Hi Bob/Eric,
I think Bob makes some good points, not least of which is safety. However, I'd like to add more fuel for thought, whilst still keeping safety in mind.
One reason we've adopted the 1kW/long ton light ship displacement,90% 'rule' (for EP sailing boats) is because most will not have an actual tow test result, so in this sense the rule becomes conservative. If in doubt do a tow test.
I like your swim analogy Bob. I'm a Masters sprinter and whilst I could a few years back swim 3,300m in the hour, I prefer the 100m sprint or rather 50m at my age - so I see air cooled EP as more anaerobic than aerobic when thinking of short term high power and/or higher boat speeds; in the sense that EP is better at short high power bursts and diesel at longer higher power continuous. When it comes to low power aerobic, EP wins every time. My point is, stick to what your system, be it body or boat, is designed for. If you don't it will come at a price.
If your e-repower is 1/3rd of the hp of the original diesel that will likely be the minimum. i.e. 0.75kW/ton light ship and assumes the original diesel was sized properly and as a sailing auxiliary in the first place. So my original 10hp diesel is happy being replaced by a 3.33hp (2.5kW) continuous (1hr) rated EP. If you want more, say up to 1.25kW/ton you are moving into the region of the low revving/heavy duty cycle diesel that long term cruiser RTW sailors use, to punch waves and attain hull speed. Having said that, 0kW/ton can also be a rule. It works for James Baldwin (Atomvoyages.com) and the Pardeys!
When designing and choosing my system, and like Bob, I had to decide on cruise and top speeds first. So, over some thousands of miles my average boat speed with my 26' LOA/21.25LWL boat has been 4 Knots and that is the overall, all time average speed of motoring and sailing. In that sense this is the speed my boat likes to do and the original diesel and the EP replacement match that. Going faster costs more in time and money and means less time under sail.
In fact one thing I haven't seen mentioned much is that dreaded thing called money and why we choose EP in the first place. If I had gone with 1kW/ton I'd be into more batteries, higher voltages, complexity and cost. So my 0.75kW loaded (0.83kW/ton light ship) sums work for me.
Another reason is my boat is a double ender and the engine room space is low volume so the form factor of the system is important. In other words I couldn't get a motor, gearbox and batteries in, even if choosing epicyclic over helical gearing. I could get a geared belt drive in but then, like a gearbox system, I'd have to choose between being a 36V or 48V system over 24V.
On the basis that the biggest AH rating battery is best in engineering (resistance) and cost terms i.e 2x260AH 8Ds is better/cheaper for me than a combination of 4 x 130s. So, due to available volume it came down to what voltage did I want to run my system at? If I wanted to hit the 1kW/ton I'd have needed a 36V rated system and what truly is the overall gain of going from an 85% to 90% hull speed, going to give me for that?
Note as my controller accepts 24V to 48V I could simply have decided only to use 30 Volts of a 36V or even a 48V bank or string and program my controller so my throttle only reaches the needed voltage. There are pros and cons here that are outside the scope of this mail.e.g. max shaft speeds for a given prop.
So to my way of thinking it comes down to the criteria you set in the first place, bearing in mind the max power you desire/need. Reducing from the 1kW to 0.75kw and direct drive fitted my rules. Mine were:
1) Cost/Simplicity/System efficiency (less batteries, smaller chargers, higher efficiency figures when compared to geared etc)
2) Quietest system possible (direct drive no gearing)
3) Clean
4) Green
5) Modular expandable system
Looking at the above we come out with the following additional 'rules' or results for my needs.
1) Sufficient power/range (with back up smallest hand held Honda genny - 1kW for me) and the least expensive cost of ownership over time. Greener overall?
2) Use the minimum number of AGM batteries with the highest AH for a series string. If the string goes down only two batteries to replace for 24v, 3 for 36v etc.. An option to add a 2 x 130AH reserve and inverter bank in the future for my 24V system. Note - now done.
3) Maximise Solar especially if your boat lives on a mooring and also it will likely work out cheaper/greener in the long run when compared to more batteries.
4) Minimise charger costs (24V generally cheaper than voltages above that)
5) Modular Expandable. When my main EP batteries die I can go to a 36 or 38 V string if I need that extra 5 or 6 volts to push the boat to a higher top speed and re-use my old 24V chargers/inverter for plenty of domestic power on a separate house string.
I hope the above gives some insight into the other 'rules' that may govern ones choices. It's not all about power and whilst I certainly mean no offence to our dear American cousins; you do have a history of over weight, over powered vehicles and boats, whilst neglecting fuel and system efficiencies. I say that after having lived in 50mph California for awhile when younger; and being appalled at the fuel use/consumption and cheap give away price of the petrol/gas beasts, compared to our UK 70mph rules that let us use less gas for higher speeds due to less weight with a realistic, if more expensive price of our fuel in the UK.
My how times change and Ca. now leads us once again to newer, greener, quieter tech. Well done America! Now all you have to do is get rid of those beautiful sounding stonking V8 cars/trucks and those heavy big V twin motorcycles ;)
Looking forward to a quieter greener electric world.
Yours - a reforming petrol head.
John.
http://john.rushworth.com/
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