Building a boat is perfect time to do it right. I would focus on Hull, standing rigging, running gear, and motor in that order. Keeping in mind that technology is moving at different speeds with certain of the items you will want on the boat.
The Boat:
The catamaran you are building is most certainly easy to push for a 56ft boat. The fact that the marine engineer or designer recommended 40hp diesel sail drives to power the boat is important to understand. For the typical owner of that boat, that is the power they thought would be appropriate. Catamarans are often over powered with the idea that a single motor will drive the boat at close to the traditional hull speed of the boat (catamarans of course may exceed hull speed).
This is what my computer model shows:
8.2 290 13920 8.8 370 17760 9.0 390 18720 10.7 670 32160 11.5 820 39360 Hull speed 10.03
Propulsion needs:
The 40hp diesel with a sail drive for this boat seems to be a good fit for the traditional thinking of power management. A 40hp diesel is likely close to a 20kW to 25kW electric motor. Power generation should be significant under sail. The sail drive will be a bit more efficient due to the placement of the propeller and its angle.
Gearing and propeller:
I question if is the 2:1 gearing with a 15 X 10 folding propeller as being the best. I do not know without running a propeller model. Here is my question to you and the readers, why do you focus of efficiency of a motor and the weight of a battery pack and then hope a 2:1 gearing will give you the best performance with a propeller that is unknown efficiency with that gearing. As Miles has pointed out, that is where efficiency is important. All the electric motors are going to be within 5% of each other ( PMAC, Induction, dc brush or dc brushless) the engineering is where it really happens.
Battery pack and Power storage/generation:
Why are you spending all that money on Lithium batteries and end up with too small a pack ( 2 X 200ah or total of 19.2kWh battery pack)? I estimate that battery pack will provide about the same range as 3 gallons of diesel. Batteries that can handle PSOC are important for cruisers and LiFePO4 would be the best. But if the cost of those batteries are such that you end up with too small a pack, I question using them as opposed to quality modern TPPL AGM carbon infused batteries. I estimate you would triple the range capacity for the same cost and likly equal the original design weight of the "diesel powered" version of the boat.
Solar
This is absolutely the best way to go for power generation besides sailing fast. Lots of space on the boat and if you need shade (just about every cruising boat does), it is easy to position.
Finally, when your think about how you are going to use the boat over the next number of years, think long term. LiFePO4 may well be the best solution when spread over a long time period. Control system, throttles systems, battery management systems, well engineered gearing and propeller, are really important.
Mike
Electric Yachts of Southern California
Posted by: mike@electricyachtssocal.com
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