More of your opinions and comments
Here’s the overflow from the bulging postbag from Yachting Monthly’s March issue:
I have been reading with interest Nigel Calders plans for his new boat. Many
years ago I set up a high tech R&D operation and the plan was to develop
high technology for the marine market. Power distribution was just one of
the products we developed and installed (although being R&D and having a
decent budget and access to some pretty impressive facilities we obviously
took it a lot further and experimented with many different permutations). As
it turned out, our marine products became control system for massive
underground pumps and our marine network system eventually permuted into
systems used in shops, airports and newspaper distribution! This finished up
being so lucrative that all development stopped on the ‘marine side’ and the
‘rest is history’ so they say. A few years ago I sold up and ‘retired’ out
of the high tech end of R&D but my love and interest in high tech yachts has
never wained. Very recently we purchased our new love, an Oyster with plans
to sail around the world in a few years time.
I would appreciate it if you could either forward this to Nigel or let me
know his e-mail address since there are a few simple not obvious ‘rules’ to
follow with this technology and I can probably help him out a bit,
particularly with regards to interference supression design, ‘common mode’
pitfalls, secondary transmission effects and ultraLF oscillation, all
problems that we had, understood and resolved one by one.
What really stopped our products from becoming viable 15-20 years ago
(despite the fact that we could earn a lot more money in other areas) was
that we desperatly needed somebody to invent the IGBT, (all we had were
MOSFETs), now that IGBTs are around and the cost is plumetting every month,
we will see so much of the marine industry change in ways you would not
Paul McRae, Cyprus (by email)
Diesel electric propulsion
I found Nigel’s article on diesel electric propulsion food for thought. Nigel correctly highlights the inability of a fixed pitch propeller to be matched to a boat except at only one set of diesign conditions, usually the maximum design speed of the boat and the maximum power/rpm of the engine. But surely this remains true whatever the transmission system that intervenes between engine and fixed pitch propeller?
An electric motor can develop maximum torque at zero rpm as Nigel says, but then a diesel connected to a fixed pitch prop always has an excess of torque available except at the ‘design point’ of the prop as well. A diesel always produces just suffiecent power/torque to overcome the load on the prop, whatever its rpm, because the engine’s governor ajusts fuel flow to just overcome the load, so I’m unclear where a diesel electric system scores in this respect.
If the prop runs at a lower rpm, then ideally it needs to have a bigger diameter, as you should go for a big a prop as possible and then find the pitch to suit. Unfortunately there probably won’t be too much room to fit a bigger diameter.
The way to match load on the prop to the engine in an efficient way is to use a propeller whose pitch is constantly variable while under way. In the perfect case the prop law curve will then match the engine power curve exactly.
I have been using a Brunton Autoprop for two seasons now. This ‘auto pitches’ to match the load. I have measured performance over all the speed range and find that the prop power is matched pretty well to the engine power over the top two thirds of the rev range. At the old cruising rpm of 2700, the boat is now 0.7 knots faster and at the same cruising speed, (6.0 kt) I used only 2300 rpm. The maximum speed of the boat (7 kts) is unchanged. A very good proof that prop matching does, in fact, occur is when motoring into a rough sea. If the boat is ‘stopped’ by a big one, then speed recovery is very rapid as the prop pitch fines off, rather than the speed not recovering befoer the next one hit when using the previous fied 3 blade prop of the same nominal size.
I’m also a bit puzzled by Fisher Panda’s claim that fuel savings of up to 78% have been reported. At six knots in calm conditions the resistance (drag) of the boat rig will be the same regardless of power source. As a diesel engine is still being used to produce the power and the horse power required will be the same in both occasions the fuel consumption should be the same. Presumably a similar fixed pitch prop is also being used, so the propulsion system must be about 3 times more efficient to get that fuel saving. I find this very difficult to understand. Can Fisher Panda tell us more!
Pat Manley (by email)
Power Struggles, YM, Jan 2006
Quite apart from the argument of whether transmission efficiency is really an important yardstick in a yacht, Nigel Calder’s analysis of efficiency is wide of the mark.
The Yanmar curve shows a conventional cubic speed/power propeller relationship. In the example, an engine rated at a maximum of 3600 rpm will deliver 58% power at 3000 rpm. The engine efficiency at this load will not be significantly different from that at 100%.
The statement that in a diesel electric system the engine is slowed down “to produce the power absorbed by the propeller and no more” implies that because of the divergence of the engine and propeller power curves in the directly coupled case power was produced which was not being used. This is not correct. In both transmission systems the power produced equals the power absorbed. The Yanmar engine power curve represents the maximum the engine could produce at the particular speed, whereas the propeller curve shows the actual power produced.
Overall, an electric transmission system is less efficient than a conventional mechanical drive, and the claimed fuel savings of 78% cannot be a like-for-like comparison. It would be interesting to see the basis of this figure.
Graeme Armstrong’s letter was just one of a number I have received along similar lines. Everything he writes is correct. Despite the wide divergence between the engine’s (maximum) power curve and the propeller’s power absorption curve in a conventional installation, the engine will not be particularly inefficient because the governor will reduce the fuel supplied to whatever is necessary to support the power absorbed by the propeller at any given engine speed. However, we should also look at the ‘specific fuel consumption at the propeller power curve’ (which I did not put in the article). This is a measure of fuel efficiency (how many lbs or grams of fuel it takes for each HP or kW of engine output at different engine speeds). You find that on many engines there is a marked variation in efficiency at different engine speeds. For example, I have in front of me the curve for a Yanmar 4JH2-TE for which the specific fuel consumption curve drops from 237 g/HP-h at 1800 rpm to 182 g/HP-h at 3200 rpm – a 25% change in fuel efficiency.
There are also other variables at work here, such as the fact that you can typically get away with a considerably smaller diesel engine in the diesel-electric installation than in the conventional installation (Solomon has always claimed you only need one quarter the HP; others are more conservative, but all are at least 25% less) which not only saves weight, but also means the engine is generally operating at a more efficient point on its power curve.
Nevertheless, as I noted in my article, “I am still having trouble getting a good understanding” of how diesel-electric can be that much more efficient than a traditional installation! There’s clearly a complex set of
factors at work, with many variables and little real world or other testing in small boat applications. The anecdotal data supporting dramatic efficiency gains comes from people with a vested interest in supporting the technology (I hope I made this clear in the article). Which is one of the reasons I’d like to get into it myself, to see if it works this well!
My thanks to Graeme and the others for writing.
In response to Nigel Calder’s desire to own an electric engine, I would caution that it is still very early days and I hope he enjoys being a high-profile guinea pig: no doubt he will get rather better support than I did when I installed a Solomon system in my 42ft catamaran .
As he rightly points out, the Solomon system is heavy (because of the 12 x batts to run the engines at 144v DC) and expensive: the genny alone costing around £8000. The one recommended to me turned out to not produce enough power to enable me to fully utilise the engines: this resulted in my having to buy another more powerful generator. Throughout this process,neither Solomon nor HFL (the generator manufacturer) offered any kind of financial help in this expensive mistake: HFL would not even take in the generator in part exchange for the new one. So much for customer relations!
Having said that, the Solomon engines have performed faultlessly over two years. Apart from not being able to use them at more than 2/3rds of their rated output, they have done everything that Solomon said they would. One important difference between their system and the ones being contemplated by Mr Calder , is that they will run for several hours off the batteries alone and also you are able to generate power whilst sailing. This makes for an important, if not vital, redundancy to the system. For coastal sailing, there is nothing to beat the delight in being able to shift anchorages and go on and off marinas with the silence of electric engines. Generators are not as reliable as a ‘normal’ diesel engine, for obvious reasons, so I would hesitate before installing a system that had a single engine and would not function when the generator breaks down (which will happen!).
At least with my system I can motor for a few hours with no genny, and if I’m ocean sailing then keeping the batteries topped up with the regenerating power of the feathering props is simple.
My catamaran has shallow rounded hulls, so installing 2 x diesels in the hulls would have put a lot of weight aft, which it was not designed for, as well as rendering the two aft cabins useless, as they would have become engine rooms. The electric engines are light and small so I was able to put the weight (ie the batteries) into the centre of the starboard hull which greatly improved the stability and balance of the boat which previously had a large heavy 65hp Isuzu truck engine under the saloon table with a heavy centre “pod”. So, for all these reasons, installing the electric engines made some sort of sense.
With a monohull and a single engine, I can see no valid reason for not having a diesel engine that powers a propeller in the traditional way, especially if you are having direct drive with no battery storage. Whilst there might be some theories about “efficiency” which make sense, there’s nothing to beat a stonking diesel in the face of 30 knots of headwind. I use the engines quite often to “nudge” the boat here or there: if I had to start the genny every time I wanted to use the engines: why not just have an engine? Anyhow, good luck Nigel, and welcome to the world of expensive beta-testing!
Oliver Stapleton (by email)
Always look out for two things when being sold something, either by a salesman, or politician. One is the word “compelling,” and the other is the inaccurate assumption.
Nigel Calder in the January issue gives 5 benefits of diesel electric propulsion they are all valid, but not in my opinion “compelling.” He might also have added a sixth which is the electric motor could be contained in a submersible steer-able and retractable pod which would increase manoeuvrability and reduce drag when sailing.
However given a normal installation a larger slower more efficient propeller could equally well be fitted to a diesel installation. The only requirement is to increase the reduction ratio of the gearbox. The reason this is not usually done is that larger shafts and props cost more, there is generally insufficient room beneath the hull to give the required clearance, and the drag would be significantly increased when sailing.
The inaccurate assumption is the weight he gives to the inefficiency of an under-loaded diesel engine. A diesel only requires a tiny amount of fuel to turn itself; the rest is injected to turn the load at a rate of about 1gph for every 20BHP actually used. There will only be a marginal increase in efficiency if you take all the power available at any speed rather than half, because you will then need to inject twice as much fuel.
The final arbiter would be to run a series of tests and note the fuel consumption of two identical boats, one fitted with conventional, and one with diesel electric. Maths is there to save expensive mistakes. I will happily bow to more specific, and accurate test results but as an approximation my generalised guesses would be as follows. Larger prop gain 5%, fully loaded diesel gain 2%. Conversion to Electricity lose 10%, re-conversion to mechanical lose 10%, wiring and control losses 3%. Net loss in efficiency of diesel electric propulsion at normal cruising speeds = 14%. Something to carefully consider with the predicted loss of red diesel.
Graham Lascelles (Isle of Wight) by email
Windscreen for Sovereign 32
I recently purchased one of the above boats with a tatty spray hood. It had the usual problem – the clear vinyl “window” was discoloured and the distortion was so great that you could only see blurred visions through it. I decided rather than replace it, I would try to fix a solid curved perspex screen. Eventually, after weeks of effort, I finally managed to create a “mould” from which a local manufacturer produced the screen. Amazingly, it’s quite a reasonable fit! Rather than dismantle the mould the thought struck me that another reader might like such a screen, which could be made by the same manufacturer, who retains the mould for the time being. If anyone is interested they can contact me on 01923 855860.
Robert Tompkins (by email)
on p47, I notice that you suggest catching rainwater from the sails, scuppers, etc. From experience, I consider that some means of catching rainwater is absolutely essential for ocean cruising. Having fitted-out a 33ft. Atlantic Clipper ketch many years ago, I sailed it to Poole Harbour before it was fully finished. No fuel or water tanks. Diesel was drawn from an old-fashioned 2-gallon petrol can (as seen on the running-boards of 1920’s cars) and water was kept in several 5-litre plastic cans. Nearing Poole Harbour, the mizzen sail became jammed and could not be lowered. In very rough weather, I leaned out about 3 feet over the stern and released the clew outhaul then wrapped the sail round the mast. So we sailed into harbour looking like a Chinese laundry! Once aground with the legs out on firm sand, the mizzen mast was soon lowered (I had mast mounts specially made to my own design to enable each mast to act as a crane for the other) then the sail was soon removed.
Although not difficult, all this palaver took much time and lots of cups of tea so drinking water was getting short. Then it started to rain. I put the washing-up bowl out into the cockpit but the amount of water collected was pathetic. Upon crawling beneath the cockpit in search of something larger, I noticed a torrent of water coming down the 2inch dia. plastic tubes from the 2 deck drains into the harbour. I quickly disconnected the pipes from the hull outlets and placed plastic buckets beneath. By the time the shower had finished in a half-hour, our water shortage was over!
Recently, I did the same trick with a motorhome awning when camping a long way from a tap! After these experiences, I would certainly not consider a cruising yacht without some means of catching rainwater. I learned this (and many other hints) 50 years ago from a little paperback book “Cruising on a Small Income”. The book suggested allowing it to rain for about 30 minutes to clear the sails or deck of salt and now I also pass the water for drinking through a Brita filter. The book it cost 2/6d (12.1/2 p) and proved invaluable. Like “2 drops of formalin (in an eye-dropper bottle from a pharmacist) will keep a pint of milk fresh for at least 48 hours”.