electic kW / diesel kW

[horry_bv]

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..

[brian eiland]

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

For instances
Diesel/Electric Propulsion for Sailboats

Welcome to the forum

[gonzo]

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.

[marshmat]

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.

[mydauphin]

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...

[Rick Willoughby]

Originally Posted by horry_bv

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..

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

[M-Sasha]

Originally Posted by mydauphin

A diesel engine boat will always find a buyer...

If I may add:

and home!

Sasha

[COOL Mobility]

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