It would help to know some basics:

Did the previous tanks work well before they failed? Was their propane use high? Why did they fail?

What is the flow rate in the shower? You can measure how long it takes to fill up a bucket of known volume.

What is temperature of the hot water when the tank is fully recovered?

How long is a shower?

How much propane is alot?

How cold is the incoming water?

if a shower is taken for 30 minutes.thay would be the 75 gallons of water.. sounds right but its not..the heater will actually give you only about 50-60 gallons of not water
because the tank is filling with cold as you use the hot
it is also mixing
so..if you take a shower for 30 minutes you will deplete the heater, then it needs to reheat the water. but the next person jumps in the shower. he will get cooler hot water
and less of it..it all boils down to the length of showers and how lon inbetween them. when your house was built
a misunderstand of a water heater system happenrd
instead of a huge heater seperat heaters for different baths would have been the way to go
this is also the reason of your high gas bill
you have a hugh vessal of water, the gas has got to keep that vessal at the temp you want it..every time you use hot water the heater has got to come on and heat up the water to thee desired temp. a larger vessal will cost more than a smaller vessal to keep warm..
my opinon..water heater to big...need to split the system upinto 2 systems,and..with the price of fuel.the way it is now. i suggest to switch to 2 50 gallon electric heaters
AND the heaters get put on timmers. if you both work
heater off 9pm-5am on 5am-8am off8am-4pm
something like that
the ages of your children also have a big factor in all of this. teen agers stay in the bathroom or on the phone
i never thought i would be telling people to go with elec water heaters. but my fuel price went up ti $4.a gallon a few mths ago. went down..but i think it will go back up

Frodo,

A 40 gal heater at set at 120 degrees should provide about a 20 minute shower @ 105 degrees with a 2.5 gpm shower head.

A 75 gal heater should do almost twice that or two showers back to back.

The amount of propane used is a function of the amount of heat loss, either through water going down the drain, out the flue or through a poorly designed or malfunctioning recirculating system.

We need some answers back from the OP.

I agree we need more info
as it stands now. they family has 5 members,i assume at least one is a teenager
the 75 gal w/h is not efficient enough for all members to shower at the same time. and we have not factored in laundry
its my position that the family need 2 w/h's
and the propane gas...that is a whole nother ball of wax.what all is gas? heating unit ? oven? cook top?
dryer? eireplace? we cant touch that subject
i my self use 150 gal a mth.heat,cook top,rinai w/h

Thanks for all the replies, and apologies for only now having this important information to add: looks like that really nice master bath shower is chewing up loads of water. Like maybe 4.5 gallons per minute! I learned from my wife that the regulator (is that what it's called) was taken off by the plumber when he installed it, so we have a very nice, but probably very expensive, shower going in there!

I don't think our showers are excessively long, and it's not 100% hot water, of course. But I suppose a "regular" shower in that bathroom, followed by a kids' shower (I didn't check, but I seriously doubt the other shower heads are putting out this much water), could put a serious dent in the Rudd's available water.

So...I think we're going to have to dial back the flow.

As for a circulator, we do have one but shut it off a couple of years ago due to the propane bills. All we use propane for his hot water and the barbecue grill. But I guess that two master bath showers per day using perhaps 35 to 40 gallons of water could drive up that propane bill.

Thanks for the input. I think we have the problem solved...or at least known.

With a 4.5 gpm shower head, you have about 20 minutes of hot water with a 75 gal tank taking into account the mix of cold and hot water.

I think that you have found your culprit. A 2.5 gpm or less shower head will solve the problems.

You might also want to consider tankless at the next replacement. They save about 38% for a non-condensing gas unit. The actual dollar savings would be higher with propane because it's so expensive. A condensing unit would give you slightly higher savings and allow you to vent with PVC pipe instead of Stainless Steel. As a side benefit, you wouldn't run out of hot water. In any case, you will want to change out the shower head.

Other things you might consider:

1) If your cold water intake is at a low temp because it's winter, you may want to turn up the water heater thermostat. Because you're mixing colder water with the hot from the tank, you need more hot to get the same shower temp. That depletes the hot water faster.

2) Doing #1 has its limits, since you don't want the hot water output too hot, because of scalding. To avoid that, you could install a tempering valve that will mix hot from the water heater with cold water from the well before sending it out to the house. That allows you to turn the water heater up much higher (150 or 160+), and then mix with cold for proper temp (120-125). That effectively expands the capacity of your tank by 25% or so, because you need only ~3 gal. of hot tank water to get 4 gal. of actual hot (adds 1 gal. of cold).

Here is some math to consider, assume cold water is 45 F, shower temp is 105 F. That is 60 degree increase. Assume WH is 60% efficient. A gallon of propane has 91600 btu's of heat, 60% is used to heat water (54960 btu). To heat a gallon of water (8.8 lbs) 60 degrees equals 8.8x60=528 btu. A gallon of propane under these conditions can provide (54960/528)= 104 gallons of shower water. At a flow rate of 4.5 g/min, that is a 23 minute shower per gallon of propane! How much is a gallon of propane?

I would definately dial the flow down. You also may be a good candidate for a drain heat recovery unit. We have a 40 g NG tank and a DHRU and take 3 or 4 (10 min showers, 2.2 g/min) back to back each morning w/o problems. I would also crunch the numbers on other more efficient WH, your spending alot of money each month to heat water.

good luck