Mast foot repair - Need overkill layup schedule

MiddayGun

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As some of you who already read my other post may know, I'm re-rigging my boat.

Its a thankless task that I'll never make back in resale value, but that's boats.
Anyway, so the new mast necessitates new mast base, so as that turned up I unbolted the old mast compression post from the deck & unscrewed the 4 screws that hold into the floor that it sits on in the bilge.

When removing the screws I noticed some mushy wood clinging to the ends of them, so I decided to open them up with a larger drill bit.

IMG_20200301_122353.jpg IMG_20200301_114353.jpg

IMG_20200301_122639.jpg IMG_20200301_122812.jpg

There's a distinctly 'sweet' smell to the wood I've drilled out & its definitely damp, poking around with a screwdriver it still seems pretty firm in there and there's no evidence of the floor being crushed, so I guess I have 2 options:~
- Fill oversize hole with epoxy and re-drill screw holes. Worry about it later on and just sail it.
- Cut out the top of the floor, dig out the block of wood, replace with new epoxy sealed wood & re-laminate.

If I go with option 2, can anyone recommend an 'overkill' layup schedule for epoxy that will be stronger than the original?

This is the only info I have on the existing layup:

image.png

Boat is 27 feet, about 2.6m tonnes (metric) and carries about 450square feet (ish) of sail.

 

SloopJonB

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If it was me I'd keep drilling bigger & bigger hoping to get to dry wood.

If no joy then I'd fill the holes with high density filled epoxy and sail it - it's a 27' so the loads aren't that big.

Otherwise you have a big, ugly job to do - the kind that is best postponed if possible.

 

MiddayGun

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I drilled almost through to the hull. (masking tape on drill bit so I didn't go too far)
The wet looking stuff is from the FWD holes, the dry from the aft. (Though its all damp)

If I have to replace all the wood then it will be a real pig of a job as the wood goes under the molded in bunks.

 

casc27

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Penetrating epoxy? Um, no. SJB's suggestions make sense so, I'd start with something like that. If you do end up replacing the whole thing (a very big job) why does it need to be stronger than the original layup schedule? Other than the water ingress the original looks to be in good condition so it doesn't appear that it was not strong enough.

 

Crash

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I'm in the SJB camp as well.  I'd drill the holes fairly large, and backfill with epoxy.  Can you tell how deep the wet wood goes?  I.e. only the first inch if wood in the forward holes are wet?  Or is it wet to the bottom of the drill bit? Heck, it nothing else, you've now made a bunch (4?  more?) of solid Epoxy pillars to help support the forces of the compression post.  If you really want to know how far horizontally the wet wood extends, you can drill smaller holes in ever expanding concentric circles till you get to dry wood.  Those small holes can then be backfilled with epoxy if you decide you can live with it.

The challenge to cutting off the top of the stringer and digging out the wet wood, it that any replacement wood should be scarfed into the original stringer on each side if the stringer is "structural" wood, which I suspect it is.  Can you tell from the shavings what kind of wood you have there?  I'm assuming it's not balsa...

It looks to me like the "flat plate" portion that the step sits on was added after, and the fiberglass covering the stringer was cut post construction to fit it?  The Flat portion seems to be a different color, and there is sealant around the edges. 

 

Zonker

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It almost looks like mat layup under the mast step. But that might imply that the 4 layers of UD were cut by the mast step. Nobody would do that would they?

For a 27' boat I'm with JonB on this one.

It's survived sailing with the damp wood so far. 

 

MiddayGun

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It almost looks like mat layup under the mast step. But that might imply that the 4 layers of UD were cut by the mast step. Nobody would do that would they?

For a 27' boat I'm with JonB on this one.

It's survived sailing with the damp wood so far. 
So from what I know of the history of the boat, when the mast was lost the first time (or maybe in a separate incident?) the original aluminum tubed compression post was damaged.
So the compression post that's on now (which is a seriously strong & heavy piece of kit) was built to replace the original. I can only guess that perhaps some of the step was cut away at this time. They almost certainly didn't have the drawings that I have as I had to ask for them from the designer especially.
Otherwise there's not much point to UD rovings across the top of a beam if you cut them in the middle.
 

@Crash I think the wood is oak. Its definitely structural, the rest of the floors are foam cored, obviously this has the timber to take the mast compression loads.

I'll take the suggestions of what most have said & sail it for this year, at the least with epoxy filled holes there is no chance of water getting in that way. And I'll just keep a close eye on it over the coming years.

The problem with this form of construction, molded liner that's then bonded into the hull mean that any water that does manage to seep in there (and I've found quite a few places it can) has no real way to escape.

(I said stronger than the original to compensate for the fact that I'd be doing the work as an enthusiastic amateur rather than a pro who does this day in day out.)

 
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SloopJonB

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The problem with this form of construction, molded liner that's then bonded into the hull mean that any water that does manage to seep in there (and I've found quite a few places it can) has no real way to escape.
Living in a rain forest taught me long ago that you can never keep water entirely out.

You can only trap it in.

 

Zonker

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Sounds like a reasonable plan. Mast compression loads are VERY ROUGHLY = the weight of the boat on a boat like this.  Say 7000 lbs ? = 31,000 N

White Oak cross grain compressive strength = 7400 kPA = 7.4 MPa

So required area to resist this = 31000 N / 7.4 KPa = 4189 mm2.  So if your mast step is 75mm in the fore/aft dimension, you only need 55mm in transverse direction to resist crushing loads (with no safety factor)

Therefore, a few holes potted with epoxy ain't gonna matter. You don't need much oak that is intact. Don't forget the glass on top helps a bit too (if there is UD still there)

 

Gouvernail

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I would fill the holes with epoxy and stuff strings of pre-wetted  glass in there until i couldn’t stuff any more in there 

considering it is apart and more available / accessible than ever before, I would lay on a bunch of layers of glass. You don’t have anything to lose by sanding the top and walls and laying on ten layers of mat. The section would spread the load and you would no longer give a rat’s ass if the wood totally composted. 
so what if you spend ten bucks on materials?? Never worrying again has to be worth $10 or even $12
 

 
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casc27

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(I said stronger than the original to compensate for the fact that I'd be doing the work as an enthusiastic amateur rather than a pro who does this day in day out.)
The composite work doesn't know if you are an amateur or a pro. It's not that complicated or difficult to do it right. Even Gouv can do it. (Sorry Gouv, couldn't help myself. Actually, Gouv has offered heaps of solid, practical advice to get people sailing again on here.) And there is plenty of information easily available to help. Surprisingly quite a good amount of that help and advice can be found right here. (Trying sooo hard not to direct you to the inexperienced couple catamaran thread to learn about boat repair...No, I won't do it. This OP seems like an okay guy...)

The Gougeon books, pamphlets etc. are one good place to start. If you truly have zero experience with this kind of thing, it will be worth the extra cost and time to do some practice before diving into your actual repairs. The worst work results come from people buying just barely enough material for the repair they need to do and then diving right in and trying to make that work. And there is plenty of poor quality work done by "professionals." (Just saying.)

 

SloopJonB

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27's advice to practice is good advice. Laminating up a few small pieces on a bench will quickly learn ya how to wet out and roll out and squeegee out a lamination without having to later grind out any mistakes.

Keep in mind that laminating fiberglass is regarded as semi-skilled labour (except high tech stuff).

BITD of the SoCal and Tampa Bay sailboat industry most of the laminating was done by pickup labour who were given a few minutes to a few hours training.

Most of those boats are still floating.

I regard basic laminating as being easier than hanging wallpaper - you don't have to match seams so perfectly.

Laminating cored construction is little more difficult than making a peanut butter sandwich.

 

MiddayGun

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Cheers guys.

This isn't quite my first (laminating) rodeo, I actually just finished a chain-plate relocation project which involved a good bit of fibreglassing.
Here :)

However working a drawing & deciding how many layers & the best approach to make myself are two different things. Hence the question about a good layup schedule.
I'll stick with the plan of seeing how it holds up over the year, I like Govs suggestion, but if I add any more thickness to the top of the foot then i'll have to shorten the compression post as its a tight fit as is.

 
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Gouvernail

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I just checked out your thread - you should be here answering questions, not asking them. :D
Dunno... anybody who uses the term “layup schedule” when describing a simple repair job reeks of what Country folks call “all hat and no cattle.”

 

MiddayGun

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Dunno... anybody who uses the term “layup schedule” when describing a simple repair job reeks of what Country folks call “all hat and no cattle.”
Doing it is simple enough, knowing what constitutes as strong enough takes experience.
I'd rather ask and look stupid than take my chances with my own uneducated guess. Pretty sure that's the opposite of 'all hat & no cattle'.

 




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