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October 12, 2001

Ask the Experts: Sea Chest

Gain confidence in your boat's seawater reliant systems by knowing the limits of your sea chest.

Q: Is there a limit to the number of items you can supply water to from one sea chest, and are there any items that should not be fed from the sea chest?

Derek Snodgrass
Islamorada, Florida

A: Yes, there is a limit to the number of items you can feed via a sea chest. It basically comes down to a problem of supply and demand. The supply to the sea chest is usually two fixed-diameter through-hull pipes symmetrical on the bottom, and only a certain amount of water can be expected to flow through it. If the requirement from the engine seawater pumps combined with items such as saltwater wash-down pumps, air conditioning pumps, water makers, etc., is more than the through-hulls can supply, the system will be starving for water and something will eventually suffer.

Usually, a new boat does not have this problem, as the builder or designer has sized the supply and demand for the boat correctly. It's when the owner, after a number of years, decides to add additional equipment or repower with larger engines, that the demand exceeds the supply. The only fix is to add larger through-hulls if possible.

But also remember that feeding all seawater systems from the sea chest is not always the way to go. Some items may perform more consistently by having a separate seawater supply. For example, when a sport-fisherman with a sea chest arrangement reverses hard for extended periods, the water to the sea chest is reduced because the through-hull strainers are designed for maximum efficiency in the forward direction.

It is not uncommon for large diesels to suck a sea chest dry for a split second during a hard back-down. Since the engine is the largest pump in the system, it gets what it needs and the others suffer. The pump that suffers the most is the small pump that loses its prime while all of this action is going on. The most common pump of this type on a sport-fishing boat is the air-conditioning pump. So now you know why after an exciting time of chasing a fish in reverse, you walk into the salon and it is just as hot inside as outside.

For this reason, I would feed self-priming pumps from the sea chest and install separate seawater pickups for the non-self-priming pumps.

Robert Ullberg is President of Ullberg Yacht Design And Engineering in Winter Park, Florida. For more information, contact him at: