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SEACOCKS and THRU-HULLS

Cleaning the space under one of the floor boards, I lightly supported myself on one of the seacocks.

The hose connection part broke and water started to flush inside at an alarming rate! HELP!

The seacock in questions is for draining the cockpit and is 1,5" in diameter. Water was therefopre rushing in at a speed of maybe 400 litres an hour. It was fortunate that this happened while we were in a marina and that I was aboard myself. I managed to temporarily block most of the rushing water, then I dived into the water and properly plugged the intake from the outside using a large wooden bung.

There are 12 holes through Scorpio's hull under the water line. One is for the propeller shaft and an other is for the rudder shaft, while the other 10 are designed to let water in or out. I have allways wondered why they build so many holes in boats. In principle I guess two holes would be enough, one for taking in water and one for draining. Or maybe you would have two hole for the outlets, separating sewage from cleaner water, but that would add up to only 3 holes instead of 10!

Scorpio's seacocks are Blake's of bronze. The body is cast bronze. Modern Blake seacocks look like this:

Blake today exploded view

However, Scorpio's seacocks are manufactured in the 70's. They differ from the one pictured as follows:
a) The hose connection on our larger seacocks (2" and 1") is not integrated into the body for a direct connection of the hose. Instead, the body connection is threaded on the outside, where a hose barb is then attached (photo below, left).
b) We have no plate on the outside of the hull, like the one shown in the exploded view above. Instead the base plate (flange) is laminated into the hull (photo below, right).
c) There is no grease nipple on any of the older seacocks.

old Blake with threaded hose connection engine intake
Stripped 2" old Blake with threads for
hose connection (cockpit drain)
1" Blake with threaded connection and hose connector
fastened, hose off (engine water intake)

A closer inspection revealed that the hose connector had broken. The photo below, left, shows the nut part (red #2) still screwed on to the body, with the pipe part (red #1) gone. (Click the image for a larger picture.) The photo in the middle shows the nut part (broken) after removal, and in my hand, right, is all that was left of the pipe part of the connector.

broken connection nut part parts of pipe

I then took apart the other 2" cockpit drain seacock and the 1" seacock for taking in cooling water to the diesel engine and discovered that the same thing was about to happen to both of them also. The photo below (left) shows that a part of the pipe had broken, but fortunately not at the joint between the pipe and the nut, as on the larger cockpit drain seacock. The remaining seacocks are of smaller size where the whole piece, including the hose connection is cast bronze. One of them I had already replaced two years earlier with a ball valve type seacock (story at the bottom of this page).

hase connection

seacock location

The cockpit drain seacock is
easily accessible under the floor
boards in the master cabin

repaired seacock

Here is the repaired cockpit drain
seacock. The material of the new
hose barb connection is bronze

What had happened this time?

We obviously have a case (actually three) of galvanic corrosion. The bodies of the seacocks are of (corrosion resistant) bronze, but the screwed-on hose connectors were probably brass. The less 'noble' brass is an alloy of copper and zinc. During the corrosion process the zinc is dissolved leaving only porous copper. Bronze is also an alloy consisting primarily of copper with, usually, tin as the main additive, but bronze is more corrosion resistant than, for instance, steel and, particularly, brass. ABYC and CE standards specificially state that below the waterline all metals have to be galvanically compatible. Brass and bronze are not galvanically compatible as brass is a much less "noble" metal than bronze. Even though the differences in potential are small, corrosion goes on for 24 hours a day and is faster in warmer waters.

At this stage it is impossible to find out if the brass connectors were fitted already with the new yacht in 1979 or if they have been added later. The fact that all three were brass, however, indicates to me that they were original. We will never know either wether the hose connectors were supplied together with the seacocks or fitted by the yard.

Cast bronze can also brake!

Once a year during haul out I use to strip the cones off the seacocks for cleaning and lubrication. Sometimes, if I have forgotten to turn the handle once in awhile during the season, a cone may jam. The best way to get it out is to drive it from the outside using a round piece of hard wood as a drift. It would be better if the force is directed only to the lower end of the cone and not to the top. A couple of years ago I used a metal pipe which caused the top to break.

cone top blew off valve body cut off
Beware of too much force
directed right at the top
The hose connector (which in this case is a part of the cast body) had to be cut off first to allow access for the cutting of the lower part off from the base. The replacement ball valve
seacock and thru-hull
after installation

The cone is cast in one piece and therefore the damage seen above is irrepairable. On the other hand, it is possible that wall thickness of some parts of the cone above has been left too thin during casting, so maybe it was a good thing this happened. However, because the cone was impossible to repair I had to replace the whole seacock. In our case the flange of the seacock is laminated into the fibre glass hull, as I have explained above, which makes the job a bit more challenging.

This particular seacock is for the drain of the kitchen sink and it is easily accessible in Scorpio's deep bilge. Therefore I decided to use an angle cutter to cut off the old seacock body flush with the inside of the hull, leaving the laminated part as it was. This solution would be impossible in the case of one of the less accessible seacocks. Then we just installed a thru-hull ('mushroom-head') fitting, bedded with Sikaflex, and connected it to a Groco ball valve seacock, all parts bronze. The old in-glassed bronze flange adds to the strength of this installation. Connecting parts like this it is important to make sure that only straight NPS thread (not tapered NPT) is used. In any case do not use NPS and NPT thread together.

Some additional observations

Today you can find CE certified plastic marine seacocks and thru-hulls on the market. The certification indicates that these are considered strong enough for these applications and they do not corrode at all.

As mentioned above, the old Blakes do not have a grease nipple, but grease nipples could probably be added to them. Regarding grease in general, it is important not to use graffite based grease because it contains carbon fibre. Carbon is high on the galvanic scale and as a result of this corrosion will occur on metals in contact with such grease.

One big problem regarding the present issue is that on the surface it is often difficult to tell bronze and brass apart. The material used for various fittings often have misleading names, for example Admiralty brass, Naval brass, Tobin bronze, Muntz metal are all brass and should not be used in salt water. Blake seacocks are today apparently made from something called inhibited brass, which has additives making them corrosion proof. Such fittings should be marked DZR (for de-zincification resistant). A detail, which could give a rough indication wether a material is bronze is an uneven surface, as bronze is cast in sand. Casting molds for brass are made of metal, resulting in a smooth surface. Note, however, that bronze can also be given a smooth and polished finish.

I recommend kicking and hitting the seacocks and other below-waterline metal fittings every time the yacht is out of the water. Take a tour around the boat and use force. If anything breaks it is preferable when it happens on dry land.

Some interesting outside links:

Brass vs. bronze
More about galvanic corrosion
Grease basics

Quality bronze seacocks and tru-hull fittings
Certified plastic marine seacocks and thru-hull fittings

[During my seacock project I was able to consult Mr Lars Ström, former R&D Manager at Nautor's Swan, who provided some of the information above and pointed me in the right direction.]


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