Bilge thru hull question

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Mac2

Mac2

Senior Member
Joined
Mar 9, 2014
Messages
418
Location
Seattle, Washington
Vessel Make
1998 RealShips Voyager
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?
 
I think you are right to be concerned about a low thru-hull. Yes both the high discharge and an anti-siphon loop will reduce capacity but I think that’s the smart thing to do. Do not install a check valve. That will also reduce capacity and introduce a new failure point in the system.
 
HopCar said:
I think you are right to be concerned about a low thru-hull. Yes both the high discharge and an anti-siphon loop will reduce capacity but I think that’s the smart thing to do. Do not install a check valve. That will also reduce capacity and introduce a new failure point in the system.
Click to expand...
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.
 
I personally don't think making the thru hull high or low will make a difference assuming top of loop is same height, but whatever works for you. The pump only "sees" the highest point. Assuming loop is sake height, possible the lower thru hull creates some siphon-pull, but would be offset by longer hose. I don't think it makes much of a difference either way.

Regardless of where you place the thru-hull, do form a foot or so above the thru hull. An anti-siphon loop is never a bad idea, but not necessary if the thru hull is at least 6 jnches above the loaded/wet waterline.

Peter
 
If the included check valve is one of those rubber ones in the pump outlet, remove it. I've seen them cause airlocks and a failure to pump once the hose is full of water and the pump goes to cycle the next time. Usually takes manually wiggling the pump to get it pumping again.
 
mvweebles said:
I personally don't think making the thru hull high or low will make a difference assuming top of loop is same height, but whatever works for you. The pump only "sees" the highest point. Assuming loop is sake height, possible the lower thru hull creates some siphon-pull, but would be offset by longer hose. I don't think it makes much of a difference either way.

Regardless of where you place the thru-hull, do form a foot or so above the thru hull. An anti-siphon loop is never a bad idea, but not necessary if the thru hull is at least 6 jnches above the loaded/wet waterline.

Peter
Click to expand...
Very helpful. Thanks. I would prefer the thru hull at the lower point (I can only drill it about two inches above the waterline). I will create a rise in the hose, about two feet above the waterline. just before the thru hull. The run of the hose will be about 20 feet.
 
rslifkin said:
If the included check valve is one of those rubber ones in the pump outlet, remove it. I've seen them cause airlocks and a failure to pump once the hose is full of water and the pump goes to cycle the next time. Usually takes manually wiggling the pump to get it pumping again.
Click to expand...

That is exactly what it is! I will take your advice. I wondered why they state in the directions that you could remove it if you want. It was a red flag for me that it was an option.

As always, this forum has come through again for me. Thank you guys!
 
Easy way to locate the holes is to get some rare earth magnets and tape one on the inside where you want the hole. Then take another one and go on the outside and get the second one near the one inside and it will stick to the hull where you want the hole, no measurements necessary.
 
If the pump will run frequently, having it high will create a water stain down the side of the boat. Won’t be as much of a visible issue if it’s near the waterline.
 
Comodave said:
Easy way to locate the holes is to get some rare earth magnets and tape one on the inside where you want the hole. Then take another one and go on the outside and get the second one near the one inside and it will stick to the hull where you want the hole, no measurements necessary.
Click to expand...

Great idea.....except I have a steel hull:)
 

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Mac2 said:
Very helpful. Thanks. I would prefer the thru hull at the lower point (I can only drill it about two inches above the waterline). I will create a rise in the hose, about two feet above the waterline. just before the thru hull. The run of the hose will be about 20 feet.
Click to expand...

Be aware of your highest waterline, the one when all tanks are full and everyone is on board. It can differ from a half load or empty waterline by several inches.
 
koliver said:
Be aware of your highest waterline, the one when all tanks are full and everyone is on board. It can differ from a half load or empty waterline by several inches.
Click to expand...
Weebles explained this above. I can create a rise in the discharge hose about two feet above the waterline.
 
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.
 
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jgwinks said:
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.
Click to expand...
As a temporary fix, you could have put a high loop on the inside of the boat, but would absolutely require a vented loop to prevent bilge pump from priming the line and creating a back-siphon into the boat.

Putting a high loop inside the boat is goof practice. Even A/C discharge lines should be configured with a loop inside the boat to give some added protection. Yea, raises the initial head height the pump must overcome, but not by much.

Peter
 
The loops were already there but the water was siphoning anyway. As a temp fix I poked a small hole in the top of the loop and that stopped the siphon. Same as a vented loop except it peed a little water when the pump ran. The water was already in the boat and not dripping where it would cause damage so no foul. I moved the outlets and replaced the hoses a week or two later
 
Sandpipers thru hulls are located 3 to 4 inches above the waterline, in the middle of the boot stripe.

There is a seacock on every thru-hull except the generator exhaust.

I believe that seacocks are required when thru-hulls are close to the waterline.
 
Probably not a good idea to close a seacock on a bilge pump outlet. If a seacock is required, it's to be used if the hose comes loose or something. I don't think any of the waterline drains on my boat have valves, apparently not required in 1973 when this antique was built.
 
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jgwinks said:
Probably not a good idea to close a seacock on a bilge pump outlet. If a seacock is required, it's to be used if the hose comes loose or something. I don't think any of the waterline drains on my boat have valves, apparently not required in 1973 when this antique was built.
Click to expand...

Who said anything about closing the seacock on bilge pumps routinely?

They are for emergencys such as you describe.

Just because something was not required in 1973 is not an excuse for not installing it; smoke and CO2 detectors are examples.
 
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From the perspective of flow, I think the lower outlet will be higher flow. The pump doesn't see the high point, only the discharge point: higher pipe pressure near the hump is recovered on the fall. If the difference is two feet between high and low choices, you are trading 2 feet more lift against two feet more hose. The two feet more hose will offer less resistance.
 
syjos said:
Sandpipers thru hulls are located 3 to 4 inches above the waterline, in the middle of the boot stripe.

There is a seacock on every thru-hull except the generator exhaust.

I believe that seacocks are required when thru-hulls are close to the waterline.
Click to expand...

Is that by choice not to have a seacock on the generator exhaust, or code?
 
jgwinks said:
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.
Click to expand...
Apologies if you know this already. Nothing sticks to Starboard (unless something new has just come out) for sealing. I know of an article by "Marine how to" that explains how Butyl tape is used for this purpose.
 
DDW said:
From the perspective of flow, I think the lower outlet will be higher flow. The pump doesn't see the high point, only the discharge point: higher pipe pressure near the hump is recovered on the fall. If the difference is two feet between high and low choices, you are trading 2 feet more lift against two feet more hose. The two feet more hose will offer less resistance.
Click to expand...

My biggest concern is siphon. My bilges are always dry. If the bilge pump comes on, something is very wrong. The high capacity bilge pump will be mounted higher then the existing bilge pumps. The Johnson bilge pump I will be installing (4000 gph) gives you a choice of 1 1/2 inch hose and 2 inch hose. I will be using the 2 inch hose for the best flow. I can put the thru hull a couple of inches above the waterline, which is now what I plan to do unless talked out of it. The thru hole will be above the waterline, but I see occasions when it will be underwater (lowering 1,500 lb dinghy to this side).
 
syjos said:
Sandpipers thru hulls are located 3 to 4 inches above the waterline, in the middle of the boot stripe.

There is a seacock on every thru-hull except the generator exhaust.

I believe that seacocks are required when thru-hulls are close to the waterline.
Click to expand...

Mine is exactly like that. (As you might expect ) I had a back siphoning event that put quite a bit of water into the bilge and it was a little scary for a few minutes while I figured out what was going on. The cockpit floor dictates where the outlets are essentially, and when fully loaded, they’re partially flooded when under way.
As long as the pumps are all working ok it’s not been a problem. But I had a failure on the fuse holder on the little pump control panel that caused the event. I was quite surprised as I test them weekly.
There was no bilge alarm in the boat at the time, but there sure is now.
 
I think ABYC requires seacocks when near the waterline and they will be underwater at so many/maximum degrees of heel. It may only be for sailboats but I no longer have the standard in it's full so I can't say for sure

My 1988 Albin had seacocks on all through hulls near the waterline.

Seacocks are just another way to secure water from entering the hull for ANY reason.

They are a good idea for vessels with crews "who get it"..... probably why ABYC recommends them at least for sailboats...but in reality, a trawler in a storm or even consistently strong winds may heel enough that it has the same effect.
 
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Article by Steve D explains

https://www.proboat.com/2010/04/the-standards-for-seacocks/

ABYC states that if thru hull is within an area above static waterline that would be submerged with a 7-degree heel, it needs a seacock. For a boat with a 15-foot beam, depending upon how that's calculated (whether hull rolls when heeled or pivots on it's keel), could mean up to 22" above static waterline, though the author mentions at least 12-inches.

Not sure it's covered in the article, but if 22-inches is the right number but thru hull is 10-inches above waterline and loops internally to 22-inches, does it meet ABYC? While better than not looping high, probably does not meet the standard since the requirement is intended to protect against a hose failure, not just back-siphoning.

Peter
 
Mac2 said:
Is that by choice not to have a seacock on the generator exhaust, or code?
Click to expand...

I don't think seacocks are required for generators. Have not had an occasion recently to read ABYC.

The waterlift muffler and Gen-Sep theoretically prevents water from getting to the generator.
 
suggest you check ABYC standards for thru-hull installation regarding height above waterline and use of seacocks etc.
 
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