Hybrid Engine Systems and Sustainability

I can't find any advice in any manual from Volvo on prop sizing -they have a propeller sizing software here http://vppn.volvo.com, but for partners only.

However, see articles from the CEO of a big Volvo dealer: http://www.marinepartsexpress.com/express/express.htm
"HOW TO SIZE PROPELLERS" and "PROP SIZES AND GEAR RATIOS"

 

That's what is interesting, you find individuals saying things like "an oversized prop is also bad for the engine because overloading the engine will eventually cause damage to the bearings and cylinder walls" and then you have manufacturers saying that such loading is recommended (when in a long haul truck, but what difference does that make?). The only problems points I know of are 1) running rich and 2) running at 100% load below peak torque rpm. For the latter, you just need to back off some. The couple of wear maps I've seen confirm this.

 

Jeremy, I have read again the first article of the hybrid systems series from Nigel Calder in PBO June 2007; he details in length the calculated and effective Power to Shaft and Power to Water for his Malo 46 yacht Nada.

Nada propeller efficiency results as follows:

Smooth water
Speed (knt)......................6...........7.........8
Power to Water (kW).........4.5........8.4......16
Power to Shaft (kW)..........8.........14.5......29
Efficiency........................56%......58%......55%

Rough water
Speed (knt)......................6...........7.........8
Power to Water (kW).........14........19.6......30
Power to Shaft (kW)..........30.........41........65
Efficiency........................47%......48%......46%

As you ascertained, there is not much variation with thrust, but Nada propeller efficiency varies between 46% - 58% depending of weather conditions.
.

 

Time to adjust the carberator on the ol diesel

 

Catalina 30 with Electric Drive

Interesting website: http://electricboatdesign.com/

"We pulled Kapowai through the marina at various boat speeds and then charted the results. We compared thrust (in pounds) at boat speed to kilowatts consumed at boat speed to get thrust per kilowatt at boat speed. We believe this is how electric boat drives should be rated for efficiency."

Here is the curve obtained, it is surprising to see how the system efficiency drops with speed.

 

Not really. Just shows that they matched the propeller for a lower speed, that's all. Given that propeller efficiency falls off either side of the optimum speed/thrust/rpm point, selecting the conditions where it delivers best efficiency is key to getting the best out of electric drive.

Often heavy sailing boat power plants are driven by a need to retain good low speed performance, so that there is an abundance of useful thrust for manoeuvring, where it is a useful safety feature. Efficiency will then tend to fall off as speed increases.

 

I would have thought that one would select maximum efficiency for cruising speed, as for maneuvering the consumption is so low and with short spikes, so you don't really mind efficiency.

Here is their assessment of motorsailing:

It is very easy for the propeller to be turning at the speed of the water column, it’s just a matter of overcoming the friction of the drive system. It takes between 50 and 100 watts to overcome this friction. Once the propeller is turning the speed of the watercolumn, the boat speeds up about 1 knot.

The addition of another 100 watts of power is enough for the propeller’s wash to fill in the low pressure area in the water behind the boat. This low pressure area is shifted from behind the boat to in front of the propeller, and the boat is being pulled forward rather than pulled backward.

The addition of some forward thrust results in apparent wind for the sailplan which fills out the sails. An electric motor is so effective that the throttle can be used to trim the sails.

It takes 2 kilowatts to speed up from 3 knots to 4.8 knots under power alone. Under sail it takes only 200 watts. Think of it as a 1,000 percent return on energy investment."

This would mean that with the little help of an aerogen to supply those 200w, in calm seas you could easily gain two knots when sailing without consuming any fossil fuel. Does that make sense?

 

If we look at the Volvo thumbnail , the engine is basically matched to the boat to cruise at high speed , 2800rpm or so.

If minor power is required say 30KW , the engine can easily produce that at 1500 RPM, with out overload

The problem is we have to spin the engine at 2500 RPM to get the prop up to a speed where it can produce 30KW.

A common solution is a CPP prop , tho a 2 speed transmission would work as well .

Lower engine RPM , speed the prop up to gain efficiency.

ZF on the shelf.

Were this done for most diesel displacement cruisers the efficiency would be very close to a hybrid electric at very minor cost.

And there would be no problems with batteries going Boeing !

 

The boat in question is a relatively heavy displacement yacht. If you've manoeuvred something like this around a restricted area, with a small motor, then you'll know that having an excess of low speed thrust is a comforting thing to have. Look at the prop. Wide chord, three blade - designed to give good low speed thrust with a relatively small diameter, at the expensive of higher speed performance and efficiency.

A lot of older sailing boats with small engines were set up like this, the engine was intended to be an auxiliary, rather than a main power plant. As such it was never intended to propel the boat for long periods at any sort of speed, just enough to get you home if the wind died or get in an out of harbour.

Think about this for a bit.

How big an aerogen would you need to get 200W from the apparent wind over the deck?

Bear in mind that the very best aerogen has an efficiency of around 35 to 45% (against the Betz Limit of about 60%) and that the power in the wind is given by 0.5 x rho x A x V³, where rho is the air density (about 1.226kg/m³), A is the swept area of the blades and V is the apparent wind speed seen by the aerogen.

If you assume an aerogen efficiency of 35% (which is reasonable) and an apparent wind (i.e wind speed - boat speed vector) of 5kts (2.57m/S) then to get 200W of electrical power you need an aerogen with a diameter of around 8.36m, which isn't really practical on a boat this size, IMHO.

 

Not surprising at all that "thrust per kilowatt at boat speed" drops as speed increases. Consider a hypothetical boat, motor and propeller where the motor and propeller are such that the ratio of the work done per second in moving the boat to the energy consumed per second by the motor is the same at any speed the boat can operate at. The work done per second equals to the thrust multiplied by the speed. The energy consumed per second equals the power input into the motor, usually expressed as kilowatts. So for the hypothetical boat:

Thrust X Speed / Power into Motor = Constant
That can be rearranged as
Thrust / Power into Motor = Constant / Speed​

Note that the Thrust / Power into Motor drops inversely proportional to the speed.

Now in reality the work done per second in moving the boat to the energy consumed per second by the motor is not the same at any speed the boat can operate at. But there will still be the inverse speed effect on the "thrust per kilowatt hour".

 

In fact, that's the basics of physics. And it seems that their prop wasn't even particularly efficient - less than 40% for speeds below 6 kts, according to their description of how the Thrust/kW graph was created:



Cheers

 

Unless the motor was 100% efficient and no shaft/bearning losses which is doubtful, the propeller efficiency was higher than your calculations.

 

True. It is not clear whether the "kW" in Thrust/kW ratio is intended as mechanical or electrical kW. If it's a mechanical kW, the numbers are in the ballpark (except for some minor losses in the shaft bearings). If it was power absorbed by the motor, then (assuming 75% motor+gear+bearings efficiency) these propeller efficiency values have to be multiplied by 1.35 , arriving to 55% max value.
Cheers

 

This thread has drifted way too far from the original topic of Hybrid Engine Systems and Sustainability. Why all the discussing of props. Props are a known factor and have not changed since WW2. Those of you that have questions about props please take the prop talk to its own thread.

Back on topic. MAN has the attached brochure which puts a Diesel Electric efficiency much higher than some have posted. According to MAN the losses are 8%- 10%. Look at page 4 of this brochure. I know some will say this is just marketing BS. I don't believe that since MAN sells to huge commercial interests that spend hundreds of millions of dollars. If MAN misses their performance promises by even 1% the lawyers will go for blood.

I like the diesel electric option but only if it is a NO BATTERIES system as MAN describes in this brochure. Batteries have absolutely no place in the propulsion of a boat. They are far too inefficient, you will always be better off replacing the batteries with diesel.

As the brochure states the diesel electric offers numerous packaging, serviceability and redundancy advantages. I won't repeat the brochure here but if you are designing a new boat that can benefit from especially the packaging benefits (SWATH) of the diesel electric I think it is a smart choice.

Now if only I could find/develop a system in the 150Kw range.

 

Props are relevant when someone compares the overall efficiency of a hybrid propulsion system to a non-hybrid system and the propeller efficiency is significantly different between the two.