Hi Cal,
With regard to your statement "I would recommend going with a 48v system. Line losses are terrible with a low voltage setup. 24v you might use for a canoe or something."
This is a tad insulting - I have a sailing yacht not a canoe! Plus I would argue the statement is misleading and partly wrong.
My reply for Patrick is this:
Overall system efficiency compared to thrust achieved is often what we look at when comparing differing systems.
With your size boat a 24V direct drive will be fine especially when using a Lynch type motor with high torque per amp/low RPM per volt. This will be lighter and cheaper than the 'recommended' 48V equivalent geared system. A gearbox or belt drive will rob you of efficiency more than the reduction in efficiency alluded to in the instance – especially when considering my boat, which I may add is similar to yours. This assumes say a cruise speed of around 4 knots in calm conditions where you can retain your standard prop (Max around 12" diameter subject to additional thrust bearing or not) which whilst higher rpm than a larger diameter/pitched prop with gearing - will save more money by not having to buy a new prop or more batteries. A two blade standard prop is more efficient than a standard 3 and if your old 12hp diesel prop is 3 blade and around 12 X 9 or 12 X 10 with a blade area of about 40 % it will be fine. 2 blade fine also. It all depends what you want and expect.
Downsides are your reduction in battery capacity due to the current being higher and voltage lower than a geared system, which affects range. It's due to the Peukert effect for lead acid batteries. But the absence of a gearbox and running the motor at peak efficiency at cruise balances that in my system. The most cost effective system is a range extended direct drive system which means the least amount of batteries, giving the lightest system with the best range. Your range extender, if you want one, would likely be a 900 or 1,600 Watt continuous output petrol generator with a charger or chargers that load the generator.
My boat is 9,000 lbs loaded displacement, 21.25 ft on the waterline and cruises at 4 Knots at less than 1,000 Watts. These days I run Lithiums for increased range with less Ah than lead tech batteries plus longer cycle life. Cost effective over time but high capital cost. I suggest AGM instead of flooded to start with. It all comes down to cost, power, safety and these are individual choices.
Don't be fooled by thinking you need a geared system with more batteries. There are pros and cons to both types of system. It comes down to how much power you actually need. 2kW measured at the battery will drive my boat at around 5 knots. For me a 'continuous rating' around 1,500 Watts is comfortable, otherwise I'd need to consider geared and higher voltages. I could go to 84V if I wanted as the motor is rated to that. When we are dealing in low powers there is no need for gearing using higher voltages, unless you want to 'overpower' your sailing boat - which many do. A motor boat is a different story.
If you choose the wet cell route you can use 24 to 48 Volt direct drive and limit prop rpm in the controller for nominal battery voltages over say 30 V. This can be done with batteries in series in 6 V steps or indeed individual cells in 2V steps.
As I've said a range extended 24V direct drive is usually the least costly, the lightest and has the best range. As you are new to this I suggest reading James Lambden's PDF book available via the Electroprop site to help the learning curve. Note the direct drive motors James refers to are the heavy direct drive motors I suspect and not the Lynch type motors and all those that came after them and due to them. To learn about the differing makes of axial flux motor that most people use in this forum see:
http://insideevs.com/power-flux-axial-flux-pmdc-motor-sample-chapter-history-lesson/
Regards,
http://John.Rushworth.com
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