Replaing Seacock Backing Plates

[jbenagh]

This spring, my launch was stopped short by a leaking sink drain backing plate. The adhesive had failed in about 25% of the plate where a small diesel leak had compromised the adhesive. A survey of all the other backing plates showed that the plywood was all pretty suspect. So this winter I would like to reaplce all the backing plates.
In the past spring, on the advice of a few boatyard interlopers, I replaced the Spartan seacock with a Groco which I'm not very happy with. Most of my Spartan seacocks are fine; I need to replace one due to numerous wear issues.
I'd like your advice on how to replace the seacocks. I've read Mainsail's great tutorial but I am looking for more specific experiences that you have.
I was unable to unscrew the sink seacock; I ended up grinding off the thru hull to remove the seacock. I'd prefer not to do that for all six additional ones this winter. It was a huge mess and I think I was creating more dust than I should have. I wondered if a Festool-like saw blade could cut off the thru hull cap allowing me to remove the threaded piece and the thru hull. Then I can put it on a vise at home and get good leverage to free up the seacock.
Mainsail's website recommends building up a fiberglass layup to make backing plates. What do people think of using the Coosa material to make backing plates? I think the cost is similar to material to make up FG plates and I think the strength is comparable to the plywood I have now. Otherwise I will follow Mainsail's process exactly, using epoxy to attach the plates to the hull.
I'll rebuild all the seacocks that are good enough, replace any that are not and replace all the thru hulls.
Is there any good reason to replace the Spartan seacocks with newer-style ones? The one I used for the galley is really difficult to operate and the height means I have to really bend the hose which makes me nervous about the tailstock strength. I assume I will run into similar problems on the other seacocks.
Jeff

[Ron M.]

I had the same experience - I removed all the seacocks with minimal effort except the galley sink cock. Could not get that son of a gun to break loose so I cut the exterior mushroom carefully without damaging the thru-hull. Replaced that one and engine intake with groco ball valves. All others brought into shop, cleaned , lapped and serviced.
I used 1/2" FRP for backing plates then re-installed.

[John Stone]

I removed all the sea clocks on the Far Reach and replaced them with Groco IBVF. Here is a link to the project log.

http://www.farreachvoyages.com/projects ... rline.html

[Maine_Buzzard]

Love the fiberglass backing plate replacement, I was too cheap to use MaineSail's exact technique.

I cut about 25 6-8" circles of cloth and laminated them on a sheet of waxed aluminum that had been curved to approximate the hull curvature. Careful stacking of cloth resulted in a flat valve surface and kept grinding to a minimum.



No need to make a huge flat sheet and then cut the corners off. The radiused edge is less likely to take a knuckle off.

If the Spartans are good, absolutely keep them. I replaced the engine seacock because of a hose size change. Will be doing the cockpit scuppers this winter.

[Jim Walsh]

Don't replace any Spartan seacock unless absolutely necessary. I needed to replace one a few years ago and mentioned it in the forum. Someone who had an old spare sold me an exact replacement. All I did was clean it up and reinstall. With regular service and a little luck those Spartan seacocks should outlast the boat. Mine are 31 years young and going strong.

[Cathy Monaghan]

Don't replace any Spartan seacock unless absolutely necessary. I needed to replace one a few years ago and mentioned it in the forum. Someone who had an old spare sold me an exact replacement. All I did was clean it up and reinstall. With regular service and a little luck those Spartan seacocks should outlast the boat. Mine are 31 years young and going strong.

I agree with Jim. Spartan seacocks, with proper maintenance, will last a very long time. If you do need to replace them and don't want to purchase new Spartans, make sure whatever you do purchase are either ALL BRONZE or ALL MARELON. Groco seacocks have stainless ball valves, not bronze.

-Cathy
CD32 Realization, #3
Rahway, NJ
Raritan Bay

[John Stone]

The Spartans are nice but very expensive. They can sometimes be lapped if they are leaking but if they are not routinely maintained than can become so scored they sometimes can't be saved. This was told to me by Spartan Marine. The nice thing about Groco IBFV two part seacocks is they can easily be replaced. The base is solid bronze. The ball valve and base have matching threads. If a ball valve goes bad, simple screw the ball valve off (the base is through bolted with the through hull threaded into it) and screw a new one on. I talked to the tech department at length about them. They sent me the test results. They far exceeded ABYC standards. I would only use Marlon if I had an aluminum boat.

[Keith]

I have a basic question about seacock backing plates. Why are they needed? There is no load like on deck hardware. My spartan seacocks appear to be factory installed and have no backing plates. I read on here that some sailor are having trouble with rotting and leaking wood backing plates. It seams that it is simply adding a weak link in the potential failure chain. So I ask again Why are they needed? Looking for feed back maybe I'm missing something.

Keith

[jbenagh]

The backing plates reduce the maximum force put on the hull at any one point by acting like a lever; the ratio of the radius of the backing plate to the radius of the outer edge of seacock is a force reducer. Also the added area spreads that force so that the pressure at any point is spread over the area of the plate.

It does surprise me that they are needed after seeing the 1/4 in plus hull thickness. Although the hull is not as rigid as you might think; have you seen it move from below when someone stands in a cockpit locker? It was a lot more than I would have expected. Perhaps if there were a bulkhead in close proximity it might be ok to forgo the backing plate.

I think the biggest weakness of the backing plates was the use of rot-susceptible material (marine ply is good but not perfect) and the available adhesives in the 1980's. MainSail recommends epoxy/glass plates but I notice that the Coosa BlueSea material has very similar mechanical properties to marine plywood and is dramatically cheaper and lighter than epoxy/glass. Epoxied to the hull, it should last forever or at least as long as the hull. I wonder if anyone has experience or other information on this product.

I've had a few commitments keep me from this project but plan to get going on it the next few weekends. I will post some pictures.

Jeff

[John Stone]

Keith,
Without a backing plate, the load is applied only to the area directly under the base of the hardware. Often it is not that much of an area. Add to that the lever arm applied to the seacock or the through hull as a result of some external hull strike and potentially the hardware could be ripped out. ABYC seacock test are pretty physical. Also, hulls are curved and the base of the seacock is not. The backing plate should be molded/shaped to the hull so the load is spread evenly and the hardware base has a flat surface to mate with. I epoxied the backing plates to the hull to make the whole thing stronger.

Plywood is not a great backing plate, it delaminates over time. A better/best backing plate is G10 premanufactured epoxy sheet. You can find it at McMaster Carr.

There may be other reasons but those are the ones that come to mind.

[Keith]

Hi Jeff thanks for the quick response. Not sure that I understand you explanation. A lever increase force proportional to the lever arms distance from the fulcrum not reduces force. I certainly understand spreading the load over a larger area but how much load are we talking about. There are 3 screws that are set in the grp on the outside of the hull it seem to me that that would be the area where the stress risers would be not on the inside of the hull where the load is already distributed by the triangular bronze plate.

Does anyone else have no backing plates on their seacocks? from the factory or otherwise?

[tjr818]

Hi Jeff thanks for the quick response. Not sure that I understand you explanation. A lever increase force proportional to the lever arms distance from the fulcrum not reduces force. I certainly understand spreading the load over a larger area but how much load are we talking about. There are 3 screws that are set in the grp on the outside of the hull it seem to me that that would be the area where the stress risers would be not on the inside of the hull where the load is already distributed by the triangular bronze plate.

Does anyone else have no backing plates on their seacocks? from the factory or otherwise?

Slante, our 1980 CD27 does not have backing plates. If I ever remove the through hulls I will install baking plates of G10 or whatever is favored at that time. I cannot envision any force from outside the hull that would affect the area we are talking about. The only force on the inside would be from the seacock levers, if they ever require so much force as to threaten the integrity of the hull, I think maintenance is LONG overdue. If we are trying to reduce the force on the hull, just how much bigger should those backing plates be

[jbenagh]

Keith,

regarding the physics: Picture yourself belly down on the hull, looking at the seacock. It looks like an inverted letter T with the tube the vertical part and the flange the horizontal part. I'm going to ignore the left side of the flange, so that it looks like the letter L. If something hits the top of the L, a torque is imparted on the L which will pivot about the corner of the L which is held on by the thru hull. The force transmitted to the tip of the horizontal leg of the L is reduced the longer that leg is; it's actully the reverse of the usual use of a lever since in this case the torque is constant so the force is reduced.

Now I understand that the there are a bunch of other factors like: it doesn't pivot around a zero-width point; the load on the flange is not at a single point but distributed; there will be elastic distortion; the left side of the flange that I ignored is attached via screws. But, to a first order, inceasing that distance using a backing plate reduces the force on the hull.

As pointed out, the effect of the plate in spreading the force over a larger area reducing the pressure on any given point of the hull is also very significant. The simplest estimate of the reduction being the ratio of area of the backing plate to the flange.

Jeff

[Keith]

Thanks Jeff,

As John stated above I will add backing plates if I ever need to change the seacocks I just don't see it as a critical must do project. Also as John state if there are heavy load from actuating your seacock lever then you are neglecting your preventive maintenance. As far as hull strikes are concerned that would either be a compressive load or a shearing load either of which a backing plate would provide no benefit. In any case I see backing plates as a belt and suspenders approach considering the thickness of our solid core hulls and the bolt pattern of the Spartan seacock. Finally no backing plate seems to be a better approach to the poor quality wood backing plates the so many owners have found to leak. It makes me wonder where/when the backing plates were added since many of our Cape Dory's didn't have then from the factory?

Keith

[adamganz]

Reviving this thread, which has been very helpful.

My CD28 has ply backing plates at all seacocks. The intake for the head has a spartan marin seacock in fine shape, but the backing plate is in rough shape - dry (which is good) splintering, etc. I unfortunately have very limited time between now and when the boat goes back in the water. My plan would be to try to replace the backing plate with minimal impact to the rest of the assembly. Does this make sense, and is this the right sequence:

1) Unscrew nuts from through ull bolts
2) Unscrew seacock
3) Remove backing plate
3) clean surface for new backing plate
4) install new backing plate
5) screw seacock back on to through hull
6) screw nuts back on to through hull bolts

....

How easy/quick can i make this?

Adam