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Sabado · 06-10-2023, 11:34

I’ve reached out to a “corrosion” guy but in the meantime I’m trying to get some clarification on what I was told today.

I was told that installing Groco bronze thru hulls on a fiberglass catamaran would lead to failed sail drives due to corrosion.

I’ve been under the belief that Groco are a very good thru hull to use. Am I wrong?

**s/v Jedi** · 06-10-2023, 11:49

Quote:

Originally Posted by Sabado

I’ve reached out to a “corrosion” guy but in the meantime I’m trying to get some clarification on what I was told today.

I was told that installing Groco bronze thru hulls on a fiberglass catamaran would lead to failed sail drives due to corrosion.

I’ve been under the belief that Groco are a very good thru hull to use. Am I wrong?

Everything is possible. If you connect shore power live to the sail drive and shore power neutral to the thru-hull fitting then you can probably see it in action.

So it all depends on how things are installed. First thing to do is find someone else to do the job because this one is talking crazy.

**Pete7** · 06-10-2023, 11:49

This could be an interesting discussion. Is this because in the US its popular to wire all the through hulls together, whilst in Europe we don't bother. Mine aren't. The only parts wired together are the rudder shaft and P bracket to the anode, along with the 12 and negative and 240v earth wires. The other 5 through hulls are isolated.

Was there a particular make of sail drive you were interested in? I thought Volvo used isolated sail drives so there is no connection from the engine block to the sail drive.

Pete

Sabado · 06-10-2023, 11:53

Quote:

Originally Posted by Pete7

This could be an interesting discussion. Is this because in the US it is a popular habit to wire all the through hulls together, whilst in Europe we don't bother. Mine aren't. The only parts wired together are the rudder shaft and P bracket to the anode, along with the 12 and negative and 240v earth wires. The other 5 through hulls are isolated.

Was there a particular make of sail drive you were interested in? I thought Volvo used isolated sail drives so there is no connection from the engine block to the sail drive.

Pete

It’s because I was told it’s a bad idea to install bronze thru hulls because I’ll need to replace both saildrives relatively soon because of corrosion.

**Pete7** · 06-10-2023, 12:17

I think Jedi's hint is there might be something else going on rather than the make of through hulls causing problems.

Sabado · 06-10-2023, 12:19

Quote:

Originally Posted by Pete7

I think Jedi's hint is there might be something else going on rather than the make of through hulls causing problems.

There isn’t a problem just the threat of one if Groco bronze thru hulls are used.

**Pete7** · 06-10-2023, 12:34

Struggling to see how an isolated piece of bronze some feet away will affect the sail drive. However, if its electrically connected to other objects and perhaps a hull anode, then an electrical leak from anything could cause a problem.

If you fit say TruDesign and the problem doesn't occur on the new drives, you might assume that the problem in the past was bronze fittings. However, that might or might not be true, it could be that since plastic doesn't conduct electricity the problem can't occur, not that the problem was bronze.

I think you are right to call in an expert to see what is going on given the prices of sail drives.

Sabado · 06-10-2023, 12:50

Quote:

Originally Posted by Pete7

Struggling to see how an isolated piece of bronze some feet away will affect the sail drive. However, if its electrically connected to other objects and perhaps a hull anode, then an electrical leak from anything could cause a problem.

If you fit say TruDesign and the problem doesn't occur on the new drives, you might assume that the problem in the past was bronze fittings. However, that might or might not be true, it could be that since plastic doesn't conduct electricity the problem can't occur, not that the problem was bronze.

I think you are right to call in an expert to see what is going on given the prices of sail drives.

Yeah, it isn’t a problem we’re having just being told on a different forum that bronze thru hulls are a very bad idea to install. I’ve just never heard that before.

Fishspearit · 06-10-2023, 12:51

Quote:

Originally Posted by Sabado

I was told that installing Groco bronze thru hulls on a fiberglass catamaran would lead to failed sail drives due to corrosion.

No, that's not true.

The anode on the sail drive will protect the sail drive from galvanic corrosion, no matter what type of through hulls you have.

Ideally your sail drive is isolated electrically. The one scenario where there can be problems is when all the bronze through hulls are bonded and the sail drive is part of the bonding system with no other external anode besides the one on the sail drive. In this case the sail drive anode is being forced to protect everything and it's not designed for it, it's only designed to protect the sail drive and will be consumed rapidly.

Sabado · 06-10-2023, 14:18

I’m copying his message below because it seems to go against everything I’ve read or been told about thru hulls. If this person is correct, I need to completely reeducate myself on thru hull best practices. Not only should I not use high end bronze but I should not use backing plates.

I wouldn’t normally post a message from another forum but I just feel this is really bad advice but I’m looking for more input.

—————message———-
So a few issues that many seem to not understand or think about when talking about thru-hulls on an L42. Firstly there are a ton of them. You are probably talking at around 20 holes in the bottom of your boat depending on options and equipment. Secondly the factory doesnt put backing plates on thru-hulls for a reason. The hull of a catamaran has a ton of flex, putting a rigid thick backing plate is either going to cause elongation of the thru-hull or a leak over time. Something like a thin fiberglass plate glassed to the inside of the hull to fair is fine, anything more then that or something you are going to 'goop' in is a bad idea.

There is nothing pulling on the outside of the thru hull why does it need a backing plate?

"Upgrading" to a groco thru-hull is equally a bad idea. You are doing it in an effort to stop corrosion of the thru hull, in doing so you are tossing a high copper content bronze anode(20 of them) to the underside of a boat that you know better then to put copper paint on the bottom of. So in 'upgrading' congrats you created a ton of galvanic action and just damaged your sail drives.

———cut of a ramble regarding a slush box———-

If you are going to stick with just normal thru-hulls i prefer a simple nylon thruhull to a barb fitting. From the barb fitting inside, place a short clear re-enforced hose that leads to a barbed on each end Stainless ball valve mounted to a bilge board, that then leads to wherever. Double clamp everything. place a wood plug tied to the thru-hull at each location according to the size of the thru-hull. This stops galvanic action, maintains hull integrity, gives the most flexible type of fitting, gives a sight glass for flow to every thru-hull, removes the need to have a backing plate in the first place as you just move the ball valve off of the thru-hull itself to a much stronger bulkhead.

Not only does your SD-60 manual say to avoid using copper paint they also state to consult a marine electrical engineer before changing the material composition of your propeller. People can do whatever they like of course. I spend every day in/on/around catamarans, I sell them, I outfit them. I went to school for a long time to specialize in this field. I have patents in this field directly related. Im strictly sharing information based on real world knowledge and problem solving many of the common issues found on these boats. If you put anything copper on the bottom of these boats you are going to have internal issues with your sail drive sooner than later. You dont have to believe that, but when you run into issues I usually try to keep at least 4 SD-60 units in stock at all times. Happy to price them for you in a few years when you are ready.

**s/v Jedi** · 06-10-2023, 16:09

Quote:

Originally Posted by Sabado

I’m copying his message below because it seems to go against everything I’ve read or been told about thru hulls. If this person is correct, I need to completely reeducate myself on thru hull best practices. Not only should I not use high end bronze but I should not use backing plates.

I wouldn’t normally post a message from another forum but I just feel this is really bad advice but I’m looking for more input.

—————message———-
So a few issues that many seem to not understand or think about when talking about thru-hulls on an L42. Firstly there are a ton of them. You are probably talking at around 20 holes in the bottom of your boat depending on options and equipment. Secondly the factory doesnt put backing plates on thru-hulls for a reason. The hull of a catamaran has a ton of flex, putting a rigid thick backing plate is either going to cause elongation of the thru-hull or a leak over time. Something like a thin fiberglass plate glassed to the inside of the hull to fair is fine, anything more then that or something you are going to 'goop' in is a bad idea.

There is nothing pulling on the outside of the thru hull why does it need a backing plate?

"Upgrading" to a groco thru-hull is equally a bad idea. You are doing it in an effort to stop corrosion of the thru hull, in doing so you are tossing a high copper content bronze anode(20 of them) to the underside of a boat that you know better then to put copper paint on the bottom of. So in 'upgrading' congrats you created a ton of galvanic action and just damaged your sail drives.

———cut of a ramble regarding a slush box———-

If you are going to stick with just normal thru-hulls i prefer a simple nylon thruhull to a barb fitting. From the barb fitting inside, place a short clear re-enforced hose that leads to a barbed on each end Stainless ball valve mounted to a bilge board, that then leads to wherever. Double clamp everything. place a wood plug tied to the thru-hull at each location according to the size of the thru-hull. This stops galvanic action, maintains hull integrity, gives the most flexible type of fitting, gives a sight glass for flow to every thru-hull, removes the need to have a backing plate in the first place as you just move the ball valve off of the thru-hull itself to a much stronger bulkhead.

Not only does your SD-60 manual say to avoid using copper paint they also state to consult a marine electrical engineer before changing the material composition of your propeller. People can do whatever they like of course. I spend every day in/on/around catamarans, I sell them, I outfit them. I went to school for a long time to specialize in this field. I have patents in this field directly related. Im strictly sharing information based on real world knowledge and problem solving many of the common issues found on these boats. If you put anything copper on the bottom of these boats you are going to have internal issues with your sail drive sooner than later. You dont have to believe that, but when you run into issues I usually try to keep at least 4 SD-60 units in stock at all times. Happy to price them for you in a few years when you are ready.

This person isn’t qualified to post that but it’s the Internet and fora are just for entertainment so I guess that’s what it is to him.

Nylon thru-hull fittings should never be used below the waterline. Unless your hull is aluminum, you can use a copper based antifouling. No, don’t use it on the sail drive.

SailingHarmonie · 06-10-2023, 17:07

Quote:

Originally Posted by Sabado

I’m copying his message below because it seems to go against everything I’ve read or been told about thru hulls. If this person is correct, I need to completely reeducate myself on thru hull best practices. Not only should I not use high end bronze but I should not use backing plates.

I wouldn’t normally post a message from another forum but I just feel this is really bad advice but I’m looking for more input.

—————message———-
So a few issues that many seem to not understand or think about when talking about thru-hulls on an L42. Firstly there are a ton of them. You are probably talking at around 20 holes in the bottom of your boat depending on options and equipment. Secondly the factory doesnt put backing plates on thru-hulls for a reason. The hull of a catamaran has a ton of flex, putting a rigid thick backing plate is either going to cause elongation of the thru-hull or a leak over time. Something like a thin fiberglass plate glassed to the inside of the hull to fair is fine, anything more then that or something you are going to 'goop' in is a bad idea.

There is nothing pulling on the outside of the thru hull why does it need a backing plate?

"Upgrading" to a groco thru-hull is equally a bad idea. You are doing it in an effort to stop corrosion of the thru hull, in doing so you are tossing a high copper content bronze anode(20 of them) to the underside of a boat that you know better then to put copper paint on the bottom of. So in 'upgrading' congrats you created a ton of galvanic action and just damaged your sail drives.

———cut of a ramble regarding a slush box———-

If you are going to stick with just normal thru-hulls i prefer a simple nylon thruhull to a barb fitting. From the barb fitting inside, place a short clear re-enforced hose that leads to a barbed on each end Stainless ball valve mounted to a bilge board, that then leads to wherever. Double clamp everything. place a wood plug tied to the thru-hull at each location according to the size of the thru-hull. This stops galvanic action, maintains hull integrity, gives the most flexible type of fitting, gives a sight glass for flow to every thru-hull, removes the need to have a backing plate in the first place as you just move the ball valve off of the thru-hull itself to a much stronger bulkhead.

Not only does your SD-60 manual say to avoid using copper paint they also state to consult a marine electrical engineer before changing the material composition of your propeller. People can do whatever they like of course. I spend every day in/on/around catamarans, I sell them, I outfit them. I went to school for a long time to specialize in this field. I have patents in this field directly related. Im strictly sharing information based on real world knowledge and problem solving many of the common issues found on these boats. If you put anything copper on the bottom of these boats you are going to have internal issues with your sail drive sooner than later. You dont have to believe that, but when you run into issues I usually try to keep at least 4 SD-60 units in stock at all times. Happy to price them for you in a few years when you are ready.

Not only is SOME of the things this guy says wrong, I think just about every single point he is making is just plain WRONG. I wonder what school he went to "for a long time." His description of what he thinks is a "proper" thru-hull and seacock would be funny, it it wasn't so dangerously stupid.

fstbttms · 06-10-2023, 17:37

“20 holes in the bottom of your boat.” 🤣

CarlF · 06-10-2023, 21:30

If your saildrives are corroding, you need to find out why. Get this test kit: https://www.boatzincs.com/corrosion-...electrode.html

I agree the Groco thruhull is no problem if properly installed. They are great seacocks. But why not switch to the Forespar OEM 93 Series seacocks? Over 20 years of using them I’ve found they last longer, require less maintenance, and work more smoothly than Groco bronze. They pass the ABYC test for side loading.