Outboard spacer for transom bang plate repair

[GW Miami]

I need some feedback on my attempt to repair the transom bang plate on my Sailfish.

I intend to replace the existing very thin aluminum 1.25 inch x 1.25 inch angle iron, with a larger beefier .125 inch thick, 1.75 inch x 1.75 inch aluminum angle iron, after ensuring that the area underneath the removed bang plate is prepped accordingly with epoxy / 5200 / other appropriate repairs are made. Note that the transom is solid and not an issue at the moment, but if I leave this any longer it will definitely become one.

That said, if I go with the thicker and wider Aluminum angle iron material, I’ll need to create a spacer for the twin outboards to be mounted onto, in order to avoid the soon to be installed new transom bang plate thickness.

With that in mind, I have drawn up four (4) options for my metal guy to fabricate from .125 inch thick aluminum sheeting.

The options range from one (1) single entire sheet with the full length and width of the twin outboards included, all the way to having two (2) separate sheets fabricated for each outboard mount location, and some in between options.

I have attached a picture of the options I have drafted up, for you guys to review and comment on. Note that the blue line represents the bottom paint water line.

Thanks in advance for your help with this request.

 

[Sdfish]

GW - have you considered a stainless steel bang plate cover? That's how I had mine done and do not regret it.

 

[GW Miami]

That’s my intent, but was thinking of going with aluminum for the angle iron, in order to match the sheet of aluminum spacer I’ll need to account for the added Bang plate thickness, to allow for the outboards to sit flush.

On my Sailfish, I only need a straight 5 feet of angle iron, as it’s a closed transom with no cut out.

Yours looks great. What type of spacer did you use? And if you don’t mind me asking, what was the cost?

Thanks for the input

 

[Sdfish]

I did not use a spacer, didnt think of it at the time unfortunately. The cost of the custom bracket was $400. I also had the the gap sealed with thicken west systems epoxy.

 

[Greyduk]

GW, There are several threads that you will find in a search for " bang plate " where the posters used 1/4 inch starboard for the outboard spacers. To me they made a good choice instead of aluminum because there would be no galvanic reaction like you would have with the aluminum being under water. Just a thought.

 

[dainlaroche]

I did just as you described and bought the same thickness aluminum plate as the angle iron. You can see pics here: https://www.greatgrady.com/threads/transom-cap-bang-plate-replacement.29251/page-2#post-187024

 

[GW Miami]

To Greyduk, I never thought about the galvanic issues, as I’ll be on a lift. But that’s a good catch. I was thinking more along the lines of avoiding the use of disimilar metals. I’m assuming the outboards are made of aluminum, but I’m not sure. Looks like a little more research is needed. Thanks

To daillaroche, I have searched this site up and down, and never noticed your post. Thanks for responding. Exactly what I wanted to see. Now I have more confidence in what I was envisioning. I have twins, so I still need to decide if I keep the plate in one large sheet, or split it into two separate plates, or the other 2 contoured options

Great feedback.

Thanks to all.

 

[GW Miami]

I did not use a spacer, didnt think of it at the time unfortunately. The cost of the custom bracket was $400. I also had the the gap sealed with thicken west systems epoxy.

Greatly appreciate the cost input, as it’s tough to guage what’s what when you slap the word Marine in front of anything. I’ve had good luck with a local metal fab shop, but I try too avoid the word Boat / Marine at all costs. Thanks again.

 

[wspitler]

Make sure to check what series aluminum is used and the heat treat. Some are highly susceptible to corrosion and others are not. 6000 series could be a good choice as I remember it. In general stronger alloy seem to be more corrosive.

 

[dainlaroche]

Make sure to check what series aluminum is used and the heat treat. Some are highly susceptible to corrosion and others are not. 6000 series could be a good choice as I remember it. In general stronger alloy seem to be more corrosive.

That is a good point. The spacer plate on mine is holding up well but the transom cap is already showing some small spots of corrosion and seems to be an inferior grade of aluminum.

 

[GW Miami]

This is the short and sweet version of what’s recommended:

Quick Reference Chart – Choosing an Aluminum Grade

	Formability or Workability	Corrosion Resistance
Alloy 5052	Good	Excellent
Alloy 6061	Good	Excellent
Alloy 6063	Good	Good
Alloy 7075	Poor	Average

This is the detailed long version:

https://www.asminternational.org/documents/10192/3456792/06787G_Sample.pdf/c4151917-99fc-46e8-a310-d5578d0af160

6061 or 6063 it is.

Thanks for the heads up Wspitler.

 

[wspitler]

This is the short and sweet version of what’s recommended:

Quick Reference Chart – Choosing an Aluminum Grade

	Formability or Workability	Corrosion Resistance
Alloy 5052	Good	Excellent
Alloy 6061	Good	Excellent
Alloy 6063	Good	Good
Alloy 7075	Poor	Average

This is the detailed long version:

https://www.asminternational.org/documents/10192/3456792/06787G_Sample.pdf/c4151917-99fc-46e8-a310-d5578d0af160

6061 or 6063 it is.

Thanks for the heads up Wspitler.

6061 T6 is readily available. T6 is a heat treat. T0 is used in aviation and is less corrosive but is more expensive and somewhat rare.

 

[Jzeno186]

I had a stainless steel bang plate made by a metal shop on the spot in 10 minutes. It’s also worth it to fiberglass over the seam on the transom while the motor is off

 

[Jzeno186]

I had a stainless steel bang plate made by a metal shop on the spot in 10 minutes. It’s also worth it to fiberglass over the seam on the transom while the motor is off

 

[GW Miami]

Looking at the stainless steel poli

I had a stainless steel bang plate made by a metal shop on the spot in 10 minutes. It’s also worth it to fiberglass over the seam on the transom while the motor is off

I’ve done more research and confirmed the outboards are made of aluminum, so the aluminum spacer can remain as is and there’s no issue with disimilar metals between the outboards and spacer plates. Also choosing 6061 will help with corrosion issues.

As for the bang plate replacement, after seeing your recent post and the pictures of the polished stainless steel bang plate in Sdfish’s post, I want to use polished stainless steel angle iron. I’d also love to have the transom glassed over, but my approach to this repair doesn’t leave room / time for it at the moment. I intend to remove the outboards just slightly off of the transom, but not derig them at all. I have access to two appropriately sized chain hoists on a gantry style beam crane to just lift off the outboards, where I would be getting enough clearance to remove the existing bang plate and make the epoxy / sealant repairs to the seam between the cap and transom hull, then subsequently insert the bang plate / spacer plates and remount the outboards. Glassing over the transom with this approach / scenario doesn’t sound like a good option. Does this sound feasible?

That said, I’m now on the hunt to find the best combination of thickness for the polished stainless steel bang plate and the aluminum spacer plates, to ensure they don’t touch each other and avoid disimilar metal issues.

So far I’ve found I can get the polished stainless steel angle iron in a .120 inch thickness and the aluminum plate in a .190 inch thickness. That leaves me about 1/16th of an inch in clearance. If that’s not enough clearance, the next size aluminum plate I can find is a 1/4 inch In thickness, which might be a bit too much. Any thoughts on these thicknesses?

Gonna make a decision one way or the other and pull the trigger by this weekend.

Again, thanks to all for the responses and input received. They definitely helped steer me in the right direction and will make a better final product for sure.

 

[Jzeno186]

I would defiantly recommend at least sealing the seam with 5200. Such a crappy design no idea why they wouldn’t glass that over. I also noticed that the nuts on the inside of the transom (bottom 2) were starting to pull through the gelocoat. I added a backing plate while the motor was off .

 

[GW Miami]

I would defiantly recommend at least sealing the seam with 5200. Such a crappy design no idea why they wouldn’t glass that over. I also noticed that the nuts on the inside of the transom (bottom 2) were starting to pull through the gelocoat. I added a backing plate while the motor was off .

I have that in mind as well. I have already purchased a pair of TH Marine backing plates for the top set of bolts, with bolt holes spaced at 12-7/8 inch on center.

In addition I’m having a pair of 1/2 inch aluminum backing plates made up for the bottom bolts, with bolt holes spaced at 9-7/8 inch on center, as they are hidden under deck. So no need to pay for the TH Marine backing plate eye candy mark up, for something that’s not visible.

It’s amazing how far down the rabbit hole one has to go, to get things done right.

 

[Jzeno186]

This is what i used

 

[Jzeno186]

this was the whole job

 

[GW Miami]

Thanks for posting pictures. It definitely helps with the project specifics and just as important, to build the confidence needed to take on the task / challenge.

Greatly appreciated.