Alan......model [water] ballasting can be a very interesting & functional capability to your vessel
1) free flooding tanks are fine....need baffle & above deck vents as the water level will only fill to the given space & you also need the initial mass of the vessel [who was that man in the bath?] a vessel will only displace its own weight story...
2) free flooding tanks are fine....but you need to remove the water ballast on comletion of your run ........or your back will suffer
3) as you have noted.......ballast discharge pump/s can be used [over board looks more attracrtive]..... however prior to dumping the ballast water...
you must close off your under water sea chest entry points
4) it certainly would not be too difficult to manufacture a number of fabricated plastic knife type isolation valves & servo opened/closed with similar water proof glands to our model submarine use
5) if you did not fancy manufacturing your own plastic knife type isolation valves ....you could use those 1/4" BSPP low pressure brass plated 90 degree isolation ball valves [WOG nomination = water, oil & gas] and similarly open close with a servo
6) the more we think the less complicated it becomes...the ball valves need not be located within the tank....but external via plastic ballast piping & so hence eliminates the need for waterproof glands etc
Whilst I have never constructed any model ballast system ....suggest you keep on similar lines to real vessels........a suction sea chest equally spaced P & S of the vessel axis...& the ballast main running the required length of the vessel on vessel centerline ....
Keep the system symmetrical & make sure you build in removable ballast hold tank tops ...sufficient to get your hand in to clean out any obstruction
- ...keep us posted...should be interesting.....Derek