View Full Version : What thruhull for 2000gph? what thuhull for 3700gph bilge pump?

01-01-2014, 09:28 PM
Hello, I have two 2000gph bilge pumps and one 3700 gph bilge pump. I plan on putting the 3700gph directly under the battery's in the engine compartment. I plan on putting one of the 2000gph in the aft cabin and the other 2000gph pump in the galley (front cabin). My boat is water tight and does not leak so there is rarely if ever any water in the hull. These pump are just for an emergency situation so I would like each pump to have its own thru hull fitting. What size thruhull do I need for the 3700gph pump? what size thruhull do I need for the 2000gph pump? do I need to use seacocks in combination with the thurhull's? I plan on using an anti syphon lupe as long as I have the room. Im kinda a boating "Noob" so any advise is very much appreciated. My boat is a 1972 35foot chris craft commander salon

01-02-2014, 07:11 AM
i use thru the hull fittings that use the same size hose as the pumps output...you mention a loop in the hose?...what you dont want is water in the hose when the pump quits pumping....if you have water in the hose that completely blocks air then when the water in the bilge get high enough to operate the pump then the pump may not be primed and will just spin...when you have air that is blocked the rising water will not push the air out...it does not have the power..some boats are built with a low spot in the hose and is embedded in the boat where you cant get to it..then you get an intermittent bilge pump failure depending on whether water is in the hose...the fix for that is to drill a very small hole in the side of the bilge pump outlet and let the air escape as the water in the boat rises..you get a little spray out of the hole when the pump operates but what the hell does that matter?everything is wet back there anyway...the smallest bit in your collection is the right size to use..
i had to do that on the small boat...on the large boat i installed them myself so no problem...on the rare occasions when you do get some water in the bilge another tip is to use a small shop vac to get all the water out after the bilge pump does its thing...of course if you trailer a boat you dont have to worry about that...just pull the plug....

01-02-2014, 08:53 PM
Thruhulls should always be equipped with a seacock for safety and maintenance purposes. Be sure to mount them well above the waterline and reinforce the hull core, then seal, seal, seal! Discharge should be the same size as the pump discharge or larger.

01-03-2014, 03:59 PM
Roger that I will use a seacock on all the bilge pump thru hulls. Next question: I know i need to mount the thruhull above the waterline but can it be mounted to high up on the hull? or is it the higher the better?

01-03-2014, 05:46 PM
Mounting the thruhulls as high as possible is great in theory but has some problems in the real world. The higher you mount the thruhull in relation to the pump discharge, the more head (the measured distance the water has to go up hill to be discharged) the less gph the pump will move. If this distance is more than 3 feet, expect a 20% loss in volume, resulting in your 3700 gph pump only moving 3100 gph. The higher you go, the less it will pump. The second problem is the more hose you have the more water will drain back into the bilge when the pump stops, which causes a wet bilge= humidity and dry rot/ mildew. Lastly, you need to maintain reasonable access to the seacock/thruhull fittings. I don't want to tear the cabin apart to install or service them. Are you sure all of your compartments aren't bilt over a common bilge?

01-04-2014, 08:38 AM
that drain back is the purpose of the shop vac if the hull is good and you only get water in the boat from an occasion screwup etc..no common bilge pumps pump 100% of the water out either.. .i have a 2 gallon one and it stores handily in the box it came in...its also handy for cleaning the hard to get to places...one other tip...i dont have the brand handy but do not use the white pumps from walmart..especially the ones with the built in auto switch...bilge pumps are something that you pay for what you get....cheap is cheap..