Plenty of horror stories about boats sinking from having the wrong type of under water fittings and valves so I did heaps of reading.
I found this useful: http://www.pbase.com/mainecruising/seacock_primer&page=1
It appears that in the European Community's Recreational Craft Directive (RCD) ISO Standard 9093-1 was rewritten (1998) to require only that fittings should be corrosion resistant for 5 years and so various boatbuilders over there have used brass rather than bronze. However, brass does corrode so bronze is the way to go. I don't think the US standards allow brass but the European standard created plenty of controversy.
This is the type of thru-hull which I wanted to buy: It is cast bronze with large flange so it is supported strongly on the inside. http://bla.com.au/default/seacock-1.html?___SID=U
However, I decided to go with a solid bronze thru-hull fitting and a bronze valve. Doesn't have the flange for strength like the better design but more affordable. Found my usual mail-order chandlery had the fitting and valves (but not the more expensive 'proper' seacocks.)
When I purchased the bronze fittings, via mail as nothing like that in town, I had a fiddle with it and found a problem. When I screwed the thru-hull section into the valve, it only went on about four turns. That is not enough to be strong or provide a proper seal. I was shocked!
On the left in the pic is the valve screwed onto the fitting with masking tape on the fitting up to the valve. On the right is with the valve off and just how little thread went into the valve.
- fitting.jpg (38.85 KiB) Viewed 2111 times
I did some more reading. I found that industrial type ball valves typically have NPT (USA) pipe threaded openings which are tapered. I found that the thru-fittings are typically NOT tapered but have parallel thread. They should not be used together as it is not safe.
See, for example, Professional Boatbuilder, the standards for seacocks http://www.proboat.com/attention-to-detail-april-1-2010.html
"Here is a through-hull fitting attached to an inline ball valve—two components that were never designed to be attached to each other. They do not constitute a proper seacock, and are not compliant with ABYC standards. Because the ball valve’s threads are tapered, and the through-hull’s are straight, they do not properly interface, and make a weak joint at best. Moreover, seacocks are typically flanged to provide adequate load distribution, and on FRP vessels, they often rest on encapsulated marine plywood, GPO 3, or some other suitable composite material backing block."
I contacted the chandlery I had purchased the fittings from, they seemed to think there was a problem with the threads but the threads did not look damaged. Maybe some tapered thread valves had got into the batch of what should be parallel thread valves?
For more on the issue of thread differences, see: http://www.pbase.com/mainecruising/seacock_primer&page=2
So, I returned the valves, got my money back except for my postage, and ordered a Marelon valve from somewhere else.
The Boat US Foundation says: "The valves and fittings must be made of bronze or Marelon®, which are not likely to break when struck accidentally with a foot or anchor. (RC Marine's Marelon® seacocks are the only plastic seacocks that meet the requirements of Underwriters Laboratories.)" http://www.boatus.com/foundation/guide/boat_7.html
The Professional Boatbuilder says: "Hardware throughout the raw-water system should be fully compatible with exposure to seawater. Usually this means bronze (brass should never be installed, because of its zinc content); however, proprietary glass-reinforced plastics such as Marelon are also well suited to seacock and raw-water applications." http://www.proboat.com/attention-to-detail-april-1-2010.html
Hopefully, my new thru-hull and valve will never give me any problems. I sure had enough problems trying to work out a suitable replacement for the old ones.
troppo