Where to start? For what it's worth, here how I
would go about it. <br><br>In order to get something
like the same horsepower as your current diesel you
will probably need a motor in the 12-15hp range. My
KTA (with whom I have no relationship, by the way,
other than being a happy customer) catalog lists two in
that range, both under $1000. Ken Koch at KTA was very
good about working with me, once I specified what I
thought I needed, in detailing out the rest of the system
and giving me a quote on the package and a very
detailed wiring diagram. He doesn't know about boats,
however, so you will have to bring your own knowledge of
your boat and your needs to the discussion<br><br>The
next big question is battery capacity. Let's say the
motor is wired for 72V. At 72V each hp will require
about 10.4A. If your normal cruising speed in this boat
requires 8hp (i.e., about 32 of those inefficient internal
combustion horses), then you will need 8X10.4 amps for each
hour of powering between charges: 83.2
amp-hours<br><br>I use Group GC2 batteries in my boat. They are 6V
245 AH and weigh 66 lbs. If you wire 12 of them
together to create a 72V battery bank, you will have 792
lbs of batteries. (You are right to be considering
the trim of the boat when you remove the tanks and
your heavy diesel. Nowadays even repowering with a new
diesel often requires adding lead ballast to a boat in
order to get the boat back down on her lines, due to
the lighter weight construction of modern diesels.)
You will also now have a 72V battery bank of 245AH.
<br><br>As a rule of thumb, you never want to draw your
battery bank down below about 80% of its rated capacity.
That means your battery bank has an effective limit of
about 200AH. At your cruising speed of 83.2A, it means
your range with this bank is about 2.5 hrs between
charges. If your boat cruises at 7kts with this power,
your range is 16.8 nautical miles.<br><br>For many
sailors who use their engines just for getting in and out
of the harbor, this would be ok. For others, it
would not. There are various means of increasing the
range, some very expensive, technical and sophisticated,
others much more plebeian. I'll stick to the
plebeian.<br><br>Higher rated batteries are one choice. Group L-16's are
rated in my catalog at 350 AH, a 43% increase in
capacity. (Because they are somewhat more specialized the
L-16's are not as cost effective as the GC2's. If you
shop you should be able to get the GC2's for around
$60 ea.; the L-16's would probably run closer to
$150.) Increasing the voltage of the system and/or the
number of the batteries is obviously another choice.
Doubling the size of the bank and using L-16's would give
you a theoretical range of about 50 mi. between
charges. It would also give you 2, 832 lbs of batteries.
At some point you confront the fact that a 14 ton
boat is one big boat and if you couple that with a
demand to go fast or far, you have defined an expensive
system. Or maybe even one that doesn't yet
exist.<br><br>Anyway, that's an illustration of some of the variables
to play with and how you might take the first steps
in envisioning what a system for your boat might
look like. BTW, Douglas Little's book "Electric
Boats", lays out many of these calculations and explains
how these systems work for the non-technical amongst
us, although his focus and experience is clearly with
smaller boats.<br><br>At some point there's a leap of
faith. Until the system is installed in your own hull
you really can only make a best guess at the exact
performance. As it turns out, for my 20' LWL, 1700 lbs launch,
I need about 1 hp to reach optimum cruising speed
and with my 245AH battery bank my range is about 45
mi.<br><br>Larry
