I replaced the jabsco accumulator tank on my yacht and the water pressure is fine except when the hot water faucet is turned on. how do I tune the pressure to be constant ?

Pressure inside tank builds up?
Follow same steps used for low pressure on following link
http://waterheatertimer.org/Low-hot-water-pressure.html

Pressure below 80 psi is fine
Gene

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I am assuming that you are referring to a calcium or rust build up in the water heater.
If you are having this issue you will need to turn off the water to the heater. You will need a garden hose, access to a outside water soucre, cold water is fine.
Shut off the gas or power to the hot water heater before starting.
Now you will need to place the garden house on the drain fitting on the water heater, placing the end of the hose outside and lower than the drain valve. Open the valve and drain the heater, you will need to open a hot water faucet to help drain the heater. I don't suggest opening the TPR ( Temp Pressures relief )valve due to the fact that they sometimes leak or don't reseal after opening them.
Hopefully the drain valve is not clogged, you should hear and see water exiting the garden hose.
If you are not getting any water from the drain valve you are going to have a challenging task.
If no water is coming out, my recommendation is to get the required tools to remove the hot water line from the top or side or the heater, also a few towels to soak up water A bucket is helpfull also.
remove the hot water line, then you will have to use a small hose to siphon the water from the tank. Once the tank is empty you can then remove the drain valve and replace it.
once you have replaced the drain valve you should ensure that it's closed and use a Gallon of CLR. Poor this into the water heater through the hot water outlet where you removed the pipe from. Now make sure the faucets are closed and replace the hot water line. You can now turn the water back on to the water heater.
After a few minutes open a hot water faucet to let out any air until water flows out, than close the faucet.
Let the water heater set for a few hours, then drain the tank!
when the tank drains, close the drain valve again and refill the water heater. Once again open a a hot water faucet and let the air out till the water heater fills. Again let sit for a hour.
At this point open a hot water faucet at the farthest point from the heater and let the water run for 5 min. , than close it and move to the next closest faucet and repeat until all faucets have been opened and run for 5 minutes.
If you still have a slow flow in a faucet, you may need to repeat the CLR step.
I also suggest that you remove all faucet screens and clean them.
If you still have a clog than you are going to have to remove or replace a piece of water line in the house. The blockage may be hard to find but if you have access to most of the pipe or it's visable you may be able to hear the blockage with the water on. Otherwise this will result in using some water line taps and a pressure guage to find the blockage.

Do you have a well and/or, do you have an expansion tank installed in the system. Alot of homes, almost all with wells and, alot now with city water, install expansion(pressurized) tanks in the line to prevent surges in pressure and temperature fluctuations. The tank would either be outside by the pump or, could be installed on or near the water heater. If these tanks go bad or, if they lose pressure, they can't provide the cushion to prevent this from happening. Also when they go bad they will become saturated with water and adding pressure to the tank may lessen the problem but, again, it can't provide the cushion.

You have a clog in the hot water line.
If problem is at one faucet, then remove aerator and clean, then disassemble lines underneath sink.

If problem is at every faucet, then that moves search close to water heater.
Look at hot water line that exits water heater and trace that line to first Tee.
Look at cold water line that enters water heater and trace that line to Tee.
Clog is located between those 2 Tees.

Buy pressure tester that connects to hose thread.

Connect pressure gauge to hot water heater drain valve.
Then conduct same test you did at faucet.
See if pressure drops at tank.

If pressure drops, then clog is on cold side of tank, and probably hot side too.
If pressure does not drop, then clog is on hot side of tank.

Suspect 1 is any place two different metals join. For example galvanized pipe and copper.
Suspect 2 is supply pipes that exit top of tank. Take apart and look inside pipes for corrosion and sediment lining pipe walls.
Suspect 3 is elbow. Disassemble pipes and look for sediment and flakes of rust.
Suspect 4 is all suspects above, each having sediment and corrosion.

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.

just curious, im a plumber by the way. do you have a water softner

start by making sure the valve that supplies hot water to the sink and dish washer is fully open. If there is low hot water pressure in any of the other kitchen/bathrooms, then trace the line to see where the pressure drops. If it is only in the kitchen, it should be an old valve, (causes restriction)Good Luck.

Try draining the system and apply air pressure (min 30psi) to each faucet. Open other hot water faucets to allow pressure to vent. Next try draining and cleaning the hot water tank.

It could be one of two problems, one simple, one serious.

• Easy fix
  

1. Turn off hot water angle stop under sink to faucet.
2. Remove supply line from hot angle stop.
3. Place end of supply line in a small bucket to catch some water.
4. Turn on cold water for a couple of seconds and backflush debris from faucet.
5. Reconnect supply line.
6. Turn on angle stop and test
   

• Complex fix

If you have steel pipes you may be experiencing reduced flow due to corrosion in your pipes. Reduced hot water pressure in the kitchen faucet is usually the first sign since this is the faucet that is most used in the home.

Repiping is a job for a professional. Another symptom you may need a repipe is orange colored or rusty water.

THE WATER IS RECIRCULATING IN THE UNIT. MAYBE CAUSED BY THE ACID. TRY INSTALLING A BACK FLOW PREVENTER