Hybrid Engine Systems and Sustainability

Does increasing propeller diameter always reduce top speed and head wind capability? Does decreasing propeller diameter always increase top speed and head wind capability?

 

Fair point, I was repeating the articles use of "oversize" to (apparently) mean over pitch (which is consistent with their top engine speed dropping).

My answer - within limits, diameter has little effect. Pitch has a big effect.

 

Thanks for the reply. You are probably correct that pitch of the propeller was increased.

Based on the charts in the article the revised conventional configuration was capable of 8.5 knots which was presumably considered sufficient.

 

With some provisos, it's generally true that increasing prop diameter and pitch and decreasing rpm tends to improve efficiency.

The provisos are that this only works effectively if the boat speed is relatively low and the additional losses incurred by either a greater reduction ratio in the transmission, or, in the case of direct electric drive, the additional copper losses that arise from the larger diameter low speed motor, don't exceed the additional power obtained by the prop efficiency gain.

My experience has been that for electric drive it's generally slightly more efficient to use a smaller diameter motor (hence lower copper losses) coupled with an efficient reduction drive. There seems to be a net efficiency advantage in this arrangement, at least for low to medium power levels. It also makes packaging the drive easier.

 

For that engine, the cruise regime should be between 2000 and 2200 RPM, because the SFC is in the near-minimum region (around 240 for a cruise-optimized prop, 270 for max-power optimized prop) and the torque curve is in the descending zone. It means that any decrease in boat speed (due to waves - so-called "added resistance") will increase the load on the propeller (torque), which in turn will want to decrease the RPM. But, since you're in the RPM range (2000-2200 RPM) where the engine torque increases as RPM decreases, the system is self-regulated and the engine easily adapts to the added resistance.

Another observation I have to make is that you guys are apparently talking about two different types of boats, a sailboat (or a motorsailer) and a powerboat, which have different motoring, weight and (often) residential electric load requirements - besides having very different possibilities for eventual battery recharging when underway. We have to be careful not to compare apples to oranges here.

Cheers

 

Daiquiri, the diesel engine working at constant rpm, I would have thought the added resistance would have increased the load on the engine until max BHP/torque for that rpm is achieved, and that RPM would start to decrease only when the engine is overloaded.

Otherwise I agree with you, I have indeed taken the cruising regime of 2100 rpm for comparison of the two systems.

 

David, I think that these are guidelines for the Hymar project only. All Volvo or Yanmar documentation still have propeller curve crossing BHP curve at max RPM, and I don't see them (nor any yacht manufacturer) giving general advice to oversize propeller pitch. After, the owner is free to do what he wants, if he knows what he is doing (and install an exhaust pipe temp. gauge!), and is ready to waive yacht/engine guarantee.

 

In order for any action by the engine control system to take place, a change in the engine RPM has to be sensed first, for a constant fuel delivery. The governor then acts and tries to restore the initial RPM by increasing the fuel flow and hence BMEP, if possible. In the region where torque curve is decreasing with increasing RPM, it should always be possible without touching the throttle, unless the boat comes to a near dead-stop by slamming into waves.

Cheers

 

So it remains that I agree that the yacht owners have the opportunity to oversize their propeller pitch to increase efficiency, but the vast majority do not do it.

With a "correctly sized" propeller which is likely to be installed on most of the yachts, if we look at NC results, a direct serial hybrid system could be more efficient than one of the latest conventional engines (Volvo Penta D2-75) with a very low SFC.

I find this very striking. As Jeremy explained, with nowaydays technology, an hybrid system is likely to be 25% less efficient than a conventional one.

 

The 8.5 knots speed is the maximum speed achieved by Nada with the 75 hp engine, and it is actually its theoretical hull speed. Whilst the conventional system is way more efficient than the hybrid system at that speed, the consumption at 8.5 knts is double of the one at 8 knots (9 -> 18 l/h), the engine HP is above the continuous rated output, and no sane yacht owner would withstand the noise, smoke, vibrations, and impression that the engine will explode.

 

It is correct that fuel consumption is not the most important factor. Especially if we talk about the pleasure boating, where fuel bills in many cases have never been a final argument when deciding which boat to buy.

There are other important things to consider, when talking about boat propulsion in general - like the simplicity and the reliability of the system. In some instances, these two can literally make a difference between life and death.

And then there is also the matter of purchase and maintenance costs, which do count even in the nautical world.

Keep It Simple principle is still imo the best way to go, when it comes to boats. Anything added in between the engine output flange and the prop is just another thing which can fail in the least appropriate moment, besides costing more to buy, maintain and repair.

Cheers

 

True. Nothing will beat a basic atmospheric diesel for reparation all over the planet.
Soon the modern yachts will be like cars, you will need an electronic diagnostic machine plugged onto the yacht can network to be able to repair them.

 

Nigel Calder's choice of a propeller which yielded a maximum engine speed of 2700 rpm was in accordance with Volvo Penta's current public guidelines. The current Volvo Penta information on the internet for the D2-75 shows a maximum engine speed of 2700-3000 rpm.

http://www.volvopenta.com/volvopent..._diesel_sailboat/enginerange/pages/d2_75.aspx

http://vppneuapps.volvo.com/ww/PIE/...=D2-75&transClassId=9&segmentId=17&lang=en-GB

 

David, indeed on the D2-75 the maximum BHP can be achieved between 2700-3000 rpm (flat curve). But still on the latest brochure from your link, the propeller curve shown is crossing at -or even slightly below- maximum rpm.
Have you seen any Volvo documentation where they would recommend sizing the propeller to achieve max. load before max RPM?

 

I don't know about Volvo, but it's easy to find manufacturer support for running other marine compatible diesels in trucks at high loads well short of max rpm. Of course they have to be careful, people would use the data to claim that the engine doesn't achieve the rated output.