How to thickness vertical columns in situ (whilst on a choppy swing mooring)

Hi all. Hoping someone can give me some advice here. I need to rebuild the mast support structure on my boat, which requires thicknessing these vertical columns flush with the perpendicular plywood locker faces, behind them, so I can sister them with reasonably beefy new columns, running from ring beam, all the way down to the keel
(see pictures, taken from front and rear, which for some reason all need to be rotated 90° clockwise for correct orientation, despite them having correct orientation on my phone!)

The cross section of the columns is around 120mm, so too thick to use a top bearing flush trim bit on a router,(assuming I could construct a jig to keep the cut in place, on a moving boat!). I was thinking I might be able to do it in two passes (using bottom bearing flush trim bit on a jig from the non locker side, for the first pass, and a top bearing flush trim bit along the face of the locker, for the second); or perhaps I could rout a dado of the correct depth down the middle of the column side, to run the bearing along, and use a top bearing flush trim bit for both passes, also from the non locker side.

Any ideas/advice here? Im not the most experienced with a router, and i cant use a plane well, either....theres no way the columns can be removed nondestructively, which would have made life a lot easier...

Many thanks.

Why do you have to remove material? I would look at removing the plywood panels and reinforcing the beams on the inside.
Why do you need to rebuild the support? They look sound.

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Originally Posted by NCArcher

Why do you have to remove material? I would look at removing the plywood panels and reinforcing the beams on the inside.
Why do you need to rebuild the support? They look sound.

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They are sound. But they are not connected to the keel, and are deforming/collapsing the thin, totally inadequate fibreglass liner they ARE resting on (see attached photos), with the result that the mast is collapsing through the coach roof whenever the shrouds are tensioned, despite the fact that the mastfoot itself has already been reinforced at the cabin roof, at some point (these columns were added at the same time as this inadequate repair). I intend to now sister them with new columns, directly connecting the ring beam under the mastfoot, to a new bulkhead I have built up from the keel. I could do this with the columns, as they are, but the width of new sistered column required will protrude into the companionway, and be too prohibitive to freedom of movement through to the forepeak. So I'm trying to minimize this problem, by taking as much material off as I can, before adding the new sistered columns, which will allow less protrusion into the companionway, and a thicker, stronger sistered column. The plywood panels are similarly only resting on a deforming, thin, fiberglass liner, (the same one these columns are resting on) so reinforcing there will not help.

It's only an additional 20mm on each side, but since the new sistered columns are likely to be 100mm square, I'd very much prefer a combined 160mm protrusion into the companionway, than a combined 200mm protrusion (it actually makes a big difference in such a narrow space: it's only a 25ft boat, with an 7.8ft beam). That's only about a 55cm gap between those columns, as they are...so I've got a choice between an impassable 350mm gap, or a barely passable 390mm gap....I'm going for the latter.

I hope I haven’t confused the issue thanks to the nautical jargon, but what I’d be doing is using a regular router bit, not a flush trim. I’d use the same method people use for levelling timber slabs with a router - for tabletops etc.

I’d use an improvised wide router base, and fix running rails on both the locker side and the non-locker side. The router base bears on the running rails, and because of its width you can move the router side to side and all around without danger of it slipping off a rail. Start with the bit projection shallow, then move it down progressively till the desired depth, which I understand to be the white painted surface.

You won’t be able to get right to the the top or bottom, I’d just do those with a carving gouge, then finish off with a flat chisel and sandpaper. There are probably more mechanical ways, but that’s quick for me.

Thanks. Yep I had wondered about this method too. So plus 1 for that idea! A jig shouldn't be too hard to make, which would keep the bit centred on the column...would just take a bit of thought as to how to secure it :-) not as quick as the flush trim method, but maybe safer?

I’d probably get a piece of 19mm thick timber and use that for the runner on the locker side - just hot-melt glued to the face.

The runner on the other side likewise just a piece of wider 19mm timber. Maybe hot- melt glued on, but maybe with a couple of discrete screws just in case.