Hi Eric,
I think that you missed my point on efficiencies. The question becomes "If I make this system more complicated by "optimizing" components for partial throttle, how much energy will I save?" Followewd by " Is it worth it?"
Let's look at what was suggested. I installed a 5.5kW drive in my 5.1 ton boat. Perhaps installing two 2.5kW drives would increase efficiency at low speeds because the motor will be operating at a more efficient power level.
So, my boat uses about 500W (about 10% power) to cruise at 3kts (50% hull speed). My ME0913 motor operates at about 70% efficiency at such low power ratings, so the shaft output should be around 350W, if the manufacturer's motor performance charts are correct. So we now know that my boat takes about 350W of shaft power to motor at 3kts. Now, let's use a smaller motor and see how much power we can save. I don't have specs for another PMAC motor tha maxes out at 2.5kW but I can see that my motor at 10% power is 70% efficient and at 20% power it is 80% efficient. To get the same 350W of shaft power, the smaller motor should use about 440W, a savings of 60W.
So if your typical cruising throttle settings are 10% power, installing two smaller motors could give you an efficiency increase of a little more than 10%, at the price of more complexity and potential losses. My range at 3kts is currently 38.1nm, if the power requirement dropped to 440W, my 3kt range would increase to 43.5nm.
Since I have installed my system, I find that I rarely motor at less than 20% power (1000W or 20A) and usually aim for 30A, 30% power or 1250-1500W. So for my usage, the motor efficiency is already at 82%. If I installed two smaller motors and only ran one, my cruising speed motor efficiency would jump to 84%. Applying the same math process, my present 1500W cruise might drop to 1470W, saving 30W or 2%.
My motoring range today at 1500W is around 18nm. If I could drop the power requirement to 1470W at the same speed, my range would increase to 18.4nm.
So, for me the increase in cruising range of 0.4nm is not worth the additional cost and complexity of having two smaller parallel drives. But everyone would have to do their own math and make their own decision about how their system works and how they actually use their boat.
Fair winds,
Eric
Marina del Rey, CA
--- In
mailto:electricboats%40yahoogroups.com, "cire" <cirejay@...> wrote:
>
> Eric,
>
> I'm onboard with your point about unnecessary complication, however I think you confuse the efficiency of a hull at 50% hull speed with the efficiency of an electric more at low power.
>
> I admit to not being up on the absolute latest in electric motors but have observed in the past that they seem to operate most efficiently at 75% power or so and least at 25% or below.
>
> What I am saying is that I see the point that Dan is making; the question that you ask about it being worth the complications and loss of efficiency created by two drives is a good one.
>
> BTW, as to the use of one large battery bank, I have a 32' sailboat – Allied Seawind II – with a single 630amphr battery bank and when I explain the Peukert effect as part of my reason for a large single bank to my sailor friends, their eyes roll back into their heads:-).
>
> eric SV Meander
>
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