I even used 1 inch hose from the head
to the holding tank
on our last boat. I had the same "bending" restriction as you, and I wanted to reduce the amount of liquid volume in the hose since I was pumping straight up to the tank.
There was no further diameter reduction because no barb fitting was inserted into the 1 inch hose. I used 1 inch "liquid tight", I think it was called , PVC electrical
conduit. I found the coupling used with this stuff was just a hair bigger than 1 1/2 inch . With PVC cement I glued one of these couplings (actually cut one in two because they were longer than needed) to each end of the hose, and then used short lengths of 1 1/2 hose to connect to the original fittings on the tank and head
. Yes, that's 2 extra hose clamp connections, but I never had a problem. The 1 inch hose was not much smaller than the ID of the 1 1/2 hose barb, so I figured it should work, and it did.
You could do the same thing with regular head hose,I expect. (I think regular PVC couplings are a bit bigger OD, so maybe the conduit couplings would work) I used the conduit because it was grey in color, which looked good in the head ( the routing run was front and center),plus it was a lot less expensive. I enjoy finding non marine
products that work well.
Never had any problem, odor
or other, over the ensuing 4 years we owned the boat.