Seacocks and mushroom thru-hull fittings

We noted electrolisis on all the mushroom thru-hull fittings, significant enought to warrant removing all seacocks and thru-hull fitting for close inspection. I was curious to find that none of the thru-hull fittings had been bonded. Experts seem to be divided regarding whether to bond or not to bond thru-hulls. We found that the level of electrolisis increased as the distance from the propeller shaft zinc increased, with the head thru-hulls in the bow having the highest metal loss, the seacocks showed similar wastage. With the flange type seacocks, the mushroom thru-hull fitting passed from outside of the hull, through the hull and plywood backing block, screwing directly into the bottom of the seacock. The threaded portion of the mushroom thru-hull fittings were cut too short, engaging 2-4 threads of the seacock, water and subsequent corrosion wasted away remaining exposed threads, was a shame, considering 3 of the 4 1-1/2" seacocks were otherwise restored to like new condition. Despite conflicting opinion, you can bet, if its under water we’re going to bond it! The next step in fighting galvanic corrosion is to install a galvanic isolator to isolate the shore power ground.

Shamrock has Spartan Marine Hardware, conical valve bronze seacocks as installed at the factory when built, we order new replacement for all today from:

We found it less expensive to replace the Spartan seacocks rather than fill all the thru-bolt mounting holes in the hull, 2 per seacock and re-drill for Groco BV series. We don’t mind disassembling the Spartan valves and servicing them at haul out, sort of a boating ritual. Besides, it nice to look inside the valve and see whats going on inside, something you can’t do with a ball valve. With the ball valve, best you can do is wrestle off the hose and look down inside and check the condition of the ball, applying grease through the hose barb realy doesn’t work as the nylon socket tends to wipe the grease from the ball as it is opened and closed.

Servicing is simple, remove the locking nut opposit the handle, backing nut and plate and slide out the plug and attached handle, degrease, clean with white vinegar, re-grease and your done. If the fit is not smooth and clean, use a fine grade valve grinding compound coating on the plug, reinsert the plug and rotate the handle back and forth 30 times, remove and clean, re-check to see if you have a clean mating surface of the plug and seacock body, if so, clean throughly, grease and reassemble, ok, maybe now your done…

In a pinch, and with a good diver, the diver can insert a wooden plug in the thru-hull fitting while the boat is in the water, sealing off the water and the Spartan seacocks can be service without hauling the boat.

The Spartan standard 1-1/2" model with the straight 1-1/2" hose barb cast into the top of the seacock, saves on buying additional fittings to adapt to scupper or galley sink hose. The engine inlet uses the 3/4" standard model with a 3/4" cast in hose barb. The head inlet and discharge use a “seacock with adapter”, instead of a hose barb, it has a male threaded barb for attaching 90 deg or straight fitting with a locking collar, both the 1-1/2" head discharge and the 3/4" seawater inlet use the seacock with adapter and 90 degree fitting.

We really liked the Groco BV series valves and the BVH with integral straight hose barb, considered the best in the industry; we ran into a problem finding hose-pipe fitting from Groco and Buck Algonquin which screwed directly into the seacock that were 1-1/2 NPT and 1-1/2 hose, our supplier did not carry the BVH Series, we could have used bushings or adapters to make them work, but by the time you add all the fittings the cost quickly rises above the Spartan valves by a fair margin. The Groco BV/BVH models require no maintenance and can be rebuilt; although I haven’t seen anyone who carries the repair kits, willing to bet they are almost as expensive as the complete seacock, probably better off replacing when they no longer seal properly.

The best buy in mushroom thru-hull fittings for us was Groco, excellent machine work, list price is $23 for the 1-1/2 and $6.5 for the 3/4" model, our distributor Svendsen’s Marine sells them below list price.

Hope you find this useful.

Best wishes,

Marty Chin, BCC Shamrock

Why not consider doing what Hinckley and Swan do and installing Marelon seacocks . I had a similar experience with well bonded seacocks.

Jim Hiller

As noted earlier the division of opinion regarding underwater metal bonding, to date Shamrock has not been bonded, everything including propeller, propeller shaft, seacocks, thru-hull fittings have shown signs of significant electrolsis. No, the parts were not corroding off and the boat was far from sinking. We are talking about pitting or metal wastage, not yet to a level to be considered unsafe, but not worth reassembling in their present condition. With the exception of the propeller shaft, which had pitting inside the cutlass bearing, which would have eventually damaged a good bearing, was replaced primarily due to being bent; if the shaft was not bent, it would have been replaced due to scoring and pitting in the cutlass area. To strech the point, with the exception of the head inlet and outlet seacock which had obvious electrolosis issues, the other seacocks, if not removed, may have lasted 20 years. We have only owned this boat since last June 2004, so we do not have a history of how often the zincs have been changed or how often if ever the seacocks were disassembled cleaned, lapped, greased.

I am told this boat sat dormant for many years, as is often the case, the zincs may have not been changed before the old one completely wasted away. Each period of time where a zinc was not present deducts from the hypothetical 20 lifespan of underwater parts. In addition, if the boat had not been hauled every two years, seacocks disassembled, cleaned and greased, this would have definately shortened their lifespan.

Seawater although less conductive than freshwater, is still a good conductor. The theory of bonding thru-hull is to electrically connect all thru-hulls together with a less nobel metal (zinc), so the zinc wastes away instead of the bronze thru-hulls. If water is a good conductor, therefore connecting all bronze thru-hulls, propeller shaft and zinc, it would seem running bonding wires inside the boat would be redundant as the water simulates the bonding wire. There are a good many boats in our area that have never been bonded with a wire, properly zinc and show now signs of electrolisis, I believe on time replacement of the zinc to be the real issue and in the case of cone valve seacocks consistent and proper maintenance to be the cure for electrolisis.

With regard to the question of Maralon (synthetic, for lack of better description, plastic) thru-hull fittings and seacocks, they seem to work well, darned if we can’t find a way from keeping them from binding and breaking off handles.

Water tanks to fuel tanks: We service a lot of boats in the San Francisco Bay Area, number one issue is high volume storage of diesel fuel and low consumption diesel engines. For recreational boaters, low fuel consumption and large tankage results in the collection of a lot of old fuel, old fuel performs poorly, subject to varnish, partially filled tanks lead to condensation and algae, fungus, bacteria growth.

Once converted to fuel storage, the aft water tank can no longer be used to store water; in our case we already have a 25 gallon main fuel tank behind the engine,in one year use, we only managed to burn off 12 gallons of fuel, it would appear that we sail more than we motor. If we convert the aft water tank to fuel, we would have to pump it dry when not cruising to minimize the volume of old fuel being stored. We noticed early on that empty water tanks make the boat somewhat tender, initial stability, we always fill both water tanks before sailing into rough weather, she settles down as solid as a rock.

We sailed on another boat to Hawaii 2 years ago, we carried a full tank of water, 8 sun-shower bladders full and 2 jerry cans of reserve water, 2 jerry cans of fuel, we arrived with 1/2 tank of water and fuel onboard. We motored 5 days out of 18 due to light Northeast Trade Winds and electrical generation. We had a very frugal crew onboard, but if given the choice, I would rather had more water than fuel.

Watermaker: We have some friends who recently returned from 2 years in Mexico, they installed a small watermaker just before departing. They were extreamly pleased to be on the hook making their own water rather than rowing back and forth carring jugs of water. When they returned, they sold the watermaker, condidering the price paid and the funds recovered from the sale, the net cost was far less than the cost of purchased water, time and fuel they would have expended in search of water. I don’t remember the unit they used, but judging from the space where they had it installed, it could have easily been installed behind either the port or starboard salon seat back.

While in favor of a small water maker, I would never consider reducing water tankage to make room for one. If the darn thing quit working and I removed a tank or converted it to carry fuel, I would have nowhere to put water.

Marty Chin, BCC Shamrock

For what it’s worth, Whitewings is completely unbonded. If I understand
galvanic corrosion correctly, you need a complete circuit for it to
occur. As all my seacocks are attached to rubber hoses, there doesn’t seem
to be a way, short of all hoses being full of water and interconnected, for
current to flow. Nigel Calder seems to support that approach and claims no
problems after years in the salt.
I do have a galvanic isolator in the shorepower system as that is an
obvious circuit through propshaft, engine, etc., if someone is dumping
stray current into the marina. A Nordic Tug across the dock from me was
doing exactly that two years ago and his underwater metals were basically
junk after one season. Aside from the usual Lake Michigan lime buildup, my
boat was fine and has stayed that way for three seasons.


If you’re a fisherman or woman needing earthworms to catch dinner, short of going to the bait store, here is another option.

Wait until it rains or wet your lawn overnight. Now, take a car battery, (2) 8 gauge cables, (2) 3’ pieces of construction rebar and go out to your lawn.

Don’t forget the hammer, drive the rebar stakes 24’ apart and connect the negative cable to one of the bars and then to the negative post of your battery, do the same for the positive post. Within minutes earth worms burried in the ground between the two posts will begin to emerge, unharmed of course, now your ready to go fishing. The moisture in the soil act as a wire (conductor) between the two bars.

Same principal applies to thru-hull fittings, even if connected inside the boat with rubber hoses, weather bonded with wire internally or not, if not wired, the water around the hull becomes the conductor. At least one major metal component, in this case the propeller shaft has a zinc attached we should be ok.

Marty Chin, BCC Shamrock


I agree, salt water becomes the ionic bridge between two metal conductors but unless the two pieces of metal are connected there is no current flow.

In the example cited, if there were no conductor between the two rebar stakes, the worms would stay in the ground. Even if, the two stakes were connected with a wire, there would be no current flow because the two stakes have the same electro-negative potential. By connecting a battery between the stakes, you are introducing an anode and cathode into the system and current flow occurs between the two stakes. A similar principle has been tested to reclaim soil that was contaminated with metals.

In the case of the prop and prop shaft, the zinc is the anode, the bronze/steel is the cathode and the salt water becomes the ionic bridge. Because the zinc is in contact with the bronze/steel, and has a lower electro negative potential than the cathode - prop and prop shaft, the zinc’s mass decreases as it is converted to ionic zinc and your prop and prop shaft are protected.

I believe the most common cause of galvanic corrosion in boats is plugging into shore power. Because the green wire is common to all boats connected to shore power, zinc anodes on all the boats become common. Therefore, if a boat next to yours does not have a zinc, the zinc on your boat also protects the boat next to you. Hence, the zinc on your boat is depleted faster than expected and galvanic corrosion occurs. This cause of galvanic corrosion can be prevented by installing a galvanic isolator or isolation transformer in the shore power circuit of your boat.

I have a basic understanding of this subject from my electro chemistry course work but would not consider myself an expert on the subject.


The best way to protect your boat is with an isolation transformer - which has a floating ground.


Kirk McAnsh