Bilge pump line airlock

[DALOW]

Boy, I don't fully agree with the video I just watched on this website. It was the video demonstration on explaining how an airlock happens in a bilge pump hose.
It was about Rule bilge pumps and avoiding dips and sags in your discharge hose line.

The air lock they created was because the hose went back down, even to or lower than the pump impeller. This I agree is a problem and you should avoid this situation.

However, the problem is; they stated that the hose line should go continually up until it reaches the through hull fitting. A much better approach is to have the line go as far up the gunnel or transom as you can go before turning around and going down to the through hull fitting. There should be a fastener at the high point of the line so it stays where you put it.

Without the high transom loop, a hole in the boat is effectively introduced at the level of the through hull fitting. In other words if you are sinking or keeled over as far as the through hull fitting, you will now have an additional way for water to enter the hull. This is exactly what you don't want.

With the high transom loop, the additional hole in the boat is effectively the peak of the transom loop. This is critical in sinking or swamping situations.

I realize that if you get to the place where water is enter the through hull fitting you are in trouble anyway, but you do not want to amplify the bad news by introducing an additional hole in the boat at the most critical time.

This is might be a small point during a nice day but of critical value if you are sinking.

For a second opinion check out http://www.yachtsurvey.com/bilge_pumps.htm . Between him and his father they have been surveying boats for decades. I have no affiliation with this guy I just have a lot of respect for him.

Thanks for hearing me
Doug

 

[alldodge]

I agree with many of their points but I'm not so sure with raising the loop. When the loop is raised this increases lift and also drops the amount of GPM/H the pump can pump. So if you raise the loop it will allow for the water to be higher before it starts coming in the boat, this also slows the amount of water being pumped out. It's a trade off in my opinion only and not a calculated model.

 

[Outsider]

If I'm healed over to the point of my bilge thru hull taking water, either 1) I'm going on over, or 2) I've taken on way more water way faster than the pump could handle, or 3) a combination of too much water and not enough pump. My manufacturer chose to place the fitting as high as possible, the hose runs direct to it from the bilge. No loops, no bends, no air locks ...

 

[airshot]

Dalow....I agree with you, all my boats have a full loop in the bilge pump line and always will. If you look closely at how manufactures install things in boats, in all of the boats I have owned over the years.....no I don't trust them. I choose to use my own level headed thinking. If the loop requires that a bigger pump be used the so be it. In 50 yrs of boating I have never had nor seen an airlock from a loop in the discharge hose.

 

[DALOW]

In response to the "When the loop is raised this increases lift and also drops the amount of GPM/H the pump can pump". If you look into the physics of a siphon, given a similar length of hose, once the hose is full, you can raise the loop up as far as you want and there is no additional pressure drop due the the siphon effect. There would be additional energy to raise the first filling of the tube but after that no difference. Not much of a deterrent.

Regarding the keeled over part, I was referring to a sailboat. and I agree that putting the through hull fitting high on the gunnel will produce the same effect but again, if you looped it down the siphon effect would allow more pumping with less effort.

 

[Watermann]

Introducing a hole? Not sure how the pressure of outside water would be able to overcome the force of the bilge pump to allow back flow of water if the boat were keeled over or sinking. My through the hull output is like 3" under the top of the gunnel, I would already be swimming by then.