Replacing Exhaust Pipes on Shamrock 20 Pilot House 1981

Ship

Well-Known Member
Supporting Captain
Dec 15, 2008
12,114
329
Atlantic City, NJ area
I went this route using 3" hose, ski boat mufflers and tips. Would not do it again since it did not seem to quiet it as much as I had hoped. I was however going to go with this 3rd setup when I was going to put a 5.8 from a late model ski boat with single 4" exhaust,these boats being super quiet. I never had trouble with corrosion either since I hung it on davits
That is what I ended up with but shorter runs of hose and I had a pair of rubber mufflers that looked crappy but did quiet it down a bit. The most sound dampening thing I did was rubber elbows on the exterior of the thru hulls.
 

Sailfree

New Member
Sep 13, 2022
4
0
Southeast Georgia USA
Thanks for all the personal knowledge and experience. It’s convinced me to choose vernatube or fiberglass. I’ll be calling the suppliers to get more information prior to ordering.

This forum is awesome!
 

pilarboat

Well-Known Member
Apr 19, 2016
370
37
Just my opinion, but some get too carried away with the concept of galvanic corrosion.


Requirements for Galvanic Corrosion:
In order for galvanic corrosion to occur, three elements are required.


  1. Two metals with different corrosion potentials
  2. Direct metal-to-metal electrical contact
  3. A conductive electrolyte solution (e.g. water) must connect the two metals on a regular basis. The electrolyte solution creates a “conductive path”. This could occur when there is regular immersion, condensation, rain, fog exposure or other sources of moisture that dampen and connect the two metals.
Mistress: You are right on "carried away." I had a very small part time business with an electrical engineer friend to investigate and solve galvanic corrosion problems. Almost every case involved problems that had gotten out of control, or ignored, or not noticed until it was too late. I like your description as it clearly defines galvanic corrosion, but not the solutions that can be tricky.

And then, another issue is the introduction of an electrical charge from a boat's batteries or an ac ground wire from outside the boat into the mix of dissimilar metals and the salt solution. Now, I'm getting too carried away again.

I'll keep check here if anyone has any bonding questions, I just might not have an answer, or maybe I will.

Just for grins: Does anyone know if there is continuity between a prop shaft and an engine block? OK, I'll answer so y'all don't have to go investigate; maybe and maybe not, but probably not.
 

Ship

Well-Known Member
Supporting Captain
Dec 15, 2008
12,114
329
Atlantic City, NJ area
"Just for grins: Does anyone know if there is continuity between a prop shaft and an engine block? OK, I'll answer so y'all don't have to go investigate; maybe and maybe not, but probably not"
If you have a drive saver, the answer is no, the maybe is because you never know if the bearing are in contact with the shaft. The old engines had a wiper or feather contactor mounted on the shaft just behind the coupling.
 

Mistress

Moderator
Staff member
Supporting Captain
Jan 22, 2005
7,530
245
San Jose, CA
Mistress: You are right on "carried away." I had a very small part time business with an electrical engineer friend to investigate and solve galvanic corrosion problems. Almost every case involved problems that had gotten out of control, or ignored, or not noticed until it was too late. I like your description as it clearly defines galvanic corrosion, but not the solutions that can be tricky.

And then, another issue is the introduction of an electrical charge from a boat's batteries or an ac ground wire from outside the boat into the mix of dissimilar metals and the salt solution. Now, I'm getting too carried away again.

I'll keep check here if anyone has any bonding questions, I just might not have an answer, or maybe I will.

Just for grins: Does anyone know if there is continuity between a prop shaft and an engine block? OK, I'll answer so y'all don't have to go investigate; maybe and maybe not, but probably not.
I'd certainly agree that a bad AC ground will f things up in a hurry...and not just your own boat in many cases. 2nd in line is picking up grounds at various locations vs. a buss block tied to a single location (not commenting on using the bond wire as a ground for electronics).

I'd have a hard time believing an SS exhaust clamp on rubber (wired or not) hose with metallic exhaust piping would be an issue. Maybe if the exhaust is routinely submerged. Rubber is a fine insulator.

As to continuity between block and shaft, my intuition would say seldom at the low resistance specs for "bonding".
 

pilarboat

Well-Known Member
Apr 19, 2016
370
37
I'd certainly agree that a bad AC ground will f things up in a hurry...and not just your own boat in many cases. 2nd in line is picking up grounds at various locations vs. a buss block tied to a single location (not commenting on using the bond wire as a ground for electronics).

I'd have a hard time believing an SS exhaust clamp on rubber (wired or not) hose with metallic exhaust piping would be an issue. Maybe if the exhaust is routinely submerged. Rubber is a fine insulator.

As to continuity between block and shaft, my intuition would say seldom at the low resistance specs for "bonding".
You are right about rubber being a good insulated, and it is. There is always a however. Ever see a hose clamp on wire hose. for no apparent reason, just break? It is usually because the wire hose is just old, or when cutting a wire hose, there is that end of the wire at the end of the hose that is not covered. The wire in a nose is not very far from the surface and wears through easily under the pressure of a hose clamp. This is why some custom wire hoses do not have any wire in the hose for 2 inches from the end of the hose. These hoses are very expensive to have custom manufactured and is the reason they are only used in production manufacturing of boats.

On the bonding of a shaft: There is no guarantee that there is a solid bond at all times between the shaft and engine because the bond is through moving gears. To this I say the solution is a collar zinc on the shaft, or better, a bonding brush riding on the shaft and connected to central bonding system. In a Shamrock, I would just use the collar zinc. This is really getting to far carried away. Ha.

The very best way to solve all this is to keep your boat insurance up to date and quit worrying about galvanic action(s). Nice talking to someone who understands the issues with this mishegoss.
 

sboat1

New Member
Dec 8, 2020
3
1
If you can ,keep 3in from the manifolds to the transom ,change the exhaust tips to 3in and flappers.Not sure why Shamrock choked down there exhaust to 2-1/2 in. as to ski boat mufflers just two installations side by side hear for your self ,they work pretty well seems to change the high frequency popping sound ,however they may be prohibitive space wise for 3in tubes.Changing to 3in is still loud but lowers the high end popping (lack of a better description) in the long run your exhaust valves will like the 3in better!for sure.
As to wire reinforced tube the tube was designed for crush resistance in very harsh applications ,such as mining, dredging,any where there is heavy industrial goings on.I use a soft tubing ,much easier to handle
As was mentioned ,the galvanized pipe really doesn't pose a big problem i have replaced them on some boats that were at least 25 + and the ends were it meets the hose,were rusted up.other than that could probably get another 10 out of them,plus there cheap!!If you go back with pipe use never seize at the connections and at the manifolds. later Sboat 1
 

coach

Well-Known Member
Supporting Captain
Mar 27, 2007
238
4
Gainesville, Georgia
I removed a supply store of Verna tubes and reducers from a 20’ walkthrough. Then changed the through hulls from 2.5” to 3”, added mufflers and 3” exhaust hose. Could not tell any difference in sound at any speed. If I did it again, I would change the 2.5” through hulls to 3” and go reinforced 3” exhaust hose from exhaust manifolds to transom. This reduces the number of below water line connections a simplifies everything. On offshore oil rigs we only use copper for wiring. Salt water and copper are short lived, as others have noted previously.
 

Mistress

Moderator
Staff member
Supporting Captain
Jan 22, 2005
7,530
245
San Jose, CA
If you can ,keep 3in from the manifolds to the transom ,change the exhaust tips to 3in and flappers.Not sure why Shamrock choked down there exhaust to 2-1/2 in. as to ski boat mufflers just two installations side by side hear for your self ,they work pretty well seems to change the high frequency popping sound ,however they may be prohibitive space wise for 3in tubes.Changing to 3in is still loud but lowers the high end popping (lack of a better description) in the long run your exhaust valves will like the 3in better!for sure.
As to wire reinforced tube the tube was designed for crush resistance in very harsh applications ,such as mining, dredging,any where there is heavy industrial goings on.I use a soft tubing ,much easier to handle
As was mentioned ,the galvanized pipe really doesn't pose a big problem i have replaced them on some boats that were at least 25 + and the ends were it meets the hose,were rusted up.other than that could probably get another 10 out of them,plus there cheap!!If you go back with pipe use never seize at the connections and at the manifolds. later Sboat 1
I don't know what wire reinforced rubber tubing was invented for and yes it adds crush resistance. But my understanding in the marine exhaust world is it avoids kinking (can pull a tighter radius without kinking) and resists sagging and panting. I recall some rule-of-thumb for the length of a given diameter of tubing to "suggest" wire reinforced be used.

Breaking up these lengths with pipe, mufflers, FRP tubes, etc. , changes the math.
 

Ship

Well-Known Member
Supporting Captain
Dec 15, 2008
12,114
329
Atlantic City, NJ area
If you can ,keep 3in from the manifolds to the transom ,change the exhaust tips to 3in and flappers.Not sure why Shamrock choked down there exhaust to 2-1/2 in. as to ski boat mufflers just two installations side by side hear for your self ,they work pretty well seems to change the high frequency popping sound ,however they may be prohibitive space wise for 3in tubes.Changing to 3in is still loud but lowers the high end popping (lack of a better description) in the long run your exhaust valves will like the 3in better!for sure.
As to wire reinforced tube the tube was designed for crush resistance in very harsh applications ,such as mining, dredging,any where there is heavy industrial goings on.I use a soft tubing ,much easier to handle
As was mentioned ,the galvanized pipe really doesn't pose a big problem i have replaced them on some boats that were at least 25 + and the ends were it meets the hose,were rusted up.other than that could probably get another 10 out of them,plus there cheap!!If you go back with pipe use never seize at the connections and at the manifolds. later Sboat 1
When the original engine was installed, a 302 was the probable engine and at that time valves would burn because of lack of back pressure in the exhaust and lead (most car engines had mufflers) . The unburnt gas carried away some excess heat. When unleaded gas was introduced, better steel and design of seats allowed for better dissipation. AND it (steel pipe) was cheap. As I've said before, 30 years says it wasn't a bad choice, it just ran it's course in time.
 


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