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Compass Marine How To | all galleries >> Welcome To >> Seacock & Thru-Hull Primer/Pre Information > Preferred & Non-Preferred Installations
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Preferred & Non-Preferred Installations

The example on the left side of the photo is the preferred method for installing a proper seacock. The preferred method includes a wide base flange for strength and the ability to through-bolt or screw the flange to the boat.

The example on the right side of the photo is the non-preferred method of installation and has a number of down falls.

The most obvious weakness is the fact that there is nothing to keep the thru-hull from possibly twisting when turning the handle to open of close the valve. The only thing holding this thru-hull to the boat is some marine sealant and that very small lock nut. The second issue, is not so obvious, but, still just as scary. A thru-hull fitting is NPS or straight thread and the bronze ball valve is NPT or tapered thread. This installation can cause an improper mismatching of thread types which you will read about bellow.

The third major issue is strength. Simply threading a ball valve onto a thru-hull fitting, without a flange, creates a weak point in which the thru-hull could possibly snap off or break if anything substantial were to hit it in rough seas. A mishap like this happened to me when a spare alternator fell off a shelf in rough weather and cracked my thru-hull fitting. This is part of the reason I wrote the blog about installing proper seacocks and this primer to show why to do it the right way.

Nikon D200
1/60s f/7.1 at 52.0mm iso800 full exif

other sizes: small medium large auto
Sven 08-Nov-2017 13:20
Folks, question from a novice... The thru hull fitting on the right - Why cant you add a "large bronze washer" below the nut to add in stability?
Guest 16-May-2012 04:50
Can I use stainless hardware to mount the flanged mount? Or should it be brass?
Guest 16-Aug-2009 15:15
WOW!!! OUTSTANDING presentation thank you for your time and effort, you've saved my life and that of my boat. With these instructions anyone can replace their seacocks and do it right! Thanks again
Guy Williamson 18-Nov-2008 21:58
How do I attach the wide base flange. Your example shows bolts coming from underneath. I prefer not to put any additional holes in my hull.
Guest 24-Sep-2008 15:25
If this is so bad why do most production boats use this method?
skagittom09-Jun-2008 17:43
Apologies for not being directly related, but I feel desparate. My 14 year old Pacific Seacraft has Apollo "bronze" flanged seacocks throughout. Great! My big problem is with the drain plugs. They appear to be of a different metallugical content than the body. (Even in the pictures of the Groco seacocks, the drain plugs like brighter than the bodies they're screwed into.)
My research into electrolysis tells me I have two dissimilar metals in a seawater electrolyte. hmmm! My observation is one of the drainplugs is 40% wasted away and two are not. (Those two do show a little brightness on their ends.) Also those two are much closer to the prop zinc and the zinc at the bottom of rudder (3ft as compared to 15ft).
I've been thinking about hanging a grouper/guppy zinc over the side, connecting the other end to the engine block (?).
Do you have any thoughts about my little dilemmas?
Thank you very much in advance.
Guest 29-Feb-2008 19:13
I think the first line should read "The example on the LEFT...."
An excellent article.