I intend to put a huge ( 3500gph) bilge midships in my 69 42'. I would rather not make an  1 1/2' thru hull to accommodate it and a reducer to an existing smaller one would defeat the purpose. Could I Y at the sink grey water  1 1/2' discharge and add the bilge discharge?  This "emergency" pump  will be 6-7" above maintenance  pump level. So it should never go on ...right ;)

If it does maybe it might squirt out the sink drain? If that happens I have more things to worry about than a wet kitchen rug! And a heads up that bilge systems have gone to defcon-5 isn't an bad thing.

Am I missing anything here or is this a no brainer...?

thx!

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this inline to stop backflow up to the sink?

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You will need to install a check valve in both lines upstream of the Y. This will prevent water from the sink from ending up in the bilge, and water from the pump coming out the sink drain. Local plumbing supply should be able to help with proper size.

Just drill the hole. Once you’ve done it you’ll feel alot less worried drilling more.

I agree with Mike, drill a separate discharge for the serious bilge pump. Any y's, check valves, or stuff like that is going to decrease the real world output of that pump. I had a serious monster pump in our 38 and it had an unimpeded 1 1/2" run about 8 feet to it's own 1 1/2" discharge. I would test the boats pumping systems in the spring by throwing 2 wide open dock hoses in the bilge. My pumps, including "Mongo", could keep up with this inflow. If you're gonna install a "Mongo", give it the best chance to hit it's rated output, and Y's & check valves are not best chance.

My OMGWe're sinking option is a 3500gph sump pump with overboard hose, a re-purposed swimming pool hose.  It is a Winter only option but it has been used in anger once for a neighbor's perforated steel hull during the Summer.

I don't understand the reasoning for being that far above the other pumps, if they can't keep up they need help and an alarm should be sounding. An alarm system calling help is probably much more valuable than the pump. One of the members in our harbor has a fully geeked boat that would let him know it was in need of various types of attention so a come save me I'm sinking alarm is within the limits of this topic and may be within the cost parameters as well.  The only boats I am aware of (2) sinking had adequate pumps but were not winterized and sank due to compromised through hull fittings.

A local member with a 45 that has done the loop had a catastrophic bow puncture and was able to first make a port, make emergency staunching of  leaks then make way to a haul out facility, no special pumps.  If you're not on hand there may not be any pump large enough to resolve the problem, 1800insurance works for me.

OTOH the Curmudgeon has a lot of experience of his own and observations of others. 

In the interest of confusion,

Tim

the OMG pump will be set as a backup 

at whatever level doesnt dont allow it to be trigger either underway or at the docknfor  day

to day bilge maintenance. What ever level that may be...

As an option for an underway hull breach, I have diverter valves on my raw water intakes, flip the valve and close the sea cock and a 7.4 crusader becomes a bilge pump. Just make sure to monitor the water level as this will move a lot of water in a hurry. Wanderer has 4 2000 GPH pumps installed to work on float switches or manual switches, and I carry 2 3500,s, 1 12 volt and 1 110 volt with 30' of 1 1/2 inch roll flat hose. These are both automatic. The 110 pump has gotten a lot of use pumping out dinghies after a nice refreshing New England down pour. (With a shorter hose)

If your really worried about water intrusion. You can get a bilge alarm that will send a signal to your smartphone. Last time I checked it was a little too pricey for me, almost $1000. But if it helps you sleep at night. 

I don’t remember remember why, but when we installed our OMG pump, Lee did not want to “Y” with another thru-hull. Recommended against it, so we used a hole for an old freshwater inlet.

I don’t have a huge bilge pump, but I do have a high water alarm. The insurance company required it. It’s in the keel a few inches above the bilge pump. It’s high enough to prevent false warnings when the water the pupms can’t get out sloshes around, but low enough to know before it’s too late. It’s just a basic float switch wired to a buzzer, I’ve thought about wiring it up to the horn some how for when I’m away from the boat. I also like the idea of using the engines as emergency pumps.  

When I bought the boat it only had 1 pump, I installed a second and drilled the through hull near the original, Stainless through hull. 

As has been said above, I would not put a Y and the required valves in a bilge pump hose, too much to go wrong.

I have also read a suggestion of not fusing your pumps. I'm not sure how I feel about that, could be an electrical hazard. Maybe just the last ditch pump to ensure it never turns off just because it's working hard.

I'd also recommend checking the size of the wire feeding the pump and ensure there is not much voltage drop, maybe 3-5%. Lower voltage will just pump less water. I've got to think there are a lot of newly installed more powerful pumps on existing wiring and fusing that were designed for the original, smaller pump.

Our secondary pump is a 3700 GPH Rule, with an Ultra Safety Switch, which includes a high water alarm. The switches are expensive, but are supposed to be very reliable for many, many years. A great pump is much less useful if the float doesn't work.

The switch looks like this:

Of course it's not installed yet, first thing this spring. For the through hull, we got lucky, the boat came with 2x 1.5" sink drains and we only needed one in the new kitchen. Last year we only had a 4.5GPH single chamber pump - not so good in an emergency! The plan is to add another pump mid ships and see if I can bring the one in the stern of the boat back to life. That'll make 4.

We also carry one of these on board to plug many different hole sizes (assuming you can locate it):

Forespar StaPlug Soft Foam Cone-Shaped Emergency Boat Plug

I would love to add a by-pass valve to at least one of my engines, that would be the ultimate in safety for long crossings.

Engine cooling pumps as emergency bilge pumps ---- This subject pops up frequently and the consensus is that it's a good idea. Well, I'm not so sure about this. However, I know that being against more bilge pumping capacity in a boat is like being against God, Country, Motherhood, Chevrolet, and Apple Pie all in one fell swoop   So, here's my take on it. For purposes of discussion, lets use the Sherwood 11095G pump found on 427F  & 327F engines. See attachment for this pumps output at various rpm's with 3 different cams. Our pump has the 10961 cam. Looking at the chart we see this pump puts out about 20 GPM at 3,000 RPM. This assumes 0 psi of discharge pressure with 1 1/16" intake and outlet porting. Oh yeah, one more small item --- a new condition pump with no end cap or cam wear and a new impellor. Just about every pump I've seen disassembled has noticeable wear both on the end surfaces and cam. Rev your 427 up to 3000 and observe the exhaust. You really think you're seeing four 5 gallon cans worth of water coming out?? Nope, nowhere near it. In addition to wear, our supply and discharge paths are not full size, plus we have restrictive elements such as oil coolers, thermostats, and 2-3 psi pressure relief valves for the pumps output to pass before it is discharged into the exhausts. I'm going to hazard a guess that a normally worn, as installed Sherwood pump in a 427 actually pumps out about 10 GPM at 3,000 RPM. This certainly is enough to handle the heat load of the engine, but 10 GPM correlates to 600 GPH (bilge pump ratings). Even when you multiply it by 2 you only have 1200 GPH, and that's at 3000 RPM.
Now, let's set our pumps up so they can either draw from their normal through hull supply, or from the bilge. First, this plumbing setup is going to include a Y or a T fitting, a couple of ball valves, & misc. nipples and/or couplings & ells. Guess what ? The ball valves you'll probably use will be a further restriction. Wait a minute, we have to have some sort of filter on the bilge inlet to prevent normal bilge detritus from clogging the pump (Yet another restriction). Now for the last thing --- In operation you have to monitor bilge water level so that you don't uncover the inlet filters and let the pumps run dry. Most impellors will self destruct in a very short time if running dry, especially at 3,000 RPM. Perhaps you're seeing my point --- For the expense, complexity, and operating cautions we don't get a real serious amount of bilge pumping capacity from engine pumps. It sounds good in theory, but in practice you can get way more emergency pumping capacity from say buying an extra 3500 GPH with about 12 feet of 1 1/2 hose, a 20 foot power cord with serious alligator clips on the ends, or for 110v use, I see some reasonably priced pumps at home improvement stores. Here's a serious pump at Lowes -- 4400 GPH at a 10' lift. If you have a reliable generator on board, $173 plus some hose money buys you a hell of an emergency pump. OK, stepping off my soapbox now and waiting for the rebuttals !
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Those are excellent points Dick. 

My question is how much water will come in through say a 6”x6” hole? That’s about the size I would expect if say a rudder was ripped out as happened to someone on lk St Clair last year. That’s the point of these big pumps right? Hit a dead head, rip out a shaft log etc.

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