Repairing Cav Plate Crack - remove lower unti?

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IronPrice

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I went a bit too shallow this weekend and the prop picked up a stone or similar and cracked the cavitation plate. So now I have a little crack and dent in the cav plate.

My plan is to tap it back into shape against a flat block, then prep and TIG weld the crack.

My question is: Do I need to remove the lower unit to weld? Is there a risk to electronic components of the engine when welding? There aren't many electronics but there is an igniton unit, sensors etc.
 
If you disconnect the battery and the harness, there should be no problem with the electronics.
 
Gonzo is wise, listen to him. If you don’t, the ground seeking will occur through the datalines and cook your ECU and any gateways and probably the voltage regulator..

Nothing can be connected …
 
You can stuff wet rag around the propshaft, but generally it doesn't matter. If you remove the prop and run it at idle in gear -exercise care- you can eyeball the centre drilling in the end of the shaft and see if it is bent. The stingray style foils often cause damage on cavvy plates, but if it sat on something unnoticed like with a falling tide or a boat wake, the propshaft could be bent as well and you can either replace/straighten it now, or rebuild the bag of gears later if it is.

I have had almost zilch to do with welding repairs on computer controlled outboards, but would imagine that the brain could fry easily. Maybe look for warnings and procedures specific to your outboard model if it has engine management, and at a minimum disconnect control harness and battery — both + & - leads as Gonzo said. Be aware of your earth placement so the welding return current is not jumping across outboard components like needle rollers.

If you list the year/model of the motor with photos of the damage you possibly might get more results.
 
Another option, avoiding this whole issue, would be to drill a small hole at the end of the crack.
You may need dye to locate the end.
Then monitor for propagation.
Pictures would help.
 
HelmutSheina said:
You can stuff wet rag around the propshaft, but generally it doesn't matter. If you remove the prop and run it at idle in gear -exercise care- you can eyeball the centre drilling in the end of the shaft and see if it is bent. The stingray style foils often cause damage on cavvy plates, but if it sat on something unnoticed like with a falling tide or a boat wake, the propshaft could be bent as well and you can either replace/straighten it now, or rebuild the bag of gears later if it is.

I have had almost zilch to do with welding repairs on computer controlled outboards, but would imagine that the brain could fry easily. Maybe look for warnings and procedures specific to your outboard model if it has engine management, and at a minimum disconnect control harness and battery — both + & - leads as Gonzo said. Be aware of your earth placement so the welding return current is not jumping across outboard components like needle rollers.

If you list the year/model of the motor with photos of the damage you possibly might get more results.
Click to expand...

The outboard is a Johnson 70HP, carburetted two stoke. Manufactured in ~2001. I don't have any photos sorry. I keep the boat in storage some distance away.

The incident happened when I was surf-launching the boat at a pebble beach. I surmise that the prop picked up a pebble and it was forced between the tip of one of the blades and the cav plate. This caused an upward bend in part of the edge of in the cav plate and a small (12mm) crack. The damage is cosmetic only, but I want to sell the boat and want to repair and repaint (I have some official Johnson touch up paint to do that with).

Fortunately, the shaft is still straight (as measured with a run-out gauge).
 
Last edited:
IronPrice said:
The outboard is a Johnson 70HP, carburetted two stoke. As far as I know the only electronics are ignition control and a few simple sensors - temp, oil etc . Manufactured in ~2001. I don't have any photos sorry. I keep the boat in storage some distance away.

The incident happened when I was surf-launching the boat at a pebble beach. I surmise that the prop picked up a pebble and it was forced between the tip of one of the blades and the cav plate. This caused an upward bend in art of the edge of in the cav plate and a small (12mm) crack. The damage is cosmetic only, but I want to sell the boat and want to repair and repaint (I have some official Johnson touch up paint to do that with).

Fortunately, the shaft is still straight (as measured with a run-out gauge).
Click to expand...
 
gonzo said:
It is the round connector somewhere on top of the engine.
Click to expand...
The outboard is a Johnson 70HP, carburetted two stoke. Manufactured in ~2001. I don't think there is any central connector, just a few plugs and some single wire connections . However, if I disconnect all electrical connections to anything external to the outboard, I should be OK, right?
 
Last edited:
BlueBell said:
Another option, avoiding this whole issue, would be to drill a small hole at the end of the crack.
You may need dye to locate the end.
Then monitor for propagation.
Pictures would help.
Click to expand...
The crack isn't spreading, but I want to repair it - so I can sell the boat. All the kids have left home so we're downsizing.
 
Won't be a big deal on an old 70 Johnno, if you've been running it in shallows it might pay to give it an impeller anyway & weld it while it's off. If the crack is stable you might be able to use an aluminium epoxy repair putty like Devcon.
itwperformancepolymers.com

Devcon® Aluminum Putty (F) - ITW Performance Polymers

Devcon Aluminum Putty (F) is an aluminum-filled epoxy putty for repairing aluminum castings, machinery, & equipment Learn more here
itwperformancepolymers.com itwperformancepolymers.com
 
IronPrice said:
The outboard is a Johnson 70HP, carburetted two stoke. Manufactured in ~2001. I don't think there is any central connector, just a few plugs and some single wire connections . However, if I disconnect all electrical connections to anything external to the outboard, I should be OK, right?
Click to expand...
I thought it was a sterndrive.
 
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