Thru-hull – A Fluid Dynamics Question

[FredV]

On my 37-C, the shower drains into a sump where a small pump moves the water to a 1/2” thru-hull just to port and slightly forward of the keel. Now, since this thru-hull is at least a couple feet below the waterline, and the sump maybe a foot below, why wouldn’t the water rise to its level, i.e. into or even above the sump?

I’m guessing there needs to be a check valve between the pump and thru-hull, but I don’t like the idea that a check valve is all there is keeping the water from overflowing into the bilge. I’m thinking that a better way to drain the shower is to pump it to the vanity drain’s thru-hull, but since the existing arrangement is original to the way the boat was built, I must be missing something in my logic.

Any help?

 

[Blaise]

Are you SURE that is the way it is plumbed? The water wouldn't just rise to "it's level" it would sink your boat. If a PO bet the farm on a check valve, he needs to be fitted for a corrective hat. I also doubt that the pump could empty the sump against the pressure. I am assumng the sump is also beneath the sole.(floor) My head which is above the waterline kept filling up with water until I put s seperate valve on the intake. By the way, on Midnight Sun (#42) the shower discharge is on the starboard side 2 inches below the toe rail.

 

[Benny17441]

The sump box may have a check valve to prevent backflow of water but the outlet hose should lead to the bilge or to a thru-hull above the waterline. If you have followed the hose and confirmed that the outlet is below the waterline I would urge to close the valve (if there is one) and route the outlet hose appropriately.

 

[FredV]

Thank you, thank you, thank you - for making me feel not so dumb after all!

Yes, that is the way it was plumbed. If you look at the Plumbing Diagram of the Hunter 37 Owner's Manual, you'll see that thru-hull, although it shows it with a hose coming from the head. Is it possible that the thru-hull was put there originally to provide seawater to the head? In that case, the PO obviously got both dumb and lazy when he re-plumbed it to the shower pump!

Either way, you've confirmed my plan to drain the shower through the port thru-hull currently used by the vanity sink. Thanks!

 

[JoeWhite]

I will have to review my setup - but I do believe that there is siphon break valve above water level and hidden in an accessible cabinet nearby.

 

[FredV]

My apologies, especially to the PO, whom I accused of being dumb and lazy. He was probably neither, although I do think he was cheap, as he disconnected almost all of the hoses before selling the boat - presumably to salvage the SS hose clamps!

Anyway, after crawling into the bilge and digging through a tangle of hoses under the head/shower area, I found another hose (and a lot of dirt!) curled up under the head floor. So it's possible that the hose from the shower drain just stretched out all by itself to lie next to the thru-hull, while the other one decided to stay curled up and out of sight.

I just can't say enough about how much fun it is to restore a 30 year old boat!

 

[coleman]

On my 37-C, the shower drains into a sump where a small pump moves the water to a 1/2” thru-hull just to port and slightly forward of the keel. Now, since this thru-hull is at least a couple feet below the waterline, and the sump maybe a foot below, why wouldn’t the water rise to its level, i.e. into or even above the sump?

That's horrifying.

My shower sump pumps uphill to a thru-hull well above the waterline. Sounds like the same way Blaise described, outlet is just below the toe rail. It's rigged similarly to the bilge pump. No check valves, so when it stops running (the small sump pump) there is a little backwash into the sump basin.

The location you describe for the outlet:

...a 1/2” thru-hull just to port and slightly forward of the keel...

That's the location of my raw water inlet for my marine head.

 

[Stu Jackson]

There is a pump between the sump and the thru hull, no? It should be: sump, check valve, antisiphon, pump, thru hull.

Pretty typical.

 

[Benny17441]

Don't know if the boat came from factory with a sump pump; it could have been an aftermarket addition by the PO. That was indeed a dumb way of connecting the outlet hose as it could have very easily sank the boat. Seems you are thinking about re-routing it to the drain of the vanity sink at the opposite side of the boat. I do not know how strong that small pump might be to push water over a longer distance and elevated outlet. It would really depend on the frequency and volume of water utilized by the use of the shower whether to route to the main bilge or to a dedicated thru-hull but I would consider drilling a new thru-hull closest to the sump-box if that were the case.