We are really down into the weeds here but it is interesting.
A prior response said:
"Now go take a look at the different pump's rated flow rate, which of course mi amigos is for a flooded head and open discharge, as published in their specs. From real life testing the rated flow rate of a pump can easily drop in 1/2 once you put suction head and discharge pressure on it."
My Spectra manual says to measure the combined discharge of the product water and the bypass water. They give required volumes for three different feed water temperatures and voltage measured at the feed pump.
Their maximum flow values are at 14.4V and 90 degree seawater. The measured values should be 2.7 GPM bypass and 0.3 GPM product for a total feed value of 3 GPM.
I took that value as the required feed water needed to meet the factory specifications and saw no need to worry about "flooded heads" or "open discharge."
Why do I need to consider the other considerations if the maximum required feed is 3 GPM? That is real life and is exactly what I measure when I tune up my system.
Another example provided to me by a civil engineer
who manages a very large municipal water system and has lived with pumps and flow considerations for over 30 years.
A fire hydrant can connect (T) to three different fire engines. Each engine
is pulling water from it at a different suction value and is flowing a different output. As long as the total suction pressure does not exceed the wall strength of the main and the total flow does not exceed that which is provided by the main pressure and diameter, and hydrant stand pipe diameter - there is no problem with three very different pumps drawing from the same source (thru-hull).
Yanmar and Spectra provided very specific flow requirements and the hydraulic calculations of flow is pretty straight forward so here I am with one thru-hull.
My professional career was measuring things and I began that career in oil
refinery operations. I consulted with several civil engineers (including the one shown above) about the need for a dedicated watermaker thru-hull. My calculations and all those of the engineers showed that one more thru-hull was not necessary because Caliber installed a really big one for just the use we are discussing here.
The easy answer is "DEDICATED THRU-HULL" but it is not necessarily the best technical answer.
My contention is that there is nothing inherently wrong with a shared water maker source (thru-hull) as long as the source can provide an adequate flow to all users.