You've only considered building a tube type heat exchanger, identical to the stock one.
Consider building a plate type exchanger out of copper sheet. A plate type exchanger is a bunch of metal sheets
bolted together with spacers. Two holes through the sheets
alternately pass the liquids between the sheets. It's simple. The area of the copper plate exchanger would be the same as the area of the tubes in the tube type exchanger (I'd make it bigger, I'm very conservative). A picture is worth a thousand of my words, but I can't find an illustration of a plate exchanger to link to.
In the manufacture of tube type exchangers, the tubes are rolled/expanded into the holes in the end plates. One of your liquids is raw sea water
, and I would not braze or solder the tube ends to the plate because of galvanic corrosion
. If I were building a tube type exchanger I might consider machining a groove inside each hole in the thick end plate and seal the tube with an o-ring. I might, that is, but first I'd trundle on down to a radiator repair shop and see what they think--I know, they solder radiators together because the liquid inside is fresh water with anti-corrosion chemicals added, but they'll know a lot about heat exchange.
In the exchanger on my Yanmar
3GM30F here, turbulent flow is maintained by making three passes through the exchanger, each pass through only a few tubes.
Heat exchangers can be made of anything, copper or stainless steel
or whatever. In a sulphuric acid manufacturing plant, they cool the sulphuric acid in heat exchangers with Teflon tubes, Teflon being just about the only thing that'll stand up to the acid. Teflon has very
poor heat transfer capability, so there are a lot
of very small tubes for a big surface area.
I'm in the camp that says 48X, assuming the same thickness of metals, because the heat transfer coefficients through the liquid films are the same in both applications. The only thing that changes is the metal. Again, if you do TIG weld the tubes to the end plates, the weld alloy must be the same as the tube/end plate metals. And generally, one of the end plates must be free floating, because of expansion from heating
This is not my $.02. This is my free advice. Think of all the blather that would have issued for $.02!!