Compression Post
A compression post is used to transfer the rigging and mast load from the deck to the keel. It is very similar to a shear wall, however instead of a shear plate a compression post shears the mudsill and wood fibers inside it. This allows for shear forces to be transmitted through the compression post without causing severe damage.
These posts were usually stepped down from the deck, and then bolted to the lower stringers. Unfortunately, the deck was not designed to bear such loads and most of the time this led to cracks in the fiberglass structure. In many cases the wood also rotted, which led to the compression post sinking into the water or even dropping out of the deck. This was very common on older Jeanneau models, including the 43DS and 39i.
One way to fix this problem is to replace the rotten wood and cement around the compression post. This can be a very difficult job, especially in the cramped confines of these boats. Alternatively, you could remove the compression post and then drill through the deck and through the mudsill to attach the new shear wall. This would allow you to use the shear wall as a step down to the keel and also make it easier to get the mast in and out of the boat.
What Is a Compression Post?
The compression post is designed to prevent the mudsill from being crushed by shear forces applied through the end posts during seismic or severe wind events. It has a plate 62 mounted to the mudsill and a plurality of cylinders 64 positioned on the plate. The cylinders have a length selected to conform to the thickness of the mudsill. Each of the cylinders also has an extended portion positioned perpendicularly to the plate.
The plate 62 is provided with a pair of holes 66 through which screws or nails may be inserted to secure the compression post to the end post. The extended portion of each cylinder is pressed fit into a hole in the plate 62. A variety of plate sizes and cylinder lengths can be stocked to accommodate a wide range of different end post thicknesses. The shear wall is then fastened to the end post using a bolt or screw secured with a hold down connector (e.g., a Leek structural fastener).
The shear wall also includes a shear plate positioned between the end post and the mudsill to reduce the force transferred to the mudsill from the shear wall. This arrangement minimizes the amount of shear that is transferred to the mudsill and allows for an extended period of time for the mudsill to recover from the shear force. It also provides a more attractive appearance than the conventional “U” shaped post saddles currently available on the market. This is particularly important for boats where the appearance is of significant importance.
