Planned modifications 11/3/09

Given the last posting and some other useful correspondence, I now plan to modify our conversion as follows:

A. Engine cooling:

As noted elsewhere in this blog, we have a problem that a blockage in the "live" cooling water intake can rapidy lead to overheating and damage to the impeller requiring prompt replacement (apart from clearing the blockage!)

However, the generator is actually cooled via a secondary, closed, water/anti-freeze circuit, itself cooled via a heat exchanger by the "live" water intake...

I intend to re-plumb so that this secondary water circuit is ALSO cooled via flow through our existing, disused, skin tank. In this case, (I hope!) if a blockage cuts the "live" intake, sufficient engine cooling will continue due to the flow through the skin tank.

The exhaust will cease being silenced so majorly - but this a minor inconvenience compared to not being able to run the generator and will only continue until it is convenient to clear the blockage and (if need be) replace the impeller - it takes the "emergency" element out of getting the "live" flow working after a fault.

What is not entirely clear at this stage is (a) whether our existing skin tank has sufficient cooling power to enable normal "flat out" running of the generator (but to have to limit max. revs during a fault condition would be far better than to have to stop running the generator altogether!) and (b) it seems the live water is also used to cool the diodes and the coils of the generator.... and I'm not sure how crucial this function is given that the diesel engine itself is kept "normally cool" - I would not expect either diodes or coils to heat up significantly even if delivering high current "by themselves" and that their cooling is included only because the whole generator unit is effectively thermally sealed (by the acoustic coccoon) from the outside world so in the long term all elements need active cooling for seriously long periods of use....

However, we never use the generator that long at a time, so if the motor itself is cooled when in use, I would suspect heating of the electrics would never be a problem. Equally, to risk trying my plan out would not be "fatal" since the F-P unit has cut-outs for the cases diodes or generator coils overheat.

If my plan works, note that the whole "risk" of blockage causing a major problem vanishes - although we will still want to operate the boat and the generator to avoid blockage since eventual repair would still be potentially necessary. But there is a big difference between being stuck with no re-charging source and about 4 hours' driving battery power left and being able to continue (without limit) until repair is convenient, merely with a louder exhaust and - possibly - with the generator running at reduced output!

B. Batteries in use and wiring:

I intend to rewire so that all power comes from our 'drive' battery stack.

One advantage of this is that all battery units will have the now-verified (from our experience) good properties of the Odyssey - type batteries we have for our drive. These are especially that they do not need re-charging attention if left longer periods (so long as they are left more-or less re-charged.) They also have the advantage of no "sudden collapse" in charge-holding properties if not maintained ideally.

For us, there are no other advantages of reduced weight, space or ballasting requirements since our conversion retained separate 'domestic' and 'start' batteries, but for others considering conversion to diesel-electric hybrid drive there could be considerable saving, both financial and of fitting/ballasting problems.

I feel it clearest to explain separately as regards the two functions I now intend to use our 'drive' batteries for:


1. Domestic needs:

In our case, and I would imagine for most boaters, domestic power requirement is never huge. I will be re-wiring so that all this power requirement in fact comes via a DC:DC 72:12 V converter from, therefore, ALL the drive batteries, but at 12V. In our case, we can use this up to a rate of 400W via our converter. What may not be obvious is that this means we have effectively 600 A-hr domestic capacity available - more than any system other than super-de-luxe I have heard of - where, yes, the truth is you would be unwise to use more than 300 A-hr before recharging - but in comparing figures our 600 A-hr as a statement is the same has having dedicated batteries of, say, 450 A-hr... the figure is the total, what you can actually sensibly use is in proportion.

For those might need domestic (temporary) currents greater than the 30A (or so) a 400W converter can provide, 'domestic' function should be wired as for 'start' function (see below) - and actually I will wire that way in any case since I see and for experiment have been able to find no disadvantage.

2. Start function:

This requires about 80A - but briefly - at 12V. One CAN actually obtain this kind of power (1kW) from all the drive batteries by using three converter units from the 72V drive batteries, but I can't see that this cost is necessary or sensible.

One can fairly easily wire so that one of the drive batteries - call it 'A' - provides all the power you'd ever need at 12V - but otherwise it comes via the converter from all the batteries. For the - and it would only ever be short-term - time 'A' is used more, the same converter can recharge the difference, from all the batteries. The exact control of this can be easily enough balanced. Mostly it simply means you recharge 'A' a bit although from my figures the extra use would hardly show...

I've not yet decided or worked out the best way to automate keeping all in balance - manual control is easy enough given suitable meters. Overall there is absolutely no theoretical problem except that 'A' had more discharge-recharge events....

But 'A' is only one of six identical units and the differences - I calculate - are small... so, maybe once a year rotate which battery is actually 'A'!