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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Proper height of a thru-hull

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Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Initially Posted - 02/18/2010 : 15:17:48

Preface - I have one thru-hull on my boat. The galley sink, ice-box drain and bilge pump hose all route to it with ball-cocks to close each of them. In the past I have had the sink drain into the bilge or ice box dependent on if I remembered which should be open or closed. The thru-hull is mounted at the water line. I installed an electric bilge pump last year and after an extended cruise found the bilge full of sea water. The sea-cock had been opened during the cruise. Due to the weight in the boat at the time the sea-cock was now below the water line when ever the boat listed even slightly. I turned on the bilge pump and it would not overcome the water pressure from the outside so I had to hand pump it dry. The cheap backflow preventer that came with the bilge pump was essentially worthless.

So I want to install a second thru-hull dedicated to the bilge pump. Based on my previous experience I want to mount it higher than the existing one. Is their a rule for how high I should, or should not, mount it? What other considerations should be taken into account? I am thinking the higher the better as long as the bilge pump can overcome the height.

Joe Wergers
Utopia
Fleet 7/Oceanside, CA
78 C25 FK/SR #381

Edited by - on

OJ
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Response Posted - 02/18/2010 : 15:29:00

My 1981 C25 had a thru-hull in the transom (and above the waterline) that was connected to the manual bilge pump . . . could you splice into that line?

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Sloop Smitten
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Response Posted - 02/18/2010 : 15:46:26

Utopia does not have a built-in manual bilge pump. I use a portable hand bilge pump.

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OJ
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Response Posted - 02/18/2010 : 16:52:16

I see . . . maybe someone here is close enough to their boat to give you dimensions(or has a picture).

In the commerical world there are formulas that take volume and rise (to name only two) into consideration when selecting pumps. Perhaps the owner's manual for your bilge pump has the specs you are looking for?

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Voyager
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Response Posted - 02/18/2010 : 17:16:14

Further to OJ's point, on Passage, the manual bilge pump ports out of a thru hull about six inches above waterline on the transom. If I were going to add one, that's precisely where I'd put it.

Since water is heavy, the height of the port above the level of the pump is inversely proportional to the amount of effort or efficiency you will get with a pump as you've already found out.

If I were to add an electric bilge pump, because power is limited, I'd use the same port placement as the manual one, because it is rarely below water line, even while heeling (because of cavitation when you're underway) and it is only about 2 feet (estimate) above the lowest point in the bilge.

Thank God I've never needed to pump a drop of water out of the bilge! I sometime used a paper towel to sop up rain water that leaked in, but no serious stuff.

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JimB517
Past Commodore

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Response Posted - 02/18/2010 : 17:19:02

I have an electric bilge pump in the keel well wired directly to the battery. I also have the manual bilge pump. The hose for the manual pump exits the transom by the (port) motor mount. The exact height is not critical, it should be well above the water line.

I have my electric pump running a seperate hose, but tees into the manual line just above the through hull. Based on the geometry, water would have to flow uphill for the electric pump to drive water through the manual pump and back into the bilge. However, I put a manual shut off valve in line to prevent this in an emergency. Back flow preventors are not recommended on bilge pump lines.

Keep in mind the hose routes from the keel well into the port cockpit locker. Then routes up almost to the top of the seat, before running downhill to the through hull. Don't just lay it on the floor.

Here is a nice photo of a factory installed through hull for bilge pump discharge, about 6 inches above the waterline on the stern.

Edited by - JimB517 on 02/18/2010 17:20:56

Dave5041
Former Mainsheet Editor

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Response Posted - 02/18/2010 : 18:16:18

Think there must be another problem if the pump could't move any water. Output does fall rapidly when pumping against a head, but not because the through hull is submerged. Inflow of water is prevented by a loop with anti siphon protection before the outflow tube attaches to the through. That said, I am also strongly in favor of a dedicated through hull for a bilge pump. Regardless of where you put it, it needs to loop well above the waterline and have anti siphon protection.

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Tom Gauntt
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Response Posted - 02/18/2010 : 19:08:19

I have two dedicated thru-hulls for bilge pump discharge. Both exit the transom above the waterline just like JimB's picture above, except one to port, one to starboard. One thru-hull is for the manual bilge pump and one thru-hull is for the electric bilge pump. I am hesitant to tie the discharge lines together because you'd need a check valve to keep bilge discharge from flowing from one to the other, so I have dedicated discharge lines and thru-hulls to avoid the use of one flooding the other. Having the thru-hull above the water line makes it very easy to check the operation of either pump. I'd be hesitant to have a bilge discharge that is routinely closed... when you need to bilge water overboard, chances are something untoward has happened/is happening.

Most electric bilge pumps have charts that show pump discharge in gallons per hour (GPH) based on standard distances(1 and 2 meters) the pump has to raise the discharge. For example, my Rule 1000 pumps a 1000 gph at zero meters lift but only 725 gph at 1 meter and 450 at 2 meters. Echo the vented loop as well. Here in Maryland, we've had a record breaking year for snowfall. Many boats have sunk at the slip because (among other things) the snow weighs down the boat and bilge discharges that are normally above the waterline now sink just below the waterline. WIthout a loop, the bilge fills up with water. An automatic bilge pump will then kick on, pump the water back out and then shut off only to have the water come right back in. A loop, especially a vented loop makes this scenario much more unlikely. Of course I see you live north of San Diego, so I doubt you really are too worried about snow!

Speaking of San Diego, I'm going to be there tomorrow. Planning a afternoon at West Marine, just looking.

Ahhhh, sunshine, no snow, the Mecca of West Marine's, and can't buy anything! Sounds like a great day!

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Sloop Smitten
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Response Posted - 02/19/2010 : 11:25:26

I did some more research on how to build the drain system for a bilge pump. Based on the comments online and here this is what I came up with. The thru-hull should be mounted so it is not submerged when the boat is listing or squatting. A loop in the line that rises above the thru-hull assures any water that may enter the line does not run directly into the bilge. A vent placed high on the loop that is routed to a position higher than the loop assures that a siphon action cannot be created if the thru-hull were to submerge. Back-flow devices are notorious for plugging and should be avoided. Corrugated drain pipe will decrease the flow-rate of a pump. Smooth hoses are better. Double-clamp all hose connections.

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Ed Cassidy
Captain

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365 Posts

Response Posted - 02/19/2010 : 11:48:12

Where does the inlet hose for the bilge pump go and how do you pump out all of the bilge? I don't have any kind of bilge pump at all , except for a hand held tube of dubious use and I'm going to install either a manual or electric pump.

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Sloop Smitten
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Response Posted - 02/19/2010 : 12:03:00

Ed,
My pump does not have an inlet hose. It mounts to the floor of bilge and suctions up water. Only hose connection is the outlet.

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Prospector
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Canada
3159 Posts

Response Posted - 02/19/2010 : 12:56:20

Similar to what ws posted above. have your wife stand next to the boat, and it should be between Nose and eye height...

On Iris, we have the bilge pump hose in the bottom of th ebilge. It runs back to the manual pump which is on the port side of the cockpit. Its outlet then goes to the through-hull shown above.

This season I should be installing an electric bilge pump alongside the manual one. I will likely not splice it into the line of the manual pump since one would likely slow the other. If I do get lazy and do that, the splice will be between the pump and through-hull, definately not ahead of it. I think the proper way to do this is to bite the bullet and put another hole in the boat.

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tbosch
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Response Posted - 02/20/2010 : 08:18:02

The P.O. put an electric bilge pump at the end of the hose that is connected to the manual bilge pump. The water seems to push through the manual pump ok.

I took the manual pump apart to see how it works. The valves are basically rubber flaps that allow water to travel in only one direction through the pump. It doesn't seem like forcing water through these valves in the intended direction would be abusive to the manual pump. Likewise, the manual pump seems to be able to pull water through the electric pump's inlet.

So, is this a reasonable solution or a bad idea that I should correct? Does anyone else have a set up like this? Any problems with it?

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 02/20/2010 : 09:41:28

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by tbosch
 The P.O. put an electric bilge pump at the end of the hose that is connected to the manual bilge pump... It doesn't seem like forcing water through these valves in the intended direction would be abusive to the manual pump. Likewise, the manual pump seems to be able to pull water through the electric pump's inlet.

So, is this a reasonable solution or a bad idea that I should correct? Does anyone else have a set up like this? Any problems with it?
<hr height="1" noshade id="quote"></blockquote id="quote">I'd say there are perhaps three reasons this is less than optimal:

1. Each pump in that configuration reduces the efficiency of the other by as much as half, from reports I've seen. (From the electric pump's point of view, the manual pump is effectively two check-valves in the system, and even one check-valve is not recommended.)

2. If you have a real problem (busted seacock, hole in the boat, etc.), you effectively have only one less-efficient pump--not two fully efficient pumps.

3. If either pump is compromised (torn diaphragm on the manual, or stuck rotor on the electric), it might keep the other from working.

Edited by - Dave Bristle on 02/20/2010 09:56:56

tbosch
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Response Posted - 02/20/2010 : 17:11:10

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle

I'd say there are perhaps three reasons this is less than optimal: ...

<hr height="1" noshade id="quote"></blockquote id="quote">

Those are 3 good reasons. I'll add this to my growing list of projects.

Thanks!

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triley
Captain

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USA
251 Posts

Response Posted - 02/28/2010 : 20:55:25

A story from a trip to the Leeward Hawaiian Islands-Nihoa and Necker- in 1980 on a Cheoy Lee 44. The one-way at the through hull of a brand new electric bilge pump failed in the middle of the night as we encountered a storm of some proportions. I was one of the watch of two on deck as we began to plough the sea rather than surf it. I went below to discover water sloshing across the cabin sole. The electric bilge pump pumped it out, but we took it in as soon as it was switched off. An interesting conundrum in the middle of the night somewhere near 100 miles to the northwest of Kauai. Changing tack solved the problem for awhile, and we closed the valve before to hull of the siphon breaker. Then we had to replace the leathers in the manual bilge pump, which hadn't been used for about ten years. Back then I was the slimmest member of the crew, so I spent an uncomfortable hour or two in the engine compartment removing the pump barrel and replacing the leathers. As close to sick as I had been at sea for awhile.

Discovered later that the owner had added 700Lbs of ballast AFTER the electirc pump had been put in and the siphon breaker was below water level when she was on starboard tack, suicking water when the through hull one way failed.
Robert Stone describes the same thing in his novel "Outerbridge Reach" . He and his wife and some friends welcomed us back on that trip, and I am sure that our experience is what he wrote about. It is something that happens more often than one would think.

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