Backgound
On a wooden fence I have several posts that are water damaged at ground level and the fence isn't holding strong anymore. The remainder of the fence is in very good condition.
I believe I can stabilise the fence with the following approach that will be cheaper to do than getting the each of the entire fence poles replaced and cemented in.
Proposed Idea
I am considering using 750mm x 75mm (2.4ft x 0.24ft) galvanised steel retaining wall posts and securing them to the existing wooden posts with galvinsed bolts (100mm x 10mm) with spring washers.
This way the 100mm bolt would be through each fence post, about 300mm (1ft) ( above ground level. 400mm (1.4ft) of the steel would be under ground and cemented in.
Question
Will this approach keep the fence strong enough? Or will the steel + bolt attached to the wood be a weakness point after a short period of time?
Is 300mm (1ft) of steel support above ground, 450mm (1.4ft) below ground enough to support the 2000mm (6.5ft) of fence post + fence above ground.
Costs
The cost of the steel is about $10 more per brand new wooden fence pole, but the effort to attach the steel is less than replacing the entire post.
Best Answer
The best fix is to replace the posts with new posts correctly set in concrete. if you want very long-life use concrete posts.
I can't say if your steel post will be strong enough, because I don't know how strong you need it to be.
sheet-steel buttresses are used to reinforce rotting power poles, so your plan is not totally unprecedented.
You will probably need two bolts 500mm apart to hold the post well. Given that the ground is likely to be soft near where the post is rotten I would go at-least half the height of the fence underground.
TLDR: disconnect the rails set the fence panels aside then rent a post hole digger and replace the posts, back-fill the holes with "instant concrete", dress the top of these foundations to slope away from the post.