Bilge thru hull question

[Mac2]

suggest you check ABYC standards for thru-hull installation regarding height above waterline and use of seacocks etc.

I should have clarified. In my thinking, why wouldn't you put a seacock on every thru hull? I have seacocks on every thru hull except my generator. Was curious why.

 

[bill.kimbell]

The three bilge outlets on my boat were factory installed just above the waterline, along with all the other water drains etc. After loading all our belongings aboard and filling the tanks, they went underwater, causing them to siphon water back in and cycle continuously. I had to move all of them higher, and as in the OP's situation about 2 feet was as high as I could go. It's worked well so far. I figure if the actual water level gets 2 feet above LWL, I've got more serious problems to deal with and the pumps will not be shutting off to allow the siphon back. I like the idea with the non countersunk outlet, I'm getting some staining so I think I'll try that. I don't remember now which type of fitting I used but if they're already countersunk I have some Starboard scraps I can make spacers out of. Should work just as well, and would work with a mushroom head too. Make the spacer a little larger than the fitting and leave the edge square or even taper it back like the flush fitting. I think that's key to getting the water to drop cleanly.

Be careful w starboard - none of the usual sealers will adhere to it.

 

[Greg QS]

I am a firm believer in using sea cocks at all discharges including bilge pumps. Lets say a bilge pump fails , you can shutoff the opening in the boat until repaired. Say you start taking on water you can shutoff you sink and ac drains that once submerged they will let more water in. Say a hose or clamp fails, quick way to stop a leak at the hull.

Sea cocks can also be advantageous when you are winterizing on a system with multiple discharge points, allowing antifreeze to be trapped in the line, and or purge each unit individually off a common pump like some ac systems. They also keep critters out during lay up.

My recent personal experience. We brough our boat up from Florida this spring. We left Florida and pull into sandy hook 4 days later, 24/7 driving.

I have them on all discharges. It was sure nice to be able to turn off the sink drains (mainly noise) on our trip home from Florida. When we hit some 8' seas I felt confident the seas were not shooting up through the drains in the heads. We had a problem with a bilge pump cycling, then failing, and high water alarms going off in the forward compartment. The seas were coming in, overcoming the loop, and filling the forward compartment. It was really nice to be able to shutoff the through hull to stop the water coming in I was then able to empty the compartment and install the backup pump onboard. The new pump also cycled so I shut it again until I could correct the backflow problem after the trip. We were 90 miles out to sea at the time. So yes valves on bilge pump discharges.

 

[OldDan1943]

I'm planning on drilling two holes in the side of my boat (not thrilled about it!) for two high capacity bilge pump outlets. Either 1 1/2 or 2 inch thru hulls. Due to a thick rub rail and equipment on the inside, the holes can be drilled a couple inches above the waterline, or two feet above the waterline. My first inclination is to go higher for if the boat starts taking on water and the thru hull becomes submerged. I can create an anti siphon loop above the water line if I go with the lower hole, but I'm concerned this could limit the bilge pumps capabilities. Suggestions?

Give consideration to discharging on opposite sides of the boat.

 

[Steve DAntonio]

Thanks for the confirmation. I bought one Johnson bilge pump (4000 gph). It says it has a built in check valve. I will not rely on that and still plan to create a anti-siphon loop, or as much of an upward curve as possible. The two inch thru hull is huge and I wish they came with a flapper to keep debris/insects etc out. I will probably put a thin piece of foam over the opening to keep the critters out.

You need more than just a high rise "loop" you need an actual anti-siphon valve too. The riser will prevent flooding, the anti-siphon valve will prevent...siphoning.

ABYC Standards do prohibit the use of check valve to prevent siphoning.

This article may be helpful https://stevedmarineconsulting.com/flooding-vs-siphoning/

 

[Steve DAntonio]

From the perspective of whether or not a seacock is needed for an above the resting WL thru-hull, if the vessel heels 7 degrees and it becomes submerged, it needs a seacock. But, there is an exception, if it is in that heel zone, i.e. above resting WL below heeled WL, you could use "reinforced" hose and get around the need for a seacock, at least for ABYC compliance. I sit on the committee that writes this standard and have been lobbying for some tome to define "reinforced" as "wire reinforced", in any event that's what I recommend if no SC is used in this zone only because it won't fit, J2006 wire reinforced hose.

 

[Mac2]

From the perspective of whether or not a seacock is needed for an above the resting WL thru-hull, if the vessel heels 7 degrees and it becomes submerged, it needs a seacock. But, there is an exception, if it is in that heel zone, i.e. above resting WL below heeled WL, you could use "reinforced" hose and get around the need for a seacock, at least for ABYC compliance. I sit on the committee that writes this standard and have been lobbying for some tome to define "reinforced" as "wire reinforced", in any event that's what I recommend if no SC is used in this zone only because it won't fit, J2006 wire reinforced hose.

Because of my large rub rail, I can only put the thru hull at the water line with a vented loop and seacock, or two feet above the water line with a high loop in the line before exiting the boat (for rain etc.). Looks better with the thru hull at the water line (under the rub rail), but I'm guessing the flow would be much better with the thru hull in the higher position. Thoughts?

 

[OldDan1943]

The gph rating of the pump is at zero pumping head.

 

[rslifkin]

The gph rating of the pump is at zero pumping head.

Yes, but most good pumps will give specs (or a graph) for flow at a higher head as well.

In general, I find that for small pumps, de-rating to 50% for small pumps and 60% for large pumps often gets you close to as-installed capacity (assuming a reasonable discharge run).

 

[Mac2]

The gph rating of the pump is at zero pumping head.

The vented loop will also reduce the gph-just not sure how much.

 

[MVDarlin]

What kind of boat is this? My factory powerboat bilge pump and sink outlets are located just above the waterline. My sailboat bulge pump outlet is located just below the deck, presumably because the sailboat heels more.

 

[Mac2]

What kind of boat is this? My factory powerboat bilge pump and sink outlets are located just above the waterline. My sailboat bulge pump outlet is located just below the deck, presumably because the sailboat heels more.

Steel trawler. I have fuel and water tanks welded to the sides. I can only get to small sections of the hull in the engine room where I am installing the bilge pump. Add to that a huge rub rail (about 10 inches wide-wraps around to become the swim step) that's about 6 inches thick. I can install the thru hull at the water line, or about 2 feet above that between ribs. If I install the thru hull at the water line, water will definitely enter the thru hull, but I can create a loop about two feet above the water line.

 

[Steve DAntonio]

Because of my large rub rail, I can only put the thru hull at the water line with a vented loop and seacock, or two feet above the water line with a high loop in the line before exiting the boat (for rain etc.). Looks better with the thru hull at the water line (under the rub rail), but I'm guessing the flow would be much better with the thru hull in the higher position. Thoughts?

The only difference between these two locations would be the resistance on the anti-siphon valve and portion of the hose between the riser and the discharge. Aesthetics are important and you don't want a dirty streak running down your hull, the vast majority, 99%, of bilge pump discharges are in the boot stripe, so I would go there with the appropriate riser and anti-siphon loop.

Bilge pump installations https://stevedmarineconsulting.com/bilge-pump-systems-design-and-installation/