Back up bilge discharge

[cemiii]

I have a back up bilge pump. I envisioned its main function to be in case of the primary pump malfunctioning. However, it certainly would assist in the event of needing greater capacity in a major incident.

When I installed the back up, rather than cutting a second thru-hull, I simply ran the new disharge through the motorwell boot. I've been trying to think this through as to whether or not it is worth rerouting to a higher position. Even in the unlikely situation that the boat took on enough water that that discharge hose was nominally below water, would the little bit of backpressure seriously impair the function?

Chris

 

[Sea Wolf]

Chris-

The efficiency of a centrifugal vane bilge pump goes down significantly with the "head" or height to which the water must be raised. Jabsco, the makers of Rule and many other lines of pumps, will have tables for this.

The outlet must be high enough to prevent reverse backflow siphoning for obvious reasons, and one has to guess where to put the through hull so that it will be high enough to avoid this, but low enough to permit maximum flow. Perhaps the height of the factory installed through hull would be a good guideline in the absence of a better "guesstimate".

What I think is important here is that dumping the water in the engine well is not really getting it's weight out of the boat, since it will then have to flow outward of the drain holes in the rear of the engine well. In heavy seas, the water may well just slop over the front of the well back into the boat, negating the secondary pumping operation entirely.

Of course, if you're flooded enough and in heavy seas, your bilge pumps won't be able to rescue the boat anyway, but I'd pump the water overboard, not into the engine well. You don't want it's weight there, period.

Many folks have built wall-like structures across the front of the engine well, thinking the low cut-out is dangerous in heavy following seas, etc.

If you plan on operating in dangerous, heavy seas, get at least a 2000 gph primary pump, with a another 3500 gph backup (the smaller pump will work better for small, normal operation), build the wall for protection and peace of mind , then get a VHF with the DSC distress function, and good safety equipment, including an EPIRB and, in cold waters, a Mustang immersion or work suit, etc.

Sorry if the answer contains information overkill, but I do get carried away at times!


Joe. :teeth :thup