IMHO, bilge pump manufacturers advise against non-return valves for two reasons.
1st. They presume that an installer will make up an assembly that will fit into the hose size and therefore, further restrict their flow figures. These figures are generally not realist anyway, being determined with no head lift
, no hose friction loss, and a fully charged battery
. Remember, they rate them in gallons PER HOUR.
To remove 4000 gallons in an hour (thats 1 1/2 oil
drums a minute) is going to require a substantial battery
bank, and un-impaired pump circumstances.
Read their fine print.
So, IMO, build a non-return valve assembly that provides FULL hose bore opening THRU the valve. Thats what I have done, but using belt driven pumps.
Also, the brass ron-return valves often used because of easy availability, have clappers that are very heavy. This is because they are designed to withstand high pressures in plumbing
systems. Remove the clapper and grind away most of the excess weight on the non-sealing side of the casting. This will reduce the resistance that the pump will have to overcome, and assist the flow.
2nd. Restrictive impediments in the discharge hose increases the risk of blockage, and creates a situation of "who dun it" in the event of a sinking.
Lawyer country, pump builders don't want to go there, if their $150 pump sunk your $200,000 + yacht.
Negate this "fun" scenario, by building proper baskets with large flow areas around your submersible pumps.
Door and window manufacturers, who build intrusion resistant products often use stainless or monel 8 x 8 per inch mesh. Thats what I use.
And, talking about lawyers, What I have opined above, works on my boat, but may provide different performance on yours.