Thanks All, I did some more research yesterday and found that the Interstate Battery store has 4D deep cycle batteries for about $200 each, with more than 1300 CCA and about 230 Ah. So I think I'm going to get two of those.
Thanks so much for all your help, advice and insight.
A 4D or 8D short of a diesel sport fishing boat with 600HP needing CA's is beyond me..
I will post this again, as I have before...
You can't compare an Interstate 4D cranking battery to a good quality true deep cycle battery. Ignore the sticker, they are LYING TO YOU when they tell you it is a deep cycle battery. This is akin to comparing the reliability of a Honda or Toyota to a Renault Le-Car, IMHE. Those "types", deep cycle vs. starting batteries, are not even in the same class of battery. The Interstate SRM-4D is made by Johnson Controls/JCI for Interstate. Interstate is a "sticker" company these days and JCI is really the Yugo of batteries, not particularly the best brand but in some markets they remain somewhat competitive. Other then their GC2 6V's I have not seen their batteries do much of anything, longevity wise, in the marine market.
I actually called interstate a few years ago to try and find out the Ah rating and plate thickness of one of their batteries. What a freaking debacle. Even their tech support had no clue. I got three different answers when I asked about the 20 hour Ah rating. Don't even attempt to get a Peukert's exponent.....



uke
If you have a battery monitor you need to know the precise 20 hour Ah rating or it will not work correctly. Buying a battery that lacks a 20 hour rating is a crap shoot without knowing the accurate specs, and Interstate refuses to publish a 20 hour rate. Even when they do the Ah capacity is NOT accurate, I know this because, I have tested three or four relatively new but broken in Interstate batteries and none of them could deliver the "claimed" 20 hour capacity under well controlled test conditions. Course with three differnt answers from three differnt people in tech support who knows what the "real" capacity is...? Probably why they refuse to publish 20 hour ratings.
Here's a good general rule of thumb. If the manufacturer does not publish a 20 hour Ah rating the battery is often not a true deep cycle battery, but then again Interstate does not market this as a true deep cycle battery they market it as a hybrid deep cycle/cranking battery. It is a truck starting battery, nothing more. The "sticker" does not change anything physically inside the battery...
There are however exceptions to the 20 hour rating guidance. NAPA sells the SAME deep cycle 12V batteries that West Marine does but NAPA does not publish the Ah rating in their specs for the battery. A simple call to the manufacturer, Deka/East Penn, and you know this. East Penn actually has a clue because they are an actual manufacturer of batteries not just a company that slaps a sticker on someone else's battery. East Penn also sells batteries to "sticker" companies like West Marine, NAPA, Duracell, O'Rielly Auto etc. etc....
Interstate labels it as a dual purpose battery or "deep cycle/cranking" these are hybrid batteries at best, that attempt to do both but almost always miss the mark when put into a true deep cycling application like a boat. They all start life on the same assembly line and at the end get a truck cranking battery sticker or a "marine deep-cycle" sticker.. The real and only value in a battery is how many cycles you will get and in deep cycling applications 4D's and 8D's fall flat when compared to a true "deep cycle" design...
One of the only true 4D deep cycle batteries I know if is the Dyno 4Dd (the small d designated deep cycle) not to be confused with the Dyno 4D which is for starting. This battery has an Ah rating of 180 Ah's and cost $268.00. Like many true deep cycle, thick plate batteries, they do not publish a CCA value. I have not seen any 4D actually deliver 230 Ah's.. That is 8D range.
If you have the physical height a set of 6V batteries in series or series parallel will last longer & give more cycles.
In the past Interstate has had some good batteries in its line up. Interestingly enough the
old U2200 6V used to be made by US Battery and was an EXCELLENT, top quality battery. Now days that battery it is made by JCI. I guess paying for the quality of the US Battery, to put the Interstate name on, left them with little margin so they outsourced it to a less expensive vendor, namely JCI.
That being said any 6V battery, good quality or lower end brand, will almost always have thicker plates than a dual purpose flooded 4D and thus last longer in deep cycle applications. I've yet to see an industrial floor scrubber machine or golf cart with 4D's or 8D's..

There is a reason for that. 6V batteries are also taller by design so sulfation can be shed off the plates and fall to the bottom of the case during equalization. A shorter battery fills up with shed plate junk faster than a shorter one. As you get into even thicker plate deep cycles like the L-16 case they are even taller.
If you have the room (height) for two or four 6V batteries they are always the better value. In the end they will cost less on an Ah/$$$ and $$$/cycles basis.
My Synopsis of Wet Cell Batteries by Grade/Quality:
Top Tier:
Rolls
I intentionally left a space here because the Rolls batteries are significantly more top tier than US Battery or Trojan..
Trojan
US Battery
Second Tier:
Dyno
Crown
Superior
Deka/East Penn
Third Tier:
Johnson Controls / Private Labels
Exide
Interstate (most of these are made by Johnson Controls)
Various Private labeled batteries
I've not even touched on getting a hernia or needing back surgery from the 4D's...
Bottom line there are only a handful of true "deep cycle" batteries in a 4D case and those are:
Dyno 4Dd Flooded
Lifeline AGM 4D
Deka GEL 4D
Every other 4D is a starting battery or "hybrid" at best.. Don't be fooled by the "sticker" as you will not see the same cycle lofe as a true deep-cycle battery.. Stickers don't make batteries "deep cycle" plate thickness and design does......