Thru Hulls

[Gadabout Guinea]

I'm going to install a second bilge pump in my boat this year and need to install a new through hull for the discharge. I'm a big fan of 316 stainless steel, but I know that some people consider bronze to be the way to go in saltwater. There's also chrome over bronze too. I'm interested in people's opinions regarding material of construction for these little suckers.

Chris

[tsheehy]

Chris,

Gemlux (www.gemlux.com) makes some nice stainless ones if you go that route. They have a new model with a flap that looks interesting. I personally used a pair of the newer UV resistant plastic models (duralon?) because I need two of the 90* bend models that would have been too expensive in stainless.

Make sure you put an anti-siphon loop in the new bilge hose if at all possible. Get the best bilge hose you can, the kind which has a smooth bore on the inside and not the ridged type.

You also might want to consider mounting the second pump and switch a little higher off the bottom of the bilge to help prevent it from getting gunked up, and to keep both pumps from cycling every time you get water in there.

Hard wire one to each battery, and make sure that they both have their own fuse. And if possible, mount one thru-hull on each side of the hull, such that if the boat is listing to one side, the other thru-hull will be out of the water.

Try to use two of the same bilge pumps so that you can swap them around (if you mount one higher than the other) by just un-clipping them from their base. I also saw a slick setup where someone installed a bus bar in the bilge with ringed terminals to interconnect the pumps, switches and battery feeds such that they could easily troubleshoot/replace/repair a new pump or switch on the water.

-- Tom

[Go Fish]

Stainless and Bronze are appropriate for different applications in boats:

Stainless should be used ONLY in above water line locations. Stainless steel gets it's corrosion resistance from a thin layer of chromium oxide that forms on its surface with exposure to oxygen in air. In the absence of oxygen, the oxide layer does not re-form if it is damaged. Corrosion is likely to occur at the damage location.

Bronze can be used above or below the waterline because it is not dependant on oxygen for corrosion resistance. In fact, some copper alloys are even more chemically inert in the absence of oxygen.

If you don't mind the funky color bronze is the way to go for boats...but it just isn't as shiny as stainless...

[DaleH]

I'm going to install a second bilge pump in my boat this year and need to install a new through hull for the discharge.
Kev's right on about using SS 'above' the waterline. FWIW Wal*Mart sells Atwood brand of cast 316 thru-hulls for ~$15 each, a price that cannot be beat! If you need to buy on-line, best prices next would be from www.surplusunlimited.com , do a search on the term 'thru-hull'. Their prices are much cheaper than GemLux, who does sell great stuff, like SS thru-hulls but with 90-degree angles and such.

One GREAT tip - if you can do it - when adding a 2nd bilge pump. Plumb this discharge out the OTHER side of the boat, so you'll have pumping capacity (read: 'room') above the waterline even if/when the boat is heeled. It should aslo be on a separate circuit, switch, and battery than the main pump.

Also, that site mentioned above sells the 2200gph Johnson bilge pump using a 1-1/8" or bigger (I forget what size ...) discharge for < $70. It was rated THE best performing bilge pump that Powerboat Reports ever tested!

[Gadabout Guinea]

Tom,

I was considering that specific one from Gem. I have their rod holders and they seem to do a nice job on their stainless. I had taken most of your other suggestions into consideration. This is actually a 2X larger pump than what I run at the bottom of the bilge and I was planning on setting it up so that it will only go on if the first one gets submerged. I hadn't considered mounting it on the opposite side from the first one which isn't a bad idea. My fuel fill is over on that side so I have to see if I can get the hose around it without creating too much head.

I had never heard that explanation of bronze vs. stainless for corrosion resistance. Both copper and stainless form protective stable oxides which prevent further corrosion. There should be enough dissolved oxygen in seawater to provide for the thin oxide layer in either material. I suspect the difference is more related to their reactivity to the chlorine content of seawater. I'll have to consult the Oracle...

Dale,

I wondered about the quality of those fittings at WalMart. Are they really 316 stainless? They seem so cheap. When I click that link, I get my old buddy Andy at Chiller City in Arizona? I am running Johnson pumps, and did get pretty good prices online.

If it stops snowing every weekend near me I may actually get this on the boat soon.

[DaleH]

Link corrected ...

As far as price or SS alloy ... I'd bet $$ they're all coming out of the same factory regardless of label on them. Just be glad they're not marked 'Yamaha' or you'd pay $60 for them ;) haha! (Just my rant against their pricing policies ...)

[Go Fish]

...I'll have to consult the Oracle...



Be sure to ask the Oracle about the kinetics of the passivation rates between the two oxide types and the thermodynamics associated with high hydrogen overpotentials at copper bronze surfaces.

BTW, chloride is very important in stainless corrosion and has a lot to do with why it is bad below the waterline.

[DaleH]

... geek :eek: !