David,
I was hoping that someone else would jump in here, since I do not have a macerator aboard. But, since they did not, I'll try to help.
The complete answer to your question would depend, to some degree, upon the pump you selected. My my gut feeling, nonetheless, is that this would
not be "OK".
First of all, I'll assume that you refer to the outlet hose and not the inlet hose. (The more typical inlet diameter is 1-1/2" rather than 1". ) Flow rate of a hose is dependent upon 3 factors: hose length, and hose diameter and inlet pressure, and changing any of these three would affect on flow rate. Presumably, hose length would remain constant between the two setups. Inlet pressure could change, based upon the pump's efficiency at the given hose diameter. But the largest contributing factor would be the hose diameter itself.
Flow calculations are dependent upon several factors, but are ultimately a factor of the cross-sectional area of the hose (or thru-hull if that is the smallest diameter). A 1" ID hose has a cross-sectional area of 0.7854 sq in. A 3/4" ID hose has a cross-sectional area of 0.4418 sq in. Therefore, reducing hose diameter from 1" to 3/4" would reduce volume approximately 56% (0.4418 / 0.7854 = 0.5625). Clearly, that would have a major impact on the efficiency of the system.
Using a SHURflow 3200 as an example (which has a1-1/2" inlet and a 1" outlet), the manufacturer's specification is: "Use 1” minimum ID hose on discharge side of pump. Connect to thru-hull fitting above highest heeled point above waterline. Vented loop installations must vent at least 10” above highest heeled point above waterline. Use stainless steel hose clamps on all sanitation connections." (The JABSCO 18590 has the same size inlet and outlet hose.)
My advise would be to follow the manufacturer's recommendations, or it's likely that you'll end up doing the install again and possibly replacing a burned out macerator pump in the process. Potentially decreasing the flow by 56% is substantial. At the very least it would make the pump work harder and it would greatly increase the likelihood of a clog. Remember what you're pumping through this!
Sorry... I know that's not what you wanted to hear. But that's what I would suggest. Might I ask which macerator pump (manuf. and model) you are installing?
Jack
