I too will make this my last post on this subject, but first and foremost I wish to offer my sincerest apologies to the OP and BR for getting way off topic here.
I took the liberty to re-read the thread, I can see clearly that I was simply just not making sense, and most likly should have just stepped back for a little until I was better prepared to offer the right opinions and advice.
Electronics are a funny little thing in my opinion, it seems to be one of the only big things in life, besides life itself, that has what we refer to as "variables" and a lot of them, the theory of electricity is something that has been openly discussed and argued for centuries, while certain facts remain a stable post in the evolution since the discovery of electricity.
To the OP, my suggestion of using an 8 guage wire was absolutely incorrect, however I remain adamant in the fact that you should check the manufactures suggestions on the usage of the recommened cable size for the motor you are purchacing, my sincerest apologies for the confusion there, you should always take the manufacturers recomendations under strong consideration otherwise you may end up with a voided warranty, or worse yet something major could happen and damage the boat or you yourself could be put in danger.
As for other things,
I do not feel I have posted anything "bad" other than the fact that I was misleading in the cable size, bad info, no, confusing info, to say the least for sure.
I won't make excuses for my reasoning, but to say it was "bad info" I would have to disagree, the argument could be made in many ways here.
Fact is when we look at posts both are at fault here,
A common practice, though I don't recommend it, is to use one as the cranking battery. That way you still only have to have three batteries but if you happen to run them down with the TM, you're screwed because the big motor is not cranking.
As far as "bad info", in your post above, you stated you would not recommend using the battery configuration in this manner, but yet you laid the plan for how it could be completed, does that not suggest that it could be used in this manner? why would you eplain how to do it if you would not suggest it?
Never ever would I even remotly suggest that the owner leave him or herself at risk by doing such a thing as using their primary source of dependency for modes of mobility up to chance, had there been an alternitive suggestion following the comment allowing the user a more viable alternitive for charging the batteries and or lessen the cost of having to purchace a third battery, then I would not give it a second thought.
One more thing I noticed in that comment made, you politely mentioned you had a degree in electronics, now, unless I missed something my classes, a "big motor" is using electricity to make power or provide a means of propulsion in this case, assuming this boat is not a hybrid, I am guessing it has an internal combustion engine, if you are as adamant about "bad info" as you state you are, then I would assume that correctly addressing what a device is used for has to have some kind of priority.
As for my math, as stated earlier, there are always variables in the field of electronics, the equasion is a direct result from formulas used for many years, the 101 that was used as an example in a previous post utilised a maximum ampere draw of 49 amps (recommended) not actual, as for the run time lots of good information there that you put in your post, but then again, it is still dependant upon variables, it was put there for the OP to have a general idea of what he or she can expect to see, not as an absolute, I did not make that clear in my post.
I am just as much at fault here, I want to be perfectly clear, I have already offered my apologies.