That's a common problem with advice "around here". It seems it's always
buy Bigger, Stronger, Faster, and damn the consequences! You might get a
bit more performance from a 24V motor, but look at all the problems it's
causing! Maybe see if you can make some agreement with a local shop that
you try a 12V motor (measure speed with GPS), and if it's not
satisfactory, trade it up for a 24V model.

If I could find someone to let me use their 12V trolling motor, that
would be great. But I really doubt that anyone would do this. May be
I can ask the marine motor service station to see if they can let me to
try their 12V trolling motor if I become their regular customer. Is
this what you have in mind?

I'm using a 12V motor (Max55T) as the PRIME "engine" for my 10ft
inflatable, and I used a smaller Max40T just as you describe on my 12-ft
skiff.

I have a feeling that a 10-ft inflatable and a 12-ft skiff are not
quite comparable to a 18-ft fiberglass semi-V boat, isn't it? My
impression is that skiff is flat bottom and can run quite efficiently
in calm water. That may have explained the reason that a small 12V
trolling motor has no problem pulling it around. And the fact that the
18-ft has a permanently mounted T-top that tends to catch wind. I have
a feeling that the trolling motor needs to have a bit more power to
pull it around.

Although it's IDEAL to have two nearly-identical batteries in series for
your 24V system, it's by no mean required. I might buy two new ones when
the older one wears out, but no reason to throw out a perfectly good
deep=cycle battery!

Good to know that the two batteries in a 24V system don't have to be
identical. This is a useful background info that may come in handy.

Two things:
First, it wouldn't be a big problem running "everything else" on a single
battery. "House" batteries (the second one) is great if you have a cabin,
with cabin lights, DC fridge, stereo running all evening, etc. But for an
18ft open boat: I'd guess you have less accessory-power on it than your
car, and your car only has one battery, right?

What you said makes sense. Seem like a three 12V batteries system will
work just like what Tom and you have described: One for starter and
"house" use, two for the 24V system, and one of the two can also be
used for starter and "house" use. I am not saying that I will surely
go this way; this is one option that I should keep in mind, especially
if I want to save some money and cut down on weight.

One thing that I don't quite understand when I use the combined power
of both 12V batteries to start the motor today. The battery-power
rating in the power gauge drops from 12V to 10V after I have cranked
the motor for four times. At that point, the combined power of the
batteries drops so low that it cannot start the motor any more. Is
this normal? Are those two batteries dying? How can I check the life
of a battery?

Second: you can still use the one 12V battery of the 24V system as your
"house" battery. It will mean it will be drawn down SLIGHTLY more than the
other one, but realistically it just won't make a difference. I'm an
Electrical Engineer: I can run the boring numbers for you if you like, or
you can just trust me on this.

Thanks for the info. Now I understand. I appreciate your help.

Jay Chan