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diesel-electric small-boat propulsion...


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What's the latest on this? (my question is prompted by the prospect of 'living' with an old single-banger diesel auxiliary). How much better a generator somewhere powering an electric motor aft?

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Not much has changed. Especially as no one as of yet has managed to change the Laws of Physics. It can be done, but it comes down to cost, space and wasted energy in conversion.

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Seems to be a great idea if you are on berth with 240 volts and use the yacht for daysails or weekenders. If cruising prepare to motor very slowly. What do you use the yacht for?

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Weekend warrior is more likely to motor home against 20kn SW, a Full time cruiser would stay anchored a couple of days till the wind shifted.

Someone who does long passages is likely to arrive fully charged. The weekender never quite gets to full charge.

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Most daysailers would be lucky to go ten miles out. Well over here where we are all gentlemen, leaving our beds when the missus has gently passed her morning wind at 11 , coming back in before dark. With 240 available the batteries would get charged overnight. I mean 60 or so miles is achievable at 4 or so knots without really massive battery banks. I still like the thought of that little assist from the prop just quietly adding a knot or so in light conditions for next to nil energy expenditure. I guess if your the type to sail in 4 knots of breeze then electric drive may be for you but if you motor when your speed drops below 5 knots maybe not.

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Some of the replies above sound like the authors are thinking of electric propulsion. But the question was about Diesel - Electric . i.e. a diesel motor driving an electric generator which in turn via a cable drives an electric motor that drives (in this case) the propeller.

 

The multiple stages mean loss of energy in the conversions. Advantages would include much more versatility in where the heaviest and noisiest part, the diesel-electric generator, could be located in the boat.

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could mean one noisy thing to take care of both propulsion and all the electric toys, probably not at the same time.

 

If you included batteries for the motor you could have different options for charging them, diesel, solar, wind,

 

can't see it being too successful in a small boat, could work well where you have more space and more non-propulsion electric load.

 

I read about a production crusing cat with two electric driove motors and a single diesel for charging

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Diesel /hydraulic would fit into the same category as diesel / electric. There was a push using diesel / hydraulic on pleasure yachts back in the 1960's / 70's that never really caught on, one of the main problems was the noise of the pump / motor that was fairly pervasive. The other was the lack of owner maintenance, it seemed that most owners thought that they were maintenance free. The plus as with diesel electric was no alignment problems and the choice to put the motor wherever it suited, you traded electrocution for oil in the bilge if it all went wrong!

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If you want to get the best efficiency out of a diesel electric system this type of prop is reportedly 5% more efficient than a conventional shafted prop, steerable as well.

Probably not the cheapest option.

 

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Diesel /hydraulic would fit into the same category as diesel / electric. There was a push using diesel / hydraulic on pleasure yachts back in the 1960's / 70's that never really caught on, one of the main problems was the noise of the pump / motor that was fairly pervasive.

Noise is certainly one consideration. But the main offender is efficiency. Hydraulics is the least efficient of all power systems. 50% of the energy is wasted just pumping fluid around the system and much of that wasted energy is heat as well as the sound. But on a working barge or Ship, there are many advantages that over rule the losses.

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Rigger, here is an idea I have had in my mind for a wee while. I assume you have seen that Dyson Bladeless Air fan. Much like the shape of the housing around the propeller in your picture. Imagine water being ejected out a slot completely around the back edge of that housing. No propeller. Water would be sucked through the hole doubling the water flow. Water would be pumped down through the shaft that rotates the entire housing and into that housing and directed out as around the trailing edge.So a modified waterjet propulsion. I wonder if anyone has tried that.

If only I had lots of money to be able to play with ideas like that. I could make a small fortune.....after starting with a large one.

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Apparently the charge from the prop is very very small unless going fast. Most yachts say 10 metres or so average 5 knots on a long trip and this would not generate significant power. You could go for a long way but you would get a lot older by the time you got there under solar power. I'm not being negative Spinner , I really like the idea of battery powered yachts just the battery technology I don't like. Lithium is heaps expensive and dangerous it seems or go for deep cycle lead acid which means carrying a lot of weight. I don't believe the sealed lead acid batteries are suitable.

A big plus is the solar panels are getting really cheap. They could be attached to the pullpit top rails and extended horizontally when you need current from them. That is when there is not much wind. Motor sailing quietly would be a treat.

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I looked into this a while back...

Motor and control system and wiring around $3000

Batteries 48 volt by 220 amp hour Sealed lead acid around $4000

Lithium Ion around $20000

Solar panels 6 by 120 watt about $2000

Then a 240 powered charger guessing up from $1000

So for around $10,000 you could have a good system that would give around 80 miles at 3 knots or 30 at 5 knots. Rough figures only as depends on the yacht. On a sunny day you could probably motor at 1 or 2 knots just on the solar panels but they don't go real well on moonlight.

 

An electric motor could be attached to the prop shaft of a diesel installation via a belt or chain allowing electric drive, diesel powered or use as a battery charger. Extra complexity unlimited!!

With lots of batteries aboard house power needs are met well. The above solar panels will give 15 amps at 48 volts so a few sunny days will top the batteries off in summer. Please don't argue these figures as I'm only going on memory and only going into this to help with a rough, real rough idea. I'm happy to be corrected but without negativity please.

You would have to budget on replacing the batteries , guessing, say every 5 years in lead acid. They would get used hard in this application.

A new diesel installation nowadays is quiet, smooth, economical and not very fumy and you don't need the solar panels nor wind generator leaving the deck for sailing! Electric drive is not a cheaper alternative but certainly has merit.

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