140 watts per kilo for 1 hour. Most of the other battery technologies described seem to fall between 70 and 100 watts for 1 hour per kilo.

The next mind numbing battery issue is charging. If you don't charge your batteries with the proper peak Amperage as part of each charging cycle you could destroy them in less than a month.

The third issue of concern to me is maximum safe point of discharge. I have an unserviced cabin in the country. I bought a 12 volt to 110 volt power converter so I could use deep cycle auto batteries to watch movies there. The salesman assured me that the battery protector would trip the circuit before the battery could be damaged. After 2 evenings the battery was a stone that refused to charge. I tried a second battery and it did the same thing becoming a total stone after only 2 evenings. ( The two batteries in question came from a 1 ton diesel truck I parted out. The batteries were in reliable daily use in the truck up to the day I removed them for use at the cabin. ) That ended movies at the cabin.

 

Too much confusion.
First of all there is no such thing as a deep cycle auto battery.
Your two batteries were old automotive ones. They were usable in the truck because all they had to do was deliver starting current for a few seconds; their capacity may already have been reduced to 10% of what is printed on the label.

If you draw more than 1/10th of a battery's rated capacity, there is a 2-way penalty: the capacity is reduced and so is the life expectancy.

I have a backup system because there are many blackouts and I don't like sitting in a cold and/or dark house.
So I use a 200 Watts solar array, 250 Ah battery, an inverter and an automatic charger that delivers 25 Amps whenever the voltage reaches 10.5 and the power grid is working. The central heating always gets its power from this system, TV and lights normally use the grid.

I use good quality car batteries, but the discharge cycle are in fact to deep, so there is a lot of wear. After 4 years I replace them; they still work but have lost so much capacity the inverter starts buzzing if I switch on the TV and lights. But I can still use them to start a car!