thruhull depth

[mark hoesman]

I would like to install a thruhull exiting the boat from the bilge. This is appproximately 3.5'below the waterline. The purpose is for direct pumpoutout from the head, after the y valve. My question, will the head pump win the battle against the backpressure?

[Don Berger]

as long as the head is above the thru-hull. Assuming you will be direct-pumping while underway, the velocity of water passing the thru-hull will create a negative pressure.Don

[Peggie Hall/HeadMistress]

Otherwise, it should work both underway and at rest. Make sure that the discharge thru-hull is aft of the intake thru-hull...if the intake is aft of the discharge, you can recycle waste. It's essential that you install a vented loop in the head discharge line that's at least 8-12" above the waterline...and in the head intake line as well if any part of the toilet is below the waterline. In a manual toilet, it goes between the bowl and the pump, not in the intake line between the thru-hull and the pump.

[Vic Willman]

Head will work best if the discharge through-hull is "just below" the waterline of the boat (2-4"). It should work with the through-hull down near the keel, but will work best when it's up nearer the waterline - less outside head pressure to overcome.

[Peggie Hall/HeadMistress]

stinkpotters on boatered. :) You're forgetting that sailboats heel...some as much as 30 degrees. A head discharge thru-hull only a few inches below the waterline when the boat is at rest will put it well out of the water when heeled to the opposite side from the thru-hull...and will put it a foot more below the waterline when heeled on the opposite tack. So IMO, he's better off putting it low enough to keep it out of the water at max heel. Otherwise, if anyone's using the head when he overtakes another sailboat, "starboard!" can take on a whole new meaning! :)

[Jose Venegas]

First, the probability of having another boat close enough to see the spectacle while underway is minimal, unless the boat is near shore where discharging is illegal.Second, the deeper the thru-hull the greater the column of water you need to pump against when emptying the head, independent of its location. The head location above the waterline only affects (reduces) the pumping effort while pumping liquid but helps little while pumping air during emptying.Third, the deeper the thru-hull, the faster the boat will sink if it malfunctions.OTOH, the location of the intake thru-hull may present a problem while underway because it may suck air if the boat is heeled in the wrong direction.