Boat Flooding on a Following Sea, Possible Bilge Pump

Dana Arenius · Post by **Dana Arenius** » Jun 4th, '97, 06:56

My computer indicates that the postings on June 3,1997 have been lost on the site.

This is a repost of an answer to Zeida's question on 6/3/97 concerning water flooding into the bilge when the boat (CD25) was subject to a 4 to 5 foot following sea. The description was flooding up to the floor boards, needed to pump a lot of water out, and that this has happened twice.

Zeida,

I think your water is coming into your bilge via reverse flow through your bilge pump. If I had to guess, I would say that your thru hull for your bilge pump is at a low outside hull location on the stern of the boat. Although the hose may be sloped upwards toward a cockpit mounted bilge pump, this may not be adquate height compared to the wave height AND force of the following sea. The thru hull fitting is submerged with a following sea and the hose is subject to filling depending on how much the fitting gets submerged and the moving force of the wave. Both the degree of submersion and moving force will tend to increase the hieght of "standing" water into the hose.

The only things which can prevent this is:

1. The thru hull is not located where it is
2. effective check valve in the pump discharge
3. closed manual valve at the thru hull
4. The mounting height of the pump is not adaquate

But there is a lot about the above that needs careful consideration:

1. If it is a new installation, try to avoid the problem all together. For instance, I put my bilge pump thru hull mounted high in the engine well side wall. It never sees a following sea or boat heeling.

2. Such installation require a VERY GOOD check valve and the integrity of function of that valve. The truth is that check valve construction varies from a simple rubber flap/plastic housing construction to a metal housing/spring version.

3. One can not generally keep a manual valve closed if you need the pump in rough conditions, i.e. a high following sea.

Dana

3. The mounting height of the pump is generally limited for most boats.

arenius@jlab.org