Hybrid Engine Systems and Sustainability

Not what I said. What I said was that the change in efficiency from cruise RPM to max power RPM (assuming that the maximum power RPM condition doesn't take the prop blades into a loading region where there is a cavitation or stall condition) will be quite modest for a fixed pitch prop. Try it in a decent prop modelling package and see.

There is certainly a fairly big drop in efficiency when the prop is run at an RPM range well below the peak efficiency point, but this is a region that isn't usually significant for most boat operation regimes.

A fixed pitch prop efficiency curve starts at zero for zero rpm, then increases to a plateau, and then stays fairly level (just a few percent variation) until the onset of either blade stall or cavitation at the upper end.

Provided the prop is correctly selected, so that it operates in that region where efficiency doesn't change much, then there's no great benefit from having variable pitch.

For example, here are some quick and dirty figures I've just modelled for a small 304mm diameter x 376mm pitch two blade prop (for a light 18ft electric river boat), running between a cruise thrust requirement of 30N for 2m/S and a maximum thrust at full power requirement of double this, 60N (also for 2m/S to simulate pushing into a headwind at the same boat speed but with double the thrust):

At 30N thrust:

RPM = 400, efficiency = 82%, power absorbed = 74 W

At 60N thrust:

RPM = 420, efficiency = 76%, power absorbed = 158W

As you can see doubling the thrust and more than doubling the power only results in an efficiency degradation of 6%.

 

Okay, now from the latest article from Nigel Calder in April/May PBO, I will try to compare fuel efficiency of diesel driven and hybrid serial engines. If anything wrong, please advise and stay constructive.

Let's take the attached curve for The Volvo Penta D2-75.
Say the yacht is at a cruising speed of 2100 rpm, its theoretical SFC will be 260 g/kWh (matched propeller curve).
You will argue that you can have a lower SFC of 250 by cruising at 2500 rpm, but I don't think most people would do this because of noise, vibrations, consumption (cube of rpm). In fact, I think that with a 3000 rpm diesel engine, most sailors tend to cruise below 2000 rpm.

Now let's take a generator that has the same characteristics and could work at 230 g/kWh, with an alternator efficiency of 90%, and state of the art electric motors/controllers with an efficiency of 90% at cruising speed.
The system SFC would be 230/0.90/0.90 = 284 g/kWh.

So the difference is less than 10%, and we haven't even considered e.g. marinization/transmission losses.

But now with this system, you will have the comfort of being able to charge house batteries with an 80% efficiency, whereas with the diesel engine, I don't think I'm too far when saying that the efficiency of a standard alternator/charger system is around 20% (Jeremy, I'm sure you'll have something to say).

When ocean passage crossing, generally the use of engines is strictly limited to battery charging around two to three hours a day (greedy autopilot/fridge), so the hybrid system would be ideal.

Plus: the above has been calculated strictly in perfect conditions, maximum propeller efficiency of say 70%, and for designed displacement.

But I don't think we often motor in perfect conditions. We use the engine e.g. in a formed sea because our windward pointing ability has been seriously reduced, or we motorsail because it is more comfortable or we want to arrive earlier, or indeed for harbor maneuvers, or as written above to charge batteries etc.

All conditions which are far from ideal conditions for which the propeller pitch has been calculated, and it is likely that the propeller efficiency hence thrust will easily be reduced by 10 to 20%.

On a cat you would put the second engine, but then same situation, the propellers efficiency would drop (if manufacturer has correctly considered max. eff. for only one engine).

With automatic variable (sorry Eric, controllable) pitch propellers, in these conditions the diesel/electric efficiency will likely be better than the diesel one.

And last, if you have a Gunboat and constantly sail above 14 knots, then you benefit from regeneration. But not for me unfortunately...

All this to say that we cannot compare diesel and diesel/electric systems only on the theoretical consumption argument. If we like the comfort (including in design) it brings, then it will soon be the way to go.

EDIT 1: I forgot to mention that you would not want to use anything other than an atmospheric diesel engine on an offshore cruiser, whereas a supercharged generator would reduce SFC.
EDIT 2: I've just looked at Volvo's curves for D2-75. Max. torque is for 1800 rpm, maybe this is the cruising rpm to choose for motoring in waves. But in this case engine SFC is 280.

 

I recently calculated prop efficiencies for a power cat in ideal weather but various conditions:

2 engines light disp. : 68% *
2 engines max disp. : 63%
1 engine light disp. : 57%
1 engine max disp. : 50%

* max. efficiency for 2 engines at this is a power cat. For a sailing cat I would have taken max. efficiency for one engine only.

Do these results surprise you Jeremy?

Wind and sea conditions, motorsailing etc can easily bring the same type of change in the propeller efficiency.

 

But nothing there addresses the fundamental point that propeller efficiency doesn't vary a lot between 50% thrust and 100% thrust, as I illustrated in the example above (in that case the prop efficiency difference was only 6% between these two conditions).

If I adjust the pitch to get optimum efficiency for the 30N cruise thrust and 60N maximum thrust example (so simulating a variable pitch prop) then we get these figures:

30N thrust case:

Efficiency = 82%, RPM = 400, pitch = 376mm, power = 74 W

60N thrust case:

Efficiency = 76.5%, RPM = 540, pitch = 300mm, power = 156.7 W

Optimising pitch gains 0.5% in efficiency, so it's simply not worth doing.

 

What prop modelling code did you use and what boat resistance data did you use?

My resistance data comes from Michlet (verified by a towing test) and my prop data comes from Javaprop (also verified by test and measurement.

 

I had carefully read the your posts and Jeremy's posts before my post.

Why didn't you respond to the question of "Can you provide a reference for it or an example of its use elsewhere? Or is it your own creation?"

 

I've used David Gerr Bp-d method, with Wf -single = 0.84 and Wf -twin = 0.88.
It is generally accepted to give good results, and these results have been confirmed by calculations from Brompton's for max efficiency and max speed.

The cat has been launched, full speed was 10% slower than calcs probably due to appendices which are difficult to apprehend in block coefficient.

 

Much as I have respect for the Gerr method (and the man himself) that isn't, by any stretch of the imagination, a prop performance modelling method. At best it's a way of approximately sizing a prop, based on a set of empirical observations that have been adjusted to fit a set of curves.

The method produces gross errors under unusual load conditions, such as when the hull resistance increases beyond that expected for some reason, or for prop load conditions that are outside the range of the empirical data used to formulate the method.

I played about with his method years ago, but quickly found so many anomalies when I applied it to my boat test data that I went back to first principles. Together with others here, we determined that Michael Hepperle's Javaprop code, when used with the right parameters, gave very much more accurate results, generally within 1 or 2% of measured results. Similarly, Michlet gives pretty good hull resistance results, certainly within about 5 to 10% of the measured tow test data for my hull.

I've probably spent a few hundred hours modelling, making and testing props in recent years, and in all that time I've never seen much of an efficiency change for a fixed pitch prop between the half thrust and full thrust condition.

 

A bit of topic change but it's for MY HYBRID, so maybe on topic....
I need backup more than ahead propulsion on my motorsailor.
I'm tempted to install a 5 bladed seagull hydrofan prop, backwards on my main prop shaft.
the prop will be working "in reverse" when I'm motorsailing ahead. I only need a little thrust, just so I'm not dragging the prop.
If I need brakes, Then full astern, spins the prop in it's designed "ahead" and most efficient thrust direction.
In adittion, at anchor in a current, the reversed prop is better oriented as a turbine generator.

any comments welcome.

 

Jeremy, thanks for the feedback. I always thought David Gerr Bp-d method to be accurate, but I will certainly have a look at Michael Hepperle's Javaprop code.

 

Another note on Nigel Calder paper in PBO April/May.

NG compares the serial hybrid and diesel engine systems on his Malö yacht Nada. Nada is 39.5ft LWL, therefore for fuel efficiency, the cruising speed should not be over 80% of theoretical hull speed, hence below 6.7 knts.

At 6.7 knts, the fuel consumption is 4.8 l/h with the conventional diesel engine, and 5.1 l/h with the hybrid system (bypassing batteries), hence a 6% difference.

What is striking is that to obtain this result with the conventional engine, NG has oversized the propeller, limiting the engine top speed to 2700 rpm instead of 3000 rpm, "to raise the bar for the hybrid systems as high as he could".

Although this allows to achieve a SFC of 235 instead of 260g/kWh at the cruising speed of 2100 rpm, this is definitely not what any standard yacht owner would do, as it is likely to overload and damage the engine when heading strong winds or motoring in waves.

With a properly sized propeller, the fuel consumption would have been 4.8x260/235 = 5.3 l/h, hence conventional diesel consumption above the hybrid system consumption.
.

 

Again, this seems to be missing the fundamental point, and focussing on specifics applied to a mismatched engine and prop combination.

Let's go through the series hybrid efficiency data and the conventional transmission data again to illustrate this.

The best alternator/generator will have an efficiency of no better than 90%, more probably it will be around 85%. The best electric motor will have about the same efficiency, 85 to 90%. There will be losses in the wiring and motor controller of around 5%.

A conventional transmission will have around 95% efficiency.

So, if we assume a level playing field, with the conventional transmission and the series hybrid system both optimised for best efficiency at normal cruise speed performance, then the conventional transmission will have an efficiency of around 95% and the series hybrid will have an efficiency of around 77%. There's no getting away from this, it's a fundamental issue associated with the efficiency of the relative components.

 

I'll post actual data from my experiment when completed.
if it doesn't work, then you told me so.
but if it does...I win because I've got the boat!

 

From Nigel Calder's article in Professional BoatBuilder April/May 2013:

"The next step was to extract a representative propeller curve from the conventional system and to compare the performance of hybrid systems to it. It is important to note that we optimized the conventional system as best we could with off-the-shelf components. In particular, we tested a small modern diesel engine operating at 3,000 rpm, as opposed to the more conventional 3,600 rpm; and we fitted a relatively high reduction gear f 2.74:1 as opposed to the more traditional 2.6:1. This slowed the peak shaft speed to 3,000/2.74 = 1,095 rpm, as opposed to a more typical 1,300+ rpm. The slower shaft speed required a larger, more efficient propeller than might typically have been fitted. Following Volvo-Penta's latest guidelines, we achieved additional efficiency gains by somewhat oversizing the propeller, limiting the engine's top speed to around 2,700 rpm. This effectively shifted the propeller curve into a more efficient part of the engine's fuel map, so we gained at both the propeller end engine end." [Boldng added]​

Given that Volvo-Penta's latest guidelines were followed I doubt that th selected propeller is likely to overload and damage the engine.

 

This bears repeating. You CAN oversize a propeller to increase efficiency without hurting the engine. Assuming that you don't add gears, it will reduce your top speed and head wind capability - but that tradeoff is worth it to many people.