Not enough hot water

I have a 50 gallon water heater. I can put about 6 inches of all hot water in the bathtub and it gets down to about 100 degrees. The starting temp is 136 degrees F and I feel that's too hot for starters. I emailed the maunfacturer and went through their checklist and assumed the dip tube was broken or cracked. Upon removal of the diptube I found it to be intact except for a small drilled hole about 3 inches down from the top. I don't feel like spending 300 bucks for a new WH but I'm getting frustrated about why I can't take a decent bath with enough hot water to cover an average person. the measurements are as follows 45"X28"X6" deep. That works out too approx 4 cubic ft of water or 30 gal of water. It seems to me that it drops off way to quick and would sure like some help
Thanks
Dave

Is this an electric or gas water heater?

That would make a differance wouldn't it!!! It's a natural gas model. Standard B vent unit. I have a gravity circ loop on it also but that was put on as an attempt to fix the problem. Didn't work!!!
Thanks

all the circulator does is get the hot water there faster (and wear out the tank faster, too). It has no effect on the volume of the water available. How cold is your incoming cold water? In the winter, that can make a significant impact on the amount of hot water you can expect from the tank. Where I live, my incoming water this time of year gets to about 35 degrees - so, after using half of the water, I'd expect it to be about 85 degrees, starting at about 135 - half way between the starting point with a delta of 100 degrees. Now, the heater is trying to heat the incoming water, so adding some heat would make it somewhat higher. Also depends on how fast you are dumping the water in the tub, a larger line will mean the water heater has less time to react to try to heat up the incoming water. A fast recovery unit might work better...my unprofessional opinion.

Well it looks as though the circ loop did have something too do with my problem. When you refreash the water heater with cold water it goes to the bottom of the tank. That's where the circ loop is connected. I will come up with a solinoid and either control it with time via X10 or by temp with a remote bulb tstat. I will also try opening the circ line valve just a little but that would be way too simple.
Thanks for the help

I may not understand how your system is laid out, but are you drawing your hot water from the bottom of the tank? I have a Laing SMC 303 recirculating device in my home, pumped, not gravity, and I have the return line going into the bottom of the tank (I replaced the drain hose bib with a nipple and tee so I can still drain the tank) The only change I made to the hot water supply system was to add the return line from the far end of the system back to the tank. There is a pump, check valve, and automatic air vent just before it gets back to the tank. My understanding of a gravity system is that the tank must be below the level of the pipe. Pretty much means in a basement.

hot water

The circulating line does not have anything to do with the problem, assuming it is installed correctly. Opening or closing the valve should not help or cure it either. This is assuming you have a properly installed check valve at the tank. If not, then that is probably your problem.

I have no check valve. I will be playing with a couple of options and the installation of a check valve is one of them. I can use the other methods for other things like being able to control the circ loop when I'm not home. or at night.
Thanks

I don't think a check valve is an option.