Mac 25 Compression Post

[patrickfarley]

I noticed that my compression post was a little loose. It will wiggle with the mast down.
I then looked under the fiberglass support for the compression post and found that the compression post is sitting on the fiberglass with no support under the fiberglass. Is this normal? I am trying to describe the area above and forward of the pivot bolt and directly below the compression post.
Can anyone post a picture of what they have?
I will post a picture when it is no longer freezing here.

 

[StinkBug]

I'll try to sneak my head into mine tonight or tomorrow and check. Send me a PM if I forget.

 

[Fred Villiard]

There is no support under the post, I cut a piece of 3/8 thick G10 to fit over the top of the trunk and bedded it with 5200, I didn't want it to come loose.

 

[THECUSCUS]

The bottom of the comp post sits on part of the interior liner, which rests on top of the keel trunk. I wouldn't necessarily worry about the comp post wiggling with the mast down. Without the compressive forces of your shrouds , the deck relaxes abit upwards. I would worry if it wiggles when the mast is up and shrouds set to their proper tension.

 

[Fred Villiard]

There is no support under the post.

I cut a piece of 3/8 G10 to fit the top, and glued it down with 5200, didn't want it to come loose. Put a L bracket, like on the top, on the bottom of the post to keep it in place.

 

[Piotr]

The bottom of the comp post sits on part of the interior liner, which rests on top of the keel trunk. I wouldn't necessarily worry about the comp post wiggling with the mast down. Without the compressive forces of your shrouds , the deck relaxes abit upwards. I would worry if it wiggles when the mast is up and shrouds set to their proper tension.

What The Cuscus said...

 

[patrickfarley]

Thank you.

I am still going to post a picture when it warms up this weekend. The liner under the post does not sit directly on the keel trunk.

 

[Doc_holiday]

Its never going to warm up. Have another winter advisory again tonight here in Va. This ain't the deal I made when I moved south. Guess I need to reevaluate my climate requirements.

 

[centerline]

the post can be shimmed to tighten it up if you dont like it as it is... there is no perfect length as each hull was slightly different when assembled.
the posts were cut to fit snugly, and the hull and liner settled a bit over time to allow the slack like you have... not all of the boats relaxed that much.

so, again, there is no perfect length for the compression post, and you can jack the cabin top up an inch and either shim the post or cut a new post for it... and adjust the rigging afterwards the accommodate the higher tabernacle...

I used an eighth inch stainless steel support plate above AND directly below the compression post for more square inches of support. the upper support shim is thru bolted to the tabernacle and acts as a backing plate, and the lower is held in place by two small screws into the fiberglass, which will keep it from falling out if it should ever get loose.

I did NOT cut the post to accommodate the shims, but just added the shims and readjusted the rigging afterwards....

I did this AFTER I found the tabernacle had NO backing plate to keep it from tearing out of the cabin top when using the mast raising system.....(the washerless nuts/bolts pulled thru) it wont tear out now without some abnormally extreme load, and taking a sizable piece of decking with it

 

[patrickfarley]

the post can be shimmed to tighten it up if you dont like it as it is... there is no perfect length as each hull was slightly different when assembled.
the posts were cut to fit snugly, and the hull and liner settled a bit over time to allow the slack like you have... not all of the boats relaxed that much.

so, again, there is no perfect length for the compression post, and you can jack the cabin top up an inch and either shim the post or cut a new post for it... and adjust the rigging afterwards the accommodate the higher tabernacle...

I used an eighth inch stainless steel support plate above AND directly below the compression post for more square inches of support. the upper support shim is thru bolted to the tabernacle and acts as a backing plate, and the lower is held in place by two small screws into the fiberglass, which will keep it from falling out if it should ever get loose.

I did NOT cut the post to accommodate the shims, but just added the shims and readjusted the rigging afterwards....

I did this AFTER I found the tabernacle had NO backing plate to keep it from tearing out of the cabin top when using the mast raising system.....(the washerless nuts/bolts pulled thru) it wont tear out now without some abnormally extreme load, and taking a sizable piece of decking with it

I like your idea. Even when I raise the mast by just lifting it up I put a lot of upward force on the mast base.

 

[THECUSCUS]

For the same reasons I had the tabernacle enlarged so it would be the same size as my backing plate, made two L brackets to thru bolt the post to the plate. It isn't moving!

 

[centerline]

I like your idea. Even when I raise the mast by just lifting it up I put a lot of upward force on the mast base.

where the bottom of the compression post sets on the liner, between the liner and the keel trunk, there was supposed to be a pad built up in there...
depending on the size of the gap, it was folded up resin saturated cloth, or if it was a really close fit, maybe nothing at all.

if you have nothing at all in there, its is still possible to take a bottle jack and jack the liner up a bit away from the keel trunk and put some bedding in there.... it will add better overall support.
figure out about how much gap you have, then just jack it apart and lay several layers of resin coated cloth in there to build it up thick enough so when you let the liner down onto it, it makes firm contact and beds in.....

then commence to shimming the post....

I think with any light built boat, everything beds in and finds its place over the years with the slack that is either available, or is created by the stretching and working of the fiberglass.... once this process has settled, the hull can be, and should be, re-tensioned properly by adding a shim to the compression post.

before jacking the cabin top you need to remember to take the bulkheads loose from the roof, so you dont rip the screws out as the roof goes up.... and you may have to move or shim them when you replace the screws IF you get the compression post too long when you shim it....

and although there would be very little reason to jack more than an inch or so, theoretically, for structural design, you cant really get the compression post too long, as when the cabintop is farther away from the hull, (more space between the two) it makes a stronger shape.... you are only limited by the existing interior fittings and the cut of the existing rigging..... DISCLAIMER; you must understand though that you cannot jack it so much that you will ever be able to get standing headroom inside the cabin:cry:, but up to 2.5 maybe 3 inches higher is do-able, but only if you were going to refit the entire boat.

I jacked my mac21 cabin top 2 inches when I was working on the compression post and tabernacle... but let it down with only a half inch worth of shims added to the post. the broader area of the 25 would allow more.

after doing so, you can even tell by walking on the cabintop WITHOUT the mast in it, that it is a much tighter hull....