5 November, 2017
Using a skin fitting for multiple uses might seem like a good idea to start with as it reduces the number of holes in the hull. This can however reduce the effectiveness of the items sharing the skin fitting.
In the vessel shown above, the seacocks’ primary use was for the cockpit drain but this one has had a manual bilge pump and then an electric bilge pump added using ’T’ pieces. Each pipe has its flow rate determined but its diameter and the pressure you can apply to it. The total flow will be restricted by the diameter of the single outlet.
The cockpit drain will be using gravity to allow water to flow out. If you now start pumping with the electric bilge pump the cockpit will not be able to drain as fast. Similarly if you have to start using the manual bilge pump because of an emergency you will have to pump even harder to force the water through the seacock to reach the full capacity of the pump. In this setup you may also start pumping water back into the cockpit.
In the example shown, the manual pump would also be restricted by the smaller diameter ’T’ piece fitted for the electric bilge pump. This could easily halve the capacity of the manual bilge pump.
An improved installation would be to fit two new above the water line skin fittings for the bilge pumps and have the seacock dedicated for the cockpit drain. Keep all pipes and skin fittings the same diameter to ensure maximum flow.