A few have asked, here on the forum and by direct message, for more details, so I thought I'd answer the questions all in one place. Thanks for all the thought people put into their replies.
OUTBOARD DETAILS
It will do 20 kW continuous, meaning for very long periods without danger of overheating. We can do 25 kW for short periods (we have not tested the limits, but at least 30 seconds). We may be able to do better, once we have more test data.
Everyone here knows that 20 kW = 26.8 HP. Mike you ask very good questions about how we can make any claims about propulsion comparisons with gas motors. We mounted our motor on a Still Water hull and took it out and tested against equivalent catamaran coach boats with 30 HP gas outboards that were permanently attached and used daily by the University of Washington. Our outboard out-accelerated the UW boats and provided higher top speed. We could beat them below 20 kW, probably around 16 kW. While we cannot say that their prop was perfect for their motor/hull, we can say that it is what is commonly used by daily, experienced boaters. So in a practical sense, our motor provides more propulsion and more acceleration than a 30 HP gas outboard, when both are used on a boat properly matched with the best available prop (in our case, our own prop).
Our voltage is 350V nominal, using our own design 6 kWh battery packs, which can be combined. (When combined, they work in parallel, to keep discharge/charge rates low.) High voltage is a trade-off, of course. We get lower motor weight, lower heat generation, lower current, higher efficiency than a low-voltage system, but we have safety issues to contend with. That's why we require our own battery pack, which is thermally controlled and which must affirmatively communicate with the motor (over CAN) before any contactors close and activate the HV system. All connections and components on the boat are IP67 rated.
PROP
The prop is aluminum three-blade, 14" diameter, 1500 RPM. The larger diameter / lower RPM enables more efficient operation. A plastic prop at this power is too flexible, so the shape becomes less efficient, and the thickness of the blades creates too much friction.
HULL RECOMMENDATIONS
Ned's boat looks like a very efficient and attractive one to try, when we can get our system out to the East Coast (which we plan to do). I also think the Boston Whaler suggestion sounds good. It will be interesting to see what performance we get. (I'll post back here when I have results.) No one seemed to like the rigid inflatable option, but maybe a high-quality one would be stiff enough to get up on plane and ride efficiently.
Thanks to all for the feedback!
Andy
---In electricboats@yahoogroups.com, <mike@...> wrote :
Andy,
Excited about what you are building. My advice? If you are going to use the boat as a test/demonstration boat, you should get a boat that is more common in the market and would provide the most representative model for the market. If you are looking at 20kW outboard, I would want to know how it pushes a 13ft Boston Whaler which is a well know and popular boat which is often powered by motors from 9.9hp to 40hp. That would be a great demo boat to use in order for the market understand the capabilities of the motor.
I also want to understand the 20kW continuous. Does it have a higher rating for short bursts? What is the voltage of the motor? The 1500 RPM is good. The special propeller also is of interest. We know of other electric outboards that over compare their capabilities because in a controlled test their special propeller will perform better than what would be a real world usage.
Outboards are rated at the propeller (thrust) which is vastly different than what inboards are rated. A 35 hp diesel (break hp) is actually about 28shp (shaft hp). That is what we all expect. My question is will your 20kW electric outboard therefore would likely be more comparable to a 28hp outboard (20000w / 746). Very impressive and I believe there is a market for such a quiet powerful motor. I would also think that a motor that would provide the thrust of a 9.9hp outboard would be even more popular.
Mike Electric Yachts of Southern California/Pacific
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