Design and make new hot water unit.

[KASHMIR]

6 years ago I repaired my hot water unit, thinking I might get a few more years out of it, so before it fails I want to make a replacement.
My plan is to make it fit into one of the rear locker, which is a little difficult to access. I have calculated that the volume of the HWS would be around 30 litres plus insulation. The biggest issue is that the tank/locker is not very deep but I will have to live with that. My plan is to run a 600w 12v element and have a tube heat exchanger that provides hot water from the engine and a tube/heat exchanger for the fresh water. The tank will be full of glycol/hot water.

The thing I cannot work out is how long to make the heat exchanger tubes. I imagine the hot water unit will run at least 65deg C (150) and I need to be least 55C from the shower. My shower uses about 4l (1 US gal) per minute.
Has anybody experience in making these things or can help me with the calculations. The HWS and tubing will be all stainless steel.

Thanks

[Lars_L]

I have made a simple excel ark that calculates the tube length. Many values have I done a guess, put in the right values. But even so, the tubes will be long and stainless tubes aren’t easy to do a coil of. It will be more practical to use small plate heat exchanger PHE.

A different solution is to have the warm water direct in the tank. Then you have a PHE for the motor and another PHE to the boat heating system.

[KASHMIR]

Thanks Lars, That was very helpful, I will be using 13mm tube so I changed that value. I will look at how it bends and what it does show is that at least 9m of tube is needed to be effective. If I cannot fit the tube or if it does not bend well I will put a brazed plate HE on the outside of the tank with a small pump to heat the bat water.

Peter