Setting up a Donzi for twin engine setup with superchargers. Each engine has two stage seawater pumps one stage feeding the closed cooling exchanger and the other stage thru an exchanger for supercharger chiller. Im assuming the pickups on the Bravo drives alone likely wont be enough, and have to do another pickup for the second stages. What are the best ways to do that? Possible to use just one pickup to feed both intercooler loops or would starve pumps?

When are relief valves necessary and best way to install them? On the engine circuit Id think the exchanger would be able to flow enough that it would really generate a pressure spike or anything from the boat getting in and out the water (maybe Im wrong) but the supercharger exhanger isnt nearly as big or tube design so I could see a sudden influx of water overpressurizing that for sure. Would the relief valve be between pickup and engine mounted pump, or between pump and device (this case intercooler exchanger) ?

Other question is after going through each exchanger should i dump water into the exhaust manifolds, or put one through the manifolds and put fittings in the transom to dump the other out of(if I did that was thinking put the discharge fittings right at the drive for cooling)? Where would possible relief valves be venting too, same type of deal?

So if I closed cool the engine dont worry about reliefs valve or water pressure at all it seems. No head gaskets to worry about etc.

Intercooler circuit I guess i just need to test the pressure rating of the core since itll be an open ended system too maybe irrelevant.

Now the question is how to get water to the systems. Should I use the drive pickup to feed both stages (dont think that will work) if add another pickup, should I use the drive pickup to feed engine or intercooler? That one will be the cleanest of the two I would think, being the intercooler exchanger be the most susceptible to clogging feed that from drive to pump to exchnger, and use surface pickup for motor? Theres a million ways to do this, have two pickups feed one strainer each engine, and both stages of pump pull from strainer... a single surface pickup feed a strainer and both intercooler loops pull from that.... do I even need a second pump stage if a surface pickup will move 30+ psi through the intercooler exchanger?

SOME stepped hulls can interfere with hull-mounted water pickups. Stepped CATS definitely are picky about placement.
On a <100 mph boat, adjusting them deeper until you have consistent pressure at speed in choppy water (and when turning) is pretty much what you have to do. On a >100mph boat, adjusting them deeper is a bandaid that can kill more than a few mph.

Do you have standard Bravo or low water pickup?

Here's my thoughts - You already have the Bravo pickups. Put a pressure gauge after each seawater pump. As you speed up, you will see the pressure steadily rise with rpm and pressure at the pickup. Watch for steady rise up to a stable pressure. If, at any point WHILE SLOWLY INCREASING SPEED you start seeing pressure drop gradually, the pumps are asking for more flow than the pickup can provide. If you start seeing either pressure gauge start bouncing erratically, then your pickup is sucking air along with the water. Trim angle and drive height ALSO come into play. You may have stable water pressure at your favorite WOT trim angle when you have 10 gallons of fuel, one person on board, and no water in the freshwater tank - but with 6 people and full fuel, you may have to trim a little differently at WOT and this may be just enough for your drive pickup to start sucking foam.

IMC

Remember this has two seawater pumps per engine. Can it really get enough water through the bravo to feed both of them? They guy making my pumps said he had to add a pickup for his so each one of the pump stages had its own.

They are older B1s 95-99 area with the thin floor gears, I dont think those had the low water pickups at all or dual pickups, pretty sure just side holes

Depending on your final set-up, you may find the need for pressure relief valves. My old closed cooling setup with a crank driven raw water pump would peg my my water pressure gauges at beyond 40 psi on hard runs (actual pressure unknown past that point), and at times caused end gaskets of heat exchangers to leak, as well as pushed a raw water hose off twice.

What exactly was your setup?

Im sure there are popoff valves on the industrial side of things that would be easy to plumb in from get go, some of the boat ones Ive seen are definitely stuff you can get from mcmaster carr for a 1/4 of the price

I am not a fan of through hull pickups as they tend to slow the boat down and you have to put more holes in your boat. If there is ample pressure before the pumps at high speed/rpm, which I think there should be, do not add pickups. Using stock pickup may also eliminate any need for pressure relief valves.

I had an IMCO lower with the slotted type pickup, it had so much pressure at speed I had to eliminate pressure relief valves and just keep adding dumps until pressure was manageable.