Replacement of Throughulls and Seackocks

Performed: Summer 2015
Edited: 10/03/2016

It is a well know fact that thoughulls and seackocks, especially those under water, should be made material and quality suitable for this environment. The most common material is bronze. There are brass alloys which are resistant to corrosion marked as DSR. The one suitable for use on boats is called CZ132 or CW602N (the number of the European standard). And then there are the composite materials such as Marlon.
Nevertheless on Lyra, like most European mass production boats, the original fittings are made of plain brass. I was not aware until lately that the European standards specify that the fittings must be resistant to the environment for 5 years only, so the manufacturers in their efforts to reduce costs install plain brass fittings which will most probably hold for these five years. When I bought Lyra she was 5 years old and everything looked just OK, but as the years passed signs of corrosion began to appear on the surface.
In addition, the installation is minimal - a hole in the boat, the throughull and a tightening nut, meaning that all mechanical load is concentrated on a rather small area.

The original fitting

When the time came for the last haulout I felt that the situation was that of an accident waiting to happen and decided to replace the underwater throughulls, and this time do it properly by selecting quality parts and also installing a backing plate to distribute the load.
Fortunately a friend was facing a similar situation on his boat, so we decided to do the work together. This proved later to be essential, or at least something that made things much easier, since there are several operations which require a person in and out of the boat at the same time.
I selected Grocco bronze fittings. These are flange type fittings which have the benefit of increased load bearing surface. Theoretically they require drilling 3 mounting holes through the hull, however Grocco also supplies special backing plates with the required threaded inserts which provide the larger load distribution area, eliminate the need to drill mounting holes in the body and increase the load bearing surface.

Installation Procedure

Remove old fittings

This part went with no special problem, apart form the fact that while applying force to dismantle one of te elbows it broke and the dizincification could be clearly seen. Then all holes should be thoroughly cleaned form the old sealing compound.

The broken part and the dizincification

Surface preparation

The backing plates require a much larger flat area, so the surface is sanded with a random orbital sander. This is preferably done with one that can be attached to a vacuum cleaner since a lot of dust is created in the process (I did nt have one and there was a lot of cleaning to be done later - luckily most of it is confined to the space under the sink and the holding tank area).

Sanded surface

Surface Adjustments

At the outside of the hull the throughull should lay flat  against the surface and touch the hull throughout its whole perimeter. With the original throughull and its small diameter nut it is not a problem, but with a thick and wide backing plate, it is the one that dictates the angle, so it must be parallel as much as possible to the outside surface. This is a trial and error process which is best done by two persons, in and out of the boat. The process is sanding, screwing in the throughull, checking and reporting to the person inside where the throughull does not touch the surface. One just has to remember that the orientation inside and outside are mirrored so if the throughull is not touching at "2 o'clock" the person outside should report "10 o'clock".

Dry Fitting

The flange is fastened to the backing plate as well as the seacock and fittings and the whole assembly installed with the throughull  to make sure that everything is ok. Make an alignment mark on the hull and the backing plate so that it is epoxied in the same orientation.

Prepare the Parts for Epoxy

All throughull parts are covered with wax to prevent the epoxy from sticking to them. I assume that there are purpose made releasing agents, however furniture wax performs well for this task.

Waxed parts

Prepare the Epoxy

Epoxy should be mixed according to manufacturer instructions and colloidal silica (also known as Aerosil or Cabosil) added to get it to a consistency of peanut butter.

Ready for epoxying


Spread the epoxy generously over the backing plate and the hull, put in place according to the alignment mark and tighten with the throughull. Clean excess epoxy. Epoxy generally cures within several hour but better wait for 24 hours.

Final Assembly

Dismantle parts from the backing plate and remove wax - hot water and soap with a final touch of Acetone did the work. Spread generously sealing compound on the throughull head surface, the outer entry to the flange thread and the inner flange surface. Tighten throughull and bolts. Clean excess compound. Note that this whole assembly is slightly larger and taller than the original assembly. This means that in a tight space (like in Lyra) you will have to pre assemble the seacock and elbow on the flange because it will not be possible to screw them on later. In Lyra I also had to cut a bit into the plastic of the locker door because the handle is slightly longer (it is also possible to cut the handle a bit shorter). Wait for the compound to cure - again generally 24 hours.

Assembled - in and out

All that is left is to put the boat back in the water. When in the water keep it on the crane straps and check for leaks - first between the throughull and the hull and then open the seacock and check threads, handle mechanism and connection to pipes.