Thru Hull

[JKC]

On my 37' I have decided to combine the 1" A/C thru hull & the 1" generator thru hull into one. The one 1.5" will feed a small water closet, to be mounted just below the waterline, which will feed the 1" line to the generator (8.5 KW Onan) and A/C pumps. In the process, I have discovered my single A/C pump servicing the 10,000 BTU forward unit and the 16,000 BTU main is rather small. So I will use it for one A/C and get another pump for the other A/C. Or should I get one larger pump for both?

But the real question is will the single 1.5" thru hull flowing into the water closet be adequate for the generator (with its pump, I do not know the GPH) and the two A/Cs (each with their own 500 GPH pump)? I realize one 1.5" hole is bigger than two 1" holes, but I wonder if there will be too much negative pressure in the water closet with three pumps competing for the same water, since the water is supplied to the closet "naturally", not pumped.

[jim rosenthal]

Well, no offense but I wouldn't do this...for one thing, these units frequently get run at the same time which means they will compete for the water supply available and they may both get starved. You are better off with two separate through-hulls, or a real sea chest and separate through-hulls coming off it. That is a formidable modification on a Hatteras, and might weaken the hull in that area. I would stick with two separate openings, two through-hulls, two sea valves, etc.
Regarding the pump, you are better off with one large pump than two small ones. All the boats I have ever seen with two AC units were able to supply enough cooling seawater with one good-size pump- there is a 500gal March and also Cal Pump makes some units that will supply that much. That should serve for the number of BTUS that you have.
If you are absolutely determined to have ONE opeing to supply both appliances, then you would need at least a 2" through-hull, maybe even larger, to supply the genset (1"), and two seawater pumps. A problem in this sort of setup is that the complicated manifolding involved renders flow less efficient; also, an obstruction to flow outside (seaweed, garbage, clogged pickup) will disable all the units which depend on this seawater intake.
If you look in the engine rooms of high-quality boats such as Hatteras or Bertram, you will generally find that each consumer of raw water has its' own through-hull and sea valve as noted above. There is a reason for this- it is simpler, easier to maintain, and only one thing will fail at a time. If you look at the cost of these appliances (genset, AC condenser/compressor units) it makes sense to me to protect them as best you can.
Also regarding the seawater supply to the AC units, if you contact a Cruisair or Marine Air Systems dealer, they will have tables to help you figure out what size pump you need to supply those two units. Generally the pump isn't the rate-limiting factor- what limits flow is the manifolding and piping arrangements.