Something like oakum

there has to be a gap; the beam slides all the way through the port hull to the star hull; there is silencing material to take up the gap; the beam is bolted through the socket and local BH with two 1/2"-13 tpi bolts that are 14" long...

absent a gap; the beam would not slide in or out

as it is; the clearances are so close we have to lubricate the silencing material to insert the beast

and also noteworthy; the beam section shown has no silencing; so t sits at the bottom of the socket and you see all the margin at just the top

 

Then I don't see your problem. If I understand it correctly you have the beam, then wrapped around it some "silencing material" (whatever that may be), then a lot of grease and then the socket. Are you afraid water will wick itself trough the greased material?

 

water will enter the socket through the gap; the silencing material is about 6" inside
since it is an aluminum beam in a glass socket; it would be subject to crevice corrosion

just need to know what sealant to use

 

Anything preventing the greased material to reach to the outside? Do you plan on disassembling often?
I would make the silencer to fill all the available gap (or fill the gap with something like greased oakum or felt) then use butyl tape under compression. Same way one seals chainplates trough the deck, a close fitting collar around the beam (plywood, G10, etc.) clamping a butyl tape gasket. If you don't want to screw into the hull, glue another plate around the socket to recieve the screws.

Another way would be to use a latex manchette under a sunbrella cover.

If you just use a sealant from a tube you have one of the following problems. Either the sealant attaches properly to both aluminium and fiberglass making dismantling extremly difficult, or it does not and seawater can enter. A sealant with good adhesive properties would be a solution if dismantling is not a priority but you must first clean both surfaces of any grease.

 

okay, this is what I am getting at..

if you use a sealant; removal becomes a bugger; the cat is designed for taking apart; how often? Say once every 5 years..

the silencer is not the solution; it is simply there to avoid the beam making bad noises in the socket

the installation lube is water based like ky, but electrical pull lube

Greased oakum sounds good...just need to be able to pull it out

 

So, 1/8" gap or less is cotton caulking? Then can I just use interlux above water seam compound or will it fall out?

The main goal of cotton would be to limit the depth of the sealant so it doesn't become impossible to remove...

 

You are not caulking anything with the cotton or oakum or felt. It's simply there as a spacer so you don't have to shove in a tube of sealant. Just filling dead space. The grease is to displace humid air so no condensation can happen. Vaseline or lanoline, not something soluble in water.
The sealant glues itself to the socket and beam. At 1/8 thickness it can be cut with a knife for removal. Choose something that has a chemical remover othervise you must sand beam and socket before reinstalling. If sanding is ok 5200 works.
I would prefer the butyl tape with compression plate myself, that can be inspected and replaced regularly.

 

Don't use cotton. It will exacerbate any crevice corrosion.
At 1/8th inch, you won't be able to push the sealant very far into the void.
A well stocked paint store will sell foam backing/packing rod. It is designed to be pressed into cracks prior to liquid caulking. Comes in a variety of diameters from 1/8 to 3 inches.

 

Can you explain the butyl tape and compression idea? Butyl tape is a real bugger to unstick unless it is more on the face. And the compression plate; is that a two piece bolt to the hull thing? Like moulded fiberglass that fits the section? The section also has the hardware tracks as it is a repurposed sailing mast.

5200 would be a nightmare and take like an hour to remove~so, no

 

this looks like the winner; now I just need to find a sealant...this stuff could sit in a maintenance locker, too as so light and it is really just a backer for the sealant

The hardware track is a point of failure and will like to catch water.

Any votes against me filling it with thickened epoxy? Weight and cost suck. But so does buying another full mast if a section fails.

on the other two beams the track is on the bottom

 

Here is an illustration for chainplates. A cure for leaky chainplates - Practical Boat Owner https://www.pbo.co.uk/practical-projects/a-cure-for-leaky-chainplates-27982

What does the designer actually specify? After all he should know best.

I would fill the masttrack with a fitting foam piece glued with 5200 or similar. Just sand it out of XPS.