thru-hull size needed for bilge

Don S/V ILLusion · Feb 16, 2023

Not really a problem with typical bilge pumps but the general rule is to size the thruhull to match the hose diameter.

Davidasailor26 · Feb 16, 2023

Like Don said the hose is probably the guiding factor. Nigel Calder’s Boatowner’s Mechanical and Electrical Manual has a table with the equivalent head heights for different hose and fitting sizes. To summarize and give an example - a wide open seacock is like adding 0.4 to 1.2 feet more hose, depending on hose size. With a 3/4” hose and fitting the fitting “costs” an extra 5” of horizontal hose. With a 2” hose and fitting the fitting “costs” an extra 1.2’ of horizontal hose.

Ralph Johnstone · Feb 16, 2023

Most likely the greatest increase to friction is the use of cheap corrugated vinyl hoses which I found factory installed in my boat when purchased. That had to go in exchange for smooth walled hose.

jssailem · Feb 17, 2023

This is an interesting question.

I do not recall it being asked before. There is a basic assumption by owners that the builder figured the size of the bilge thru hull would be adequate. Boater owners would connect a hose to the size of the thru hull then just pump away using on board power either electric or manual.

Xylem is a major producer of pumps and hoses for the marine industry. They provide a quick guide to there products here:

https://www.xylem.com/siteassets/brand/rule/document/rule-quick-reference-flyer.pdf

They specify the hose size for their pumps. The capacity of the pumps is clarified by this statement.

Electric bilge pumps are rated by their capacity, which is measured in gallons per hour, under open flow conditions with no restrictions to the discharge.( Xylem)

Your boat should have the thru hull above the water line to eliminate “restrictions to the discharge “. Starting at the thru hull measthe diameter of the thru hull. Attach a hose of that size to the thru hull and a pump that has a discharge to the hose size. This will match the design discharge envisioned by the boat designer.
Example: Thru hull OD =3/4” Hose ID =3/4” Pump discharge OD=3/4”

If you start at the pump and work towards the thru hull then you may find that you will need to change the thru hull size to fit your pump.

Open ocean racing rules usually have pump discharge capacity requirements specified. These pumps systems are usually manual designs and use hoses that are lead over the side of the cockpit (no thru hull).

You may consider calculating the capacity of your bilge and maybe a footprint of the cabin to see the volume of water you might be concerned about. This could guide you to the pump size or number of pumps.

Our boats are designed to keep the water on the outside of the hull. Under normal use they do a pretty good job. There is no better motivated person than a sailor with a bucket and unexpected water in his/her boat.

Benny17441 · Feb 17, 2023

The Bilge pump is not a safety issue, 500GPH is plenty to meet its function. These are small pumps to automatically discharge incidental water that collects in the bilge. These pumps are bench tested and rated, but in the real world with discharge hoses, strainers and lift loads they rarely produce 1/2 of their rated capacity. If you need more than a 500GPH pumping capacity you may need a sump pump running on 120V. A manual gusher pump is the standard for added safety in a boat. I have found that safety is better improved by periodic checking and maintenance of the thru hulls, hoses and clamps.

Ralph Johnstone · Feb 17, 2023

Benny17441 said:
The Bilge pump is not a safety issue,

@Benny17441 , your words should be emblazoned on every bilge pump installed.
They are NOT intended to handle a breached hull. They handle small amounts of leakage (gland, rainwater, leaky hose) when you are away from your boat. Unless you have a generator, you're not going to have a large 120V sump pump in your bilge. If you have a hull breach, you better find the leak and staunch it or else :

There is no other answer for a sinking boat. A hand pump may equal a 12V pump but neither is going to save you.

Remember, you're not sitting in your living room,

Tally Ho · Feb 17, 2023

I keep meaning to get a length of hose, a barb to join it to my engine water intake hose, and stick a bulk junk filter on the end…in the event of a major leak/hole in the hull, I would hook the emergency “pump hose” on the engine intake hose, fire up the diesel and hope I can pump enough water out to get the hole plugged and maybe run the boat up on a beach or maybe back to the marina where the crew is waiting with the travel lift in position to catch my boat.

Man, I hope I never need to do that…I bet the yard crew will be on break when I come in hot hoping for a lift out of the water…and boat will sink right there in the travel lift well.

Greg

dlochner · Feb 17, 2023

obxjeff said:
lots of opinions on how many and the capacity of bilge pumps but I can't find any information on the volume a thru-hull can expel.

what good is a 3000gph centrifugal pump if the thru-hull can only let 500gph out....

I know.... lots of factors like pressure and lift but as a general rule how do you size the thru-hull diameter to the pump or the pump to the thru-hull?

Read the instructions and data sheet carefully. The 3000gph figure is for the pump at ZERO head, that is basically straight through the largest diameter hose the pump was designed to accommodate. No bilge pump is installed with zero head because that would put out the outlet below the waterline.

Practically the actual capacity of the pump will be much lower because the pump must push the water up and out and all kinds of factors affect the flow rate including hose design, height, bends and turns, the bend radius who knows what else. A good pump should provide some of this actual information not just the marketing hype of 3000 gph.

Davidasailor26 · Feb 17, 2023

dlochner said:
Read the instructions and data sheet carefully. The 3000gph figure is for the pump at ZERO head, that is basically straight through the largest diameter hose the pump was designed to accommodate. No bilge pump is installed with zero head because that would put out the outlet below the waterline.

Practically the actual capacity of the pump will be much lower because the pump must push the water up and out and all kinds of factors affect the flow rate including hose design, height, bends and turns, the bend radius who knows what else. A good pump should provide some of this actual information not just the marketing hype of 3000 gph.

Plus the pumps are often rated at 13.6 V, not the 12.8 or less you’re likely to see if you’re not charging the batteries when running the pump. That could be worth another 10% or so.

Ralph Johnstone · Feb 17, 2023

Tally Ho said:
Man, I hope I never need to do that…I bet the yard crew will be on break when I come in hot hoping for a lift out of the water…and boat will sink right there in the travel lift well.

Ralph Johnstone · Feb 19, 2023

Tally Ho said:
I keep meaning to get a length of hose, a barb to join it to my engine water intake hose, and stick a bulk junk filter on the end…in the event of a major leak/hole in the hull,

I've already applied for a patent on this design but have not heard back yet. I think that was back in 2005. Maybe they lost my address.

dlochner · Feb 19, 2023

Tally Ho said:
I keep meaning to get a length of hose, a barb to join it to my engine water intake hose, and stick a bulk junk filter on the end…in the event of a major leak/hole in the hull, I would hook the emergency “pump hose” on the engine intake hose, fire up the diesel and hope I can pump enough water out to get the hole plugged and maybe run the boat up on a beach or maybe back to the marina where the crew is waiting with the travel lift in position to catch my boat.

Bronze Service Adaptor w/ Valve| GROCO

GROCO Bronze service adaptor /valve combo allows for easy access to water lines for maintenance and winterization

www.groco.net

Dalliance · Feb 19, 2023

Tally Ho said:
I keep meaning to get a length of hose, a barb to join it to my engine water intake hose, and stick a bulk junk filter on the end…
Greg

That piece of emergency hose has been on my list too.

I have two automatic bilge pumps wired to separate batteries: primary on a cycle counter & secondary with an alarm, but as very well stated elsewhere in this thread, those little bilge pumps aren’t really about safety. They’re about property protection, preventing the boat from sinking at the dock and becoming the featured “Claim #____” in Boat US Magazine if something goes wrong. On that tangent BTW, I may be the only guy on my dock who actually closes every seacock when I leave the boat.

Anyway, I do have two manual pumps on board, and buckets of course, but being out there solo more often than not, manual pumps are not ideal when I would likely be occupied with multiple things. If it’s bad enough to require emergency pump capacity, the MMSI distress button on my VHF might be the more important thing to have. Additional pump capacity might buy time though.

Dalliance · Feb 19, 2023

dlochner said:
Bronze Service Adaptor w/ Valve| GROCO

GROCO Bronze service adaptor /valve combo allows for easy access to water lines for maintenance and winterization

www.groco.net

Brilliant. I recently added 3-way valves with antifreeze hoses to my water system and head to streamline winterizing and had actually been toying with the idea of adding something similar to the engine side of my raw water strainer, though capped on the antifreeze intake side. A longer hose with a bilge strainer would work with that configuration too.

Tally Ho · Feb 19, 2023

Ralph Johnstone said:
View attachment 213129

I've already applied for a patent on this design but have not heard back yet. I think that was back in 2005. Maybe they lost my address.

I knew I read about that idea somewhere!
I better hurry up and install something before I have to pay you a royalty!

Greg

thru-hull size needed for bilge

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SBO Weather and Forecasting Forum Jim & John

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