Good point Fred- but these run through the bilge so no light to enter. Copper sounds rugged and would flow well until it began to get marine growth. On our boat- and most I suspect- the supply and discharge hoses are too buried to actually remove one end and set up a circular flush.
Man- on the cross section of hose I posted, I had made every attempt I could mechanically to clean the hose. Straightened curves, high pressure water, etc.
I was amazed it had THAT much obstruction
1983 Present 42 Sundeck
Twin Lehman 135's